CN115523778B - Cylindrical loop heat pipe - Google Patents

Abstract

The invention provides a cylindrical loop heat pipe, which includes an evaporator, a condenser and a pipeline. The liquid absorbs heat and evaporates in the evaporator, and then the steam enters the condenser through the steam pipeline to condense and release heat. The heat-released liquid passes through The liquid line enters the evaporator to evaporate, forming a cycle. It is characterized in that the evaporator includes a shell with a circular cross-section and a liquid pipeline located in the center of the shell. The liquid pipeline is covered with a deputy capillary wick, and a capillary wick is arranged between the deputy capillary core and the evaporator shell. The capillary core is connected to the evaporator shell. A steam channel is provided at the connection point of the shell. The loop heat pipe evaporator of the present invention includes a shell with a circular cross-section, so that it is suitable for a circular heat dissipation environment. In addition, in order to reduce the impact of bubbles on the stability of the heat pipe during operation and increase the suction performance of the capillary wick, the liquid pipe of the heat pipe is Add a secondary capillary core inside to achieve the desired effect.

Description

A cylindrical loop heat pipe

Technical field

The present invention relates to a heat pipe technology, in particular to a loop heat pipe, which belongs to the field of F28d15/02 heat pipes.

Background technique

Heat pipe technology is a heat transfer element called a "heat pipe" invented by George Grover of the Los Alamos National Laboratory in the United States in 1963. It makes full use of the principle of heat conduction and phase change media. The rapid heat transfer property of the heat pipe can quickly transfer the heat of the heating object to the outside of the heat source. Its thermal conductivity exceeds that of any known metal.

Heat pipe technology has been widely used in aerospace, military and other industries before. Since it was introduced into the radiator manufacturing industry, people have changed the design ideas of traditional radiators and got rid of the single cooling mode that simply relies on high air volume motors to obtain better heat dissipation effects. The use of heat pipe technology enables the radiator to achieve satisfactory heat exchange effects, opening up a new world in the heat dissipation industry. At present, heat pipes are widely used in various heat exchange equipment, including the field of nuclear power, such as waste heat utilization of nuclear power.

Loop heat pipe refers to a closed loop heat pipe. It generally consists of an evaporator, condenser, liquid reservoir, and steam and liquid pipelines. Its working principle is: applying a thermal load to the evaporator, the working medium evaporates on the outer surface of the evaporator capillary core, and the generated steam flows out from the steam channel into the steam pipeline, and then enters the condenser to be condensed into liquid and supercooled, and the reflux liquid passes through the liquid. The pipeline enters the liquid storage chamber to replenish the evaporator capillary core, and so on, and the circulation of the working fluid is driven by the capillary pressure generated by the evaporator capillary core without external power. Since the condensation section and the evaporation section are separated, the loop heat pipe is widely used in the comprehensive application of energy and the recovery of waste heat.

In order to solve the problem that the heat transfer of traditional heat pipes is limited by the long distance and the orientation of the cold and heat sources, Maidanik et al. of the National Academy of Sciences proposed the concept of loop heat pipes based on traditional heat management theory in 1971, and designed and processed the first loop heat pipe in 1972. A set of loop heat pipes. In the following ten years, loop heat pipes have continued to develop in China. In 1985, Maidanik et al. applied for a patent for this heat pipe in the United States. This automatic heat transfer device that relies on capillary force to drive working fluid circulation has been called "Heat pipe", "Heat pipe with separate channels" and "Antigravitational heatpipe". It was not until 1989 that the loop heat pipe was first used in spacecraft heating. In the control system, it attracted widespread international attention and was eventually named "Loop heat pipe", which is called "loop heat pipe" in the domestic industry. After the 1990s, loop heat pipes have attracted widespread attention from relevant scholars and space vehicle thermal control design workers from various countries due to their advantages. Many countries have invested a lot of money in research. Loop heat pipes with various structural forms and using different working fluids have continued to be developed. Presented at relevant academic conferences. Research on loop heat pipes mainly includes three aspects: experimental research and analysis, mathematical modeling and application research.

The thermal conductivity of the LHP system depends largely on the heat exchange performance between the condenser and the heat sink. Most of the early research on LHP was in the context of space applications. The condenser mainly releases heat to the space heat sink in the form of radiation. Therefore, the structure of embedding the condenser pipeline into the condenser plate is commonly used. Simple casings can also be used in ground experiments. Type condenser, a constant temperature bath is used to simulate a heat sink, and a pump drives the refrigerant medium (such as water, ethanol, etc.) to circulate in the casing to cool the condenser.

The evaporator is the core component of LHP. It has two important functions: absorbing heat from the heat source and providing working fluid circulation power. After decades of improvement and development, the currently more common structural form is that the evaporator body mainly includes an evaporator shell, a capillary wick and a liquid guide tube. The axial groove outside the capillary core is called the vapor groove (Vapor groove), and the inside of the capillary core is the liquid pipeline (Liquid core or Evaporator core).

The capillary wick is the core component of the evaporator. It provides working fluid circulation power, provides a liquid evaporation interface, and realizes liquid supply. At the same time, it blocks the steam generated outside the capillary wick from entering the liquid reservoir. Capillary cores are generally formed by molding micron-sized metal powder through processes such as pressing and sintering to form micron-sized pores.

In the capillary core test conducted by the applicant, NaCl, gC ₃ N ₄ and a mixture of gC ₃ N ₄ and NaCl were used as pore-forming agents. The suction performance of gC ₃ N ₄ and NaCl was between that of NaCl and gC ₃ N ₄ is used as a pore-forming agent alone, taking into account the machining problem, but the inappropriate ratio of gC ₃ N ₄ will cause the capillary core to break easily during processing, so g-C ₃ N ₄ and NaCl were selected to mix the capillary core, and Through a large number of experiments, the appropriate mixing ratio is determined to solve the problem of capillary core breakage. The present invention also provides a method for preparing a loop heat pipe.

Contents of the invention

The present invention aims to provide a low-cost loop heat pipe with a capillary core that is not easily broken, so as to improve the promotion and commercial application of heat dissipation from heat sources.

In order to achieve the above objects, the technical solutions of the present invention are as follows:

A cylindrical loop heat pipe includes an evaporator, a condenser and a pipeline. The liquid absorbs heat and evaporates in the evaporator, and then the steam enters the condenser through the steam pipeline to condense and release heat. The heat-released liquid enters the liquid pipeline through the liquid pipeline. Evaporation is carried out in the evaporator to form a cycle, which is characterized in that the evaporator includes a shell with a circular cross-section and a liquid pipeline located in the center of the shell. The liquid pipeline is covered with a deputy capillary wick, the deputy capillary core and the evaporator shell. A capillary core is provided between them, and a steam channel is provided at the connection between the capillary core and the shell.

Preferably, the condenser is a shell-and-tube heat exchanger, with the cold source traveling on the shell side and the steam traveling on the tube side. The heat exchanger includes a cold source inlet and a cold source outlet provided on the shell, The steam pipeline adopts a serpentine arrangement.

Preferably, the liquid pipeline is connected to a liquid reservoir, and the liquid reservoir is connected to the evaporator.

Preferably, the capillary core is an annular structure, and a plurality of steam channels are provided on the outer wall, and the steam channels extend along the length direction of the capillary core.

Preferably, the axis of the capillary core is parallel to the axis of the steam channel.

Compared with the prior art, the present invention has the following advantages:

1) The loop heat pipe evaporator of the present invention includes a shell with a circular cross-section, so that it is suitable for a circular heat dissipation environment, and in order to reduce the impact of bubbles on the stability of the heat pipe during operation and increase the suction performance of the capillary wick, the heat pipe is A secondary capillary wick is added to the liquid pipeline to achieve the desired effect.

2) In terms of capillary core performance: the new pore-forming agent gC ₃ N ₄ used in the present invention will evaporate (500°C) during the capillary core sintering process to form flaky pores. Compared with using NaCl as the pore-forming agent, this The porosity and permeability formed by the pore-forming agent are larger, and the pore size is larger. The large pores can reduce the resistance when the working fluid flows, increase the evaporation area, facilitate the overflow of steam, and increase the ultimate power of the heat pipe. This capillary core can reduce weight by about 30% compared with nickel-based capillary cores of the same size, which is beneficial for aerospace heat dissipation.

3) The present invention provides a new capillary core preparation method, which improves production efficiency through various steps and process optimization.

4) Assembly: The evaporator shell and the capillary core adopt an interference fit, and the interference degree is 0.4mm; the evaporator shell and the aluminum saddle also adopt an interference fit. This combination method can reduce the impact of contact thermal resistance, reduce heat leakage from the evaporator to the liquid storage chamber, and increase the limit of the heat pipe.

5) Evaporator deputy capillary wick: A 500-mesh metal mesh is used to fill the gap between the liquid pipeline and the liquid flow channel. As a deputy capillary wick, it can assist the suction of the heat pipe and increase the circulation flow of the return liquid. At the same time, the secondary capillary core can filter out non-condensable gases in the return liquid, reducing disturbance to the performance of the capillary core and reducing axial heat leakage.

Description of the drawings

Figure 1 is a vacuum hot-pressing sintering furnace for sintering capillary cores of the present invention;

Figure 2 is a sintering temperature diagram of the sintered capillary core of the present invention;

Figure 3 is a schematic diagram of the capillary core structure of the present invention;

Figures 4-1 to 4-3 are cross-sectional and radial cross-sectional views of the capillary core of the present invention;

Figure 5 is a schematic structural diagram of the evaporator shell of the present invention;

Figure 6 is an aluminum saddle shell model of the present invention;

Figure 7 is an assembly diagram of the condenser of the present invention;

Figure 8 is a cleaning flow chart;

Figure 9 Schematic diagram of evaporator shell welding;

Figure 10 Capillary core welding flow chart.

Detailed ways

The specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

The invention discloses a loop heat pipe, which includes an evaporator, a condenser and a pipeline. The liquid absorbs heat and evaporates in the evaporator, and then the steam enters the condenser through the steam pipeline to condense and release heat. The heat-released liquid passes through the liquid pipeline. Enters the evaporator for evaporation, thus forming a cycle.

Preferably, the liquid pipeline is connected to a liquid reservoir, and the liquid reservoir is connected to the evaporator.

The evaporator structure is shown in 4-3. The evaporator includes a shell with a circular cross-section and a liquid pipeline located in the center of the shell. The liquid pipeline is covered with a deputy capillary wick. A capillary wick is provided between the deputy capillary core and the evaporator shell. The connection between the capillary core and the shell Set up steam channels.

When the loop heat pipe is left standing, the capillary core is in a wetted state, and liquid ammonia mainly exists between the evaporator, liquid reservoir and liquid pipeline. When the evaporator is heated, the liquid is heated and begins to nucleate boil. This phenomenon mainly occurs on the liquid film infiltrated on the outer surface of the capillary core. The trace gas generated enters the steam chamber through the steam channel, and finally enters the condenser through the steam pipeline. Cooling is carried out, and the cooled liquid working fluid enters the liquid storage chamber through the liquid pipeline to participate in the next cycle. The secondary capillary core assists the main capillary core in suction. The effective aperture of the secondary capillary core is larger than that of the main capillary core, and the resistance is smaller during the suction process. The suctioned working medium is replenished to the capillary core along the radial direction to participate in the circulation; when the circulating liquid Returning to the liquid storage chamber and entering the capillary wick, the auxiliary capillary wick in the liquid storage chamber blocks and destroys bubbles in the return liquid, reducing its impact on the operational stability of the heat pipe. When the heat load continues to increase, the amount of gas increases, and the liquid film on the surface of the capillary core gradually evaporates, forming a layer of gas film between the evaporator shell and the capillary core, wrapping the outer surface of the capillary core. At this time, the thermal resistance of the heat pipe is relatively small. Small, the internal air pressure is larger, the gas circulation speed is faster, and the startup time is shorter, which is called the optimal operating state of the heat pipe. When the heat load continues to increase, the gas-liquid interface invades the inside of the capillary core, and the gas is in a superheated state, that is, the phenomenon of burning out. At the same time, the heat leakage from the evaporator to the liquid storage chamber is serious, and the heat exchange capacity of the evaporator decreases. It can be considered that This power is the limit power.

The structure of the condenser is shown in Figure 9. The condenser is a shell-and-tube heat exchanger. The cold source travels through the shell side and the steam travels through the tube side. The heat exchanger includes a cooling unit arranged on the shell. The steam pipeline adopts a serpentine arrangement for the source inlet and the cold source outlet.

The loop heat pipe preparation method includes the following steps:

1. Preparation of capillary core

1. Preparation of gC ₃ N ₄ (carbon nitride)

1) Preparation of carbon nitride: use urea as the raw material, sintering in a muffle furnace, the heating rate is 4.9-5.1°C/min, preferably 5°C/min, the temperature is raised to 490-510°C, preferably 500°C, and the temperature is maintained for 170 -190min, preferably 180min, to generate carbon nitride. The generated carbon nitride is then subjected to cooling treatment, preferably to room temperature, preferably 20°C. The above-mentioned heating rate is an improvement point of the present invention. Through experiments, it is found that when the heating rate is around 5°C/min, the output of gC ₃ N ₄ is optimal. When it exceeds 5.1°C/min, the output of gC ₃ N ₄ will decrease with the increase in temperature. decreases as the rate increases. When it is lower than 4.9°C/min, although the output does not change significantly, the efficiency decreases significantly.

2) Thermal stripping of carbon nitride: first raise the temperature of the muffle furnace, and when the temperature reaches 440-460°C, preferably 450°C, then open the furnace and put in the carbon nitride in step 1), and peel for 25-35 minutes, preferably 30 minutes , layering gC ₃ N ₄ .

It should be noted that step 1) is mainly the process of high-temperature reaction of urea to generate carbon nitride. This process is a chemical reaction. Step 2) thermal stripping is to layer the carbon nitride to form a sheet structure, and then screen it. point. This process is a physical process. This process requires placing room temperature carbon nitride directly into a high temperature environment to complete the peeling.

3) Grind the gC ₃ N ₄ after thermal peeling, and then use a vibrating sieve to screen out gC ₃ N ₄ powder with a mesh size of 200-400 mesh for later use. This application prefers manual grinding, mainly to avoid damaging the layered structure of gC ₃ N ₄ .

2. Grinding of NaCl

First, a ball mill is used to grind the NaCl particles, with the QT-300 planetary ball mill being preferred. During grinding, periodic forward and reverse ball milling is used, the forward and reverse time is 40-50min, preferably 45min, the interval time is 4-8min, preferably 5min, and the total ball milling time is 5-7h, preferably 6h, to ensure the grinding effect. After the ball milling is completed, the NaCl particle size is mainly distributed between 100-500 mesh, and there are very few NaCl particles below 500 mesh. NaCl powder with a particle size of 200-400 mesh (37-74 μm) is screened through an oscillating sieve.

3. Powder ratio

gC ₃ N ₄ , NaCl powder and nickel powder are prepared in a certain mass ratio, preferably gC ₃ N ₄ and NaCl powder occupy a total of 5% wt-35% wt, preferably 10% wt-30% wt; or preferably gC ₃ N ₄ 5 %-20%, NaCl powder 5%-20%, and the rest is nickel powder. For example, 10% wtg-C ₃ N ₄ + 10% wtNaCl + 80% wtNi are mixed on a ball mill for 50-70 minutes, preferably 60 minutes, and then the mixed powder is put into a dryer for drying.

The macroporous structure formed by gC ₃ N ₄ can increase the suction rate of the capillary core. NaCl is dissolved and formed into pores after ultrasonic cleaning after machining, which can reduce the breakage of the capillary core during the machining process. The combination of the two pore-forming agents can significantly increase the porosity of the capillary core to 83%, which can significantly increase the suction limit.

4. Cold press forming

A press is used for bidirectional pressurization, and the molding pressure (set applied force) is 10-25Mpa. As a preference, each group of molding pressure adopts a pressure value. The specific molding pressure and the corresponding capillary core group are shown in Table 1.

Table 1 Preparation scheme of nickel-based dual-aperture capillary core

5. Capillary core sintering

The sintering parameters of this experiment are set as follows: After the powder is pressed and molded, it is sintered. The temperature rise rate is 10℃/min to 700℃. The holding time is 60min. Then the temperature starts to decrease. The cooling rate is 70℃/h. The sintering is carried out in a vacuum environment. Vacuum degree Maintained below 10 ^-4 Pa. Using a heating rate of 10°C/min can reduce the formation of sintering necks during the sintering process, reduce the number of small pores, and avoid oxidation of graphite inside the furnace body. Cooling is natural cooling, which helps maintain the service life of the furnace and strengthens the capillary core strength.

Preferably, the sintering temperature curve is as shown in Figure 2, including a heating stage, a heat preservation stage and a cooling stage.

6. Ultrasonic cleaning

After the sintering of the capillary core is completed, ultrasonic cleaning is required to dissolve the NaCl particles in the capillary core to obtain pores and then obtain a dual-pore structure.

Preferably, weigh before cleaning to calculate the mass of gC ₃ N ₄ volatilized during the sintering process. Weigh again after cleaning and drying to calculate the mass of salt washed away. Weigh to determine whether cleaning is complete.

Cleaning method: Clean with warm water at 50-60℃ for 7-8 hours, then change the water, continue the above steps, clean 4 times in total, and then put it in a drying box to dry at 120℃ for 48 hours.

2. Surface processing of capillary core

1. Processing of steam channels and liquid pipelines

As a preference, the capillary core sample sintered when preparing the loop heat pipe is 210mm. According to the length of the aluminum saddle, the required capillary core length is 180mm and the diameter is 15.4mm; there are a total of 6 steam channels with a size of 1x1x172mm; The liquid pipeline diameter is 6mm and the hole depth is 172mm. The capillary core is surface treated by turning, grooving is performed by wire cutting, and deep holes are drilled.

The internal structure and overall structure of the capillary core are shown in Figure 3. The capillary core is an annular structure, and multiple steam channels are provided on the outer wall, and the steam channels extend along the length direction of the capillary core. Preferably, the axis of the capillary core is parallel to the axis of the steam channel.

Preferably, the capillary wick at the liquid inlet position of the evaporator is not provided with a steam channel, and the steam channel occupies 80-90% of the length of the capillary wick. The long steam channel helps the gas to escape from the heated parts in time, preventing the formation of overheated gas layer and the bubbles inside the capillary core that cannot be eliminated in time and disrupt the internal operating status.

Preferably, along the flow direction of the fluid in the evaporator, the flow area of the steam channel becomes larger and larger.

Preferably, along the flow direction of the fluid in the evaporator, the flow area of the steam channel increases to a larger and larger extent.

Through the above settings, the need for continuous heating and flow of steam can be met, so that the steam has a more sufficient flow space, and the failure of the heat pipe evaporator caused by blockage is avoided.

Preferably, the length of the steam channel in the length direction of the capillary wick is L. Along the length direction of the capillary wick, the flow area of the initial position of the steam channel is S, and the distance from the initial position of the steam channel is l. The flow area s of the location has the following rules:

s=S+w*S*(l/L) ^f , where f and w are coefficients, meeting the following requirements:

1.13<f<1.23,0.24<w<0.30.

Preferably, as l/L increases, f and w gradually increase.

Preferably, 1.17<f<1.19, 0.26<k<0.28.

The above empirical formula is also the result of a large number of experimental studies in this application. It is an optimized structure for the area distribution of the steam channel. It is also an invention point of this application and is not common knowledge in this field.

2) Processing of secondary capillary core

In order to reduce the impact of bubbles on the stability of the heat pipe during operation and to increase the suction performance of the capillary wick, a preferably 450-550 mesh, and further preferably a 500 mesh metal mesh (as a secondary capillary core) is added to the liquid pipe of the heat pipe. achieve the desired effect. The installation of the auxiliary capillary core and capillary core is shown in Figure 4-3.

As a preferred option, the thickness of each layer of metal mesh is 0.13mm, and the gap width between the liquid pipeline and the liquid channel is 1.4125mm. 9-10 layers can be wound around the liquid pipeline and then inserted into the liquid channel. In the liquid storage chamber (Inner diameter is 15mm), about 45 layers need to be filled. The size and shape of the cut metal mesh are shown in Figure 4-1.

As a preference, the auxiliary capillary wick in the evaporator should fill the entire space between the liquid pipeline and the liquid flow channel, and the auxiliary capillary wick in the liquid reservoir should fill the entire space between the liquid flow channel and the evaporator shell. the space between. The liquid pipeline that goes deep into the capillary core has a deputy capillary core, and there is also a section of deputy capillary core in the liquid storage chamber, which can be clearly seen in Figure 4-2. The technical effect is to remove bubbles in the reflux liquid, reduce disturbances during operation, and assist the main capillary wick suction to enhance suction performance.

The arrangement of the liquid pipeline in the capillary core is to enable liquid to be supplied to the capillary core uniformly along the axial direction. Otherwise, the liquid supply resistance from the liquid reservoir to the capillary core in the axial direction is very large, which can easily lead to insufficient liquid supply, causing an axial temperature difference in the capillary core, and even local burning out. Set up a liquid guide pipe to directly introduce the refluxing supercooled liquid into the center of the evaporator. On the one hand, the coldness carried by the refluxing liquid can be used to balance the radial heat leakage of the evaporator through the capillary wick; on the other hand, when the liquid pipeline is The heat leakage of the evaporator generates bubbles or accumulates non-condensable gas. The supercooled liquid flowing out from the liquid guide tube can cool and eliminate the bubbles by relying on the coldness it carries, and at the same time, rely on its own flow to remove these non-condensable gases. Or bubbles are pushed out of the liquid pipeline to prevent air locks on the inner surface of the capillary core and improve the operating stability of the evaporator.

3. Preparation of evaporator shell

Processing technology: the diameter is Use a 304 stainless steel rod to drill, first use a center drill to drill a center hole on one end face for positioning. Fix the other end, then use a drill bit and add cutting fluid to drill (the rotation speed is 70-100r/min), and use the same processing method for the other end. After drilling the hole, turn the outer surface of the steel pipe. First, use a speed of 320r/min for turning. When there is still 20-30 wires left from 15mm, increase the speed to 500r/min or 600r/min to treat the surface. Polish to make the surface smooth.

As a preferred option, the length of the evaporator shell is 250mm, the outer diameter is 17mm, the inner diameter is 15mm, and the wall thickness is 1mm. A hole with a diameter of 6.35mm is drilled 20mm away from the right side of the evaporator shell to inject ammonia gas. (Note: When the pore-forming agent contains NaCl, the capillary core should be processed first and then cleaned to ensure greater strength during processing). The specific structure is shown in Figure 5.

In order to prevent the reverse flow of steam, the evaporator shell and the capillary core adopt an interference fit in the radial direction (for example, the evaporator shell and capillary core are 15.4mm/15mm respectively). After drying the capillary core, place it in the press. The capillary wick is pressed into the evaporator shell. A section of 1/4-inch pipe with a length of 40mm and a wall thickness of 1mm is welded at the liquid ammonia filling port. A section of 1/8 pipe is welded at the 1/4 pipe to connect to the inlet of the filling platform.

4. Preparation of aluminum saddle

In order to increase the contact area between the cylindrical heat pipe and the heat source, an aluminum saddle is set on the evaporator shell. Figure 6 shows the aluminum saddle model. A through hole is provided in the middle of the aluminum saddle for inserting the evaporation end of the cylindrical heat pipe. At the bottom of the aluminum saddle is a flat surface, which is connected to the heat source. By setting up the aluminum saddle, the contact area between the cylindrical heat pipe and the heat source is increased, further improving the heat exchange efficiency.

5. Design of condenser

The condenser adopts a partition-type counter-flow heat exchanger, and the internal steam pipeline adopts a serpentine arrangement to increase the contact area with the cooling water and increase the heat transfer. The cooling water inlet method is right in and left out, and the steam is left in and right out. way to cool down. Figure 7 is a schematic diagram of the condenser structure.

Length design of steam pipeline in condenser:

Suppose the ultimate load is φ. Under the ultimate load, the inlet working fluid temperature of the condenser is t ₁ and the outlet temperature is t ₂ . The inlet temperature of the cold source working fluid is t ₃ and the outlet temperature is t ₄ .

Calculate the area of the steam pipeline in the condenser according to the following formula:

φ＝kA ₃ Δt (1)

In the formula, k is the heat transfer coefficient; A ₃ is the heat exchange area (outer wall) of the steam pipeline; Δt is the average temperature difference between hot and cold fluids.

The average temperature difference is obtained by the following formula:

In the formula, Δt _max is the maximum temperature difference between hot and cold fluids, and Δt _min is the minimum temperature difference between hot and cold fluids.

The heat transfer coefficient is obtained from the following formula:

In the formula, d ₂ and d ₁ are the outer diameter and inner diameter of the steam pipeline respectively; λ _s is the thermal conductivity of the steam pipeline material (304 stainless steel pipe); h is the convection heat transfer coefficient of the outer wall of the steam pipe.

Find the P _r at the cold source working fluid and the corresponding temperature, and substitute it into the turbulent forced convection heat transfer correlation equation (4) in the tube groove and combine it with equation (5) to get h:

N _u ＝0.023R _e ^0.8 P _r ^0.3 (4)

In the formula, R _e is the Reynolds number of the cooling fluid in the condenser λ _l is the thermal conductivity of the cooling fluid; d _e is the equivalent diameter of the annular space through which the cooling fluid in the condenser passes, that is, the condenser jacket The difference between the inner diameter of the pipe d ₃ d ₃ and the outer diameter of the steam line d ₂ d ₂ .

6. Steam pipelines and liquid pipelines

As a preferred option, 1/8-inch 304 stainless steel pipes are used for steam pipelines and liquid pipelines. Preferably, the depth of the liquid pipeline into the evaporator is 240 mm.

7. Packaging

Impurities such as grease and chips will remain in the shell plate during processing. The components of the LHP need to be cleaned and pre-processed before packaging.

1. Heat in an air dryer at 150 degrees Celsius for 30 minutes to remove any organic and inorganic impurities in the production process.

2. Alkaline cleaning. Use sodium acetate solution at a temperature of 50°C and clean for 20 minutes. The pH test value is around 9, or use a neutral degreaser.

3. Cleaning. Rinse with boiling water and repeat this process three to four times.

4. Pickling. The aluminum shell is prone to oxidation after being left for a long time, which will affect the heat transfer performance of the LHP. The matrix needs to be cleaned before brazing. Use citric acid to pickle the aluminum saddle and stainless steel shell. Add citric acid to adjust the pH to around 5 and the temperature to 40 -50°C, clean for 20 minutes to remove the oxide layer.

5. Cleaning. Rinse with boiling water and repeat this process three to four times. Finally rinse with deionized water.

6. Drying. Incubate at 110°C for 20 minutes in a drying oven.

Install after cleaning.

1. First vacuum braze the evaporator shell and the aluminum saddle.

2. Then insert the capillary core into the welded body. The specific operations are as follows:

Before placing the capillary core into liquid nitrogen, dry the capillary core to prevent the capillary core from freezing and expanding due to cold and causing the capillary core to rupture. Place it in liquid nitrogen for 60 minutes. Under the action of a press (preferred pressure value is 0.1t), the capillary core (preferably diameter 15.4mm) is pressed into the evaporator shell (preferably inner diameter 15mm) to ensure interference fit. The capillary wick evaporation end leaves a 10mm gap at one end of the evaporator shell and a 60mm gap at one end of the liquid storage chamber. The evaporation end exceeds one side of the aluminum saddle by 10mm, and the liquid storage chamber end exceeds the other side of the aluminum saddle by 40mm. The steam pipeline and the evaporation end are connected by welding, and the liquid pipeline reaches 234mm deep into the evaporator liquid storage chamber and is connected by argon arc welding. The length of steam pipeline and liquid pipeline are both 850mm.

8. Welding

There are multiple solder joints on the entire loop heat pipe.

1. There is one solder joint at each evaporator inlet and outlet, and a total of two solder joints at the 1/4-inch pipe transition.

2. Use laser welding to weld the stainless steel wire mesh and the outer wall of the liquid conduit together and insert them into the deep hole.

3. The selection of liquid pipelines and steam pipelines is generally 304 stainless steel pipes with smooth inner walls and rust-free. The selected liquid and steam pipelines are all 1/8-inch (3.175mm) stainless steel pipes. Argon arc is used where the steel pipes are connected to the evaporator. For welding, 1/4 pipe over-welding is used at the connection between the steam pipeline and the liquid pipeline. It is preferred to use argon arc welding, including 4 welding points.

4. The loop heat pipe filling port is a 1/4 (6.35mm) round port about 30mm away from the end of the liquid reservoir. Weld the filling pipeline and the round filling port together through argon arc welding, and then use a 1/8 The pipe is connected to the 1/4 pipe through argon arc welding to connect the filling port for the filling operation.

5. The evaporator shell and the aluminum saddle are connected by vacuum brazing. According to the melting point of the matrix, the following two solders are selected and welded in a vacuum hot-pressing sintering furnace. When the temperature reaches the solder temperature, the heating is stopped immediately. , take out the mother body after it cools down.

6. The stainless steel sheets at both ends of the evaporator shell are shown in Figure 9. After the capillary core is pressed in, argon arc welding is performed.

9. Leak detection

First weld the steam pipeline, liquid pipeline and evaporator, use pneumatic equipment to detect the exhaust of the welded heat pipe in the water, check whether there is air leakage at the welding joint, and whether the pipeline is smooth. If it is smooth, install the condenser and then weld. , finally complete the overall leak detection of the loop heat pipe, and it will be considered qualified if there are no obvious bubbles emerging.

1. Check for leaks and blockages

First, weld the welding points of the evaporator inlet and outlet to the steam pipeline and liquid pipeline respectively, and use a water bath and air compressor to detect leaks in the loop heat pipe. The specific operations are:

(1) Leakage inspection: Gas enters through the filling port, seal the exits of the liquid pipeline and steam pipeline, and observe in a water bath whether there is air leakage at the evaporator inlet and outlet solder joints and the evaporator surface.

(2) Check for clogging: Gas enters through the filling port, seal the liquid pipeline outlet, put the steam pipeline outlet into the water bath, and observe whether there are bubbles coming out of the steam pipeline outlet during inflation, so as to know whether the steam pipeline and capillary core are smooth. .

10. Perfusion

1.) Preparation before perfusion

The ice machine prepares ice, checks whether the filling equipment is operating normally, weighs the empty mass heat pipe, and fills the heat pipe with dry air for drying.

2.) Vacuum infusion

It is constructed using solenoid valves, gas mass flow meters, PLC and other components to perform automated perfusion.

1. Vacuuming (PLC solenoid valve segmented vacuuming)

Vacuuming consists of two parts: one is to pump the air in the perfusion pipeline; the other is to pump the air in the loop heat pipe (which can heat the entire heat pipe to expand the internal gas and remove the gas and non-condensable gas in the capillary core and pipeline) )

Use mechanical pumps, vacuum pumps, Edward molecular pumps, etc. to perform segmented vacuuming operations, with a vacuum degree of 10-1Pa.

Preferably, during the process of vacuuming the air in the loop heat pipe, a hot air gun needs to be used to heat the entire LHP pipeline in order to extract the non-condensable gas. The heating method adopts indirect heating. When heating again, the vacuum degree displayed by the vacuum pump will no longer increase with heating, indicating that the vacuum has been pumped to the limit value. Stop heating, close the valve, and end vacuuming. After the heat pipe cools down, start perfusion. .

2. Perfusion (pressure difference perfusion method) (ammonia working fluid: boiling point: -33°C, critical temperature: 132°C)

Start the perfusion program, open the ammonia cylinder pressure valve (pressure 6Mpa), maintain constant pressure, turn on the flow monitoring equipment, and keep the flow rate at about 28L/min.

Volume calculation method:

Fluke (data acquisition instrument) is used to collect the data on the flow indicator in real time. Finally, the volume under the outlet pressure of the corresponding ammonia cylinder is calculated and summed, and finally converted to the corresponding volume under liquid ammonia to meet the filling requirements for packaging. Filling port, if not, continue to complete filling.

Quality calculation method:

Weigh the empty heat pipe before filling, weigh it again after filling, calculate the corresponding volume of liquid ammonia (filling fluid) based on the mass difference, compare it with the target filling rate, and finally complete the packaging work.

Note: Fluke and thermocouple wire (temperature measurement device) can be used to measure the temperature of the heat pipe after perfusion, find the density of liquid ammonia at the corresponding temperature, convert the volume, and then calculate the perfusion rate.

3.) Ammonia exhaust packaging

Use air compressors, filters, etc. to blow out the residual ammonia from the perfusion system and blend it into deionized water, then test the startup performance of the loop heat pipe, and finally use hydraulic shears and electromagnetic fuses to complete the final packaging operation of the loop heat pipe.

Although the present invention has been disclosed above in terms of preferred embodiments, the present invention is not limited thereto. Any person skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, the protection scope of the present invention should be subject to the scope defined by the claims.

Claims (6)

1. The cylindrical loop heat pipe comprises an evaporator, a condenser and a pipeline, wherein liquid absorbs heat and evaporates in the evaporator, then steam enters the condenser through a steam pipeline to condense and release heat, and the exothermic liquid enters the evaporator through a liquid pipeline to evaporate so as to form a cycle; the capillary core at the liquid inlet position of the evaporator is not provided with a steam channel, and the steam channel occupies 80-90% of the length of the capillary core; the flow area of the steam channel is larger and larger along the flow direction of the fluid in the evaporator; the flow area of the steam channel is increased along the flow direction of the fluid in the evaporator; the length of the steam channel in the length direction of the capillary core is L, along the length direction of the capillary core, the flow area of the steam channel at the initial position is S, and the flow area S at the distance L from the initial position of the steam channel is as follows:

s＝S+w*S*(l/L) ^f wherein f, w are coefficients, satisfying the following requirements:

1.13<f<1.23,0.24<w<0.30。

2. the loop heat pipe of claim 1 wherein the condenser is a shell and tube heat exchanger, the cold source is in a shell pass, the steam is in a tube pass, the heat exchanger comprises a cold source inlet and a cold source outlet arranged on a shell, and the steam pipeline adopts a serpentine arrangement.

3. The loop heat pipe of claim 1 wherein the liquid line is connected to a reservoir, the reservoir being connected to an evaporator.

4. The loop heat pipe of claim 1 wherein the wick is a ring-like structure having a plurality of vapor channels disposed on an outer wall, the vapor channels extending along a length of the wick.

5. The loop heat pipe of claim 1 wherein the axis of the wick is parallel to the axis of the vapor channel.

6. The loop heat pipe manufacturing method as recited in any one of claims 1 to 5, wherein the loop heat pipe manufacturing method includes manufacturing of a wick, comprising the steps of:

1. preparation of carbon nitride: 1) Preparing carbon nitride: taking urea as a raw material, sintering in a muffle furnace, heating to 490-510 ℃ at a heating rate of 4.9-5.1 ℃/min, and preserving heat for 170-190min to generate carbon nitride; then cooling the generated carbon nitride; 2) Heat-stripping carbon nitride: firstly, heating the muffle furnace, when the isothermal temperature reaches 440-460 ℃, opening the furnace, putting the carbon nitride in the step 1), stripping for 25-35min, and layering the carbon nitride;

2. grinding NaCl: firstly, grinding NaCl particles by adopting a ball mill, wherein the grinding is carried out by adopting periodic forward and reverse ball milling, the forward and reverse rotation time is 40-50min, the interval time is 4-8min, the total ball milling time is 5-7h, the particle size of NaCl after ball milling is finished is mainly distributed in 100-500 meshes, and the NaCl powder with the particle size of 200-400 meshes is screened by a vibrating screen;

3. powder proportioning: mixing carbon nitride, naCl powder and nickel powder according to a certain mass ratio, wherein the carbon nitride accounts for 5% -20%, the NaCl powder accounts for 5% -20% and the balance is nickel powder on a ball mill for 50-70min, and then putting the mixed powder into a dryer for drying;

4. cold press molding: adopting a press machine to carry out bidirectional pressurization, wherein the molding pressure is 10-25Mpa;

5. sintering a capillary core: sintering the powder after compression molding, heating to 700 ℃ at a heating rate of 10 ℃/min, keeping the temperature for 60min, then starting cooling at a cooling rate of 70 ℃/h, and maintaining the sintering vacuum degree at 10 ℃ in a vacuum environment ^-4 Under Pa;

6. ultrasonic cleaning: after the capillary core is sintered, naCl particles in the capillary core are dissolved by ultrasonic cleaning to obtain pores, so that a double-aperture structure is obtained.