High-liquid-filling-rate loop thermosiphon with gradient silk screen structure evaporator

Abstract

The invention discloses a high-liquid-filling-rate loop thermosiphon with an evaporator of a gradient silk screen structure. The evaporator is of a tubular structure and comprises a hollow tube body. A plurality of layers of silk screens are arranged on the inner wall of the tube body, and meshes of the silk screens are increased layer by layer in the direction from the inner wall of the tube body to the center of the tube cavity; the pipe body and the screen mesh adjacent to the pipe body are tightly attached to each other and fixedly connected, and every two adjacent layers of screen meshes are tightly attached to each other and fixedly connected. The loop thermosiphon comprises the evaporator, a steam pipeline, a tubular condenser and a liquid pipeline, wherein the evaporator, the steam pipeline, the condenser and the liquid pipeline are sequentially connected end to end to form a closed loop. By using the gradient wire mesh evaporator and using the self-wetting fluid as the heat transfer working medium, the size of bubbles generated after phase change of the heat transfer working medium is reduced when the bubbles are separated from the evaporator, the bubble separation frequency is increased, the circulating power of the loop thermosiphon is enhanced, and the heat transfer performance of the loop thermosiphon is improved.

Description

Technical Field

[0001] The invention relates to the technical field of unpowered passive heat transfer, in particular to a high-liquid-filling-rate loop thermosiphon with a gradient wire mesh structure evaporator. Background Art

[0002] With the rapid development of microelectronics technology, especially in the application of 5G communications and high-performance electronic devices, the structure of electronic devices has become more compact and the functions have become increasingly powerful. However, the miniaturization of devices has led to the aggravation of heat dissipation problems. The high heat flux density of the equipment and the need for long-term stable operation make it difficult for traditional heat dissipation methods to meet the requirements.

[0003] The traditional two-phase loop heat pipe (LHP) has been widely used in the field of high heat flux heat dissipation due to its simple structure, no need for external power, easy processing and low cost. The basic principle of this technology is to use the phase change of the working fluid to achieve heat transfer between the heat source and the cold source. Its advantage is that it can drive the natural circulation of the internal working fluid through gravity, thereby achieving the purpose of efficient heat dissipation. However, the traditional low-filling rate loop heat pipe has the problems of slow internal flow speed and low heat transfer efficiency. Especially at low heat flux, the defects such as difficulty in starting and large loop thermal resistance seriously limit its application.

[0004] In contrast, the high-filling-rate loop thermosyphon can effectively improve the heat transfer efficiency of the evaporator by increasing the filling rate of the working fluid in the loop. Its higher circulation velocity helps to quickly transfer heat and avoid the drying of the evaporator, thus having a higher critical heat flux and lower thermal resistance under high heat flux.

[0005] As the core component in the loop thermosyphon that is in direct contact with the heat source, the performance of the evaporator directly affects the overall heat transfer effect of the thermosyphon. Existing evaporator surfaces usually use micro-nanostructure surfaces (such as microchannels, micropores, etc.) to improve the heat exchange efficiency, so that it has a higher critical heat flux density and a lower superheat. However, the traditional micro-nanostructure preparation processes such as mechanical processing, chemical etching, electrochemical deposition, and photolithography have problems such as high cost, complex process, and poor durability, which limit its popularity in practical applications. Therefore, while maintaining efficient heat transfer, it is imperative to design a simple, low-cost, and durable evaporator structure.

[0006] In addition, existing heat transfer fluids are generally water or traditional refrigerants. The advantage of water as a heat transfer fluid is its large specific heat and latent heat, which makes it suitable for heat dissipation applications with high heat flux density, but its high viscosity and surface tension lead to large flow resistance and are prone to temperature fluctuations. Although traditional refrigerants have lower surface tension and viscosity and can achieve relatively stable temperature fluctuations, they have high operating pressures and cannot withstand extremely high heat flux densities, which poses a hidden danger to the safety and maintenance of the system. Therefore, seeking a fluid that can overcome the defects of water and improve heat transfer performance at the same time has become an important research direction. Summary of the invention

[0007] To this end, the technical problem to be solved by the present invention is to provide a high-liquid-filling-rate loop thermal syphon with a gradient wire mesh structure evaporator. By using a gradient wire mesh evaporator and using a self-wetting fluid as a heat transfer medium, the size of the bubbles generated after the phase change of the heat transfer medium when they detach from the evaporator is reduced, and the bubble detachment frequency is increased, thereby enhancing the circulation power of the loop thermal syphon and improving the heat transfer performance of the loop thermal syphon.

[0008] In order to solve the above technical problems, the present invention provides the following technical solutions:

[0009] An evaporator with a gradient wire mesh structure is a tubular structure, comprising a hollow tube body; the inner wall of the tube body is provided with multiple layers of wire mesh, and the mesh openings of the wire mesh increase layer by layer in the direction from the inner wall of the tube body to the center of the tube cavity; the tube body and the wire mesh adjacent thereto, as well as the two adjacent layers of the wire mesh, are closely attached to each other and fixedly connected. The gradient wire mesh structure optimizes the nucleation and detachment of bubbles in the evaporator. The arrangement of the multiple layers of wire mesh from dense to sparse not only increases the contact area between the heat transfer medium and the wire mesh, and increases the number of nucleation sites, but also reduces the resistance of the bubbles moving to the tube cavity by increasing the mesh openings layer by layer, so that the bubbles generated by the vaporization of the heat transfer medium can quickly detach from the evaporator, thereby improving the heat transfer efficiency.

[0010] The above-mentioned evaporator with a gradient wire mesh structure has three layers of wire mesh, namely a first wire mesh, a second wire mesh and a third wire mesh; the first wire mesh, the second wire mesh and the third wire mesh are arranged in sequence in the direction from the inner wall of the tube body to the center of the tube cavity; the mesh ratio of the first wire mesh to the second wire mesh, and the mesh ratio of the second wire mesh to the third wire mesh are both 1.2 to 2.0.

[0011] The number of wire mesh layers will affect the performance of the evaporator. When the number of wire mesh layers is greater than 3 or equal to 2, the evaporator can still work, but the efficiency will decrease. This is because the more wire mesh layers there are, the greater the flow resistance of the heat transfer medium inside the evaporator, and the more difficult it is for the bubbles generated when the heat transfer medium vaporizes to detach from the surface of the evaporator; and the more wire mesh layers there are, the higher the manufacturing and maintenance costs, and it is also easy to cause problems such as material waste; when the number of wire mesh layers is small, there will be problems such as insufficient heat transfer area in the evaporator and insufficient bubble nucleation sites.

[0012] The mesh count difference between adjacent screen layers should be controlled within a reasonable range. The mesh count ratio of the larger mesh to the smaller mesh in two adjacent screen layers should be between 1.2 and 2.0. When the mesh count difference between adjacent screen layers is too large, the sudden increase in aperture will destroy the continuity of bubble detachment. After the bubbles are nucleated in the bottom high-mesh screen, when they move upward to the low-mesh layer, they will lose support due to the sudden increase in aperture and are easily retained or merged between layers. If the mesh count difference is too small, it will lead to insufficient bubble nucleation points, making it difficult to effectively improve the heat transfer efficiency. At the same time, it will weaken the gradient effect, resulting in a blocked channel for bubbles to detach from the evaporator, affecting the detachment efficiency.

[0013] In the above-mentioned evaporator with gradient wire mesh structure, the mesh number of the first wire mesh is 250-200 meshes, the mesh number of the second wire mesh is 200-150 meshes, and the mesh number of the third wire mesh is 150-100 meshes.

[0014] In the above-mentioned evaporator with a gradient wire mesh structure, the mesh number of the first wire mesh is 200 meshes, the mesh number of the second wire mesh is 150 meshes, and the mesh number of the third wire mesh is 100 meshes.

[0015] In the above-mentioned evaporator with a gradient wire mesh structure, the tube body and the wire mesh adjacent thereto, as well as the two adjacent layers of the wire mesh, are fixed by vacuum sintering;

[0016] During vacuum sintering, the wire mesh is first placed into the tube cavity of the tube body, and then a carbon rod is inserted into the tube cavity of the tube body; the carbon rod presses the wire mesh, and makes the two adjacent layers of the wire mesh, and the tube body and the wire mesh adjacent to it close to each other; finally, sintering is performed under vacuum conditions. Vacuum sintering not only improves the strength and stability of the connection between the evaporator and the wire mesh, but also prevents the deformation of the wire mesh from causing changes in the size of the "bubble channel" and the ability to limit the size of the bubble, so that each layer of the wire mesh can fully play the role of providing nucleation sites for the heat transfer medium, limiting the size of the bubble, and promoting the detachment of the bubble.

[0017] A high-liquid filling rate loop thermosiphon with a gradient wire mesh structure evaporator comprises the above-mentioned evaporator with a gradient wire mesh structure, a steam pipeline, a tubular condenser and a liquid pipeline, wherein the evaporator, the steam pipeline, the condenser and the liquid pipeline are connected end to end in sequence to form a closed loop, and the loop is filled with a heat transfer medium; at room temperature and in a non-working state, the liquid filling rate of the heat transfer medium in the loop is greater than 0.6. In the evaporator of a traditional two-phase thermosiphon, the heat transfer medium and the heat source are mainly exchanged through pool boiling, and the heat exchange capacity is relatively low; while in the evaporator of a high-liquid filling rate thermosiphon, the heat transfer medium can exchange heat with the heat source in the form of liquid phase convection and evaporation and boiling of the liquid phase, and the bubbles generated during boiling can accelerate the circulation of the heat transfer medium and promote the movement of the cooled heat transfer medium in the condenser to the evaporator.

[0018] The high-filling rate loop thermosyphon with gradient wire mesh structure evaporator mentioned above, the heat transfer medium is a water-based self-wetting fluid. The water-based self-wetting fluid has low surface tension and good wettability, can form component concentration gradient and surface tension gradient, and promote the reflux of condensed liquid to the evaporation section under the action of the "Marangoni" effect. In addition, the low surface tension of the self-wetting fluid helps to reduce the nucleation size of the bubble, and the gradient wire mesh evaporator reduces the bubble detachment size and increases the bubble detachment frequency. The two complement each other, further enhancing the circulation power of the loop thermosyphon, while enhancing the replenishment of the liquid in the evaporator and delaying the occurrence of drying.

[0019] In the above-mentioned high liquid filling rate loop thermosyphon with gradient wire mesh structure evaporator, the heat transfer medium is one of the aqueous solutions containing alcohols in the following concentration ranges:

[0020] a) an aqueous solution of n-butanol, wherein the mass fraction of n-butanol is 1 wt% to 7 wt%;

[0021] b) an aqueous solution of n-pentanol, wherein the mass fraction of n-pentanol is 0.1 wt% to 2 wt%;

[0022] c) an aqueous solution of n-heptanol, wherein the mass fraction of n-heptanol is 0.01 wt % to 0.1 wt %.

[0023] n-Butanol, n-pentanol and n-heptanol are all high-carbon alcohols with low solubility in water. When the mass fraction of high-carbon alcohols in the aqueous solution is too high, stratification may occur between the alcohols and water, resulting in increased viscosity of the solution and decreased uniformity, which affects the fluidity and heat transfer performance of the heat transfer medium 5.

[0024] The high-liquid filling rate loop thermosyphon with gradient wire mesh structure evaporator mentioned above, when manufacturing the high-liquid filling rate loop thermosyphon, after assembling the evaporator, the steam pipeline, the condenser and the liquid pipeline into a closed loop and completing the loop leak detection, the heat transfer medium is filled into the loop; when filling the loop with the heat transfer medium, the pressure in the loop is first pumped down to 0.1-0.5Pa, and then filling. The method of first pumping the loop into a vacuum and then filling the heat transfer medium is conducive to ensuring that there is no non-condensable gas in the loop to interfere with the circulation process of the heat transfer medium.

[0025] In the high liquid filling rate loop thermosiphon with gradient wire mesh structure evaporator, the evaporator, the steam pipeline, the condenser and the liquid pipeline are all made of copper.

[0026] The evaporator with gradient wire mesh structure was prepared as follows:

[0027] S1, washing the screen and the tube body;

[0028] S2, drying the screen and the tube under vacuum conditions;

[0029] S3, placing the wire mesh into the lumen of the tube body, and then inserting a carbon rod into the lumen of the tube body; the carbon rod presses the wire mesh tightly, and makes the two adjacent layers of the wire mesh, and the tube body and the wire mesh adjacent thereto, close to each other;

[0030] S4, performing vacuum sintering; after vacuum sintering, a fixed connection is formed between the tube body and the wire mesh adjacent thereto, and between two adjacent layers of the wire mesh.

[0031] When washing, 0.5 mol / L citric acid solution, anhydrous ethanol and deionized water were used in sequence;

[0032] In step S2, the temperature during vacuum drying is 50°C and the drying time is 2h;

[0033] In step S4, the temperature during vacuum sintering is 900° C. and the sintering time is 1 hour.

[0034] By first evacuating the loop and then filling it with the heat transfer medium, the non-condensable gases in the loop can be removed, thereby reducing the adverse effects of these non-condensable gases on the circulation process of the heat transfer medium.

[0035] The technical solution of the present invention achieves the following beneficial technical effects:

[0036] 1. The high liquid filling rate loop thermosyphon in the present invention adopts a gradient wire mesh evaporator, and three layers of copper wire mesh from dense to sparse are sintered on its inner surface. The reasonable design of the number of layers and aperture of the wire mesh ensures that the nucleation density and detachment frequency of bubbles in the evaporator are effectively controlled. The aperture of the first wire mesh is small, which increases the contact area between the liquid phase working medium and the wire mesh and increases the number of nucleation sites. The provision of the second screen and the third screen increases the heat transfer area inside the evaporator and promotes the evaporation of the heat transfer medium; and the mesh holes of the first screen, the second screen and the third screen form a gradient from dense to sparse, and form a V-shaped channel, which reduces the resistance when the bubbles detach and improves the bubble detachment efficiency, thereby enhancing the "bubble pump" phenomenon, promoting the circulation of the heat transfer medium, and enabling the evaporator to replenish the liquid heat transfer medium in time, avoiding the evaporator from drying up, and improving the heat transfer efficiency; in addition, the mesh counts of the second screen and the third screen are set within a reasonable range, which can prevent the size of the bubbles from being too large when they detach, and the heat transfer medium in the evaporator produces more and smaller bubbles, which is also conducive to promoting the circulation of the heat transfer medium in the loop thermal siphon and facilitating the transfer of heat from the evaporation section to the condensation section. And from the overall point of view, the three-layer wire mesh structure in the present invention has a higher pore density, and the wire mesh with a higher pore density is also more hydrophilic, which is conducive to "attracting" more liquid heat transfer medium into the evaporator, and is also conducive to preventing the evaporator from drying up and achieving a higher critical heat flux density. Compared with the low-filling rate loop thermosyphon, the high-filling rate loop thermosyphon of the present invention has a higher circulation flow rate at high heat flux density.

[0037] 2. The working fluid used in the high-filling rate loop thermosyphon in the present invention is a water-based self-wetting fluid, which is a n-butanol solution with a mass fraction of 1wt% to 7wt%, a n-pentanol solution with a mass fraction of 0.1wt% to 2wt%, or a n-heptanol solution with a mass fraction of 0.01wt% to 0.1wt%. These water-based self-wetting fluids have a low surface tension, and their surface tension increases with the increase of temperature when a certain temperature is reached. During operation, this fluid can form a component concentration gradient and a surface tension gradient, and under the action of the "Marangoni" effect, it promotes the reflux of the condensed liquid to the evaporation section, which has the effect of accelerating the circulation of the heat transfer working fluid. In addition, the low surface tension of the self-wetting fluid helps to reduce the nucleation size of the bubble, and the gradient wire mesh evaporator reduces the size of the bubble detachment and increases the frequency of the bubble detachment. The two complement each other, further enhancing the circulation power of the loop thermosyphon, while enhancing the replenishment of the liquid in the evaporator and delaying the occurrence of the drying phenomenon. The experimental test results show that the critical heat flux density of the high liquid filling rate thermosyphon with gradient wire mesh structure provided by the present invention can reach 500W / cm 2The evaporation thermal resistance is much lower than that of a high-liquid-filling thermosyphon with the same specifications that has an evaporator with a smooth inner wall and uses water as the heat transfer medium. At the same time, the preparation and assembly process of the high-liquid-filling thermosyphon with a gradient wire mesh structure provided in the present invention is simple to operate, low in cost, and can operate stably for a long time. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 A schematic diagram of the structure of the evaporator in Example 1 of the present invention;

[0039] Figure 2 Scanning electron microscope image of the gradient wire mesh evaporator of the present invention;

[0040] Figure 3 A partial magnified view of a scanning electron microscope image of the gradient wire mesh evaporator of the present invention;

[0041] Figure 4 A structural diagram of a self-wetting fluid high filling rate loop thermosiphon having a gradient wire mesh structure evaporator in Example 2 of the present invention;

[0042] Figure 5 A graph showing the relationship between the evaporation thermal resistance of the loop thermosyphon in Example 2 of the present invention and a high-liquid-filling-rate loop thermosyphon with a conventional smooth-wall evaporator and water as the heat transfer medium and the heat flux density;

[0043] Figure 6 Visualized images of the evaporator outlets of the loop thermosyphon in Example 2 of the present invention and a high-liquid-filling-ratio loop thermosyphon with water as the heat transfer medium having a conventional smooth-wall evaporator.

[0044] The reference numerals in the figure are as follows: 1-evaporator; 101-first wire mesh; 102-second wire mesh; 103-third wire mesh; 104-tube body; 2-steam pipeline; 3-condenser; 4-liquid pipeline; 5-heat transfer medium. DETAILED DESCRIPTION

[0045] Example 1

[0046] like Figure 1As shown, in this embodiment, an evaporator 1 with a gradient wire mesh structure is provided. The evaporator is tubular, including a hollow tube body 104, and three layers of wire mesh are arranged on the inner wall of the tube body 104, namely, a first wire mesh 101, a second wire mesh 102 and a third wire mesh 103. In the direction from the inner wall of the tube body 104 to the tube cavity, the first wire mesh 101, the second wire mesh 102 and the third wire mesh 103 are arranged in sequence, and the mesh aperture of the first wire mesh is smaller than the mesh aperture of the second wire mesh, and the mesh aperture of the second wire mesh is smaller than the mesh aperture of the third wire mesh. The tube body 104 and the first wire mesh 101 adjacent thereto, the first wire mesh 101 and the second wire mesh 102, and the second wire mesh 102 and the third wire mesh 103 are all closely attached to each other and fixed by vacuum sintering.

[0047] In this embodiment, the mesh number of the first screen is 200 meshes, the mesh number of the second screen is 150 meshes, and the mesh number of the third screen is 100 meshes.

[0048] By designing a hierarchical structure of wire meshes with different mesh sizes, a gradient effect can be achieved during the operation of the evaporator, thereby optimizing the formation and flow of bubbles and improving the overall heat transfer efficiency.

[0049] The first screen 101 is the screen closest to the heat source, and the heat of the heat source is mainly transferred to the heat transfer medium through the first screen 101. The first screen 101 has a smaller aperture and a higher mesh count, which is beneficial to increase the contact area between the liquid phase heat transfer medium and the screen, and increase the nucleation density of bubbles.

[0050] The mesh count of the second and third wire meshes is lower than that of the first wire mesh. On the one hand, the mesh diameter of the second and third wire meshes is larger than that of the first wire mesh, and the closer to the tube cavity of the evaporator, the larger the mesh diameter of the wire mesh, which helps to reduce the resistance encountered by the bubbles in the process of moving to the tube cavity, so that the bubbles can pass through the wire mesh layer more quickly. The rapid detachment of the bubbles not only improves the transfer efficiency of the heat flow, but also increases the overall flow stability of the evaporator; on the other hand, the mesh count of the second and third wire meshes is not too large, which can play the role of limiting the size of the bubble detachment, and the heat transfer medium in the evaporator produces more and smaller bubbles, which is conducive to promoting the circulation of the heat transfer medium in the loop thermal siphon, avoiding the poor flow of the heat transfer medium caused by the retention of bubbles in the evaporator, further leading to the drying up of the evaporator, temperature fluctuations and reduced efficiency. In addition, the second and third wire meshes themselves can also increase the heat transfer area in the evaporator, provide nucleation sites and promote the vaporization of the heat transfer medium.

[0051] In this embodiment, the mesh ratio of the first screen to the second screen is 1.3, and the mesh ratio of the second screen to the third screen is 1.5. If the mesh difference between the screen layers is too large, the continuity of the bubble detachment will be destroyed. After the bubble is nucleated in the bottom layer of high-mesh screen, when it moves upward to the low-mesh layer, it will lose support due to the sudden increase in pore size and easily stay or merge between layers; while if the mesh difference between the two adjacent screen layers is too small (the mesh of the layer with a larger mesh in the two adjacent screen layers is reduced relative to the existing mesh), it will lead to insufficient bubble nucleation points, making it difficult to effectively improve the heat transfer efficiency, and at the same time weaken the gradient effect, resulting in a blocked bubble detachment channel, affecting the detachment efficiency.

[0052] In some other embodiments, the mesh number of the first screen 101 can be other values ​​within the range of 250 to 200 meshes, the mesh number of the second screen can be other values ​​within the range of 200 to 150 meshes, and the mesh number of the third screen can be other values ​​within the range of 150 to 100 meshes. The mesh number difference between adjacent screen layers should be controlled within a reasonable range, that is, the mesh number ratio of two adjacent screen layers (screen with a larger mesh number / screen with a smaller mesh number) can be other values ​​within the range of 1.2 to 2.0.

[0053] In this embodiment, the tube body 104, the first wire mesh 101, the second wire mesh 102 and the third wire mesh 103 are all made of copper. When preparing the evaporator 1, the first wire mesh 101, the second wire mesh 102 and the third wire mesh 103 with different mesh sizes are sintered to the inner surface of the tube body 104. The specific preparation steps are as follows:

[0054] S1. Washing the oil stains on the surface of the screen and the tube body; during washing, washing is carried out using 0.5 mol / L citric acid solution, anhydrous ethanol and deionized water in sequence.

[0055] S2. Dry the screen and the tube under vacuum conditions; the vacuum drying temperature is 50° C. and the drying time is 2 h.

[0056] S3, placing the wire mesh into the lumen of the tube body, and then inserting a carbon rod into the lumen of the tube body; the carbon rod presses the wire mesh and makes the two adjacent layers of the wire mesh, and the tube body and the wire mesh adjacent thereto, tightly attached.

[0057] S4, vacuum sintering: sintering the assembly of the carbon rod, the wire mesh and the tube body in a vacuum environment, the sintering temperature is 900°C, and the sintering time is 1 hour. After the sintering is completed, the product is obtained.

[0058] The vacuum sintering fixation ensures the firmness of fixation between two adjacent layers of wire mesh and between the wire mesh and the tube body 104. During vacuum sintering, the wire mesh and the tube body form a tight bond through atomic diffusion, and no oxidation reaction occurs. The two adjacent layers of wire mesh and the wire mesh and the tube body 104 can be evenly bonded, avoiding the unevenness that may exist when other fixing methods such as welding or gluing are used; in addition, in this embodiment, vacuum sintering after inserting the carbon rod is not easy to cause local deformation of the material, which is conducive to ensuring the stability of the wire mesh structure and preventing the wire mesh deformation from causing changes in the size of the "bubble channel" and the ability to limit the bubble size, so that each layer of wire mesh can fully play the role of providing nucleation sites for the heat transfer medium, limiting the bubble size, and promoting bubble detachment.

[0059] For the convenience of characterization, the layered gradient screen surface was prepared on a copper plate using the same preparation method. Figure 2 This is a scanning electron microscope image of the overall surface profile obtained after the first screen 101, the second screen 102 and the third screen 103 in this embodiment are sintered on the surface of the copper plate. Figure 3 for Figure 2 It can be seen from the figure that the mesh of the screen prepared by the method in this embodiment is uniform and the mesh is not deformed.

[0060] The evaporator 1 in this embodiment can be used to make a loop thermosyphon. In the loop thermosyphon, the heat transfer medium (aqueous solution of high carbon alcohol) is heated and boiled at the evaporator 1, and a plurality of small bubbles are generated. After these small bubbles are generated, they will detach from the inner surface of the evaporator 1. Since the inner wall of the tube body of the evaporator 1 is processed with a gradient wire mesh structure, the denser copper wire mesh at the bottom can provide more nucleation sites for the boiling process, and the gradient wire mesh from dense to sparse can form a V-shaped channel, which can promote the detachment of bubbles and limit their detachment size, that is, the evaporator 1 in this embodiment can play a role in reducing the detachment size of the heat transfer medium bubbles and increasing the detachment frequency of the bubbles. The "bubble pump" phenomenon of bubbles with more and smaller volumes is more prominent, that is, it can better promote the flow of liquid heat transfer medium in the loop thermosyphon, and the circulation velocity of the loop thermosyphon is accelerated, which is more conducive to the transportation of heat from the evaporation section to the condensation section to achieve the heat dissipation process. In addition, the pore density of the three-layer wire mesh in this embodiment is relatively high overall, and the wire mesh with high pore density has extremely strong hydrophilicity, which is conducive to "attracting" more liquid heat transfer medium into the evaporator, preventing or at least delaying the drying up of the inside of the evaporator.

[0061] Compared with the porous structure and multi-layer microchannel structure sintered from metal powder, the gradient wire mesh structure on the inner surface of the evaporator in this embodiment can avoid the problems of large flow resistance and uneven bubble nucleation caused by factors such as irregular void distribution, complex flow path and flow dead zone in the sintered porous structure, and can also solve the problems of complex manufacturing process and poor structural durability of the multi-layer microchannel. It has advantages in bubble nucleation and detachment performance, flow resistance, manufacturing process, structural stability, scope of application and ease of maintenance.

[0062] Example 2

[0063] In this embodiment, a high-liquid filling rate loop thermosyphon in which the heat transfer medium is a self-wetting fluid is provided, comprising an evaporator 1, a steam pipeline 2, a condenser 3, a liquid pipeline 4 and a heat transfer medium 5. The evaporator in this embodiment is exactly the same as the evaporator in Example 1. The evaporator 1, the steam pipeline 2, the condenser 3 and the liquid pipeline 4 are all made of copper.

[0064] like Figure 4 As shown, in this embodiment, the evaporator 1, the steam pipeline 2, the condenser 3 and the liquid pipeline 4 are connected end to end and welded together to form a closed loop. In the loop, the evaporator 1 and the condenser 3 are located on opposite sides. The loop is filled with a heat transfer medium 5.

[0065] When installing and arranging the loop thermosyphon in this embodiment, the outer wall of the evaporator 1 should be close to the heat source, and a cooling water jacket should be installed outside the condenser 3. The cooling water in the cooling water jacket will provide coldness for the condenser 3, so that the condenser 3 can play a condensation role. In addition, the horizontal height of the evaporator 1 should be lower than the horizontal height of the condenser 3, so that the bubbles generated by the vaporization of the heat transfer medium at the evaporator 1 can move to the condenser 3 in time and condense and liquefy at the condenser 3.

[0066] During the operation of the loop thermosyphon, the liquid heat transfer medium 5 inside the evaporator 1 absorbs heat, a part of which is converted from liquid to gaseous, which is manifested as a large number of bubbles generated in the evaporator 1, and the bubbles contain the vaporized heat transfer medium 5. Under the action of gravity difference and buoyancy, the gaseous medium pushes the liquid medium to move along the steam pipeline toward the condenser 3, forming a "bubble pump" effect, thereby pushing the heat transfer medium 5 to circulate continuously in the loop. When the gas-liquid two-phase medium reaches the condenser 3, under the action of cooling water, the gaseous medium condenses into liquid medium and returns to the evaporator 1 along the liquid pipeline 4, and circulates again.

[0067] It should be noted that, in this embodiment, the heat transfer medium 5 is an aqueous solution of n-butanol, in which the mass fraction of n-butanol is 1wt%. The aqueous solution of n-butanol is a water-based self-wetting fluid. After reaching a certain temperature value, the surface tension of the self-wetting fluid increases with the increase of temperature. During the operation of the loop thermosyphon, the self-wetting fluid (heat transfer medium) in the loop can form a component concentration gradient and a surface tension gradient, which promotes the reflux of the condensed self-wetting fluid to the evaporator under the action of the "Marangoni" effect; at the same time, the water-based self-wetting fluid has a lower surface tension, which can reduce the nucleation size of the bubble.

[0068] In some other embodiments, the heat transfer medium 5 can also be other water-based self-wetting fluids, such as an aqueous solution of n-pentanol (the mass fraction of n-pentanol in the solution is 0.1wt% to 2wt%), or an aqueous solution of n-heptanol (the mass fraction of n-heptanol in the solution is 0.01wt% to 0.1wt%). It is also possible to select an aqueous solution of n-butanol with other mass fractions as the heat transfer medium, as long as the mass fraction of n-butanol in the n-butanol aqueous solution is within 1wt% to 7wt%. N-butanol, n-pentanol and n-heptanol are all high-carbon alcohols, and their solubility in water is relatively low. When the mass fraction of high-carbon alcohols in the aqueous solution is too high, stratification may occur between the water and the solution, resulting in an increase in the viscosity of the solution, and a decrease in uniformity, which affects the fluidity and heat transfer performance of the heat transfer medium 5.

[0069] In the preparation process of the high-carbon alcohol aqueous solution used as a heat transfer medium, in order to ensure the accuracy of the mass fraction, a high-precision electronic balance should be used to weigh it. During the preparation, first put the beaker into the electronic scale to clear the data, use a rubber-tipped dropper to drop the high-carbon alcohol into the beaker, weigh the corresponding mass, and then add the corresponding mass of deionized water to the beaker. Since the miscibility of alcohol and water is not very good, and the density of high-carbon alcohol is smaller than that of deionized water, stratification will occur, and the high-carbon alcohol solution will float on the water surface. Then, ultrasonic vibration is used to dissolve the two solutions. When there is no stratification in the mixed solution, the preparation is completed.

[0070] It should be noted that after the loop thermosiphon in this embodiment is welded and assembled, and before filling with the self-wetting fluid heat transfer medium, it is necessary to conduct a leak test on the loop at a pressure of 1 to 5 MPa to ensure that the loop is well sealed before filling with the heat transfer medium. When filling with the heat transfer medium, the entire closed loop of the loop thermosiphon is first vacuumed, and the internal pressure of the loop thermosiphon is pumped to 0.1 to 0.5 Pa by a molecular pump, and then the heat transfer medium is filled. After filling (non-working state and room temperature), the filling volume of the heat transfer medium (filled in liquid form) accounts for more than 60% of the total volume of the entire closed loop, that is, the liquid filling rate of the heat transfer medium in the loop is greater than 0.6.

[0071] In this embodiment, the filling rate of the heat transfer medium in the loop is 0.8.

[0072] Furthermore, the heat transfer performance of the self-wetting fluid high-filling rate loop thermosyphon with gradient wire mesh structure evaporator in Example 2 was tested and compared with the high-filling rate loop thermosyphon with traditional smooth wall evaporator and water as working fluid. The filling rate and the specifications and dimensions of each component of the loop thermosyphon used as the comparison are exactly the same as those of the loop thermosyphon provided in Example 2.

[0073] During the test, a DC power supply was used with multiple electric heating rods and a copper heating block as the heating system. The input heat was adjusted by adjusting the input power to simulate the heat source in actual applications. A constant temperature chiller was used as the cooling system, with an output volume flow of 1L / min and a constant temperature cooling water of 18°C. An Agilent 3972A data acquisition instrument was used to measure the temperature of each measuring point on the loop with a sampling frequency of 1Hz. A high-speed camera was used to capture images at the evaporator outlet to compare the difference between the bubble detachment size and the bubble detachment frequency.

[0074] like Figure 5 The figure shows the relationship between the evaporation thermal resistance of two loop thermosyphons and the heat flux density. In the figure, "this example" represents the loop thermosyphon in Example 2, and the "comparison group" represents the high-liquid filling rate loop thermosyphon with a traditional smooth wall evaporator and water as the heat transfer medium. Figure 6 Visualization of the outlet of the thermosyphon evaporator for two loops.

[0075] Depend on Figure 5 It can be seen that compared with the high-filling rate loop thermosyphon with water as the working fluid in the traditional smooth-wall evaporator, the heat transfer performance of the self-wetting fluid high-filling rate loop thermosyphon with gradient wire mesh structure evaporator in Example 2 has been greatly improved. Among them, the critical heat flux density has been increased by 85W / cm 2 (ΔCHF=85W / cm 2 ), up to 500W / cm 2 The evaporation thermal resistance decreased by 32.6% (ΔRe = 32.6%).

[0076] from Figure 6 It can be seen that compared with the smooth surface evaporator loop thermosyphon with pure water as the working fluid, the loop thermosyphon in Example 2 has a smaller bubble volume and a larger number of bubbles at the evaporator outlet, proving that the gradient wire mesh structure evaporator combined with the self-wetting fluid can reduce the bubble detachment size and increase the bubble detachment frequency, thereby enhancing the driving effect on the heat transfer working fluid, and the "bubble pump" phenomenon is more prominent, which accelerates the circulation flow rate in the loop and improves the heat transfer performance.

[0077] In addition, after more than 20 cycles of testing, the heat transfer performance of the gradient wire mesh structure evaporator in the present invention did not decrease significantly.

[0078] Obviously, the above embodiments are merely examples for the purpose of clear explanation, and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived therefrom are still within the scope of protection of the claims of this patent application.

Claims

1. An evaporator with a gradient wire mesh structure, characterized in that: The invention is a tubular structure, comprising a hollow tube body (104); the inner wall of the tube body (104) is provided with multiple layers of wire mesh, and the mesh openings of the wire mesh increase layer by layer in the direction from the inner wall of the tube body (104) to the center of the tube cavity; the tube body (104) and the wire mesh adjacent thereto, as well as the two adjacent layers of the wire mesh, are tightly attached to each other and fixedly connected.

2. The evaporator with a gradient wire mesh structure according to claim 1, characterized in that: The wire mesh has three layers, namely a first wire mesh (101), a second wire mesh (102) and a third wire mesh (103); the first wire mesh (101), the second wire mesh (102) and the third wire mesh (103) are arranged in sequence in a direction from the inner wall of the tube body (104) to the center of the tube cavity; the mesh ratio of the first wire mesh (101) to the second wire mesh (102), and the mesh ratio of the second wire mesh (102) to the third wire mesh (103) are both 1.2 to 2.

0.

3. The evaporator with a gradient wire mesh structure according to claim 2, characterized in that: The mesh number of the first screen (101) is 250-200 meshes, the mesh number of the second screen (102) is 200-150 meshes, and the mesh number of the third screen (103) is 150-100 meshes.

4. The evaporator with a gradient wire mesh structure according to claim 3, characterized in that: The mesh number of the first wire mesh (101) is 200 meshes, the mesh number of the second wire mesh (102) is 150 meshes, and the mesh number of the third wire mesh (103) is 100 meshes.

5. The evaporator with a gradient wire mesh structure according to claim 1, characterized in that: The tube body (104) and the wire mesh adjacent thereto, as well as two adjacent layers of the wire mesh, are fixed by vacuum sintering; During vacuum sintering, the wire mesh is first placed into the lumen of the tube body (104), and then a carbon rod is inserted into the lumen of the tube body (104); the carbon rod presses the wire mesh and makes the two adjacent layers of the wire mesh, as well as the tube body (104) and the wire mesh adjacent thereto, tightly adhere to each other; finally, sintering is performed under vacuum conditions.

6. A high liquid filling rate loop thermosyphon with a gradient wire mesh structure evaporator, characterized in that: It comprises an evaporator (1) with a gradient wire mesh structure as claimed in claim 1, a steam pipeline (2), a tubular condenser (3) and a liquid pipeline (4), wherein the evaporator (1), the steam pipeline (2), the condenser (3) and the liquid pipeline (4) are connected end to end in sequence to form a closed loop, and the loop is filled with a heat transfer medium (5); at room temperature and in a non-working state, the filling rate of the heat transfer medium (5) in the loop is greater than 0.

6.

7. The high liquid filling rate loop thermosyphon with gradient wire mesh structure evaporator according to claim 6, characterized in that: The heat transfer medium (5) is a water-based self-wetting fluid.

8. The high liquid filling rate loop thermosyphon with gradient wire mesh structure evaporator according to claim 7, characterized in that: The heat transfer medium (5) is one of the aqueous solutions containing alcohols in the following concentration ranges: a) an aqueous solution of n-butanol, wherein the mass fraction of n-butanol is 1 wt% to 7 wt%; b) an aqueous solution of n-pentanol, wherein the mass fraction of n-pentanol is 0.1 wt% to 2 wt%; c) an aqueous solution of n-heptanol, wherein the mass fraction of n-heptanol is 0.01 wt % to 0.1 wt %.

9. The high liquid filling rate loop thermosyphon with gradient wire mesh structure evaporator according to claim 6, characterized in that: When manufacturing the high liquid filling rate loop thermal syphon, after assembling the evaporator (1), the steam pipeline (2), the condenser (3) and the liquid pipeline (4) into a closed loop and completing the loop leak detection, the heat transfer medium (5) is filled into the loop; when filling the loop with the heat transfer medium (5), the pressure in the loop is first pumped down to 0.1-0.5 Pa before filling.

10. The high liquid filling rate loop thermosyphon with gradient wire mesh structure evaporator according to any one of claims 6 to 9, characterized in that: The evaporator (1), the steam pipeline (2), the condenser (3) and the liquid pipeline (4) are all made of copper.

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