**3. Structure design of loop heat pipe**

#### **3.1 Structural design of capillary cores**

The structure size of capillary core has an important effect on the performance of loop heat pipe. For example, the length of capillary core has a significant effect on the heat transfer capacity of loop heat pipe. When the length of the capillary core is long, the synergy between the temperature and the flow field becomes longer with the increase of the length of the capillary core, but when the capillary core is too long, the flow resistance of the working fluid in the capillary core is further increased. It will result in less fluid flowing through the capillary core in unit time and affect the heat transfer rate of the loop heat pipe. The structural size of the capillary core was prepared with reference and in the second chapter of this paper. The final size of the capillary core was 100 mm in length and 20 mm in diameter. The 3D model of capillary wick is as shown in **Figure 13**.

The capillary core is not only a simple cylindrical structure, its structure is relatively complex, the outside is a vapor trough for the passage of vapor, and the internal storage tank for liquid working fluid. The curved liquid surface of vapor liquid

**35**

*The Recent Research of Loop Heat Pipe*

voir is shown in **Figure 15**.

*Schematic diagram of capillary core structure.*

**Figure 13.**

**3.2 Design of evaporator and liquid tank**

and liquid reservoir is shown in **Figure 15**.

*DOI: http://dx.doi.org/10.5772/intechopen.85408*

phase transition of the working fluid is in the capillary core, the liquid working fluid enters through the liquid channel, and the vaporization at the curved liquid level is derived from the vapor tank. The results show that the ratio of the depth to width of the capillary vapor channel is 1:1, and the effect is the best when the length of the groove is 75 mm, the depth and width are 2 mm, and the number of grooves is 6. In the third chapter, the capillary core suction ability of different inner diameter liquid storage channel is studied. The capillary core suction performance is the best when the inner diameter is 8 mm. The final structure of the evaporator and reser-

Evaporator and liquid accumulator are the main components of loop heat pipe. Especially the evaporator, which contains capillary core, is the place where the liquid working fluid changes to the gas working medium in the loop heat pipe. However, the vapor generated in the capillary core must prevent its reverse flow into the liquid reservoir. In the evaporator, there is a liquid lead pipe in the capillary core, which is connected to the vapor pipeline, and the liquefied working fluid is directly introduced into the liquid channel inside the capillary core. The structure of the evaporator is shown in **Figure 14** below. Structural diagram of evaporator

In the design of loop heat pipe evaporator and liquid accumulator, the most important thing is to ensure the positive heat conduction of the loop heat pipe, and the most important thing is to do the sealing work well. The seal of evaporator and liquid accumulator means that there is a certain pressure bearing capacity in isolation from the outside environment, and more important is to prevent the diffusion (heat leakage) of the gas working fluid from the evaporator to the liquid accumulator. The heat transfer within the loop heat pipe is not strictly unidirectional, but the heat transfer in the loop heat pipe is not strictly unidirectional. Most of the external heat input in the capillary core makes the working fluid gasification to participate in the loop heat pipe circulation, only a part of the heat into the liquid reservoir in the form of heat conduction, this part of the heat called is a heat leak. Heat leakage will lead to excessive temperature and abnormal increase of pressure of the liquid accumulator, which will affect the normal operation of the working fluid in the loop heat pipe.

*The Recent Research of Loop Heat Pipe DOI: http://dx.doi.org/10.5772/intechopen.85408*

*Recent Advances in Heat Pipes*

with the increase of pressure, so the capillary suction speed is porous wicks (30 kN) > porous wicks (40 kN) > porous wicks (50 kN) > porous wicks (60 kN). The total suction mass of the porous wicks with different cold forming pressures is porous wicks (30 kN) > porous wicks (40 kN) > porous wicks (50 kN) > porous

The structure size of capillary core has an important effect on the performance of loop heat pipe. For example, the length of capillary core has a significant effect on the heat transfer capacity of loop heat pipe. When the length of the capillary core is long, the synergy between the temperature and the flow field becomes longer with the increase of the length of the capillary core, but when the capillary core is too long, the flow resistance of the working fluid in the capillary core is further increased. It will result in less fluid flowing through the capillary core in unit time and affect the heat transfer rate of the loop heat pipe. The structural size of the capillary core was prepared with reference and in the second chapter of this paper. The final size of the capillary core was 100 mm in length and 20 mm in diameter. The 3D model of capillary wick is as shown in

The capillary core is not only a simple cylindrical structure, its structure is relatively complex, the outside is a vapor trough for the passage of vapor, and the internal storage tank for liquid working fluid. The curved liquid surface of vapor liquid

wicks (60 kN), which is consistent with the porosity test results.

*Pore diameter distribution (a), (b), (c), and (d) are 30, 40, 50, and 60 kN, respectively.*

**3. Structure design of loop heat pipe**

**3.1 Structural design of capillary cores**

**34**

**Figure 13**.

**Figure 12.**

**Figure 13.** *Schematic diagram of capillary core structure.*

phase transition of the working fluid is in the capillary core, the liquid working fluid enters through the liquid channel, and the vaporization at the curved liquid level is derived from the vapor tank. The results show that the ratio of the depth to width of the capillary vapor channel is 1:1, and the effect is the best when the length of the groove is 75 mm, the depth and width are 2 mm, and the number of grooves is 6. In the third chapter, the capillary core suction ability of different inner diameter liquid storage channel is studied. The capillary core suction performance is the best when the inner diameter is 8 mm. The final structure of the evaporator and reservoir is shown in **Figure 15**.

#### **3.2 Design of evaporator and liquid tank**

Evaporator and liquid accumulator are the main components of loop heat pipe. Especially the evaporator, which contains capillary core, is the place where the liquid working fluid changes to the gas working medium in the loop heat pipe. However, the vapor generated in the capillary core must prevent its reverse flow into the liquid reservoir. In the evaporator, there is a liquid lead pipe in the capillary core, which is connected to the vapor pipeline, and the liquefied working fluid is directly introduced into the liquid channel inside the capillary core. The structure of the evaporator is shown in **Figure 14** below. Structural diagram of evaporator and liquid reservoir is shown in **Figure 15**.

In the design of loop heat pipe evaporator and liquid accumulator, the most important thing is to ensure the positive heat conduction of the loop heat pipe, and the most important thing is to do the sealing work well. The seal of evaporator and liquid accumulator means that there is a certain pressure bearing capacity in isolation from the outside environment, and more important is to prevent the diffusion (heat leakage) of the gas working fluid from the evaporator to the liquid accumulator. The heat transfer within the loop heat pipe is not strictly unidirectional, but the heat transfer in the loop heat pipe is not strictly unidirectional. Most of the external heat input in the capillary core makes the working fluid gasification to participate in the loop heat pipe circulation, only a part of the heat into the liquid reservoir in the form of heat conduction, this part of the heat called is a heat leak. Heat leakage will lead to excessive temperature and abnormal increase of pressure of the liquid accumulator, which will affect the normal operation of the working fluid in the loop heat pipe.

**Figure 14.** *Evaporator structure profile.*

**Figure 15.**

*Structural diagram of evaporator and liquid reservoir.*

The heat leakage is mainly transmitted through the heat conductivity of the capillary core and the outer wall of the loop heat pipe, which is difficult to avoid. However, the heat leakage should be considered in the preparation of the loop heat pipe. Limit the adverse effects of heat leakage to a lower range of effects. If vapor enters the tank, it will cause the temperature and pressure of the tank to rise, which will lead to the failure of the heat pipe operation. When the vapor is running in the opposite direction, the heat transfer efficiency of the loop heat pipe will be seriously affected, which will cause the gas accumulation in the liquid accumulator and the capillary seriously. The thin core leads to the failure of forward heat and mass transfer in the loop heat pipe.

In order to prevent the reverse operation of vapor, an inner step stainless steel outer wall is designed for evaporator and liquid accumulator in order to reduce welding, and the capillary core is clamped in one direction, and a clasp structure is installed at the capillary core and step. There are three gaskets on the outer wall of the evaporator and liquid accumulator. Layer by layer protection reduces the reverse flow of heat vapor along the inner wall, improves the heat transfer energy of the return heat pipe, and simulates the failure of the heat pipe operation. Evaporator wall thickness as thin as possible to reduce thermal resistance to facilitate the capillary core to absorb external heat. At the vapor outlet, the capillary core is clamped by thread connection, and a plum flower gasket is installed between the bolt structure and the capillary core, and the vapor produced by the thermal reaction flows out of the pore between the pores of the plum flower gasket. This junction can be reused, just loosen the bolt structure to replace the capillary core. The specific size is obtained from the size of the capillary core. As shown in **Figure 15**.

## **3.3 Condenser optimization**

In the loop heat pipe system, the condenser is responsible for the rapid transfer of heat from the evaporator to the outside world. After heated vaporization of the working medium in the evaporator, the hot vapor enters the condenser through the gas pipeline, and the heat is exchanged with the outside in the condenser to

**37**

**Figure 16.**

*Condenser structure (including two heat dissipation modes).*

*The Recent Research of Loop Heat Pipe*

**3.4 Loop heat pipe assembly**

*DOI: http://dx.doi.org/10.5772/intechopen.85408*

dissipate heat towards the outside world, and the vapor moves towards the liquid storage device after the condenser becomes a liquid. Therefore, the condenser must have sufficient undercooling to ensure that the working fluid can be completely condensed into liquid. At present, the cooling methods of various equipment are water cooling or air cooling. It is found that the vapor in the evaporator is usually difficult to be condensed into liquid due to the lack of cooling power. So there are a lot of heat pipes in the loop. Adopt water cooling. In order to further improve the

The loop heat pipe condenser prepared in this paper is shown in **Figure 16**. The condenser adopts a cylindrical tube structure with an inlet and an outlet to circulate low temperature alcohol or cold water, and the cylinder pipe is a cooling pipe. Considering the overall size of the loop heat pipe, the length of the condenser is set to 200 mm. However, the heat transfer length of 200 mm is insufficient and the vapor in the condenser is cooled completely into a liquid. Faced with this situation, there are usually two solutions: one is to put fins on the outside of the pipe, the other is to make the pipe into a coil. After the experiment, it was found that the efficiency of the first scheme was also insufficient when the first scheme was running at high power. Therefore, the coiled tube alcohol cooling was used in the end. However, as the final cooling scheme, we should pay attention to the length of coil should not be too long, too long pipe length will lead to excessive resistance in operation of the working fluid, which will affect the forward operation of the loop heat pipe.

The connection between evaporator and condenser in heat pipe is vapor pipeline and liquid pipeline. In order to reduce the flow resistance of the medium, the more smooth the inner, the better. In this paper, stainless steel tube is selected and the inner polishing is done. The vapor liquid phase change process is involved in the operation of the loop heat pipe, and the sealing property is very high. Therefore, stainless steel is used in all parts of the loop heat pipe, and the connection between each part is argon arc welding. All stainless steel components (including liquid accumulators and evaporators) should be cleaned according to the stainless steel cleaning method before final use to improve the performance of the loop heat pipe. The loop heat pipe also needs to be equipped with a new belt valve. The working fluid filling mouth of the door is filled and sealed with heat pipe working fluid. After welding the loop heat pipe, it is necessary to pick up the leakage of the system, inflate the system with air compressor from the filling port, and place the system

cooling temperature, alcohol is chosen as the cooling medium.

#### *The Recent Research of Loop Heat Pipe DOI: http://dx.doi.org/10.5772/intechopen.85408*

*Recent Advances in Heat Pipes*

**Figure 15.**

**Figure 14.**

*Evaporator structure profile.*

*Structural diagram of evaporator and liquid reservoir.*

The heat leakage is mainly transmitted through the heat conductivity of the capillary core and the outer wall of the loop heat pipe, which is difficult to avoid. However, the heat leakage should be considered in the preparation of the loop heat pipe. Limit the adverse effects of heat leakage to a lower range of effects. If vapor enters the tank, it will cause the temperature and pressure of the tank to rise, which will lead to the failure of the heat pipe operation. When the vapor is running in the opposite direction, the heat transfer efficiency of the loop heat pipe will be seriously affected, which will cause the gas accumulation in the liquid accumulator and the capillary seriously. The thin core leads to the failure of forward heat and mass transfer in the loop heat pipe. In order to prevent the reverse operation of vapor, an inner step stainless steel outer wall is designed for evaporator and liquid accumulator in order to reduce welding, and the capillary core is clamped in one direction, and a clasp structure is installed at the capillary core and step. There are three gaskets on the outer wall of the evaporator and liquid accumulator. Layer by layer protection reduces the reverse flow of heat vapor along the inner wall, improves the heat transfer energy of the return heat pipe, and simulates the failure of the heat pipe operation. Evaporator wall thickness as thin as possible to reduce thermal resistance to facilitate the capillary core to absorb external heat. At the vapor outlet, the capillary core is clamped by thread connection, and a plum flower gasket is installed between the bolt structure and the capillary core, and the vapor produced by the thermal reaction flows out of the pore between the pores of the plum flower gasket. This junction can be reused, just loosen the bolt structure to replace the capillary core. The specific

size is obtained from the size of the capillary core. As shown in **Figure 15**.

In the loop heat pipe system, the condenser is responsible for the rapid transfer of heat from the evaporator to the outside world. After heated vaporization of the working medium in the evaporator, the hot vapor enters the condenser through the gas pipeline, and the heat is exchanged with the outside in the condenser to

**36**

**3.3 Condenser optimization**

dissipate heat towards the outside world, and the vapor moves towards the liquid storage device after the condenser becomes a liquid. Therefore, the condenser must have sufficient undercooling to ensure that the working fluid can be completely condensed into liquid. At present, the cooling methods of various equipment are water cooling or air cooling. It is found that the vapor in the evaporator is usually difficult to be condensed into liquid due to the lack of cooling power. So there are a lot of heat pipes in the loop. Adopt water cooling. In order to further improve the cooling temperature, alcohol is chosen as the cooling medium.

The loop heat pipe condenser prepared in this paper is shown in **Figure 16**. The condenser adopts a cylindrical tube structure with an inlet and an outlet to circulate low temperature alcohol or cold water, and the cylinder pipe is a cooling pipe. Considering the overall size of the loop heat pipe, the length of the condenser is set to 200 mm. However, the heat transfer length of 200 mm is insufficient and the vapor in the condenser is cooled completely into a liquid. Faced with this situation, there are usually two solutions: one is to put fins on the outside of the pipe, the other is to make the pipe into a coil. After the experiment, it was found that the efficiency of the first scheme was also insufficient when the first scheme was running at high power. Therefore, the coiled tube alcohol cooling was used in the end. However, as the final cooling scheme, we should pay attention to the length of coil should not be too long, too long pipe length will lead to excessive resistance in operation of the working fluid, which will affect the forward operation of the loop heat pipe.
