RU2640888C1 - Intensive steam condenser with contrast and gradient wetting - Google Patents

Abstract

FIELD: heating system.

SUBSTANCE: intensive steam condenser with a contrast and gradient wetting is made in the form of a cooled cylinder, on the outer surface of which alternating transverse annular strips with hydrophobic coating with a gradient wetting angle and strips with hydrophilic coating are applied. Moreover, the wetting angle of the surface with the hydrophobic coating decreases from the line of the maximum value of the wetting angle to the line of the minimum value of the wetting angle.

EFFECT: invention allows to increase the intensity of condensation due to the application of special coatings with gradient wetting, and due to the reduction in the hydraulic resistance during the flow of a two-phase flow along the condensation surface.

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Description

The invention relates to the field of mini- and microsystems that are used in energy and transport and can be used in devices for cooling electronics. The invention relates to the field of intensification of heat transfer during condensation inside pipes and channels, as well as condensation on surfaces located in the volume of steam.

Significant progress has been made in methods of intensifying heat transfer during boiling and evaporation. In evaporators, heat transfer coefficients can reach values up to 100-300 kW / (m ² ⋅K) and higher.

At the same time, much less progress has been achieved in heat transfer intensification methods during steam condensation. Various methods are used to intensify heat transfer during film condensation, for example, increasing the steam velocity or finning the heat transfer surface. For example, in the case of in-pipe condensation with intensifiers in the form of longitudinal ribs, the average heat transfer coefficients can reach values of the order of 10 kW / (m ² ⋅ K), approximately an order of magnitude less. As a result, the steam condenser is the largest part of mini- and microsystems and can be an order of magnitude, and in some cases two orders of magnitude, superior to the evaporation part in mass and dimensions.

Very effective is such a method of intensifying heat transfer as the use of water repellents for the transition from the film mode of condensation to drip. Drip condensation occurs on solid surfaces with relatively low surface energy. To create such surfaces, various deposition or ion doping technologies are used, for example, chemical, electrochemical deposition, plasma evaporation methods and others. To obtain drop condensation, thin-layer organic compounds or polymer coatings with low surface energy, diamond-like or ion-doped coatings based on aluminum and copper, nanoscale structures and others are used.

However, the drip mode of condensation cannot be maintained for a long time due to clogging or destruction of hydrophobic coatings. This limits the practical application of drip condensation, the heat transfer rate of which is 15–20 times higher than with film condensation.

It is known in the art to use hydrophobic surfaces to enhance heat transfer during boiling.

A known method of intensification of heat transfer during boiling on a smooth surface [RF patent No. 2542253, 03/18/2013, B05D 1/00, F28F 13/00], in which to ensure the intensification of heat transfer during boiling on a smooth cooled surface form many hydrophobic areas located in a checkerboard pattern okay.

A known method of manufacturing a cooling system of electronic and microelectronic equipment [RF patent No. 2581342, 06/06/2014, F15D 1/02, B82 B 3/00, B82Y 40/00], including applying to the surface of the microchannel nanostructured regions with hydrophobic properties in the form of strips across single-phase or two-phase coolant flow.

A device is known for forming a brook fluid flow in a flat micro- or mini-channel [RF patent No. 2588917, 12/15/2014, F28C 3/06, F28D 5/02, F28F 13/18, H05K 7/20, H01L 23/44] in which a hydrophobic nanocoating is deposited on the surface of the substrate along the channel on both sides of the electronic fuel element, which can significantly reduce the hydraulic resistance of the walls and coolant.

All these technical solutions are designed to intensify heat transfer in evaporators.

For the intensification of heat transfer in cooling systems that are in practical use, finned ribs of various shapes, surface ordered and disordered roughness, partial perforation, and much more are used.

The closest technical solution, which can be considered as a prototype, is described in the article [Kabov O.A., Marchuk I.V. and Legros J-C. Conjugated heat transfer at flow condensation in minichannel with longitudinal fins, Second Int. Conference on Microchannels and Minichannels, Ed. S.G. Kandlikar, June 17-19, 2004, Rochester, N.Y., ASME, NewYork, pp. 641-648 (2004)] and is widely used by a number of industrial companies (eg, EuroHeatPipe S.A.). The article investigates condensation in a cylindrical cooled condenser (in-tube condenser) with longitudinal ribs of various shapes and sizes. Ribs of two types are considered: trapezoidal with rounded vertices and ribs having the shape of curves of the Adamek's parametric family (Adamek's parametrical family). The ribs are evenly spaced around the perimeter. The characteristic height and pitch of the ribs are application specific. The ribs significantly increase the condensation surface, and also contribute to the inhomogeneous distribution of the liquid around the perimeter of the pipe, including due to the action of capillary forces. Capacitors of this type are used both in terrestrial conditions and in microgravity (heat pipes, loop heat pipes).

The disadvantages of the design described above

1. The production of ribs requires a significant deformation of the walls of the pipes and surfaces. This applies both to the case of condensation inside the pipes and channels, and condensation on surfaces located in the volume of steam, for example shell-and-tube heat exchangers with packages of horizontal pipes. In general, this leads to an increase in the metal consumption of the capacitor and an increase in its weight.

2. With a fixed inner diameter of the pipe or channel, the ribs significantly reduce the bore for the two-phase flow, which can significantly increase the hydraulic resistance of the channel. With a fixed external diameter of the pipe during condensation on surfaces located in the volume of steam, the ribs increase the external diameter, which may be unacceptable from the point of view of the assembly technology of the apparatus. There are technologies for manufacturing ribs by reducing the inner diameter of the pipes, but this leads to an increase in hydraulic resistance. The objective of the invention is to increase the efficiency of the condenser by increasing the condensation intensity due to the use of special coatings with gradient wetting and reducing hydraulic resistance during a two-phase flow along the condensation surface. The objective of the invention is also to reduce the weight and metal structure.

The problem is solved due to the new design of the steam condenser, which allows you to create a stable film-drop condensation mode.

The problem is solved in that in an intensive condenser of steam with contrast and gradient wetting, made in the form of a cooled cylinder with a structured surface, according to the invention, a nano- or microcoating is applied to the outer surface of the cylinder. The coating is applied in such a way that the transverse annular bands of the hydrophobic coating with the gradient wetting angle alternate with the annular transverse stripes of the hydrophilic coating, and the wetting angle of the surface with the hydrophobic coating decreases from the line of the maximum value of the contact angle to the line of the minimum value of the contact angle so that the droplets move along a surface with a hydrophobic coating occurs in a direction perpendicular to the main flow of condensate over a surface with a hydrophilic coating iem.

The essence of the technical solution is illustrated by drawings.

In FIG. 1 shows a cross section of a pipe (condenser channel).

In FIG. 2 shows a side view and a diagram of fluid movement on the proposed surface.

In FIG. 3 shows a diagram of the movement of condensate droplets on a surface with a wetting gradient.

Where 1 is the pipe (pipe wall); 2 - steam; 3 - a stream of condensate; 4 - surface with a hydrophilic coating; 5 - surface with a hydrophobic coating with gradient wetting; 6 - microdrops of condensate; 7 - direction of movement of droplets on a hydrophobic surface; 8 - direction of flow of condensate streams; 9 - line of the maximum value of the wetting angle; 10 - direction of the wetting gradient; 11 - cooling liquid; 12 - liquid condensate in the lower part of the pipe; 13 - line of the minimum value of the wetting angle; g is the direction of gravity; F is the force acting on the drop on the surface with a gradient of the wetting angle; F ₁ , F ₂ - the resulting surface tension forces; σ ₁ , σ ₂ - surface tension with maximum and minimum values of the wetting angle; θ ₁ , θ ₂ - the maximum and minimum wetting angle.

The condenser consists of a pipe 1, through which coolant 11 circulates, located inside the casing (not shown). Refrigerant condensation occurs between the pipe and the casing, i.e. on the outside of the pipe.

A special coating is applied along the outer surface of the pipe in the form of transverse strips so that a plurality of annular strips with a hydrophobic surface 5 with contrast and gradient wetting are formed, which alternate with annular strips with a hydrophilic surface 4, as shown in FIG. 2.

On the surface with a hydrophobic coating, a non-uniform contact wetting angle takes place. The wetting angle decreases from the line of the maximum value of the contact angle 9 (center of the hydrophobic strip) to the line of the minimum value of the contact angle 13 (periphery), i.e. toward a hydrophilic coated surface as shown in FIG. 2.

Currently, there are many technologies for the production of surfaces with a special molecular coating and with contrast and gradient wetting [W. Choi, A. Tuteja, J.M. Mabry, R.E. Cohen, G.H. McKinley. A modified cassie-baxter relationship to explain contact angle hysteresis and anisotropy on non-wetting textured surfaces, J. Colloid Interface Sci. 339 (2009) 208-216; A. Dubov, A. Mourran, M. Moeller, O. Vinogradova. Contact angle hysteresis on superhydrophobic stripes, J. Chem. Phys. 141 (2014) 074710; N. Guillot, M. Chapelle. Lithographied nanostructures as nanosensors, Journal of Nanophotonics, Vol. 6, (2012) 064506 (28pp); H. Lan, Y. Ding, 2012. Ordering, positioning and uniformity of quantumdot arrays. NanoToday 7 94 123; Biswas A, Bayer IS, Biris AS, Wang T, Dervishi E, Faupel F (2012). Advances in top-downand bottom-upsur face nanofabrication: techniques, applications and future prospects. AdvColloid Interface Sci 170 (1-2): 2-27].

Coatings can be obtained, for example, by treating the surface with a chemical method (applying a monolayer of molecules of another substance) so that a region with a nanoscale roughness and a higher contact angle of contact appears on the surface.

The thickness of the structures depends on the type of surface and does not significantly affect the thermal resistance. It should be noted that the thickness of the coatings can be on the order of tens of nanometers or several micrometers, i.e. the coating practically does not reduce the bore for the two-phase flow and does not increase the outer diameter of the pipes.

In zones with a hydrophilic coating, where the surface is completely wettable, film condensation occurs, and in zones with a hydrophobic coating, droplet condensation occurs.

The device operates as follows. Coolant 11 is pumped through the pipe, steam 2 - through the annular space (between the pipe and the condenser casing). Heat is transferred from one coolant to another through the surface of the pipe.

On surfaces with a hydrophobic coating 5, droplet condensation with a very high heat transfer rate is realized, which is confirmed in a number of works [Hu, HW, Tang, GH, Niu, D. Experimental investigation of condensation heat transfer on hybrid wettability finned tube with large amount of noncondensable gas . (2015) International Journal of Heat and Mass Transfer, 85, pp. 513-523; Mahapatra, P.S., Ghosh, A., Ganguly, R., Megaridis, C.M. Keydesign and operating parameters for enhancing drop wisecondensation through wettability patterning (2016) International Journal of Heat and MassTransfer, 92, pp. 877-883; K. Rykaczewski, A.T. Paxson, M. Staymates, M.L. Walker, X. Sun, S. Anand, S. Srinivasan, G.H. McKinley, J. Chinn, J.H. Scott, K.K. Varanasi. "Drop wise condensation about flow surface tension fluids on omniphobic surfaces," SciRep. Mar 5, 4: 4158, Nature (2014); Richard W. Bonner III. "Dropwise condensation on surfaces with graded hydrophobicity," ASME 2009 Heat Transfer Summer Conference collocated with the Inter PACK09 and 3rd Energy Sustainability Conferences, Paper No. HT2009-88516, pp. 491-495, (2009); Nenad Miljkovic, Ryan Enright, Youngsuk et al. Jumping-Droplet-Enhanced Condensation on Scalable Superhydrophobic Nanostructured Surfaces // Nano Lett., 13 (1), 2013. pp. 179-187].

On surfaces with a hydrophilic coating 4, liquid transport occurs.

When droplet condensation is realized, the surface with a hydrophobic coating is covered with microdrops of condensate. The surface between the microdroplets is covered with an ultrathin condensate film having a very low thermal resistance.

The hydrophobic coated surface strip has two regions located on both sides of the line of maximum contact angle 9, which is also the axis of symmetry of the strip. In each region there is an inhomogeneous contact wetting angle, decreasing towards the adjacent strip with a hydrophilic surface.

The nature of the droplet movement from the line of the maximum value of the angle of contact (center of the hydrophobic surface) to the line of the minimum value of the angle of contact (periphery) is determined by the properties of the liquid, the properties of the substrate, the coefficient of surface tension, the size of the drop, the angle of contact, and the gradient of the angle of contact in the transverse direction of the hydrophobic surface. In areas with maximum and minimum contact angles (θ ₁ , θ ₂ ), surface tension forces, σ ₁ , σ ₂ , act tangential to the surface of the liquid. The resulting surface tension forces, F ₁ , F ₂ acting on a hydrophobic surface, depend on the projection of the surface tension forces, σ ₁ , σ ₂ . The value of the force acting on a drop on a surface with a gradient of the wetting angle is determined from the relation F = F ₁ -F ₂ , and the larger the difference in the angles θ ₁ , θ ₂ , the greater the value of the force driving the drop F. Thus, due to the gradient contact angle wetting along the perimeter of the microdrops of condensate 6 there is a wetting gradient 10, which moves the microdrops of condensate in the direction 7 to the hydrophilic surface, which provides a significant intensification of droplet condensation, FIG. 3. Having reached the edge of the hydrophilic surface, the microdrops merge with the condensate stream 3 and flow down the pipe in direction 8. The condensate is transported by gravity, FIG. 3.

It is important to note that the movement of microdroplets occurs in a direction perpendicular to the main stream 12 of liquid and vapor condensate, which leads to the smallest possible droplet size.

The proposed steam condenser can be made not only of highly thermally conductive materials, but also from a wide range of other materials, such as glass, plastic, crystalline silicon, etc. This fact allows us to use the proposed steam condenser in various kinds of mini- and microsystems in chemical, pharmaceutical and food industry.

Steam condenser power adjustment is carried out by simple adjustment of the condenser wall temperature, surface length, pipe size.

The device can operate in normal gravity and microgravity. In microgravity conditions, the orientation of the device can be arbitrary. Under normal gravity conditions, the device position is recommended when the axis of the steam condenser pipe is perpendicular to the direction of the gravity vector. In this case, the maximum condensation intensity will be achieved.

It is known that under microgravity conditions the length of smooth-tube capacitors should be 2-3 times longer than in terrestrial applications. This is due to the implementation of the annular flow regime and makes the use of light thin-walled smooth pipes in space applications inefficient. EFFECT: invention allows to obtain high heat transfer intensity in an annular flow regime and significantly increase the efficiency of condensation in space.

Claims (1)

Intensive steam condenser with contrast and gradient wetting, made in the form of a cooled cylinder with a structured surface, characterized in that a nano- or microcoating is applied to the outer surface of the cylinder so that the transverse annular bands with a hydrophobic coating with a gradient angle of wetting alternate with annular transverse stripes with a hydrophilic coating, and the wetting angle of the surface with a hydrophobic coating decreases from the line of the maximum value of the contact angle to the line of the minimum value of the wetting angle so that the movement of microdrops on the surface with a hydrophobic coating occurs in a direction perpendicular to the main flow of condensate on the surface with a hydrophilic coating.

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