A method for reducing the internal thermal resistance of a loop heat pipe evaporator

Abstract

The present invention relates to a method for reducing the internal thermal resistance of a loop heat pipe evaporator, comprising: (1) machining a steam channel on the surface of a ceramic capillary wick, and then preparing a metal slurry layer on the surface of the steam channel by coating a metal slurry; (2) sintering the ceramic capillary wick coated with the metal slurry layer to obtain a metal transition layer; (3) heating the ceramic capillary wick sintered with the metal transition layer and a metal shell to 100-400 °C, and then assembling the ceramic capillary wick sintered with the metal transition layer into the metal shell by means of hydraulic pressure to reduce the internal thermal resistance of the loop heat pipe evaporator.

Description

Technical Field

[0001] The present invention relates to a method for reducing the internal thermal resistance of a loop heat pipe evaporator, belonging to the technical field of thermal control. Background Art

[0002] As an efficient phase change heat transfer device, the loop heat pipe has been widely used in the thermal control systems of electronic devices, aerospace and other fields due to its advantages such as long transmission distance, flexible space layout, and high heat transfer efficiency.

[0003] The loop heat pipe relies on the evaporation and condensation of the working fluid to achieve heat transfer, mainly including several parts such as an evaporator, a gas pipeline, a condenser, a liquid pipeline, and a liquid reservoir. The evaporation of the working fluid mainly occurs inside the evaporator. In addition, the capillary action of the internal porous wick provides the driving force for the fluid circulation of the system. Therefore, the structural design of the evaporator is of great significance to the heat transfer efficiency of the loop heat pipe. Due to the easy closure of surface micropores during the processing of the metal wick and the high thermal conductivity that is prone to cause heat leakage, and the relatively poor compatibility and poor thermal stability of the polymer core working fluid, in recent years, ceramic cores have become the focus of research.

[0004] The contact thermal resistance between the heating surface of the evaporator and the wick is an inevitable factor affecting the heat transfer efficiency of the loop heat pipe. How to optimize the contact thermal resistance has always been a difficult problem in the field of loop heat pipes. In particular, due to the large brittleness of ceramic materials, the wick is usually completed by thermal assembly, but thermal assembly is also difficult to better solve the contact thermal resistance problem between the ceramic core and the metal shell. Summary of the Invention

[0005] Aiming at the above problems, the present invention aims to provide a method for reducing the internal thermal resistance of a loop heat pipe evaporator. The wick of the evaporator is made of ceramic material, and the surface of the ceramic core in contact with the heating surface is metallized to increase the contact area by using the ductility of the metal and reduce the contact thermal resistance.

[0006] The method for reducing the internal thermal resistance of the loop heat pipe evaporator includes:

[0007] (1) Processing steam channels on the surface of the ceramic wick, and then preparing a metal slurry layer on the surface of the steam channels by coating the metal slurry;

[0008] (2) Sintering the ceramic wick coated with the metal slurry layer to obtain a metal transition layer;

[0009] (3) After heating the ceramic wick sintered with the metal transition layer and the metal shell to 100 - 400 °C, assembling the ceramic wick sintered with the metal transition layer into the metal shell by hydraulic pressure to reduce the internal thermal resistance of the loop heat pipe evaporator.

[0010] Preferably, the metal paste composition comprises: 75 wt% to 85 wt% metal powder, 2 wt% to 5 wt% organic binder which is at least one of polyvinyl alcohol and polyvinyl butyral; preferably, the metal powder is at least one of low melting point metals silver, zinc, and lead.

[0011] Also preferably, the metal paste further contains 10 wt% to 28 wt% (preferably 15 wt% to 20 wt%) of other substances; the other substances are at least one of water and ethanol.

[0012] Preferably, the thickness of the metal paste layer is 0.5 mm to 5 mm, preferably 0.5 mm to 2 mm. Within the preferred thickness range, while ensuring the reduction of contact thermal resistance, more micropore blockages are avoided.

[0013] Preferably, before sintering, the metal paste layer is dried, the drying temperature is 30 to 50 °C, and the time is 2 to 16 hours; preferably, the drying time is 2 to 6 hours.

[0014] Preferably, the sintering atmosphere is an inert atmosphere, the sintering temperature is preferably 300 to 1000 °C, and the holding time is preferably 10 to 60 minutes; preferably, the inert atmosphere is an argon atmosphere, and the heating rate of sintering is 0.5 to 5 °C / minute, more preferably 0.5 to 2 °C / minute.

[0015] Preferably, the heating atmosphere is an air atmosphere; the heating time is 1 to 10 minutes; preferably, the heating rate is 2 to 5 °C / minute.

[0016] Preferably, the hydraulic pressure is 2 MPa to 20 MPa, and the pressure holding time is 5 seconds to 30 seconds.

[0017] Preferably, the material of the ceramic capillary core is at least one of silicon nitride, alumina, zirconia, and silicon carbide; the ceramic capillary core is a single-aperture porous ceramic capillary core or a gradient-distributed porous ceramic capillary core; preferably, the shape of the steam channel is triangular, rectangular, or trapezoidal.

[0018] On the other hand, the present invention also provides a loop heat pipe evaporator prepared according to the above method, comprising: a metal shell, a ceramic capillary core assembled inside the metal shell, and a metal transition layer connected between the metal shell and the ceramic capillary core.

[0019] Beneficial effects:

[0020] The present invention successfully prepares a loop heat pipe evaporator with a small contact thermal resistance. Introducing a metal transition layer at the interface between the heating surface of the metal shell and the ceramic capillary wick can well solve the problem of excessive contact thermal resistance. The ductility of the metal can greatly reduce the poor contact between the ceramic capillary wick and the metal shell, such as gaps. At the same time, the high thermal conductivity of the metal increases the heat transfer efficiency from the metal heating surface to the ceramic capillary wick, and at the same time meets the use temperature of the evaporator above 300°C. These favorable factors provide good conditions for reducing the thermal resistance of the evaporator and improving the heat transfer efficiency of the loop heat pipe. In addition, according to the difference in the thermal expansion coefficients of the metal and the ceramic capillary wick, the thermal resistance of the evaporator is further reduced by thermal assembly, and the thermal resistance of the evaporator is reduced by 2% to 25%. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 is the front view of the disk-shaped evaporator of the silicon nitride-stainless steel loop heat pipe in Example 1;

[0022] Figure 2 is the top view of the disk-shaped evaporator of the silicon nitride-stainless steel loop heat pipe in Example 1;

[0023] Figure 3 is the front view of the cylindrical evaporator of the silicon nitride-stainless steel loop heat pipe in Example 2;

[0024] Figure 4 is the top view of the cylindrical evaporator of the silicon nitride-stainless steel loop heat pipe in Example 2;

[0025] Wherein, 1 - stainless steel shell, 2 - silicon nitride capillary wick, 3 - metal coating, 4 - steam channel. DETAILED DESCRIPTION OF THE INVENTION

[0026] The present invention will be further described below through the following embodiments. It should be understood that the following embodiments are only used to illustrate the present invention and do not limit the present invention.

[0027] In the present disclosure, a loop heat pipe evaporator with an extremely small contact thermal resistance is obtained by successfully introducing a metal transition layer at the interface between the heating surface of the metal shell of the loop heat pipe evaporator and the ceramic capillary wick.

[0028] The following examples illustrate the preparation method of the loop heat pipe evaporator provided by the present invention.

[0029] Grooves are made on the surface of the ceramic capillary wick to obtain steam channels. Then, according to the shape and size of the capillary wick, the metal shell is processed. The material of the ceramic capillary wick is preferably at least one of silicon nitride, alumina, zirconia, and silicon carbide. The pore structure of the ceramic capillary wick can be at least one of a single pore size uniformly distributed pore structure, a double pore size uniformly distributed pore structure, a gradient pore structure, and a multi-scale laminated pore structure, preferably one of the gradient pore structure and the multi-scale laminated pore structure. The shape of the ceramic capillary wick can be at least one of circular, rectangular, disc-shaped, and oval. The cross-section of the steam channels on the outer wall surface of the ceramic capillary wick along the axial and radial directions can be at least one of triangle, rectangle, trapezoid, and other designable shapes. The material of the metal shell can be at least one of stainless steel, copper, and aluminum. The outer shape of the metal shell can be at least one of flat plate, circular, and cylindrical. The thickness of the metal shell can be 0.1 - 5 mm, preferably 0.5 - 2 mm.

[0030] A layer of metal paste is coated on the protrusions on the surface of the ceramic capillary wick to obtain a metal paste layer, and then it is dried in an oven. Among them, the composition of the metal paste includes: 75 - 85% metal powder, 2 - 5% organic matter, and 15 - 20% other substances. The metal powder is at least one of low melting point metals silver, zinc, and lead. The coating thickness of the metal paste is preferably 0.5 - 5 mm, more preferably 0.5 - 2 mm. The drying temperature is preferably 30 - 50 °C, and the drying time is preferably 2 - 16 h, more preferably 2 - 6 h.

[0031] The dried ceramic capillary wick is placed in a sintering furnace and sintered in an inert atmosphere. After the heating regime ends, it is cooled with the furnace. Among them, the heating rate can be 0.5 - 5 °C / min, preferably 0.5 - 2 °C / min. The sintering temperature can be 300 - 1000 °C, and the holding time can be 10 - 60 min.

[0032] The sintered ceramic capillary wick coated with metal and the metal shell are placed in a muffle furnace and heated in an air atmosphere. Subsequently, the heated metal shell is fixed on a limit tooling table, and the heated capillary wick is assembled into the metal shell by hydraulic means. Among them, the heating rate can be 2 - 5 °C / min. The heating temperature can be 100 - 400 °C, and the holding time can be 1 - 10 min. The hydraulic pressure can be 2 - 20 MPa, preferably 2 - 10 MPa. The holding pressure time can be 5 - 30 s, preferably 10 - 30 s.

[0033] After the traditional technical method completes the assembly of the ceramic capillary wick and the metal shell, due to the limitation of processing accuracy, the gap between the metal shell and the ceramic capillary wick is inevitable, thus increasing the thermal resistance of the evaporator. In the present invention, the loop heat pipe evaporator prepared by a specific method well overcomes the problem of excessive contact thermal resistance caused by insufficient accuracy of the capillary wick and the metal shell in the traditional assembly method.

[0034] The following are further examples to illustrate the present invention in detail. It should also be understood that the following examples are only used to further illustrate the present invention and should not be construed as limiting the protection scope of the present invention. Some non-essential improvements and adjustments made by those skilled in the art based on the above content of the present invention fall within the protection scope of the present invention. The specific process parameters and the like in the following examples are also only an example within a suitable range, that is, those skilled in the art can make selections within a suitable range through the description in this article, rather than being limited to the specific values in the following examples.

[0035] Example 1:

[0036] Steam channels are machined on the surface of the disc-shaped silicon nitride ceramic capillary core. A silver paste with a silver powder content of 80% is evenly coated on the convex platform of the outer surface of the ceramic capillary core, and the thickness of the silver paste is 0.5 mm. The ceramic capillary core coated with silver paste is placed in an oven for drying. The drying temperature is 40 °C and the drying time is 4 h. Then, the dried ceramic capillary core is placed in a muffle furnace. In an air atmosphere, it is heated to 550 °C at a heating rate of 1 °C / min, and held for 15 min, and then cooled with the furnace. The silver-coated silicon nitride capillary core and the stainless steel shell are placed in a muffle furnace and heated to 250 °C at a heating rate of 4 °C / min, and held for 5 min. Then, they are taken out, and the stainless steel shell is quickly fixed on the limit tooling table. The capillary core is placed directly above the stainless steel inner cavity. By means of hydraulic pressure, the capillary core is assembled into the stainless steel shell so that the capillary core is in close contact with the stainless steel inner cavity. The hydraulic pressure is 5 MPa and the pressure holding time is 20 s. The schematic diagrams of the disc-shaped evaporator of the silicon nitride-stainless steel loop heat pipe prepared in this Example 1 are as shown in Figure 1 and Figure 2 shown. The prepared evaporator is used in a stainless steel / water loop heat pipe for heat transfer performance testing. In the heat load range of 10 - 100 W, the thermal resistance of the evaporator is reduced by 3% - 10%.

[0037] Example 2:

[0038] The preparation process of the cylindrical evaporator of the silicon nitride-stainless steel loop heat pipe in this Example 2 refers to Example 1, except that the shapes of the silicon nitride ceramic capillary core and the metal shell are cylindrical. The schematic diagrams of the cylindrical evaporator of the silicon nitride-stainless steel loop heat pipe prepared in this Example 2 are as shown in Figure 3 and Figure 4 shown; The prepared evaporator is used in a stainless steel / water loop heat pipe for heat transfer performance testing. In the heat load range of 10 - 100 W, the thermal resistance of the evaporator is reduced by 5% - 15%.

Claims

1. A method for reducing the internal thermal resistance of a loop heat pipe evaporator, characterized in that, Including: (1) Processing steam channels on the surface of a ceramic capillary wick, and then preparing a metal paste layer on the surface of the steam channels by coating the metal paste; (2) Sintering the ceramic capillary wick coated with the metal paste layer to obtain a metal transition layer; (3) After heating the ceramic capillary wick sintered with the metal transition layer and the metal shell to 100 - 400 °C, assembling the ceramic capillary wick sintered with the metal transition layer into the metal shell by hydraulic pressure to reduce the internal thermal resistance of the loop heat pipe evaporator.

2. The method according to claim 1, wherein The composition of the metal paste includes: 70wt% - 85wt% metal powder, 2wt% - 5wt% organic binder; the metal powder is at least one of low melting point metals silver, zinc, lead, and the organic binder is at least one of polyvinyl alcohol and polyvinyl butyral.

3. The method according to claim 2, characterized in that, The metal paste further contains 10wt% - 28wt% of other substances; the other substances are at least one of water and ethanol.

4. The method according to claim 1, wherein The thickness of the metal paste layer is 0.5 mm - 5 mm.

5. The method according to claim 4, wherein The thickness of the metal paste layer is 0.5 mm - 2 mm.

6. The method according to claim 1, wherein Before sintering, the metal paste layer is dried, and the drying temperature is 30 - 50 °C, and the time is 2 - 16 hours.

7. The method according to claim 6, characterized in that, The drying time is 2 - 6 hours.

8. The method according to claim 1, characterized in that, The sintering atmosphere is an inert atmosphere, the sintering temperature is 300 - 1000 °C, and the holding time is 10 - 60 minutes.

9. The method according to claim 8, wherein The inert atmosphere is an argon atmosphere, and the heating rate of sintering is 0.5 - 5 °C / minute.

10. The method according to claim 9, characterized in that, The heating rate of sintering is 0.5 - 2 °C / minute.

11. The method according to claim 1, wherein The heating atmosphere is an air atmosphere; the heating time is 1 - 10 minutes; the heating rate is 2 - 5 °C / minute.

12. The method according to claim 1, characterized in that, The hydraulic pressure is 2 MPa - 20 MPa, and the pressure holding time is 5 seconds - 30 seconds.

13. The method according to any one of claims 1 to 12, characterized in that, The material of the ceramic capillary wick is at least one of silicon nitride, alumina, zirconia, and silicon carbide; the ceramic capillary wick is a single pore size porous ceramic capillary wick or a gradient distribution porous ceramic capillary wick.

14. The method according to claim 13, wherein The shape of the steam channel is triangular, rectangular or trapezoidal.

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