JP2005098694A - Heat exchanging system between solid and fluid - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a heat exchanging system with low cost and high heat transfer efficiency. <P>SOLUTION: This system for exchanging heat between a solid and a fluid comprises a solid such as metal having a surface in contact with a fluid. A porous layer formed of a group of one or more types of particles having a diameter of 100nm or less selected from copper oxide (CuO), carbon C and alumina Al<SB>2</SB>O<SB>3</SB>and including a number of pores is formed on the surface. The solid has a second surface in contact with a second fluid having a temperature different from a first fluid to separate the first fluid and the second fluid to prevent them from being mixed. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention belongs to a heat exchange system between a solid and a fluid.

    The transfer and transfer of thermal energy can be broadly divided into three transfer phenomena: thermal radiation, heat conduction, and heat transfer. In engineering applications, sensible heat is generated by some kind of heat medium. And using heat transport and transformation and heat transmission into other media to increase thermal efficiency is not only energy saving, but is now a significant on a global scale. It has become an issue.

  It is an object of the present invention to provide a heat exchange system that exhibits an epoch-making high efficiency at a low cost with respect to convective heat transfer in a heat transfer phenomenon.

  Conventionally, the heat transfer phenomenon is Re (Reynolds number) representing the dynamic characteristics of fluid motion, Pr (Prandtl number) characterizing the dynamic physical properties of the working fluid, and Nu (representing heat transfer between the working fluid and the solid surface. (Nosselt number), thermal correlation with Gr (Grasshof number) representing the thermal flow characteristics when the working fluid is a buoyancy driven flow due to temperature difference or density difference, etc. Great results have been achieved in terms of transmission rate estimation.

However, many of these experimental correlation equations based on these dimensionless numbers are based on the temperature distribution and velocity distribution of the fluid against the background of the boundary layer theory. According to the present invention, the heat transfer mechanism in the laminar boundary bottom layer, which is handled only by molecular heat conduction on the fluid side in the conventional theory, is greatly improved, and the heat transfer rate is increased.
In convective heat transfer, the system of the present invention can be used to transfer heat from a hot solid to a cold fluid if the hot source is on the solid side, and conversely, if the hot source is on the fluid side, It is also applicable when transferring heat from a fluid to a low-temperature solid.

In order to solve the problem, the heat exchange system of the present invention is:
In a system for exchanging heat between a solid such as a metal and a fluid such as water or air, the system includes a solid having a surface in contact with the fluid, and the surface is composed of a group of particles having a diameter of 100 nm or less. A porous layer including pores is formed.
Examples of the particle group include copper oxide CuO, carbon C, and alumina Al ₂ O ₃ . The particles are usually spherical, but are not limited thereto. For example, in the case of carbon, the particles may be a single-layer or multilayer tube, and in the case of carbon nanotubes, the diameter may be about 100 nm or less.

  As described above, according to the present invention, the heat transfer rate is remarkably improved by simply treating the solid surface in contact with the target fluid for heat transfer with nanoparticles or forming a layer having a large number of nano-sized pores. Therefore, it is useful in all fields that require heat exchange systems and heat transfer in air conditioners and water heaters.

Example 1
[Experimental device]
A conceptual diagram of an apparatus used in an experiment for confirming the effect of the system of the present invention is shown in FIG. The experimental apparatus mainly comprises a cylindrical sealed chamber 1, a cooling chamber 2, a temperature control unit 3, a temperature monitor 4, a target fluid circulation pump 5, a cooling water circulation pump 6, and a cooling water tank 7. Installed indoors.

  The sealed chamber 1 is made of hard vinyl chloride and has a cylinder 8 with an inner diameter of 100 mm and a height of 100 mm, a SUS304 lid plate 9 that is liquid-tightly fixed to the upper end surface thereof, and a SUS304 liquid-tightly fixed to the lower end surface. The bottom plate 10 is constructed. The lid plate 9 and the bottom plate 10 have a disk shape with a thickness of 10 mm. On the lower surface of the cover plate 9, a disc-shaped copper transmission plate 11 having an outer diameter of 99.5 mm and a thickness of 1.5 mm is inserted through a silicon grease (model: AK-100, Taiwan Plowstar) and a screw (not shown). It is fixed with. A panel heater 12 having an outer diameter of 100 mm and a capacity of 80 W is mounted on the upper surface of the lid plate 9 via silicon grease (same as above). The thickness of each silicon grease is about 0.05 mm, and the distance from the panel heater 12 to the transmission plate 11 is a region of heat conduction (heat transfer inside the solid). The cover plate 9, the transmission plate 11, and the panel heater 12 all have a through hole with a diameter of 10 mm at the center, and the pipe 13 is used for air expansion and volume expansion / pressure relief at the time of volume expansion of the target fluid. Closely fitted. The outer surface of the cylinder 8 and the upper surface excluding the center of the panel heater 12 are covered with a heat insulating material 25.

  Two temperature sensors 14 and 15 made of a platinum resistor are loaded inside the cover plate 9, and one temperature sensor 14 is connected to the temperature control unit 3 to automatically control the cover plate 9 to a predetermined temperature by PI. The other temperature sensor 15 is connected to the temperature monitor 4 and used to monitor the temperature inside the lid plate 9. A temperature sensor 16 made of a platinum resistor thin film having a thickness of about 0.9 mm is embedded in the boundary between the lid plate 9 and the transmission plate 11, and the temperature monitor 4 is used to monitor the temperature of the upper surface of the transmission plate 11. Connected with. Also, a total of five temperature sensors 17 to 21 are inserted into the cylinder 8 at equal intervals in the vertical direction, and further, one temperature sensor 22 is embedded in the bottom plate 10, so that the water in the cooling water tank 7 is submerged. Another temperature sensor 23 is immersed in each of them, and both are connected to the temperature monitor 4.

On the other hand, the cooling chamber 2 is installed under the hermetic chamber 1 so as to support the hermetic chamber 1, and a base 24 made of SUS304 on the upper surface is in close contact with the bottom plate 10. The inside diameter of the cooling chamber 2 is 100 mm, and the inside of the chamber is forcibly circulated by the cooling water circulation pump 6 in a state where the cooling water having a temperature of 27.7 ° C. sent from the cooling water tank 7 is always filled during the experiment. The sealed chamber 1 is forcibly circulated so as to be sucked in from the upper portion and discharged from the lower portion by the target fluid circulation pump 5 in a state filled with tap water.
In the above experimental apparatus, when the panel heater 12 was energized so that the temperature of the cover plate 9 was 50 ° C., it was confirmed that the temperature of each part reached a steady state after 40 minutes.

  Next, copper oxide CuO particles (manufactured by Nanophase Technologies, USA, average particle diameter determined from measurement of SSA (specific surface area) based on BET method = 16 to 32 nm, almost spherical) and nitric acid are mixed to prepare a paste. This was applied to the entire lower surface of the transmission plate 11, and washed with water after drying. A thin layer recognized to be derived from copper oxide nanoparticles was formed on the coated lower surface. When this layer was observed with a scanning electron microscope (hereinafter referred to as SEM), it had many pores with a diameter of 100 nm or less.

[Experimental procedure and results]
The transmission plate 11 on which the porous layer derived from the copper oxide nanoparticles is formed is incorporated into the experimental apparatus again, and the panel heater 12 is energized so that the temperature of the lid plate 9 becomes 50 ° C. or 45 ° C. The temperature of each part was measured every minute, and the process was terminated when 20 minutes passed from the steady state where the temperature of each part was constant. For comparison, the temperature was measured under the same conditions before the porous layer was formed on the transmission plate 11. The measurement results are shown in FIG. Each temperature on the graph in the figure is an average value of 20 measured values, the horizontal axis indicates the position of the temperature sensor, and the numerical value on the horizontal axis is the distance from the bottom plate 10. In the figure, [copper plate + CuO particle 01] is data when the set temperature of the cover plate 9 is set to 50 ° C. using the transmission plate 11 in which the porous layer derived from the copper oxide nanoparticles is formed, Copper plate + CuO particle 02] is data obtained by repeated measurement under the same conditions in order to confirm reproducibility. [Copper plate + CuO particles 03] is data when the set temperature is set to 45 ° C. And [Copper plate only 01], [Copper plate only 02] and [Copper plate only 03] were obtained under the same conditions as above except that nitric acid alone was used instead of the mixed liquid of copper oxide nanoparticles and nitric acid. Data.

  First, since the data of [copper plate only 01] and [copper plate only 02] almost overlap, it is recognized that the method of the present invention has reproducibility. It should be noted that the heat transfer from the transmission plate to the water at a position of [80 mm] to [20 mm] can be confirmed as a clear difference in water temperature depending on the presence or absence of nanoparticles. On the other hand, the heat transfer from the lower layer water [20 mm] in the sealed chamber to the bottom plate may be the same condition in all experiments including cooling water, and naturally, the bottom plate temperature corresponds to the water temperature.

[Heat transfer coefficient]
The above-mentioned data are all based on the state where the temperature of each part is steady. Further, as can be seen from the data, the fluid temperature in the sealed chamber indicates the average temperature of the upper surface temperature of the lid plate 9 and the lower surface temperature of the bottom plate 10, and there is no heat loss from the sealed chamber, and the middle portion of the sealed chamber It is proved that a heat insulating layer is formed in the fluid portion because the fluid region is isothermal. Therefore, the heat flow that has passed through the upper surface of the sealed chamber with a diameter of 100 mm is convectively transferred to the intermediate fluid layer at the upper layer of the sealed chamber, transported through the intermediate layer without heat loss, and convectively transferred to the bottom plate at the lower layer. In this case, it can be said that the magnitude of the heat flow passing through the upper and lower layer portions is the same. This experiment also reveals that the heat transfer mechanism exists constantly.

The temperature of the transfer plate, sealed indoor water temperature, and bottom plate are TC, TwM, and TB, respectively, and the heat flux and heat transfer coefficient from the transfer plate of area S to water are q _upper , α _CuTop , and those from the lower layer water to the bottom plate _Where q _lower and α _bottom are the following:
q _upper * S = α _CuTop * (TC-TwM) * S
q _lower * S = α _bottom * (TwM-TB) * S
As described above, in the steady state, the heat flow is the same: q _upper * S = q _lower * S.
The following equation holds.
α _CuTop / α _bottom = (TwM−TB) / (TC−TwM)

FIG. 3 shows the values of α _CuTop / α _bottom for each experimental case based on FIG. Under the condition where the rotational speed of the pump 5 is constant, the flow conditions are considered to be the same, so the heat transfer rate α _bottom from the lower layer water to the bottom plate may be considered unchanged. Therefore, it can be considered that the value of α _CuTop / α _{bottom shows} the characteristic of α _CuTop as it is, that is, an index of the relative increase rate of the heat transfer coefficient from the upper transfer plate to the water.

It can be seen from FIG. 3 that even if the set temperature of the lid is different from 50 ° C. and 45 ° C., there is no difference in the value of α _CuTop / α _bottom . It can be said that this proves the justification of the inference regarding the evaluation of the heat transfer coefficient, including the reproducibility of the data.
Surprisingly, the heat transfer coefficient from copper to water is 60% higher than the characteristics of a single transfer plate due to the formation of a porous layer derived from a small amount of copper oxide CuO nanoparticles on the surface of the transfer plate (copper plate). These are also improving. Therefore, based on the concept of heat transfer based on the fluid boundary layer theory, it can be considered that the porous layer itself has a great influence on the temperature boundary layer. And it is speculated that the effect will not depend greatly on the base metal and the chemical species of the nanoparticles used.

-Example 2-
The temperature was measured under the same conditions as [Copper plate + CuO particles 01] in Example 1 except that the transmission plate 11 was treated with carbon nanotubes having a diameter of 20 to 30 nm and a length of 5 to 10 μm instead of the copper oxide nanoparticles. The temperature measurement result is shown in FIG. 4 as [copper plate + C tube 01]. When the surface of the transmission plate 11 was observed with an SEM, a layer having a large number of pores having a diameter of 100 nm or less was formed. In addition, a small amount of carbon nanotubes maintaining the original shape were adhered.

Example 1 except that the transmission plate 11 was treated with aluminum oxide Al ₂ O ₃ particles (manufactured by Nanophase Technologies, USA, average particle size 27 to 56 nm, almost spherical) instead of the copper oxide nanoparticle layer. The temperature was measured under the same conditions as copper plate + CuO particles 01]. The temperature measurement results are shown in FIG. 4 as [copper plate + Al2O3 particles 01]. When the surface of the transmission plate 11 was observed with an SEM, a layer having a large number of pores having a diameter of 100 nm or less was formed.

The temperature was measured under the same conditions as [Copper plate + CuO particles 01] or [Copper plate only 01] in Example 1 except that the material of the transmission plate 11 was changed to brass instead of copper. The temperature measurement results are shown in FIG. 4 as [brass plate + CuO particles 01] or [brass plate only 01].
The temperature of the transmission plate 11 was measured under the same conditions as [Copper plate + CuO particles 01] or [Copper plate only 01] in Example 1 except that aluminum was used instead of copper and aluminum oxide Al ₂ O ₃ particles were treated with caustic soda. did. The temperature measurement results are shown in FIG. 4 as [aluminum plate + Al2O3 particles 01] or [aluminum plate only 01].
Note that [copper plate only 01] and [copper plate only 02] are copies of the data of FIG. 3 for comparison.

As is apparent from the figure, the difference in the experimental water temperature depending on the presence or absence of the porous layer derived from the nanoparticles can be confirmed regardless of the material of the transmission plate and the chemical species of the nanoparticles used. The reason why the results of the copper transmission plate and the carbon nanotube (copper plate + C tube) showed a relatively low water temperature is thought to be because the carbon nanotube was less likely to form a porous layer on the transmission plate than the other nanoparticles. In any case, the reproducibility of the formation of the porous layer was confirmed, and there was no phenomenon that the degree of formation deteriorated or the heat transfer rate deteriorated each time the experiment was repeated.
Next, the result of calculating the heat transfer coefficient using the same calculation formula as in Example 1 is shown in FIG. As far as this series of data is concerned, the value of the combination of the brass transmission plate and the copper oxide nanoparticles (brass plate + CuO particles) was the highest, that is, the heat transfer coefficient was the best. The value shows a tremendous improvement of 80% or more compared to the transmission plate alone.

-Comparative example-
The transmission plate 11 and the temperature sensor 16 were removed from the experimental apparatus of Example 1. The carbon nanotubes were added to the water in the sealed chamber 1 at a concentration of 0.01 g / L. That is, the carbon nanotube-containing water is in contact with the lower surface of the lid plate 9. Others were measured under the same conditions as [Copper plate + CuO particles 01] in Example 1. As a control, the temperature was also measured when no carbon nanotubes were added to water. These temperature measurement results are shown in FIG. 6 as [Add C tube] and [None], respectively.

As can be seen from FIG. 6, the water temperature of [None] rather than [Add C tube] was about 0.6 ° C. higher. By the way, carbon nanotubes did not dissolve in water, and at the end of the experiment, a phenomenon was observed in which the particles were stuck together and solidified.
against α _CuTop / α _bottom = 0.3667 in the case of only water also for values of _{α CuTop / α bottom, α CuTop} / α bottom = 0.3576 in the case of carbon nanotube-containing water is small somewhat. Therefore, it is recognized that the heat transfer coefficient is rather lowered by adding carbon nanotubes.

Example 3
[Experimental device]
As shown in FIG. 7, a hot water tank 28 and a cold water tank 29 were prepared. Further, upper and lower two-layer flow paths 31 and 32 were formed in a rectangular parallelepiped box 30 made of a hard vinyl chloride resin having a thickness of 10 mm. The flow paths 31 and 32 have a uniform cross section each having a height of 15 mm and a width of 310 mm in the flow direction, and the upper flow path 31 and the lower flow path 32 are partitioned by a copper plate 33 having a thickness of 1.5 mm. Yes. The flow paths 31 and 32 have an inlet at one end in the flow direction and an outlet at the other end.

  The box 30 was tilted so that the entrance was down and the exit was up, and hot water was supplied to the entrance of the upper flow path 31 and cold water (both tap water) was supplied to the entrance of the lower flow path 32 by pumps 34 and 35. And the warm water and cold water which came out of the exit of the flow paths 31 and 32 flowed down to the hot water tank 28 and the cold water tank 29, respectively, and it adjusted with the valve | bulb of omission so that the flow volume might become the same. Temperature sensors 36, 37, 38, 39 were installed at a distance of L = 1.8 m upstream (near the entrance) and downstream (near the exit) of the flow paths 31, 32, and the water temperature was automatically measured. After the inlet temperature of hot water and cold water reached a steady state, it was further measured every 10 seconds for 10 minutes, and the average value of the measured values (30 data items) over at least 5 minutes was taken as the measurement result.

Heat is transferred from the upper layer of hot water to the lower layer of cold water through the copper plate 33, and the temperature of the hot water after passing through 1.8 m decreases, and conversely, the temperature of the cold water increases. Each temperature difference is determined by the heat conduction due to the physical properties of the copper plate 33 itself separating the upper and lower two layers of fluid, the heat transfer from the hot water to the copper plate 33, and the heat transfer from the copper plate 33 to the cold water. The higher the heat transfer rate, the greater the temperature difference.
In the experiment, the copper plate 33 was treated only with nitric acid [copper plate only], both sides of the copper plate 33 were treated with the same copper oxide nanoparticle-containing paste as in Example 1 [both sides nanoparticles], the hot water side of the copper plate 33 4 treatments were performed on the copper oxide nanoparticles containing paste [warm surface nanoparticles], and on the other hand, only the cold water side of the copper plate 33 was treated with copper oxide nanoparticles containing paste [cold surface nanoparticles] It was.

[Experimental result 1]
The experimental results when the flow velocity u = 4.41 cm / sec is shown in FIG. 8 (Heat exchange Nano vs Cu-only graph F). In all four conditions, the hot water inlet temperature (Hinlet) and cold water (Cinlet) are almost the same.
By comparing [Double-sided nanoparticles] and [Copper plate only], there is a clear difference in the temperature difference between the inlet and outlet of both hot and cold water. Since the flow rate and heat transfer area (0.31mx 1.8m) of both are the same, this difference can be considered to have been caused by the difference in heat transfer rate from hot water to copper plate and from copper plate to cold water.
The result in the case of [copper plate only] can be estimated by the conventional experimental correlation (Churchill &Ozoe's equation for laminar flow in the duct), and the calculated value and experimental value are quite accurate (calculated outlet temperature and experimental value). The difference between the two was 0.36 ° C. On the other hand, in the experimental value of [double-sided nanoparticles], the heat transfer coefficients from hot water to copper and from copper to cold water were improved to 1.8 times the calculated values.
The results of [warm surface nanoparticles] and [cold surface nanoparticles] indicate that the heat transfer coefficient is improved when the porous layer derived from the nanoparticles is formed.

FIG. 9 (the rate of change in water temperature due to heat exchange U = 4.4 cm / s) shows the above results in a columnar graph. Based on the inlet temperature difference (Inlet) of hot water and cold water, the ratio of the temperature difference of hot water (Hot = H _inlet −H _outlet ) and that of cold water (Cool = C _outlet −C _inlet ) is shown as (Hot / Inlet) and (Cool / Inlet). [1- (Outlet) / (Inlet)] incorporates the outlet temperature difference (Outlet) of hot water and cold water, and the sum of (Hot / Inlet) and (Cool / Inlet) is [1- (Outlet) / ( Inlet)] value. Conceptually speaking, the value of [1- (Outlet) / (Inlet)] indicates the degree of improvement in heat exchange, that is, heat transfer, and (Hot / Inlet) and (Cool / Inlet) are the respective contributions. It will be suggested.

  In the case of [Double-sided nanoparticles] and [Copper plate only], the values of (Hot / Inlet) and (Cool / Inlet) are almost the same. Therefore, it is considered that the theory of heat exchange can be applied, and it can be determined that the above-described estimation using the experimental correlation equation is also effective.

[Experimental result 2]
FIG. 10 (Heat exchange Nano vs. Cu-only graph E) shows the experimental results at a flow velocity around u = 8 cm / sec. Because of the high flow rate, the four conditions of hot water inlet temperature (H _inlet ) and cold water (C _inlet ) are difficult to control and have slightly different conditions.
FIG. 11 (the rate of change in water temperature due to heat exchange U = about 8 cm / s) shows a columnar graph similar to the experimental result 1. The experimental results under each condition show exactly the same tendency as the results when the flow velocity u = 4.41 cm / s. The difference between the calculation result and the experimental result in the case of [copper plate only] by the above-mentioned experimental correlation equation plus turbulent flow (Petukhov's modified equation for turbulent flow in the duct and Churchill &Ozoe's equation for laminar flow) It matched with good accuracy within 0.02 ° C. On the other hand, the experimental value of [Double-sided Nanoparticle] showed that the heat transfer coefficient from hot water to copper and from copper to cold water was improved 2.1 times the calculated values.
FIG. 11 also suggests four types of experimental conditions. This is because [single-sided nanoparticles] is intermediate between [copper plate only] and [double-sided nanoparticles].

[Loss associated with fluid flow]
When considering a general heat exchange system, for example, fins are often considered to increase the heat transfer area. However, they have a negative factor that energy loss associated with fluid flow increases. Therefore, an increase in friction loss of the flow path due to the porous layer is also indispensable for evaluating the entire system.

  In this experiment, we intended to measure the friction loss, but the conventional experimental correlation (Prandtle-Karman formula for smooth pipe universal resistance and Colebrook's approximation considering equivalent sand roughness) In the calculation based on), only a loss of about 0.9 mm water column was generated with respect to the flow path length of 1.5 m, and only a visual judgment with a manometer that it was actually 1.0 mm water column or less could be made. Regarding the evaluation of the friction loss depending on the presence or absence of the porous layer, an experiment in which the flow rate is further increased must be carried out. However, the surface on which the porous layer is formed is a painted surface, and in the calculation based on the Moody diagram that introduces equivalent sand roughness, the coefficient of friction is higher than that of a smooth tube. It should be noted that results have been obtained that result in only an increase of less than 3%.

Example 4
The previous examples are where the fluid is water. In this example, the fluid is air. A test plate 40 obtained through the same steps as [copper plate + CuO particles 01] and [copper plate only] in Example 1 is shown in a plan view in FIG. 12, and as a front view in FIG. Fixed obliquely (elevation angle α = 30 degrees). In addition, a temperature sensor 42 of a platinum resistor thin film was attached to the back surface of the test plate 40, and a temperature sensor 43 was attached near the outlet of the air duct 41. Then, hot air (117 ° C.) was blown onto the test plate 40 at a wind speed of 0.5 m / s, and it was confirmed that the hot air did not enter the back side of the test plate 40. As a result of measuring the temperature of the test plate 40 that reached a steady state by the temperature sensor 42, the temperature of the [copper plate only] was 57.5 ° C., and the temperature of the [copper plate + CuO particles 01] was 59.2 ° C.

It is a conceptual diagram of the experimental apparatus which confirms the effect of this invention. It is a graph which shows the result of Example 1 measured using the said apparatus. It is relative evaluation of the heat transfer coefficient computed based on the said graph. It is a graph which shows the result of Example 2 measured using the said apparatus. It is relative evaluation of the heat transfer coefficient computed based on the said graph. It is a graph which shows the result of the comparative example measured using the thing except a transmission board from the said apparatus. It is a conceptual diagram of another experimental apparatus which confirms the effect of this invention. It is a graph which shows the experimental result 1 of Example 3 measured using the said apparatus. It is a graph which shows the water temperature change rate of the experimental result 1. It is a graph which shows the experimental result 2 of Example 3 measured using the said apparatus. It is a graph which shows the water temperature change rate of the experimental result 2. It is a top view of another experimental apparatus which confirms the effect of this invention. It is also a front view.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Sealing chamber 2 Cooling chamber 3 Temperature control part 4 Temperature monitor 5, 6 Circulation pump 7 Cooling water tank 8 Cylinder 9 Cover plate 10 Bottom plate 11 Transmission plate 12 Panel heater 13 Pipe 14-23 Temperature sensor 24 Stand 25 Insulation material

Claims (6)

In a system for exchanging heat between a solid and a fluid,
A heat exchange system comprising a solid having a surface in contact with a fluid, and a porous layer including a large number of pores having a diameter of 100 nm or less formed of a group of particles having a diameter of 100 nm or less.   The heat exchange according to claim 1, wherein the solid has a second surface in contact with a second fluid having a temperature different from that of the fluid, and is partitioned so that the first fluid and the second fluid are not mixed. system.   The heat exchange system according to claim 1 or 2, wherein the solid is a metal. The heat exchange system according to any one of claims 1 to 3, wherein the particle group is one or more selected from copper oxide CuO, carbon C, and alumina Al ₂ O ₃ .   The heat exchange system according to any one of claims 1 to 4, wherein the solid is a metal, and the particle group is made of the same metal as the solid or an oxide thereof. The heat exchange system according to any one of claims 1 to 5, wherein the fluid is water or air.

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