RU2793671C2 - Heat transfer wall of a heat exchanger and method for forming of the coating to intensify heat transfer of a heat transfer wall of a heat exchanger - Google Patents

Abstract

FIELD: heat exchange.

SUBSTANCE: group of inventions relates to a heat transfer wall of a heat exchanger and a method for forming of a coating to intensify heat transfer of a heat transfer wall of a heat exchanger. The base of the heat transfer wall of the heat exchanger is made of an aluminum-containing material and provided with said coating, which is a ceramic coating obtained by microarc oxidation of the surface of the said base. The surface layer of the coating is made in the form of a capillary-porous structure with vaporization centers, characterized by the geometric configuration of the porous framework, which ensures its complete permeability and the absence of deactivation of vaporization centers. The coating is made with a thickness at which the pores formed in it have are through pores. The pore size of the coating is comparable to the critical nucleation size at nucleate boiling or exceeds it with the surface layer porosity of not less than 6%, but not more than 12%, and the contact angle of the surface layer for water is less than 40 degrees.

EFFECT: increase in the intensity of heat transfer during boiling, stabilization of the heat transfer coefficient over time.

8 cl, 8 dwg, 1 tbl, 10 ex

Description

Technical solutions relate to the field of heat transfer, namely, to the structural elements of heat exchange equipment, and can be used in the development of heat exchange devices, in particular, for refrigeration and cryogenic installations with a spiral heat exchanger, used, for example, in the energy sector and related industries, in the chemical industry in large-scale natural gas liquefaction plants.

A heat transfer wall of a heat exchanger is known (description of the patent RU 2508497, published on February 27, 2014), containing a metal base of a curvilinear configuration, made with the possibility of forming a tube for pumping the coolant, the surface of which, characterized by a large radius of curvature of the bend and washed by the refrigerant, is equipped with fins made of wire wound in the form of a spiral along the pipe and fixed on the specified surface by soldering.

This analog cannot solve the technical problem of creating a spiral-wound heat exchanger that provides high efficiency, reduced weight and dimensions of the heat exchanger. Its shortcomings are due to the traditional approach of constructive implementation with a limited resource for increasing the intensification of heat transfer during boiling.

Known heat transfer wall of the heat exchanger (patent document TW 201438834, published 10/16/2014, abstract). The known wall is taken as the closest analogue. The wall is made as part of a metal base made of a material containing aluminum, or magnesium, or titanium, and a ceramic coating obtained by microarc oxidation of the surface of said base.

Given the closest analogue can not be achieved by the solution of the technical problem of creating a highly efficient spiral heat exchanger, providing high thermal efficiency, reducing the weight and dimensions of the heat exchanger.

The ceramic coating obtained by microarc oxidation of the surface of the metal base does not meet the required characteristics in order to ensure a high intensity of heat transfer. There is no information on the achieved parameters of heat transfer intensification.

A known method of forming a coating to intensify heat transfer on the heat transfer wall of the heat exchanger (description to the patent RU 2727406, published 21.07.2020). A patented method provides for the formation of a porous coating of nanoparticles, namely, the formation of a coating of aluminum oxide nanoparticles on a flat surface. For this purpose, a solution of nanoparticles is applied to the surface, namely, a colloidal solution of nanoparticles in water is prepared, then it is sprayed onto a flat surface at time intervals of 10 seconds, during the indicated time intervals the surface is heated to a temperature of 250 ° C and carried out evaporation of water droplets at atmospheric pressure in air.

The development of the above method is aimed at obtaining a layer with high capillary properties on the surface of the product. The developers indicate that the method can be used in the manufacture of products containing heat-exchange surfaces with micro- and nano-relief to intensify heat transfer.

However, this method does not provide a solution to the technical problem of creating a spiral heat exchanger that provides high efficiency, reduced weight and dimensions of the heat exchanger.

This method provides a coating that is susceptible to destruction. The coating does not have mechanical stability and maintain its integrity. The capillary-porous structure of the coating is not optimized in order to achieve a high intensity of heat transfer during nucleate boiling with a stabilized heat transfer coefficient.

A known method of forming a coating to enhance heat transfer on the heat transfer wall of the heat exchanger (patent document TW 201438834, published 10/16/2014, abstract). The above method is taken as the closest analogue. The method consists in the fact that a metal base made of a material containing aluminum, or magnesium, or titanium is immersed in an electrolyte, in which the surface is oxidized by means of microarc oxidation (MAO), forming a ceramic coating.

The disadvantages of this method include the lack of optimization of the implementation of the actions of the method, which prevents the achievement of the necessary characteristics of the formed coating in order to ensure a high intensity of heat transfer during nucleate boiling and stabilization of the heat transfer coefficient.

The development of the method is aimed at increasing the heat transfer capacity of the pipe. The resulting capillary-porous coating is able to intensify heat transfer during the flow of one- and two-phase media due to turbulence of the boundary layer, swirl of the flow, the action of capillary forces in it, an increase in the centers of vaporization and variable porosity.

However, the developed method does not provide a solution to the technical problem of creating a spiral-wound heat exchanger that provides high efficiency, reduced weight and dimensions of the heat exchanger.

The method provides a coating that is susceptible to destruction. The coating does not have mechanical stability and maintain its integrity. The capillary-porous structure of the coating is not optimized in order to achieve a high intensity of heat transfer during nucleate boiling with a stabilized heat transfer coefficient.

The development of the proposed technical solutions is aimed at overcoming the technical problem of creating a spiral-wound heat exchanger, providing high efficiency, reducing the weight and dimensions of the heat exchanger, by achieving a technical result.

The technical result is:

- increasing the intensity of heat transfer during boiling;

- stabilization of the heat transfer coefficient over time.

The technical result is achieved by a heat transfer wall of the heat exchanger containing a metal base, while the base is made of an aluminum-containing material and provided with a coating to intensify heat transfer of the heat transfer wall of the heat exchanger - a ceramic coating obtained by microarc oxidation of the surface of the specified base, the surface layer of the coating is made in the form of a capillary-porous structure with vaporization centers, which is characterized by the geometric configuration of the porous framework, which ensures its full permeability and the absence of deactivation of vaporization centers, the coating is made with a thickness at which the pores formed in it have a through character, while the pore size of the coating is comparable to the critical embryonic size during nucleate boiling or more than it at the next porosity, the porosity of the surface layer is not less than 6%, but not more than 12%, and the wetting angle of the surface layer for water is less than 40 degrees.

In the wall, the thickness of the coating, at which the pores formed in it have a through character, is from 7 to 35 μm, including the indicated values of the interval.

In the wall, the pore size of the coating, comparable to the critical nucleation size at nucleate boiling or more, is 100 nm or more, but not more than 10 μm.

The technical result is achieved by a method of forming a coating for intensifying heat transfer of the heat transfer wall of the heat exchanger, including processing the metal base, in which the metal base is processed from an aluminum-containing material, by carrying out microarc oxidation of the surface of the metal base in the electrolyte, which forms a ceramic coating, while processing is carried out in the anodic cathode mode with a frequency of voltage pulses, with an anode current density, with a ratio of the cathode component of the current density to the anode component of the current density, using an electrolyte, which produces a surface layer of the coating in the form of a capillary-porous structure with vaporization centers, characterized by a geometric configuration of the porous frame, providing its full permeability and the absence of deactivation of vaporization centers, with the porosity of the surface layer - not less than 6%, but not more than 12%, and the wetting angle of the surface layer for water - less than 40 degrees, the oxidation time is chosen taking into account the resulting coating thickness, at which the formed in it, the pores are inherent in a through character, with a pore size of the coating comparable to the critical seed size during nucleate boiling or more than it at the specified porosity.

In the method as an electrolyte, processing in which the surface layer of the coating is obtained in the form of a capillary-porous structure with vaporization centers, characterized by the geometric configuration of the porous framework, ensuring its full permeability and the absence of deactivation of vaporization centers, with a surface layer porosity of at least 6%, but not more than 12%, and the wetting angle of the surface layer for water is less than 40 degrees, an electrolyte is used that has the ability to form polymer anions, with elements present in their composition capable of forming insoluble oxides, which, embedding in the coating composition, provide an increase in linear dimensions of the coating being formed, namely, a silicate-alkaline electrolyte with a content of KOH from 1 to 6 g/l and Na ₂ SiO ₃ from 2 to 20 g/l and/or a phosphate electrolyte with a content of Na ₅ P ₃ O ₁₀ from 10 to 40 g/l, including the specified values of the intervals, or a phosphate electrolyte also including the addition of KOH or Na ₂ SiO ₃ .

In the method, during processing, the temperature of the electrolyte is maintained from 15 to 55°C, including the indicated values of the interval.

In the method, when processed in the anode-cathode mode with a frequency of voltage pulses, with an anode current density, with a ratio of the cathode component of the current density to the anode component of the current density, by which the surface layer of the coating is obtained in the form of a capillary-porous structure with vaporization centers, characterized by the geometric configuration of the porous frame, ensuring its full permeability and the absence of deactivation of vaporization centers, with a porosity of the surface layer - not less than 6%, but not more than 12%, and the wetting angle of the surface layer for water - less than 40 degrees, use a frequency from 50 to 3000 Hz, density anode current from 3 to 50 A/dm ² , the ratio of the cathode component of the current strength to the anode component of the current strength from 0.8 to 1.1, including the indicated values of the intervals.

In the method, the oxidation time is chosen taking into account the resulting thickness of the coating, at which the pores formed in it have a through character, with a pore size of the coating comparable to the critical seed size during nucleate boiling or more than it at the specified porosity, namely, choose from 1.5 to 40 minutes, including the specified interval values.

The essence of the technical solution is illustrated by the following description and the attached figures.

On FIG. 1 shows a photograph showing the morphology of the surface of the coating obtained by microarc oxidation of a metal base from a material (AD-31) containing aluminum in an acid electrolyte - surface view.

On FIG. Figure 2 shows a photograph showing the surface morphology of a coating obtained by microarc oxidation of a metal base from an aluminum-containing material (AD-31) in an alkaline electrolyte - a surface view.

On FIG. 3 is a photograph showing the surface morphology of a coating obtained by microarc oxidation of a metal base from a material (D16-T) containing aluminum in an alkaline electrolyte - a surface view.

On FIG. 4 is a photograph showing the surface morphology of a coating obtained by microarc oxidation of a metal base from an aluminum-containing material (D16-T) in a phosphate electrolyte - a surface view.

On FIG. Figure 5 shows a decrease in the contact angle of the surface in the case of a coating 30 μm thick obtained by microarc oxidation of a metal base from a material containing aluminum (D16-T) in an alkaline electrolyte compared to the case of the absence of the specified coating: a) the contact angle of the coating surface; b) contact angle of the surface of the metal base.

On FIG. Figure 6 shows the dependences of the total heat transfer coefficient on the temperature of the coolant at the inlet to the working sections at Re=1000 for a pipe with a coating obtained by microarc oxidation in a silicate-alkaline electrolyte until its thickness reaches 109–124 μm, and for a pipe made of the same material without a coating.

On FIG. Figure 7 demonstrates the influence of the parameters of coatings obtained by microarc oxidation on the intensification of heat transfer in a falling film on a stack of horizontal pipes in the nucleate boiling mode.

On FIG. Figure 8 demonstrates the influence of the conditions of coating formation on the intensification of heat transfer - the dependences of the relative value of the heat transfer coefficient on the temperature of the heat carrier at the inlet to the working sections of pipes, which are provided with coatings obtained by microarc oxidation under various conditions, with various parameters, are shown.

The thermal efficiency of the device is a key factor in the compactness (reduction in weight and dimensions) of the heat exchanger. An increase in thermal efficiency is due to a decrease in metal consumption and, consequently, the cost of a heat exchanger while maintaining its thermal power. In turn, the increase in thermal efficiency is provided by intensifying the heat transfer process - by increasing the intensity of heat transfer during boiling, with the stabilization of the heat transfer coefficient.

A significant advantage of the practical use of coatings formed by the MAO method for heat transfer is their high wear resistance and strength, elasticity, and the possibility of forming coatings on long objects by the MAO method. The spiral wound heat exchangers of modern large-scale natural gas liquefaction plants use long pipes that are wound in layers around a central core. The MAO method, having a high adaptability potential to be integrated into the technological process of modifying the surface of long pipes, can be effectively used in the technology of creating spiral heat exchangers. The high strength characteristics of MAO coatings are capable of maintaining the integrity of the coating during coiling of heat exchange tubes.

The proposed technical solutions are aimed at achieving the implementation of efficient heat transfer during nucleate boiling. The bubbly boiling mode is practically preferable compared to the evaporation mode and the film boiling mode.

The differences between the proposed solutions in comparison with the prior art, through which a technical result is achieved and a technical problem is solved, lie in the production and use of a ceramic MAO coating with characteristics, namely, with physicochemical (wettability) and geometric (roughness, porosity) factors, causing during the implementation of the process to obtain high densities of active (working) centers for the formation of steam bubbles, which leads to an increase in the intensity of heat transfer and stabilization of the heat transfer coefficient. The surface layer of the coating for intensifying heat transfer is made in the form of a capillary-porous structure with vaporization centers, characterized by the geometric configuration of the porous framework, which ensures its full permeability and the absence of deactivation of vaporization centers. The coating is made with a thickness at which the pores formed in it have a through character. That is, the coating is made with a thickness that ensures that the process of pore overgrowth is not triggered during microarc oxidation. The pore size of the coating is comparable to the critical nucleation size at nucleate boiling or more than it at a given porosity. The porosity of the surface layer is not less than 6%, but not more than 12%, and the wetting angle of the surface layer for water is less than 40 degrees. In the method of obtaining MAO-coating for the intensification of heat transfer by the heat-transfer wall of the heat exchanger, the metal base is processed in an electrolyte. As the metal base, a base of an aluminum-containing material is used. The treatment is carried out in the electrolyte by microarc oxidation of the surface of the metal base. The treatment is carried out in the anode-cathode mode with a frequency of voltage pulses, with an anode current density, with a ratio of the cathode component of the current density to the anode component of the current density, using an electrolyte, which produces a surface layer of the coating in the form of a capillary-porous structure with vaporization centers, characterized by a geometric configuration of the porous frame, ensuring its full permeability and the absence of deactivation of vaporization centers, with the porosity of the surface layer - not less than 6%, but not more than 12%, and the contact angle of the surface layer for water - less than 40 degrees. The oxidation time is chosen taking into account the resulting thickness of the coating, at which the pores formed in it have a through character, with a pore size of the coating comparable to the critical seed size or more of it at the specified porosity.

In contrast to a smooth surface, the porous surface of the coating during heat exchange in the boiling and evaporation modes is characterized by a lower required temperature difference, a higher heat transfer coefficient as a result of the presence of a large and stable over time number of active centers of vaporization, stable changes in the diameter and frequency of detachment of steam bubbles. The centers mentioned above are microscopic vapor bubbles in the pores.

Boiling conditions include:

- heating the liquid to saturation temperature;

- the presence of centers of formation of steam bubbles;

- a constant supply of heat to create a temperature difference.

From the above conditions, it can be seen that in order to implement the desire to increase the intensity of heat transfer - to increase the removed heat fluxes, the heat transfer coefficient during boiling - it is necessary, in particular, to achieve high densities of active (working) centers for the formation of vapor bubbles.

The occurrence of a high density of active centers for the formation of vapor bubbles and, as a result, the achievement of high values of the removed heat fluxes and/or the heat transfer coefficient is characteristic of developed nucleate boiling, which is observed in the region of relatively large values of the temperature difference. In the region of small values of the temperature difference, the density of active centers for the formation of vapor bubbles is low. Accordingly, the values of the removed heat fluxes and/or the heat transfer coefficient are low.

On the other hand, regardless of the magnitude of a specific temperature difference, the following prerequisites are significant for obtaining a high density of active centers for the formation of steam.

It is known that under fixed temperature conditions, that is, regardless of the temperature difference range, the average number of active, active, boiling centers is determined both by the physicochemical factor - local adhesion characteristics (local wettability), and by the geometric factor - surface relief, characterized by the presence of features in the form of depressions, depressions, scratches, that is, attributes that are characteristic of a developed surface (D.A. Labuntsov “Physical foundations of energy. Selected works on heat transfer, hydrodynamics, thermodynamics”, paragraph “Approximate theory of heat transfer with developed nucleate boiling ”, M.: MPEI Publishing House, 2000, 388 p.).

The above source shows that the ability of a surface area to vaporization is characterized by the value of the energy barrier (thermodynamic potential increment) for the transition from a system without a vapor phase to a system with a viable nucleus, described by the expression:

where ΔФ - increment of thermodynamic potential;

V is the volume of the bubble;

S is the total surface of the bubble;

Sw is the surface of the solid body occupied by the bubble;

ρ _v - vapor density;

σ - surface tension at the vapor-liquid interface;

- удельный (на единицу массы) термодинамический потенциал Гиббса для жидкой фазы;

- specific (per unit mass) Gibbs thermodynamic potential for the liquid phase;

- удельный (на единицу массы) термодинамический потенциал Гиббса для паровой фазы;

- specific (per unit mass) Gibbs thermodynamic potential for the vapor phase;

θ is the value of the local contact angle.

The second term of this expression, which is the work to create the interface, depends on the physico-chemical (θ) and geometric (Sw/S) factors of the surface element. It can be seen that for fixed θ and S, the work done on the formation of new surfaces will be the smaller, the larger part Sw of the bubble surface S is in contact with the solid phase. Thus, surface features such as depressions, depressions, scratches are potential centers of boiling to a greater extent than even areas and various protrusions.

The conditions for the onset of vapor formation on the surface depend on the value of the local contact angle (local wettability) θ. For the cases of θ values commonly encountered in practice, when the value of the latter is determined in the range 0<θ<π, the barrier value ΔФ is set by the joint influence of both the geometric factor Sw/S and the wettability level itself 9.

The dimensions of the depressions in which nuclei can appear are comparable to the sizes of viable nuclei during bulk effervescence, which determine the scale embryonic size, which is the critical size, by the expression:

where l ₀ - scale embryonic size;

σ - surface tension at the vapor-liquid interface;

ρ _v - vapor density;

- удельный (на единицу массы) термодинамический потенциал Гиббса для жидкой фазы;

- specific (per unit mass) Gibbs thermodynamic potential for the liquid phase;

- удельный (на единицу массы) термодинамический потенциал Гиббса для паровой фазы.

is the specific (per unit mass) Gibbs thermodynamic potential for the vapor phase.

Potential centers for the formation of vapor bubbles, which are features of surface irregularities in the form of depressions, become active centers (working) at a certain temperature difference, if their dimensions are comparable to the critical embryonic dimensions.

On the other hand, individual depressions, scratches, and other surface irregularities should be considered as centers for the formation of vapor bubbles, in relation to which the local wettability is reduced, that is, the values of θ are close to π/2. An increase in θ can be due to various reasons, for example, starting with surface contamination and ending with local mechanical stresses. The local wetting angle θ (microscopic), which determines the measure of the energy interaction of liquid, solid and vapor and is a physicochemical characteristic, is associated with the average wetting angle θ ₀ (macroscopic), which is a more complex characteristic that also takes into account the nature and structure of small-scale surface roughness , ratio:

where θ is the value of the local contact angle;

θ ₀ - the value of the average contact angle;

ξ is the reciprocal of the roughness coefficient.

The value of ξ is equal to the ratio of the areas of a smooth surface to the area of a relief surface, taking into account its scale relief, ξ<1. Thus, at θ<π/2 the microscopic contact angle is greater than the macroscopic one - θ>θ ₀ , and at θ>π/2 θ<θ ₀ .

As a rule, when measuring wettability, the contact angle θ ₀ is measured - a macroscopic characteristic. Measurements of θ ₀ on metal surfaces, in particular for water and some organic liquids, show values from 30 to 50 degrees. Therefore, θ>θ ₀ , and it is quite possible that the values of θ in these cases are quite close to π/2. In this regard, on such metal surfaces, significant overheating of the liquid is not required to start vaporization.

The main conclusion from the above: the average number of operating centers for the formation of vapor bubbles at a certain, fixed, temperature difference is mainly determined by local adhesion characteristics (local wettability θ) and surface topography. The density of working centers is proportional to the average number of surface features such as depressions with dimensions comparable to the critical seed size per surface unit. In this case, the proportionality coefficient depends on the average level θ.

It is known (E.D. Shchukin, A.V. Pertsov, E.A. Amelina, "Colloid Chemistry", Moscow, Moscow University Press, 1982, 348 p., p. 100) that wetting of solids by liquids is influenced by the state of the surface of a solid body, in particular microgeometry (roughness). An increase in the real surface area of a solid body leads to a corresponding increase in the contribution to the wetting energy of the solid-liquid and solid-gas interfaces. It was noted (S.S. Voyutsky, "Course of colloidal chemistry", Moscow, Publishing house "Chemistry", 1976, 512 p., pp. 159-160) the effect of roughness on the wettability characteristic, which is expressed in the fact that the greater the roughness, the more pronounced surface properties that attract or repel water. It is believed that if at a contact angle (macroscopic) less than 90°, a smooth surface is hydrophilic, then in the presence of roughness, the liquid penetrates into the depressions of the surface in the same way as it is absorbed into the capillaries. Roughness improves wettability. If, relative to a smooth surface, the specified angle is greater than 90°, the liquid does not penetrate into the recesses of the surface. In this case, the surface roughness provides a deterioration in wettability. For a hydrophilic surface, the presence of roughness imparts greater hydrophilicity, and in the case of a hydrophobic surface, the opposite effect takes place. On the other hand, the contact angle can be affected by the conditions of surface formation. When developing the proposed technical solutions, it was established that if the smooth surface of an aluminum plate, usually covered with an Al ₂ O ₃ oxide film, is hydrophilic, well wetted by water (the contact angle is 40°), then the surface of the Al ₂ O ₃ oxide coating, formed by the method MAO, being a developed surface, under certain conditions for the implementation of MAO can be characterized by both improved wettability, with a contact angle of less than 40°, and degraded wettability, with a contact angle of more than 40°. It is possible to control the wettability characteristic by choosing the conditions for the formation of a coating during microarc oxidation.

Porous coatings, characterized by a developed surface, the necessary wettability and, as a consequence, a significant number of potential centers for the formation of vapor bubbles, and their active, under certain conditions, characteristics, are promising for use in order to intensify heat transfer. Porous coatings make it possible to obtain a developed bubble boiling regime even at low temperature differences, to increase heat transfer coefficients in almost the entire range of temperature differences, and also to increase the critical heat flux corresponding to the first crisis. Said MAO coating refers to porous coatings. The porosity of the MAO coating is an immanent property determined by the nature of the process. The porous aluminum oxide coating is characterized by a developed surface and a large number of micropores (microcavities). It is possible to control the porosity by choosing the conditions for the formation of the coating, but it is impossible to obtain an MAO coating that does not contain pores.

When using this coating in order to intensify heat transfer, heat is supplied to increasing vapor bubbles in pores (microcavities) due to high thermal conductivity not through a liquid film characterized by high thermal resistance, but, for example, from crystalline particles of α-Al ₂ O ₃ and γ-Al ₂ O ₃ fine crystalline matrix. The presence of a large number of microcaverns covered by a certain range with their realized shape and the variability of parameters at a given coating thickness are one of the factors that contribute to providing favorable conditions for the nucleation and growth of vapor bubbles, changing the boiling microcharacteristics, reducing the separation diameter and increasing the frequency of detachment of vapor bubbles, local reduction wettability in the zone of microcaverns and, as a result, a significant increase in the intensity of heat transfer. In fact, the conditions for creating a heat-releasing surface distributed over a heat-releasing surface close to contrasting wettability are realized. The shape of the pores (microcavities) of the MAO coating and the wettability close to contrast ensure the absence of conditions for the deactivation of vaporization centers.

задаваемого выражением (2). Для не очень больших перегревов (что относится к предлагаемым техническим решениям) величину

в выражении (2) можно заменить на r(T_W-T_S)/T_S(r - теплота парообразования), таким образом,

At a given temperature difference T _W -T _S, the geometric features of a solid surface (roughness, depressions, depressions) become real boiling centers if their dimensions are of the order of the scale germ size

given by expression (2). For not very large overheating (which refers to the proposed technical solutions), the value

in expression (2) can be replaced by r(T _W -T _S )/T _S (r is the heat of vaporization), thus,

пузырей, обеспечивают высокие гарантии отсутствия проникновения жидкости вглубь в отличие от центров меньших размеров. На основании этого первые должны обеспечивать более существенную производительность в работе, чем последние. Для малых углублений имеется возможность возникновения моментов, когда поверхность раздела пар-жидкость проникает в их глубь, что выводит их из работы, деактивирует, на некоторое время. В связи с чем такие центры не могут вносить значительного вклада в интенсификацию теплообмена. Кроме того, роль в интенсификации может играть также геометрическая конфигурация углублений (пор, микрокаверн). Предпочтительно в целях большей интенсификации теплообмена наличие углублений типа пор, микрокаверн с ответвлениями и образованием внутренних пространств (кратерообразные поры), сообщающихся с внешним пространством посредством устья, которое визуализируется на представленных фотографиях (Фиг. 1-4) с оплавленными краями. Такая геометрическая конфигурация оптимальна для нахождения менисков пор у их верхних краев и гарантированно должна препятствовать деактивации центров парообразования.Obviously, the detachment of vapor bubbles formed in relation to individual pores should optimally occur on the menisci, mostly at the upper edges, without penetration of the liquid into the depths. A small part of the cavity filled with steam acts as a bubble sequence generator. The frequency of separation of bubbles for different sizes of depressions is not the same. The general trend is that the centers of boiling (pores, microcavities), the size of which exceeds the embryonic size

bubbles, provide high guarantees that liquid does not penetrate deep into, unlike centers of smaller sizes. Based on this, the former should provide a more significant performance in work than the latter. For small recesses, there is the possibility of occurrence of moments when the vapor-liquid interface penetrates into their depth, which takes them out of operation, deactivates them, for a while. In this connection, such centers cannot make a significant contribution to the intensification of heat transfer. In addition, the geometric configuration of depressions (pores, microcaverns) can also play a role in intensification. Preferably, in order to intensify heat transfer, the presence of recesses such as pores, microcaverns with branches and the formation of internal spaces (crater-shaped pores) communicating with the external space through the mouth, which is visualized in the presented photographs (Fig. 1-4) with melted edges. Such a geometric configuration is optimal for finding the pore menisci at their upper edges and should definitely prevent the deactivation of vaporization centers.

In connection with ensuring efficient heat transfer, one more feature of MAO coatings should be noted.

It is known that the coating obtained by microarc oxidation on the surfaces of a metal base containing aluminum is characterized by high wear resistance in atmospheric conditions, in various corrosive environments, including chemically aggressive solutions and vapors. In addition, the coating has high mechanical resistance - fracture resistance, high resistance to cracking when exposed to factors of various nature. This contributes to the stabilization of the heat transfer coefficient, high intensity of heat transfer. These qualities of the MAO coating on aluminum are attractive in the following practical aspect.

Natural gas contains trace amounts of mercury. Liquefied natural gas producers need to remove mercury from it before cooling. Since the mercury present in the gas, aggressively acting on aluminum heat exchangers used in gas liquefaction plants, contributes to their gradual destruction and a decrease in heat transfer efficiency. The MAO coating on aluminum is not subject to the aggressive action of mercury, retains its integrity and, thus, contributes to a high intensity of heat transfer and stabilization of the heat transfer coefficient.

The MAO process is characterized by a complex mechanism that includes several stages with the transition to each subsequent stage, accompanied by a change in the structure of the oxide layer, with increasing voltage over time. At the first stage, an oxide film is formed in a non-sparking mode, and a non-porous barrier (transition) layer is formed. After that, the process passes into the stage of the spark mode with small short-lived microdischarges, which provide the appearance of pores. It is believed that the discharges occur as a result of the breakdown of gas-vapor plugs formed in micropores during the electrolysis of water and boiling of the electrolyte at the bottom of the pores. The second stage is replaced by the stages of microarc and arc discharge. Microarc and arc discharges lead to the heating of the oxide region adjacent to the breakdown channel up to 1000÷2000°C; A further increase in voltage leads to an increase in the power of arc discharges and a decrease in their number, the nature of their movement over the surface changes, and the released power is sufficient to burn through the coating to the metal base. Significant energy released in the discharge channel ensures the ejection of the entire plasma into the electrolyte, which leads to the formation of local crater-like macrodefects in the coating. The transition from one stage to another is due to changes in the electrical and thermal parameters of microplasma processes.

It is believed that the growth of MDO coverage includes three stages. At the first stage, the material in the microdischarge channels is heated to a temperature of about ¹⁰⁴ K, and the anionic components of the electrolyte are drawn into the channels. In parallel with this, aluminum and its alloying impurities are melted, introduced into the channels and oxidized. At the second stage, the oxidation products are ejected from the channel to the surface of the coating, increasing the local thickness of the coating. At the third stage, the channels cool down, and the oxidation products are deposited on the walls. The molten oxide is ejected onto the surface of the coating, on which the processes of solidification and recrystallization occur under the action of the electrolyte. As a result, the metal hydroxide decomposes to oxide and complex complex compounds. The sequence and intensity of these processes depend on the current density and the power of the discharges, which are determined by the thickness of the oxide layer.

In the case of the MAO mode, on the surface of the metal base (anode), there is a movement of a few discharges as a result of their irregular damping and the ignition of new ones. Oxidation proceeds unevenly. During a microarc discharge at any point on the surface, due to the high temperature in the discharge channel, the adjacent region of the coating with a developed structure of micropores is heated. The channel in which the discharge is burning dominates in the heated region for a certain period of time. By thermally activating neighboring regions, the discharge facilitates its transition to another pore after some time. During this time, electrochemical interaction of the electrolyte with the base metal through the barrier layer at the bottom of the pores occurs in neighboring pores, which is thermally accelerated, and provides local growth of the coating area with its formation deep into the metal base. After the transition of the microarc discharge to another place, the pore in which it worked out turns into a channel with a crater-shaped melted mouth. This appearance is due to the fact that its surface at the interface with the electrolyte is under the influence of the hot cathode region of the discharge, while at the metal base-coating interface, the temperature in the discharge is insufficient for melting. The surface morphology of the coating (in particular, the size and number of pores) formed by the MAO method is determined by the formation modes used by the electrolytes and the metal base itself. Nevertheless, all MAO coatings are characterized by the presence of melted areas and mouths of pores emerging on the surface, which are channels for electrical breakdowns.

In the MAO mode, when the conditions for the course of ordinary electrode processes change and thermal, diffusion, electrophysical processes and plasma-chemical reactions occur, the formation of crystalline phases in the oxide layer, the modification of the surface layer by electrolyte components, and the formation of films containing oxides, spinels, and salts can occur. By selecting the composition of the electrolyte and the modes of formation of the coating, it is possible to obtain compounds both containing and not containing matrix elements.

Taking into account the composition of the formed coating, electrolytes can be divided into two groups. One group includes oxide-forming, these include electrolytes that have the ability to form polymer anions - aluminates, silicates, tungstates, phosphates. The anions of these electrolytes include elements capable of forming insoluble oxides, which are incorporated into the coating composition. This gives an increase in linear dimensions during the formation of the coating. The second group includes electrolytes that are not capable of forming insoluble oxides. These include acid solutions, hydroxide solutions (sodium, potassium). When using these electrolytes, the formed modified layer is deepened into the metal base.

When using a specific electrolyte for MAO coating, the following should be taken into account:

- properties of the electrolyte components - their solubility, degree of dissociation, hydrolysis, polymerization, which determines the nature of the interaction at the electrolyte-base interface;

- the presence in the electrolyte of insoluble or slightly soluble additives that affect the processes of thermolysis and the MAO mode, the phase composition of the coating;

- change in the form of complex anions and the kinetics of the process of coating formation, the chemical composition of anions as a result of a decrease in pH in local areas at the anode;

- influence on the chemical composition of the coating of the nature of the interaction of ions with each other;

- physical and chemical properties of the formed compounds;

- the presence of alloying impurities in the composition of the metal base.

As a rule, the electrolyte composition is selected experimentally. The selection criterion is the presence of elements in the composition of anions, minimization of the ignition time of microarc discharges at a given current density.

To form MAO coatings, it is necessary to maintain a given electrolyte temperature, using, for example, a bath with a water-cooled jacket. The temperature regime ensures the proper operation of the electrolyte.

Important electrolyte parameters taken into account in process design are the composition and concentration of electrolyte components.

Coatings obtained on aluminum or its alloys, as a rule, have a three-layer structure with a sequential arrangement of layers on a metal base: a barrier (transition) layer, an inner (working) layer with low porosity, a technological (outer) layer, high porosity, which is controlled through the choice of modes of formation and electrolyte. These three-layer coatings are characterized by an uneven distribution of embedded electrolyte components, variable composition.

To implement the proposed method, alkali-silicate and/or phosphate electrolytes are used.

The identification of a suitable electrolyte was carried out experimentally by testing the alkaline silicate, acid and phosphate electrolytes, taking into account the geometric factor (obtained coating morphology) and the physico-chemical factor (wettability).

The components used in the alkaline silicate electrolyte are KOH and Na ₂ SiO ₃ . Potassium hydroxide is an electrically conductive component. The specified hydroxide affects the aggressiveness of the electrolyte, which manifests itself in the etching of the emerging coating and the surface of the metal base. On the other hand, potassium hydroxide provides electrical conductivity, and therefore the concentration of potassium hydroxide is taken from 1 to 6 g/l. Sodium silicate is a component that provides an increase in the linear dimensions of the formed coating, affects wear resistance, and also determines the ignition time of a microdischarge. The concentration of sodium silicate is taken from 2 to 20 g/L.

The component of the phosphate electrolyte - Na ₂ P ₃ O ₁₀ is taken at a concentration of 10 to 40 g/l. During the preparation of the electrolyte, the specified component provides a component that determines the required electrical conductivity during the MAO process, affects the coating being formed and the surface of the metal base, as well as a component whose function is to increase the linear dimensions of the coating being formed. Phosphate electrolyte may optionally include the addition of KOH or Na ₂ SiO ₃ .

For the preparation of electrolytes, we used distilled water and the corresponding commercially available reagents of the “chemically pure” brand.

The proposed method uses the anode-cathode mode, as the most optimal for the formation of a coating on a metal base from a material containing aluminum. This mode is characterized by the ability to control the ratio of the amplitudes of the cathode and anode currents, due to which it is possible to obtain coatings with the required thickness, their structure, and control their porosity. For aluminum alloys, as determined experimentally, alkaline-silicate electrolyte and phosphate electrolyte with oxidation modes are optimal when using the anode-cathode mode: anode current density from 3 to 50 A / dm ³ at a ratio of the cathode component of the current density to the anode component of the current density I _k /I _a from 0.8 to 1.1. In the indicated ranges of parameter values, the porosity and hardness of the formed coatings practically do not change, but the rate of oxidation and the thickness of the formed coating change. An increase in the anode current density and, as a consequence, a decrease in I _k /I _a leads to an increase in the rate of oxidation.

To implement the proposed method, the shape of the voltage pulses is used, both rectangular and sinusoidal.

The use of pulsed currents has the following advantages. At the moment of applying both cathodic and anode pulses, the coating breakdown occurs and an arc appears at the breakdown point. At the breakdown point, the cathode pulse exposes a portion of the surface of the metal base, but does not form a coating by itself. The specified area is exposed before the arrival of the anode pulse. Upon arrival, the anode pulse does not expend its energy on breakdown. The cathode pulse, not directly participating in the formation of the oxide of the formed coating, indirectly contributes to its growth.

To implement the proposed method, a pulse frequency of 50 to 3000 Hz is used.

The choice of frequency affects the intensity and density of microdischarges. Microdischarges arising during the MAO process affect the formation of coatings and their morphology. An increase in frequency leads to an increase in the thickness of the coatings. At a frequency of 50 Hz or more, many microcracks and relatively large diameter pores form in the coatings. At a frequency of 1000 Hz, a microporous coating structure is formed with mouths emerging on the surface, surrounded by solidified products ejected to the surface in a molten state. The specified frequency range is acceptable in order to form the required coverage. At frequencies less than 50 Hz, the resulting microdischarges are ineffective. At frequencies above 3000 Hz, formation occurs at an accelerated rate, and formation of an excessive thickness of the coating may occur, leading to overgrowth of the pores. In addition, globules of α-Al ₂ O ₃ , which are hydrophobic, can be ejected onto the coating surface, in particular, when using an alkali-silicate electrolyte, which, in combination with a significant thickness obtained, at which pores are overgrown, leads to a deterioration in the wettability of the coating surface. .

The following Table selectively presents the measurement data of the characteristics, the obtained parameters of the coatings, with the conditions for their formation on prototypes. Coating characteristics (thickness, composition, surface morphology) were determined on a Hitachi S-3400N scanning electron microscope using energy dispersive X-ray spectroscopy (EDX). Based on the energy spectrum of the radiation, conclusions were obtained about the qualitative and quantitative composition of the sample. The contact angle of the surface was measured on a KRUSS DSA 100 setup using the sessile drop method.

In addition to determining the characteristics of coatings obtained by MAO on a metal base from an aluminum-containing material (alloys) under various oxidation conditions, experiments were carried out to demonstrate the suitability of the obtained coatings for the purpose of intensifying heat transfer.

With regard to the coating samples, the parameters of which are indicated in the Table, in Fig. 1-4 are photographs showing the surface morphology of the coating

for sample II (Fig. 1), the surface morphology of the coating for sample I (Fig. 2), the surface morphology of the coating for sample 111 (Fig. 3) and the surface morphology of the coating for sample IV (Fig. 4) obtained under different oxidation conditions .

On FIG. 5 shows the wettability parameters of the coating surface of sample III in comparison with the wettability of the metal substrate.

Coatings with the surface morphology shown in the photographs are obtained on a metal base from an aluminum-containing material, in particular, AD-31, D16-T alloys, by means of microarc oxidation of the specified base, as a rule, contain layers: a surface layer and a solid oxide layer, located between the metal base and the surface layer. The surface layer is a porous layer containing mullite and various chemical complexes formed as a result of electrochemical reactions. The solid oxide layer contains modifications of α-Al ₂ O ₃ and γ-Al ₂ O ₃ .

In the photographs shown in Fig. 1-4 shows the coating surface of samples I-IV (see table) with open pores and roughness, the pore diameter is 20 µm (see Fig. 2, pore opening), can reach as much as 40 µm (sample I, sample II ), or less, up to 1-2 microns (see Fig. 3 and 4). The diameter of internal pores formed during oxidation can range from 100 to 200 nm. The porosity of the surface layer reaches the highest values - 24-37% for sample I with a coating thickness of 109-124 microns and minimum values of 6-8% for sample II with a coating thickness of 26.1 microns, the pore density for these samples is 2 × 10 ⁴ cm ^-2 , which is significantly lower than the density of pores in the coating of samples III and IV, which is, respectively, 4×10 ⁵ cm ^-2 and 1.4×10 ⁶ cm ^-2 . In this case, the porosity of the surface layer of the coating of samples III and IV is 6–10% and 8–12%, respectively, with a coating thickness of 30 μm. Based on the above data, it can be concluded that in the case of a thick coating (sample I) provided by a longer oxidation time, due to the long process, partial and/or complete pores, in particular pores emerging from the surface, are overgrown. Coating sample I is characterized by high porosity of the surface layer. Comparing the coating data for samples I and III, the conditions for obtaining which differ in the duration of oxidation and, as a result, the resulting coating thickness, determined by the oxygen boundary, we can conclude that a longer process provides lower pore densities, which, respectively, for the sample I - 2×10 ⁴ cm ^-2 , for sample III - 4×10 ⁵ cm ^-2 and a large porosity of the surface layer, constituting, respectively, for sample I - 24-37%, and for sample III - 6-10%. Thus, in the case of thicker coatings, more prolonged oxidation, the formation of rather large pores and subsurface cavities along with small and medium ones takes place. Since the production of oxide can occur in the pores themselves, this can lead to complete overgrowth of relatively small pores and partial overgrowth of medium-sized pores, in particular their outlets. Therefore, in the photograph of the surface (see Fig. 2) one can see the roughness and the exit hole of a relatively large size, about 20 microns in diameter, in the form of a crater with a melted mouth, which can be an exit hole from the subsurface cavity. Visible pores are only mouths of plasma channels. Along with them, the coating contains large cavities, which, judging by the large porosity values, are larger than visible pores. The presence of large cavities can be explained by the merging of several randomly oriented arc channels formed during microarc oxidation. In contrast to the coating of samples I and III, the coating of sample II with a thickness comparable to that of sample III was obtained in an acidic electrolyte. Judging by the photograph of the surface of this coating (see Fig. 1), as well as based on the data presented in the Table, the use of an acid electrolyte for oxidation leads to low porosity with a low pore density, but relatively large, with a diameter of 5 to 40 microns, with a non-uniform surface distribution. With regard to the wettability data (see Table), the coatings of samples I and sample II compared to the coating of sample III, as well as the coating of sample IV, are characterized by poorer wettability. With respect to the coatings of samples I and II, the contact angle is 39-50° and 48-52°, respectively, while the contact angle of the coating of sample III is 32-34° (see Fig. 5), and the coating of sample IV is 30 -36° (see table). The worse wettability of the sample I coating relative to the wettability of the sample III coating can be associated with longer oxidation and, accordingly, obtaining a thicker coating with overgrown pores, with the ejection to the surface, in particular, particles of solid oxide Al ₂ O ₃ , which are hydrophobic, which in together, provides a deterioration in wettability by increasing the contact angle from 40°, which corresponds to amorphous alumina. In accordance with the above, the formation of the MAO coating occurs in three stages, while at the second stage, the oxidation products are ejected from the channel to the surface, leading to a local increase in the coating thickness, and at the third stage, the channel cools down with precipitation of the oxidation products on the walls, solidification and recrystallization of the molten oxide on the surface, therefore, with an increase in the duration of oxidation, the mass of particles of solid oxide Al ₂ O ₃ that are hydrophobic can reach a value that generally leads to hydrophobization of the coating surface and deterioration of wettability. With a shorter oxidation time and, as a result, a smaller thickness of the resulting coating, for example, as in the case of coating sample III or coating sample IV, the mass of particles of solid oxide Al ₂ O ₃ ejected onto the surface may not reach a critical value, leading to surface hydrophobization coating as a whole, and lead to the creation of a surface-distributed coating close to contrast wettability. At the same time, the specified shorter oxidation also corresponds to the resulting coating thickness, at which the pores formed in it are characterized by a through character, with a coating pore size comparable to the critical seed size or more at the specified porosity.

The wettability of the coating of sample II, characterized by a contact angle of 48-52°, is probably due to the use of an acid electrolyte with a tendency to form a coating of low porosity in it - 6-8%, with a pore density of 2×10 ⁴ cm ^-2 , with its growth deep into the metal base , with surface modification towards its hydrophobization. The use of an acid electrolyte is not conducive to obtaining the required coating parameters in order to intensify heat transfer. Regarding the geometric features of the coatings of samples III and IV, the photographs (see Fig. 3 and 4) show the surfaces of these coatings with regular roughness and open pores with a diameter of 1 to 3 μm, located at distances from each other from 15 to 25 μm. The porosity of the surface layer of the sample III coating is 6-10%, the porosity of the surface layer of the sample IV coating is slightly larger, but comparable, and is 8-12%. The pore density for coating sample III is 4×10 ⁵ cm ^-2 and the pore density for coating sample IV is 1.4×10 ⁶ cm ^-2 . Thus, in the case of thinner coatings, less prolonged oxidation, and the use of alkaline silicate or phosphate electrolytes, small pores, hundreds of nanometers in diameter, and medium-sized, several micrometers in diameter, less than 10 μm, open pores are formed. The capillary-porous structure with active centers of vaporization of the surface layer of the coating corresponds to the geometric configuration of its porous framework, which ensures its complete permeability, with a thickness that ensures the absence of pore overgrowth during oxidation, the absence of deactivation of vaporization centers.

The effectiveness of coatings (see Table) for the purpose of intensifying heat transfer can be judged by the experimental data presented in Fig. 6-7.

On FIG. 6 shows the data on the overall heat transfer coefficient depending on the temperature of the coolant - hot water at the inlet to the working section of the pipe with a coating of sample I with a thickness of 109-124 microns (see Table). The temperature range of hot water at the entrance to the working areas of 43-46°C correspond to heat fluxes of not more than 10 ⁴ W/m ² , for which the main mechanism of heat removal is evaporation. The overall heat transfer coefficients for the reference pipe (uncoated) and the coated pipe sample I are almost the same. In the temperature range of the coolant 48-50°C, as shown by visual observations, on the uncoated pipe, the transition to the bubble boiling mode begins, which corresponds to heat fluxes (10÷15)×10 ³ W/m ² . When the temperature of hot water at the inlet to the working sections is more than 51-53°C, the heat fluxes on the surface of the working sections increase to 2×10 ⁴ W/m ² . In this region of developed nucleate boiling, the heat transfer coefficients on the coated pipe of sample I are significantly reduced compared to the uncoated pipe.

The results of the study of heat transfer intensification in a flowing film on a package of horizontal pipes with coatings of samples I - IV are shown in Fig. 7. Relevant data on heat transfer coefficients related to heat transfer coefficients on uncoated pipes for the respective materials are shown in FIG. 8. For correct comparison, the data are given for the same coolant temperatures at the inlet, because due to the difference in heat transfer coefficients in the falling film on experimental samples with different coatings, the heat fluxes on their surfaces differ significantly. At low heat fluxes q<1×10 ⁴ W/m ² in the evaporation mode, the heat transfer coefficients on an uncoated pipe and on all pipes provided with coatings of samples I - IV are almost the same, and weakly depend on the heat flux (see Fig. 7). At high heat fluxes in the nucleate boiling mode (q>15-20 kW/m ² ), there is both a deterioration in heat transfer in some pipes provided with a coating compared to an uncoated pipe, and an intensification of heat transfer.

On FIG. Figure 8 shows that the heat transfer coefficients on pipes coated with samples I and II decrease by 30-40%, and the greatest intensification of heat transfer on a pipe coated with sample III reaches a 3-fold value. The surface morphology of the sample II coating is shown in FIG. 1. Its thickness is significantly reduced compared to the coating of sample I (see Table). However, this did not lead to an intensification of heat transfer; on the contrary, the heat transfer coefficients when using this coating in the nucleate boiling mode decreased significantly compared to an uncoated pipe. This coating is a continuous rather loose layer. On the surface (see Fig. 1) there are separate rare areas with a porous structure, which can be formed during the coating process as a result of the interaction of the base metal and acidic electrolyte. The greatest intensification of heat transfer in the falling film was obtained in the case of a coated pipe of sample III. The area characteristic of the evaporation mode, in which the heat transfer coefficient does not depend on the heat flux for this coating, is observed only at low heat fluxes q<5000 W/m ² . At higher heat fluxes, the nucleate boiling regime is observed. On FIG. 3 shows a photograph of the surface morphology of this coating. The surface has a regular roughness with rare pores about 2 µm in diameter located at distances of 15 to 20 µm from each other.

Based on the above, the method for forming a coating for intensifying heat transfer of the heat transfer wall of the heat exchanger in the general case includes the following steps: processing a metal base made of an aluminum-containing material by conducting microarc oxidation of the surface of the metal base in an electrolyte and forming a ceramic coating. The treatment is carried out in the anode-cathode mode with a frequency of voltage pulses, with an anode current density, with a ratio of the cathode component of the current density to the anode component of the current density, using an electrolyte, which produces a surface layer of the coating in the form of a capillary-porous structure with vaporization centers, characterized by a geometric configuration of the porous frame, ensuring its full permeability and the absence of deactivation of vaporization centers, with the porosity of the surface layer - not less than 6%, but not more than 12%, and the contact angle of the surface layer for water - less than 40 degrees. The duration of treatment - the oxidation time is chosen taking into account the resulting thickness of the coating, in which the pores formed in it are inherent in a through character, with a pore size of the coating comparable to the critical embryonic size during nucleate boiling or more than it at the specified porosity.

In this case, an electrolyte is used as an electrolyte, which has the ability to form polymer anions, with elements present in their composition capable of forming insoluble oxides, which, embedding in the coating composition, provide an increase in the linear dimensions of the coating being formed. In particular, a silicate-alkaline electrolyte is used with a KOH content of 1 to 6 g/l and Na ₂ SiO ₃ from 2 to 20 g/l and/or a phosphate electrolyte with a Na ₅ P ₃ O ₁₀ content of 10 to 40 g/l , including the specified ranges, or a phosphate electrolyte also including the addition of KOH or Na ₂ SiO ₃ . Regarding processing in the electrolyte by oxidation, it is preferable to use the anode-cathode pulsed mode, with the following parameters: pulse frequency from 50 to 3000 Hz; anode current density from 3 to 50 A/dm ² ; the ratio of the cathode component of the current strength to the anode component of the current strength from 0.8 to 1.1, including the specified values of the intervals. The duration of treatment is preferably between 1.5 and 40 minutes.

Thus, the proposed heat transfer wall of the heat exchanger is made in the composition of a metal base from an aluminum-containing material and a ceramic coating to intensify heat transfer of the heat transfer wall of the heat exchanger obtained by microarc oxidation of the surface of the specified base. The surface layer of the coating is implemented in the form of a capillary-porous structure with vaporization centers. This structure is characterized by the geometric configuration of the porous framework, which ensures its complete permeability and the absence of deactivation of the centers of vaporization. The coating is made with a thickness at which the pores formed in it have a through character. In this case, the pore size of the coating is comparable to the critical nucleation size at nucleate boiling or more than it at the next porosity of the surface layer. The porosity of the surface layer is not less than 6%, but not more than 12%. The contact angle of wetting of the surface layer for water is less than 40 degrees.

In particular, the thickness of the coating, at which the pores formed in it have a through character, is from 7 to 35 μm, including the indicated interval values. The pore size of the coating, comparable to or greater than the critical seed size, is 100 nm or more, but not more than 10 μm.

The coating contains a surface layer (technological layer), which in this case is a working layer. The specified layer is the most remote from the metal base. The coating has a solid oxide layer located between the surface layer and the metal base, which is a product of the oxidation of the metal base in the form of a fine-grained matrix of α-Al ₂ O ₃ and γ-Al ₂ O ₃ . The coating also contains a barrier (intermediate) layer located on the surface of the metal base. The intensification of heat transfer is determined by the structure of the surface layer.

The heat transfer wall of the heat exchanger functions as follows, realizing efficient heat transfer with high heat transfer coefficients that are stable over time.

When the liquid flows around the wall coating, the capillary-porous structure is replenished with liquid with the geometric configuration of the porous framework with vaporization centers, which ensures its complete permeability. Liquid flows into the pores under the action of surface tension forces. Heat is supplied from the crystalline particles of α-Al ₂ O ₃ and γ-Al ₂ O ₃ of the fine-crystalline matrix of the solid oxide layer, the process of nucleation and growth of vapor bubbles proceeds in the pores until the separation diameter is reached in the superheated near-wall liquid layer. When steam bubbles break off under the action of the Archimedes force with a branched system of interconnected pores - capillary channels - steam is evacuated.

As information confirming the possibility of implementing the method with the achievement of a technical result, we present the following examples of implementation. Example 1

To implement a method for producing a coating for intensifying heat transfer of a heat transfer wall of a heat exchanger, the metal base of which is made of an aluminum-containing material, an installation is used, which includes a power source, a bath filled with electrolyte, in which the metal base being processed is placed.

A pipe with an outer diameter of 10 mm and a length of 100 mm, a pipe wall thickness of 1.5 mm from an alloy D16T is taken as a processed base. The tube is placed in an electrolytic bath, it acts as a working electrode. The coating is applied to the outer surface. As an auxiliary electrode (counter electrode), a titanium tube coated with iridium is used, setting it on the axis of the base being processed. To do this, a tooling is made that provides the required axial location of the counter electrode and electrolyte circulation in relation to the outer surface of the pipe being processed. As an electrolyte, processing in which the surface layer of the coating is obtained in the form of a capillary-porous structure with vaporization centers, characterized by the geometric configuration of the porous framework, ensuring its full permeability and the absence of deactivation of vaporization centers, with a porosity of the surface layer - not less than 6%, but not more 12%, and the wetting angle of the surface layer for water is less than 40 degrees, an electrolyte is used that has the ability to form polymer anions, with elements present in their composition capable of forming insoluble oxides, which, embedding in the coating composition, provide an increase in the linear dimensions of the formed coatings. A bath with a volume of 800 l is filled with an electrolyte with the composition (the components are dissolved in distilled water): KOH 2 g/l, Na ₂ SiO ₃ 5 g/l. Due to the volume of the electrolyte ensure the absence of heating of the electrolyte as a result of the process and maintaining a constant temperature of 25°C.

For processing purposes, an anode-cathode mode is used with a frequency of voltage pulses, with an anode current density, with a ratio of the cathode component of the current density to the anode component of the current density, by which the surface layer of the coating is obtained in the form of a capillary-porous structure with vaporization centers, characterized by the geometric configuration of the porous frame , ensuring its full permeability and the absence of deactivation of vaporization centers, with the porosity of the surface layer - not less than 6%, but not more than 12%, and the wetting angle of the surface layer for water - less than 40 degrees.

The base is processed using an auxiliary electrode in the form of a pipe in the specified electrolyte in the specified mode, when an alternating electrical voltage is applied to the metal base being processed and the counter electrode from the condition of alternately performing the function of the cathode and anode and alternately flowing between them, respectively, the anode and cathode currents with the formation of surface of the treated base of the oxide coating. Oxidation is carried out in the MAO mode. The working and auxiliary electrodes are energized from a thyristor power source. The voltage is applied in the form of rectangular pulses with a frequency of 200 Hz and an equal duration of the anode and cathode pulses, each of which is equal to half the period. The ratio of the cathode component of the current to the anode component of the current is equal to one. The anode current density is 10 A/dm ² . The duration of oxidation is chosen taking into account the thickness of the coating obtained, at which the pores formed in it have a through character, with a pore size of the coating comparable to the critical germinal size during nucleate boiling or more than it at the specified porosity, namely, 7 minutes are chosen. During the specified time, the thickness of the surface (technological) layer is increased and, in parallel, a solid oxide layer is formed. The maximum anode voltage, reaching in the MAO mode, is 520 V. The maximum cathode voltage is 140 V. The thickness of the resulting coating is 12 μm.

If the fins used in the first of the above analogs can lead to an increase in heat transfer intensity up to 2.5 times (heat transfer coefficient), then the resulting MAO coating provides an increase in intensity by 3 times.

Example 2

To implement the method of obtaining a coating for intensifying heat transfer on the heat transfer wall of the heat exchanger, the metal base of which is made of an aluminum-containing material, an installation is used, which includes a power source, a bath filled with electrolyte, in which the metal base being processed is placed.

A pipe with an outer diameter of 10 mm and a length of 100 mm, a pipe wall thickness of 1.5 mm from an alloy D16T is taken as a processed base. The tube is placed in an electrolytic bath, it acts as a working electrode. The coating is applied to the outer surface. As an auxiliary electrode (counter electrode), a titanium tube coated with iridium is used, installing it on the axis of the base being processed. To do this, a tooling is made that provides the required axial location of the counter electrode and electrolyte circulation in relation to the outer surface of the pipe being processed. As an electrolyte, processing in which the surface layer of the coating is obtained in the form of a capillary-porous structure with vaporization centers, characterized by the geometric configuration of the porous framework, ensuring its full permeability and the absence of deactivation of vaporization centers, with a porosity of the surface layer - not less than 6%, but not more 12%, and the wetting angle of the surface layer for water is less than 40 degrees, an electrolyte is used that has the ability to form polymer anions, with elements present in their composition capable of forming insoluble oxides, which, embedding in the coating composition, provide an increase in the linear dimensions of the formed coatings. A bath with a volume of 800 l is filled with an electrolyte with the composition (the components are dissolved in distilled water): KOH 2 g/l, Na ₂ SiO ₃ 5 g/l. Due to the volume of the electrolyte ensure the absence of heating of the electrolyte as a result of the process and maintaining a constant temperature of 30°C.

For processing purposes, an anode-cathode mode is used with a frequency of voltage pulses, with an anode current density, with a ratio of the cathode component of the current density to the anode component of the current density, by which the surface layer of the coating is obtained in the form of a capillary-porous structure with vaporization centers, characterized by the geometric configuration of the porous frame , ensuring its full permeability and the absence of deactivation of vaporization centers, with the porosity of the surface layer - not less than 6%, but not more than 12%, and the wetting angle of the surface layer for water - less than 40 degrees.

The base is processed using an auxiliary electrode in the form of a pipe in the specified electrolyte in the specified mode, when an alternating electrical voltage is applied to the metal base being processed and the counter electrode from the condition of alternately performing the function of the cathode and anode and alternately flowing between them, respectively, the anode and cathode currents with the formation of surface of the treated base of the oxide coating. Oxidation is carried out in the MAO mode. The working and auxiliary electrodes are energized from a thyristor power source. The voltage is applied in a sinusoidal form with a frequency of 50 Hz and an equal duration of the anode and cathode pulses, each of which is equal to half the period. The ratio of the cathode component of the current to the anode component of the current is equal to one. The anode current density is 15 A/dm ² . The duration of oxidation is selected taking into account the thickness of the coating obtained, at which the pores formed in it are of a through nature, with a pore size of the coating comparable to the critical seed size during nucleate boiling or more at the indicated porosity, namely, 5 minutes are selected. During the specified time, the thickness of the surface (technological) layer is increased and, in parallel, a solid oxide layer is formed. The maximum anode voltage, reaching in the MAO mode, is 500 V. The maximum cathode voltage is 130 V. The thickness of the resulting coating is 10 μm.

If the fins used in the first of the above analogs can lead to an increase in heat transfer intensity up to 2.5 times (heat transfer coefficient), then the resulting MAO coating provides an increase in intensity by 3.2 times.

Example 3

To implement the method of obtaining a coating for intensifying heat transfer on the heat transfer wall of the heat exchanger, the metal base of which is made of an aluminum-containing material, an installation is used, which includes a power source, a bath filled with electrolyte, in which the metal base being processed is placed.

A pipe with an outer diameter of 10 mm and a length of 100 mm, a pipe wall thickness of 1.5 mm from the AD31 alloy is taken as the base to be processed. The tube is placed in an electrolytic bath, it acts as a working electrode. The coating is applied to the outer surface. As an auxiliary electrode (counter electrode), a titanium tube coated with iridium is used, setting it on the axis of the base being processed. To do this, a tooling is made that provides the required axial location of the counter electrode and electrolyte circulation in relation to the outer surface of the pipe being processed. As an electrolyte, processing in which a surface layer of the coating is obtained in the form of a capillary-porous structure with vaporization centers, characterized by the geometric configuration of the porous framework, ensuring its full permeability and the absence of deactivation of vaporization centers, with a porosity of the surface layer - not less than 6%, but not more 12%, and the wetting angle of the surface layer for water is less than 40 degrees, an electrolyte is used that has the ability to form polymer anions, with elements present in their composition capable of forming insoluble oxides, which, embedding in the coating composition, provide an increase in the linear dimensions of the formed coatings. A bath with a volume of 800 l is filled with an electrolyte with the composition (the components are dissolved in distilled water): KOH 1 g/l, Na ₅ P ₃ O ₁₀ 10 g/l. Due to the volume of the electrolyte ensure the absence of heating of the electrolyte as a result of the process and maintaining a constant temperature of 25°C.

For processing purposes, an anode-cathode mode is used with a frequency of voltage pulses, with an anode current density, with a ratio of the cathode component of the current density to the anode component of the current density, by which the surface layer of the coating is obtained in the form of a capillary-porous structure with vaporization centers, characterized by the geometric configuration of the porous frame , ensuring its full permeability and the absence of deactivation of vaporization centers, with the porosity of the surface layer - not less than 6%, but not more than 12%, and the wetting angle of the surface layer for water - less than 40 degrees.

The base is processed using an auxiliary electrode in the form of a pipe in the specified electrolyte in the specified mode, when an alternating electrical voltage is applied to the metal base being processed and the counter electrode from the condition of alternately performing the function of the cathode and anode and alternately flowing between them, respectively, the anode and cathode currents with the formation of surface of the treated base of the oxide coating. Oxidation is carried out in the MAO mode. The working and auxiliary electrodes are energized from a thyristor power source. The voltage is applied in a sinusoidal form with a frequency of 50 Hz and an equal duration of the anode and cathode pulses, each of which is equal to half the period. The ratio of the cathode current component to the anode current component is 0.8. The anode current density is 5 A/dm ² . The duration of oxidation is selected taking into account the thickness of the coating obtained, at which the pores formed in it are of a through nature, with a pore size of the coating comparable to the critical seed size during nucleate boiling or more than it at the indicated porosity, namely, 10 minutes are selected. During the specified time, the thickness of the surface (technological) layer is increased and, in parallel, a solid oxide layer is formed. The maximum anode voltage, reaching in the MAO mode, is 500 V. The maximum cathode voltage is 190 V. The thickness of the resulting coating is 7 μm.

If the fins used in the first of the above analogs can lead to an increase in heat transfer intensity up to 2.5 times (heat transfer coefficient), then the resulting MDO coating provides an increase in intensity by 3.5 times.

Example 4

To implement the method of obtaining a coating for intensifying heat transfer on the heat transfer wall of the heat exchanger, the metal base of which is made of an aluminum-containing material, an installation is used, which includes a power source, a bath filled with electrolyte, in which the metal base being processed is placed.

A pipe with an outer diameter of 10 mm and a length of 100 mm, a pipe wall thickness of 1.5 mm from an alloy D16T is taken as a processed base. The tube is placed in an electrolytic bath, it acts as a working electrode. The coating is applied to the outer surface. As an auxiliary electrode (counter electrode), a titanium tube coated with iridium is used, installing it on the axis of the base being processed. To do this, a tooling is made that provides the required axial location of the counter electrode and electrolyte circulation in relation to the outer surface of the pipe being processed. As an electrolyte, processing in which a surface layer of a coating is obtained in the form of a capillary-porous structure with vaporization centers, characterized by a porous framework configuration that ensures its complete permeability and the absence of deactivation of vaporization centers, with a surface layer porosity of at least 6%, but not more than 12 %, and the wetting angle of the surface layer for water is less than 40 degrees, an electrolyte is used that has the ability to form polymer anions, with elements present in their composition capable of forming insoluble oxides, which, embedding in the coating composition, provide an increase in the linear dimensions of the coating being formed . A bath with a volume of 800 l is filled with an electrolyte with the composition (the component is dissolved in distilled water): Na ₅ P ₃ O ₁₀ 20 g/l. Due to the volume of the electrolyte ensure the absence of heating of the electrolyte as a result of the process and maintaining a constant temperature of 18°C.

For processing purposes, an anode-cathode mode is used with a frequency of voltage pulses, with an anode current density, with a ratio of the cathode component of the current density to the anode component of the current density, by which the surface layer of the coating is obtained in the form of a capillary-porous structure with vaporization centers, characterized by the geometric configuration of the porous frame , ensuring its full permeability and the absence of deactivation of vaporization centers, with the porosity of the surface layer - not less than 6%, but not more than 12%, and the wetting angle of the surface layer for water - less than 40 degrees.

The base is processed using an auxiliary electrode in the form of a pipe in the specified electrolyte in the specified mode, when an alternating electrical voltage is applied to the metal base being processed and the counter electrode from the condition of alternately performing the function of the cathode and anode and alternately flowing between them, respectively, the anode and cathode currents with the formation of surface of the treated base of the oxide coating. Oxidation is carried out in the MAO mode. The working and auxiliary electrodes are energized from a thyristor power source. The voltage is applied in the form of rectangular pulses with a frequency of 1000 Hz and an equal duration of the anode and cathode pulses, each of which is equal to half the period. The ratio of the cathode component of the current to the anode component of the current is 0.9. The anode current density is 3 A/dm ² . The duration of oxidation is selected taking into account the thickness of the coating obtained, at which the pores formed in it are inherent in the through character, with the pore size of the coating comparable to the critical seed size during nucleate boiling or more at the specified porosity, namely, 15 minutes are selected. During the specified time increase the thickness of the surface (technological) layer and simultaneously form a solid oxide layer. The maximum anode voltage, reaching in the MAO mode, is 520 V. The maximum cathode voltage is 150 V. The thickness of the resulting coating is 12 μm.

If the fins used in the first of the above analogs can lead to an increase in heat transfer intensity up to 2.5 times (heat transfer coefficient), then the resulting MDO coating provides an increase in intensity by 3.1 times.

Example 5

To implement the method of obtaining a coating for intensifying heat transfer on the heat transfer wall of the heat exchanger, the metal base of which is made of an aluminum-containing material, an installation is used, which includes a power source, a bath filled with electrolyte, in which the metal base being processed is placed.

A pipe with an outer diameter of 10 mm and a length of 100 mm, a pipe wall thickness of 1.5 mm from an alloy D16T is taken as a processed base. The tube is placed in an electrolytic bath, it acts as a working electrode. The coating is applied to the outer surface. As an auxiliary electrode (counter electrode), a titanium tube coated with iridium is used, installing it on the axis of the base being processed. To do this, a tooling is made that provides the required axial location of the counter electrode and electrolyte circulation in relation to the outer surface of the pipe being processed. As an electrolyte, processing in which the surface layer of the coating is obtained in the form of a capillary-porous structure with vaporization centers, characterized by the geometric configuration of the porous framework, ensuring its full permeability and the absence of deactivation of vaporization centers, with a porosity of the surface layer - not less than 6%, but not more 12%, and the wetting angle of the surface layer for water is less than 40 degrees, an electrolyte is used that has the ability to form polymer anions, with elements present in their composition capable of forming insoluble oxides, which, embedding in the coating composition, provide an increase in the linear dimensions of the formed coatings. A bath with a volume of 800 l is filled with an electrolyte with the composition (the components are dissolved in distilled water): KOH 2 g/l, Na ₂ SiO ₃ 2 g/l, Na ₅ P ₃ O ₁₀ 30 g/l. Due to the volume of the electrolyte ensure the absence of heating of the electrolyte as a result of the process and maintaining a constant temperature of 15°C.

For processing purposes, an anode-cathode mode is used with a frequency of voltage pulses, with an anode current density, with a ratio of the cathode component of the current density to the anode component of the current density, by which the surface layer of the coating is obtained in the form of a capillary-porous structure with vaporization centers, characterized by the geometric configuration of the porous frame , ensuring its full permeability and the absence of deactivation of vaporization centers, with the porosity of the surface layer - not less than 6%, but not more than 12%, and the wetting angle of the surface layer for water - less than 40 degrees.

The base is processed using an auxiliary electrode in the form of a pipe in the specified electrolyte in the specified mode, when an alternating electrical voltage is applied to the metal base being processed and the counter electrode from the condition of alternately performing the function of the cathode and anode and alternately flowing between them, respectively, the anode and cathode currents with the formation of surface of the treated base of the oxide coating. Oxidation is carried out in the MAO mode. The working and auxiliary electrodes are energized from a thyristor power source. The voltage is applied in the form of rectangular pulses with a frequency of 1500 Hz and an equal duration of the anode and cathode pulses, each of which is equal to half the period. The ratio of the cathode component of the current to the anode component of the current is 0.9. The anode current density is 8 A/dm ² . The duration of oxidation is chosen taking into account the thickness of the coating obtained, at which the pores formed in it have a through character, with a pore size of the coating comparable to the critical germinal size during nucleate boiling or more at the indicated porosity, namely, 8 minutes are chosen. During the specified time, the thickness of the surface (technological) layer is increased and, in parallel, a solid oxide layer is formed. The maximum anode voltage, reaching in the MAO mode, is 480 V. The maximum cathode voltage is 185 V. The thickness of the resulting coating is 15 μm.

If the fins used in the first of the above analogs can lead to an increase in heat transfer intensity up to 2.5 times (heat transfer coefficient), then the resulting MAO coating provides an increase in intensity by 3 times.

Example 6

To implement the method of obtaining a coating for intensifying heat transfer on the heat transfer wall of the heat exchanger, the metal base of which is made of an aluminum-containing material, an installation is used, which includes a power source, a bath filled with electrolyte, in which the metal base being processed is placed.

A pipe with an outer diameter of 10 mm and a length of 100 mm, a pipe wall thickness of 1.5 mm from an alloy D16T is taken as a processed base. The tube is placed in an electrolytic bath, it acts as a working electrode. The coating is applied to the outer surface. As an auxiliary electrode (counter electrode), a titanium tube coated with iridium is used, installing it on the axis of the base being processed. To do this, a tooling is made that provides the required axial location of the counter electrode and electrolyte circulation in relation to the outer surface of the pipe being processed. As an electrolyte, processing in which the surface layer of the coating is obtained in the form of a capillary-porous structure with vaporization centers, characterized by the geometric configuration of the porous framework, ensuring its full permeability and the absence of deactivation of vaporization centers, with a porosity of the surface layer - not less than 6%, but not more 12%, and the wetting angle of the surface layer for water is less than 40 degrees, an electrolyte is used that has the ability to form polymer anions, with elements present in their composition capable of forming insoluble oxides, which, embedding in the coating composition, provide an increase in the linear dimensions of the formed coatings. A bath with a volume of 800 l is filled with an electrolyte with the composition (the component is dissolved in distilled water): Na ₅ P ₃ O ₁₀ 40 g/L. Due to the volume of the electrolyte ensure the absence of heating of the electrolyte as a result of the process and maintaining a constant temperature of 35°C.

For processing purposes, an anode-cathode mode is used with a frequency of voltage pulses, with an anode current density, with a ratio of the cathode component of the current density to the anode component of the current density, by which the surface layer of the coating is obtained in the form of a capillary-porous structure with vaporization centers, characterized by the geometric configuration of the porous frame , ensuring its full permeability and the absence of deactivation of vaporization centers, with the porosity of the surface layer - not less than 6%, but not more than 12%, and the wetting angle of the surface layer for water - less than 40 degrees.

The base is processed using an auxiliary electrode in the form of a pipe in the specified electrolyte in the specified mode, when an alternating electrical voltage is applied to the metal base being processed and the counter electrode from the condition of alternately performing the function of the cathode and anode and alternately flowing between them, respectively, the anode and cathode currents with the formation of surface of the treated base of the oxide coating. Oxidation is carried out in the MAO mode. The working and auxiliary electrodes are energized from a thyristor power source. The voltage is applied in the form of rectangular pulses with a frequency of 3000 Hz and an equal duration of the anode and cathode pulses, each of which is equal to half the period. The ratio of the cathode current component to the anode current component is 1.05. The anode current density is 20 A/dm ² . The duration of oxidation is selected taking into account the thickness of the coating obtained, at which the pores formed in it are of a through nature, with a pore size of the coating comparable to the critical seed size during nucleate boiling or more than it at the indicated porosity, namely, 3 minutes are selected. During the specified time, the thickness of the surface (technological) layer is increased and, in parallel, a solid oxide layer is formed. The maximum anode voltage, reaching in the MAO mode, is 610 V. The maximum cathode voltage is 120 V. The thickness of the resulting coating is 15 μm.

If the fins used in the first of the above analogs can lead to an increase in heat transfer intensity up to 2.5 times (heat transfer coefficient), then the resulting MAO coating provides an increase in intensity by 3.9 times.

Example 7

To implement the method of obtaining a coating for intensifying heat transfer on the heat transfer wall of the heat exchanger, the metal base of which is made of an aluminum-containing material, an installation is used, which includes a power source, a bath filled with electrolyte, in which the metal base being processed is placed.

A pipe with an outer diameter of 10 mm and a length of 100 mm, a pipe wall thickness of 1.5 mm from an alloy D16T is taken as a processed base. The tube is placed in an electrolytic bath, it acts as a working electrode. The coating is applied to the outer surface. As an auxiliary electrode (counter electrode), a titanium tube coated with iridium is used, installing it on the axis of the base being processed. To do this, a tooling is made that provides the required axial location of the counter electrode and electrolyte circulation in relation to the outer surface of the pipe being processed. As an electrolyte, processing in which the surface layer of the coating is obtained in the form of a capillary-porous structure with vaporization centers, characterized by the geometric configuration of the porous framework, ensuring its full permeability and the absence of deactivation of vaporization centers, with a porosity of the surface layer - not less than 6%, but not more 12%, and the wetting angle of the surface layer for water is less than 40 degrees, an electrolyte is used that has the ability to form polymer anions, with elements present in their composition capable of forming insoluble oxides, which, embedding in the coating composition, provide an increase in the linear dimensions of the formed coatings. A bath with a volume of 800 l is filled with an electrolyte with the composition (the components are dissolved in distilled water): KOH 6 g/l, Na ₂ SiO ₃ 15 g/l. Due to the volume of the electrolyte ensure the absence of heating of the electrolyte as a result of the process and maintaining a constant temperature of 45°C.

For processing purposes, an anode-cathode mode is used with a frequency of voltage pulses, with an anode current density, with a ratio of the cathode component of the current density to the anode component of the current density, by which the surface layer of the coating is obtained in the form of a capillary-porous structure with vaporization centers, characterized by the geometric configuration of the porous frame , ensuring its full permeability and the absence of deactivation of vaporization centers, with the porosity of the surface layer - not less than 6%, but not more than 12%, and the wetting angle of the surface layer for water - less than 40 degrees.

The base is processed using an auxiliary electrode in the form of a pipe in the specified electrolyte in the specified mode, when an alternating electrical voltage is applied to the metal base being processed and the counter electrode from the condition of alternately performing the function of the cathode and anode and alternately flowing between them, respectively, the anode and cathode currents with the formation of surface of the treated base of the oxide coating. Oxidation is carried out in the MAO mode. The working and auxiliary electrodes are energized from a thyristor power source. The voltage is applied in the form of rectangular pulses with a frequency of 500 Hz and an equal duration of the anode and cathode pulses, each of which is equal to half the period. The ratio of the cathode current component to the anode current component is 1.1. The anode current density is 30 A/dm ² . The duration of oxidation is selected taking into account the thickness of the coating obtained, at which the pores formed in it are of a through nature, with a pore size of the coating comparable to the critical seed size during nucleate boiling or more than it at the indicated porosity, namely, 1.5 minutes are selected. During the specified time, the thickness of the surface (technological) layer is increased and, in parallel, a solid oxide layer is formed. The maximum anode voltage, reaching in the MAO mode, is 620 V. The maximum cathode voltage is 150 V. The thickness of the resulting coating is 12 μm.

If the fins used in the first of the above analogs can lead to an increase in heat transfer intensity up to 2.5 times (heat transfer coefficient), then the resulting MAO coating provides an increase in intensity by 3.9 times.

Example 8

To implement the method of obtaining a coating for intensifying heat transfer on the heat transfer wall of the heat exchanger, the metal base of which is made of an aluminum-containing material, an installation is used, which includes a power source, a bath filled with electrolyte, in which the metal base being processed is placed.

A pipe with an outer diameter of 10 mm and a length of 100 mm, a pipe wall thickness of 1.5 mm from an alloy D16T is taken as a processed base. The tube is placed in an electrolytic bath, it acts as a working electrode. The coating is applied to the outer surface. As an auxiliary electrode (counter electrode), a titanium tube coated with iridium is used, installing it on the axis of the base being processed. To do this, a tooling is made that provides the required axial location of the counter electrode and electrolyte circulation in relation to the outer surface of the pipe being processed. As an electrolyte, processing in which the surface layer of the coating is obtained in the form of a capillary-porous structure with vaporization centers, characterized by the geometric configuration of the porous framework, ensuring its full permeability and the absence of deactivation of vaporization centers, with a porosity of the surface layer - not less than 6%, but not more 12%, and the wetting angle of the surface layer for water is less than 40 degrees, an electrolyte is used that has the ability to form polymer anions, with elements present in their composition capable of forming insoluble oxides, which, embedding in the coating composition, provide an increase in the linear dimensions of the formed coatings. A bath with a volume of 800 l is filled with an electrolyte with the composition (the components are dissolved in distilled water): KOH 5 g/l, Na ₂ SiO ₃ 20 g/l. Due to the volume of the electrolyte ensure the absence of heating of the electrolyte as a result of the process and maintaining a constant temperature of 55°C.

For processing purposes, an anode-cathode mode is used with a frequency of voltage pulses, with an anode current density, with a ratio of the cathode component of the current density to the anode component of the current density, by which the surface layer of the coating is obtained in the form of a capillary-porous structure with vaporization centers, characterized by the geometric configuration of the porous frame , ensuring its full permeability and the absence of deactivation of vaporization centers, with the porosity of the surface layer - not less than 6%, but not more than 12%, and the wetting angle of the surface layer for water - less than 40 degrees.

The base is processed using an auxiliary electrode in the form of a pipe in the specified electrolyte in the specified mode, when an alternating electrical voltage is applied to the metal base being processed and the counter electrode from the condition of alternately performing the function of the cathode and anode and alternately flowing between them, respectively, the anode and cathode currents with the formation of surface of the treated base of the oxide coating. Oxidation is carried out in the MAO mode. The working and auxiliary electrodes are energized from a thyristor power source. The voltage is applied in the form of rectangular pulses with a frequency of 500 Hz and an equal duration of the anode and cathode pulses, each of which is equal to half the period. The ratio of the cathode current component to the anode current component is 1.1. The anode current density is 50 A/dm ² . The duration of oxidation is chosen taking into account the resulting thickness of the coating, at which the pores formed in it are inherently through in nature, with a pore size of the coating comparable to the critical germinal size during nucleate boiling or more than it at the specified porosity, namely, 2 minutes are selected. During the specified time, the thickness of the surface (technological) layer is increased and, in parallel, a solid oxide layer is formed. The maximum anode voltage, reaching in the MAO mode, is 600 V. The maximum cathode voltage is 140 V. The thickness of the resulting coating is 20 μm.

If the fins used in the first of the above analogs can lead to an increase in heat transfer intensity up to 2.5 times (heat transfer coefficient), then the resulting MDO coating provides an increase in intensity by 3.8 times.

Example 9

To implement the method of obtaining a coating for intensifying heat transfer on the heat transfer wall of the heat exchanger, the metal base of which is made of an aluminum-containing material, an installation is used, which includes a power source, a bath filled with electrolyte, in which the metal base being processed is placed.

A pipe with an outer diameter of 10 mm and a length of 100 mm, a pipe wall thickness of 1.5 mm from an alloy D16T is taken as a processed base. The tube is placed in an electrolytic bath, it acts as a working electrode. The coating is applied to the outer surface. As an auxiliary electrode (counter electrode), a titanium tube coated with iridium is used, setting it on the axis of the base being processed. To do this, a tooling is made that provides the required axial location of the counter electrode and electrolyte circulation in relation to the outer surface of the pipe being processed. As an electrolyte, processing in which a surface layer of the coating is obtained in the form of a capillary-porous structure with vaporization centers, characterized by the geometric configuration of the porous framework, ensuring its full permeability and the absence of deactivation of vaporization centers, with a porosity of the surface layer - not less than 6%, but not more 12%, and the wetting angle of the surface layer for water is less than 40 degrees, an electrolyte is used that has the ability to form polymer anions, with elements present in their composition capable of forming insoluble oxides, which, embedding in the coating composition, provide an increase in the linear dimensions of the formed coatings. A bath with a volume of 800 l is filled with an electrolyte with the composition (the components are dissolved in distilled water): KOH 5 g/l, Na ₂ SiO ₃ 5 g/l, Na ₅ P ₃ O ₁₀ 40 g/l. Due to the volume of the electrolyte ensure the absence of heating of the electrolyte as a result of the process and maintaining a constant temperature of 40°C.

For processing purposes, an anode-cathode mode is used with a frequency of voltage pulses, with an anode current density, with a ratio of the cathode component of the current density to the anode component of the current density, by which the surface layer of the coating is obtained in the form of a capillary-porous structure with vaporization centers, characterized by the geometric configuration of the porous frame , ensuring its full permeability and the absence of deactivation of vaporization centers, with the porosity of the surface layer - not less than 6%, but not more than 12%, and the wetting angle of the surface layer for water - less than 40 degrees.

The base is processed using an auxiliary electrode in the form of a pipe in the specified electrolyte in the specified mode, when an alternating electrical voltage is applied to the metal base being processed and the counter electrode from the condition of alternately performing the function of the cathode and anode and alternately flowing between them, respectively, the anode and cathode currents with the formation of surface of the treated base of the oxide coating. Oxidation is carried out in the MAO mode. The working and auxiliary electrodes are energized from a thyristor power source. The voltage is applied in the form of rectangular pulses with a frequency of 2000 Hz and an equal duration of the anode and cathode pulses, each of which is equal to half the period. The ratio of the cathode component of the current to the anode component of the current is 1. The anode current density is 35 A/dm ² . The duration of oxidation is selected taking into account the resulting thickness of the coating, at which the pores formed in it are inherently through in nature, with a pore size of the coating comparable to the critical germinal size during nucleate boiling or more than it at the specified porosity, namely, 3 minutes are selected. During the specified time, the thickness of the surface (technological) layer is increased and, in parallel, a solid oxide layer is formed. The maximum anode voltage, reaching in the MAO mode, is 580 V. The maximum cathode voltage is 135 V. The thickness of the resulting coating is 22 μm.

If the fins used in the first of the above analogs can lead to an increase in heat transfer intensity up to 2.5 times (heat transfer coefficient), then the resulting MDO coating provides an increase in intensity by 3.6 times.

Example 10

To implement the method of obtaining a coating for intensifying heat transfer on the heat transfer wall of the heat exchanger, the metal base of which is made of an aluminum-containing material, an installation is used, which includes a power source, a bath filled with electrolyte, in which the metal base being processed is placed.

A pipe with an outer diameter of 10 mm and a length of 100 mm, a pipe wall thickness of 1.5 mm from an alloy D16T is taken as a processed base. The tube is placed in an electrolytic bath, it acts as a working electrode. The coating is applied to the outer surface. As an auxiliary electrode (counter electrode), a titanium tube coated with iridium is used, installing it on the axis of the base being processed. To do this, a tooling is made that provides the required axial location of the counter electrode and electrolyte circulation in relation to the outer surface of the pipe being processed. As an electrolyte, processing in which the surface layer of the coating is obtained in the form of a capillary-porous structure with vaporization centers, characterized by the geometric configuration of the porous framework, ensuring its full permeability and the absence of deactivation of vaporization centers, with a porosity of the surface layer - not less than 6%, but not more 12%, and the wetting angle of the surface layer for water is less than 40 degrees, an electrolyte is used that has the ability to form polymer anions, with elements present in their composition capable of forming insoluble oxides, which, embedding in the coating composition, provide an increase in the linear dimensions of the formed coatings. A bath with a volume of 800 l is filled with an electrolyte with the composition (components are dissolved in distilled water): Na ₂ SiO ₃ 10 g/l, Na ₅ P ₃ O ₁₀ 30 g/l. Due to the volume of the electrolyte ensure the absence of heating of the electrolyte as a result of the process and maintaining a constant temperature of 20°C.

For processing purposes, an anode-cathode mode is used with a frequency of voltage pulses, with an anode current density, with a ratio of the cathode component of the current density to the anode component of the current density, by which the surface layer of the coating is obtained in the form of a capillary-porous structure with vaporization centers, characterized by the geometric configuration of the porous frame , ensuring its full permeability and the absence of deactivation of vaporization centers, with the porosity of the surface layer - not less than 6%, but not more than 12%, and the wetting angle of the surface layer for water - less than 40 degrees.

The base is processed using an auxiliary electrode in the form of a pipe in the specified electrolyte in the specified mode, when an alternating electrical voltage is applied to the metal base being processed and the counter electrode from the condition of alternately performing the function of the cathode and anode and alternately flowing between them, respectively, the anode and cathode currents with the formation of surface of the treated base of the oxide coating. Oxidation is carried out in the MAO mode. The working and auxiliary electrodes are energized from a thyristor power source. The voltage is applied in a sinusoidal form with a frequency of 50 Hz and an equal duration of the anode and cathode pulses, each of which is equal to half the period. The ratio of the cathode current component to the anode current component is 1.1. The anode current density is 5 A/dm ² . The duration of oxidation is selected taking into account the thickness of the coating obtained, at which the pores formed in it have a through character, with a pore size of the coating comparable to the critical seed size during nucleate boiling or more than it at the specified porosity, namely, 40 minutes are selected. During the specified time, the thickness of the surface (technological) layer is increased and, in parallel, a solid oxide layer is formed. The maximum anode voltage, reaching in the MAO mode, is 490 V. The maximum cathode voltage is 110 V. The thickness of the resulting coating is 35 μm.

If the fins used in the first of the above analogs can lead to an increase in heat transfer intensity up to 2.5 times (heat transfer coefficient), then the resulting MDO coating provides an increase in intensity by 3.4 times.

Claims (8)

1. The heat transfer wall of the heat exchanger, containing a metal base, characterized in that the base is made of an aluminum-containing material and provided with a coating to intensify heat transfer of the heat transfer wall of the heat exchanger - a ceramic coating obtained by microarc oxidation of the surface of the specified base, the surface layer of the coating is made in the form of a capillary-porous structure with centers of vaporization, characterized by the geometric configuration of the porous framework, ensuring its full permeability and the absence of deactivation of the centers of vaporization, the coating is made with a thickness at which the pores formed in it have a through character, while the pore size of the coating is comparable to the critical embryonic size during nucleate boiling or more than it with the following surface layer porosity of not less than 6%, but not more than 12%, and the wetting angle of the surface layer for water is less than 40 degrees. 2. The wall according to claim 1, characterized in that the thickness of the coating, at which the pores formed in it have a through character, is from 7 to 35 microns, including the indicated interval values. 3. The wall according to claim 1, characterized in that the pore size of the coating, comparable to the critical nucleation size during nucleate boiling or more, is 100 nm or more, but not more than 10 μm. 4. A method for forming a coating for intensifying heat transfer of a heat transfer wall of a heat exchanger, including processing a metal base, characterized in that the metal base is processed from an aluminum-containing material by conducting microarc oxidation of the surface of the metal base in the electrolyte, thereby forming a ceramic coating, while processing is carried out in the anode - cathode mode with a frequency of voltage pulses, with an anode current density, with a ratio of the cathode component of the current density to the anode component of the current density, using an electrolyte, which produces a surface layer of the coating in the form of a capillary-porous structure with centers of vaporization, characterized by the geometric configuration of the porous frame, ensuring its complete permeability and the absence of deactivation of vaporization centers, with a porosity of the surface layer of at least 6%, but not more than 12%, and a wetting angle of the surface layer for water of less than 40 degrees, the oxidation time is selected taking into account the resulting coating thickness, at which its pores have a through character, with a pore size of the coating comparable to the critical nucleation size at nucleate boiling or more than it at the indicated porosity. 5. The method according to claim 4, characterized in that, as an electrolyte, processing in which a surface layer of the coating is obtained in the form of a capillary-porous structure with vaporization centers, characterized by the geometric configuration of the porous framework, ensuring its full permeability and the absence of deactivation of vaporization centers, with surface layer porosity not less than 6%, but not more than 12%, and the wetting angle of the surface layer for water is less than 40 degrees, an electrolyte is used that has the ability to form polymer anions, with elements present in their composition capable of forming insoluble oxides, which, embedding in the composition of the coating, provide an increase in the linear dimensions of the coating being formed, namely, a silicate-alkaline electrolyte with a KOH content of 1 to 6 g/l and Na ₂ SiO ₃ from 2 to 20 g/l and/or a phosphate electrolyte with a content of Na ₅ P ₃ O ₁₀ from 10 to 40 g/l, including the specified values of the intervals, or phosphate electrolyte, which also includes the addition of KOH or Na ₂ SiO ₃ . 6. The method according to claim. 4 or 5, characterized in that during processing, the temperature of the electrolyte is maintained from 15 to 55°C, including the indicated values of the interval. 7. The method according to p. 4, characterized in that when processing in the anode-cathode mode with a frequency of voltage pulses, with an anode current density, with a ratio of the cathode component of the current density to the anode component of the current density, by which the surface layer of the coating is obtained in the form of a capillary- a porous structure with vaporization centers, characterized by the geometric configuration of the porous frame, ensuring its full permeability and the absence of deactivation of vaporization centers, with a surface layer porosity of at least 6%, but not more than 12%, and a surface layer wetting angle for water of less than 40 degrees, use voltage pulse frequency from 50 to 3000 Hz, anode current density from 3 to 50 A/dm ² , the ratio of the cathode component of the current strength to the anode component of the current strength from 0.8 to 1.1, including the indicated values of the intervals. 8. The method according to claim 4, characterized in that the oxidation time is chosen taking into account the resulting thickness of the coating, in which the pores formed in it have a through character, with a pore size of the coating comparable to the critical germ size during nucleate boiling or more than it at the specified porosity , namely, choose from 1.5 to 40 minutes, including the specified values of the interval.

Images