**4. Heat transfer experiment of loop heat pipe**

In this paper, the heat transfer test bench of loop heat pipe is built, including heating module, cooling module, temperature collecting module, etc. The performance of loop heat pipe is studied experimentally. The loop heat pipe test platform consists of three modules: heating module, cooling module and temperature collecting module. Heating module has two sets, one is controlled power heating,

**41**

**Figure 22.**

*Location of thermocouple distribution.*

*The Recent Research of Loop Heat Pipe*

be used.

alcohol as a cooling agent.

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

the other is constant temperature heating. The output power is controlled by puss power supply, and the loop heat pipe evaporator is heated by resistance wire.

Constant temperature heating using cast copper heating block. There are grooves on the surface of cast copper heating block for heating loop heat pipe evaporator. The cast copper heating block is controlled by PID and the temperature is monitored by internal thermocouple. This heating method can only guarantee the outer wall temperature of the loop heat pipe evaporator without knowing the heating power of the loop heat pipe. According to the situation, these two heating methods should

The experimental platform needs a cooling module to condensate the loop heat pipe condenser. The condenser is connected to the DC20-20 type low temperature constant temperature tank, and the flow velocity of the constant temperature tank is 20 L/min. The lowest temperature can be −20°C, so this tank can be filled with

The data acquisition module is used to collect the local temperature of the heat transfer experiment of the loop heat pipe, and the performance of the loop heat pipe is analyzed by the variation of each temperature. The data collector uses Fluke2638A to collect data, and Fluke2638A has three chucks, each of which has 20 channels, which can collect 60 temperature points at the same time. K-type Ω thermocouple is used to collect temperature transmission data to data collector. The thermocouple is made of TT-K-36 type Ω temperature measuring line, and MES thermocouple welding machine is used to weld the temperature measurement line. The temperature range of the thermocouple was −200–260°C, and the measurement error was ±0.5°C. In order to ensure the detection accuracy, each K thermocouple is connected to the data collector channel and the ice water is mixed to zero. The distribution of thermocouple is shown in **Figure 22**, (1) is the outlet temperature of liquid pipeline, (2) is the inlet temperature of the liquid reservoir, (3) is the inlet temperature of the evaporator, (4) is the temperature at the heating point, (5) is the inlet temperature of the vapor pipe, (6) is the intermediate temperature of the vapor pipe, (7) and (8) are the temperature at the inlet and outlet of the condenser, and (9) are the temperature in the middle of the liquid pipeline. In addition,

the condenser inlet and outlet temperatures need to be measured.

*Eva 1* is the temperature of evaporator, *eva 2* is another temperature of evaporator, *s line 1* is the temperature of vapor line near the evaporator, *s line 2* is the temperature of vapor line near the condenser, *conin* is the temperature of the condenser inlet, *conout* is the temperature of the condenser outlet, *waterin* is the temperature

**4.1 Study on start-up characteristics of loop heat pipe**

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

*Recent Advances in Heat Pipes*

**40**

**Figure 20.**

**Figure 21.**

**Figure 19.**

*Drawing of vacuum line.*

in this paper uses Edward molecular pump to pump the vacuum in the pipeline. In the vacuum process, the pressure in the pipeline can be lowered by 6 × 10<sup>−</sup><sup>2</sup>

In this paper, the heat transfer test bench of loop heat pipe is built, including heating module, cooling module, temperature collecting module, etc. The performance of loop heat pipe is studied experimentally. The loop heat pipe test platform consists of three modules: heating module, cooling module and temperature collecting module. Heating module has two sets, one is controlled power heating,

device diagram is shown in **Figure 21**.

*Loop heat pipe vacuum perfusion test bench.*

*Schematic diagram of vacuum perfusion pipeline.*

**4. Heat transfer experiment of loop heat pipe**

Pa. The

the other is constant temperature heating. The output power is controlled by puss power supply, and the loop heat pipe evaporator is heated by resistance wire. Constant temperature heating using cast copper heating block. There are grooves on the surface of cast copper heating block for heating loop heat pipe evaporator. The cast copper heating block is controlled by PID and the temperature is monitored by internal thermocouple. This heating method can only guarantee the outer wall temperature of the loop heat pipe evaporator without knowing the heating power of the loop heat pipe. According to the situation, these two heating methods should be used.

The experimental platform needs a cooling module to condensate the loop heat pipe condenser. The condenser is connected to the DC20-20 type low temperature constant temperature tank, and the flow velocity of the constant temperature tank is 20 L/min. The lowest temperature can be −20°C, so this tank can be filled with alcohol as a cooling agent.

The data acquisition module is used to collect the local temperature of the heat transfer experiment of the loop heat pipe, and the performance of the loop heat pipe is analyzed by the variation of each temperature. The data collector uses Fluke2638A to collect data, and Fluke2638A has three chucks, each of which has 20 channels, which can collect 60 temperature points at the same time. K-type Ω thermocouple is used to collect temperature transmission data to data collector. The thermocouple is made of TT-K-36 type Ω temperature measuring line, and MES thermocouple welding machine is used to weld the temperature measurement line. The temperature range of the thermocouple was −200–260°C, and the measurement error was ±0.5°C. In order to ensure the detection accuracy, each K thermocouple is connected to the data collector channel and the ice water is mixed to zero.

The distribution of thermocouple is shown in **Figure 22**, (1) is the outlet temperature of liquid pipeline, (2) is the inlet temperature of the liquid reservoir, (3) is the inlet temperature of the evaporator, (4) is the temperature at the heating point, (5) is the inlet temperature of the vapor pipe, (6) is the intermediate temperature of the vapor pipe, (7) and (8) are the temperature at the inlet and outlet of the condenser, and (9) are the temperature in the middle of the liquid pipeline. In addition, the condenser inlet and outlet temperatures need to be measured.

#### **4.1 Study on start-up characteristics of loop heat pipe**

*Eva 1* is the temperature of evaporator, *eva 2* is another temperature of evaporator, *s line 1* is the temperature of vapor line near the evaporator, *s line 2* is the temperature of vapor line near the condenser, *conin* is the temperature of the condenser inlet, *conout* is the temperature of the condenser outlet, *waterin* is the temperature

**Figure 22.** *Location of thermocouple distribution.*

of water of the condenser inlet, *waterout* is the temperature of water of the condenser outlet*, l line 1* is the temperature of the liquid line near condenser, *l line 2* is the temperature of the liquid line near chamber, *CC 1* is the temperature of chamber, *CC 2* is another temperature of chamber, *steamout* is the temperature of vapor in the outlet of evaporator.

The startup of LHP can be divided into the following four processes: (1) after the evaporator is heated, the heat is transferred to the working fluid and vaporized. Because of the barrier of the capillary core sintering structure, the vapor can only enter into the vapor pipe through the vapor trough; (2) the vapor enters the condenser to cool through the vapor pipe, (3) after condensing into the liquid in the condenser, the liquid working fluid is pumped back into the liquid storage room because of the capillary force of the capillary core; (4) the working fluid of the reflux flows back into the evaporator through the permeability of the capillary core. In this paper, it is found that the minimum starting power of the loop heat pipe is 5 W.

**Figure 23** shows the starting heat transfer characteristics of the loop heat pipe at 5 W startup. It can be seen from the diagram that the loop heat pipe operates steadily after a period of time (3500 s), and the temperature of each part remains constant. In the early stage of the experiment, the evaporator and vapor line begin to rise steadily, the internal working fluid of the loop heat pipe evaporator absorbs the external heat, the internal working fluid reaches the saturation temperature and begins to vaporize, and the vapor enters the vapor pipeline, which leads the internal temperature of the pipeline to increase. Vapor passes through the vapor line and enters the condenser, so the inlet temperature of the condenser jumps. Through the condenser enough gas refrigerants are condensed into the liquid line, so the liquid tube is filled with liquid. The line temperature is low. After that, the liquid working fluid is reflowing back to the liquid accumulator and evaporator under the action of the capillary core to complete the forward circulation of the loop heat pipe. At the initial stage of starting the loop heat pipe, there was a small fluctuation in the temperature at the evaporator. The temperature first decreased and then increased. This was caused by the liquid working fluid began to condensate and reflux, and it also marked the formal start of the loop heat pipe. The starting time of the loop heat pipe is 1000 s.

#### **4.2 Study on heat transfer characteristics of loop heat pipe**

The threshold value for the initial start-up of the loop heat pipe is 5 W. When the heating power of the loop heat pipe evaporator is increased, the temperature of each part of the loop heat pipe is different. In order to further understand the heat transfer performance of the loop heat pipe prepared in this paper, the heating power of the loop evaporator is gradually increased, and the relationship between the temperature of each part of the loop heat pipe and the heating power is discussed. In this paper, the heat transfer experiment of loop heat pipe with different heating power has been carried out from 5 to 1 W interval. The experimental scheme is carried out according to the above experimental scheme. The external cold source temperature is −5°C when the heat transfer is heated by the transverse power heating method. The temperature curve of heating power 8W and 10W with −5°C heat sink is as shown in **Figures 24** and **25**.

With the increase of evaporator power, the loop heat pipe can continue to operate normally, and its operation law is similar to that of each part of the heat pipe heated at 5 W. The temperature inside the evaporator and vapor line rises first and then remains stable. The inlet temperature of the condenser is also stable after rising sharply at the beginning, and the temperature difference between the inlet of the condenser and the inlet of the condenser has been maintained, indicating that the loop heat pipe has been in a positive operating state. The experimental results

**43**

**Figure 25.**

*The Recent Research of Loop Heat Pipe*

in **Figures 26** and **27**.

**Figure 23.**

**Figure 24.**

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

*5 W heating power loop heat pipe temperature curve.*

*8 W heating power loop heat pipe temperature curve (−5°C heat sink).*

*10 W heating power loop heat pipe temperature curve (−5°C heat sink).*

show that the liquid line temperature of the loop heat pipe is approximately the same under different heating power, which indicates that the working fluid can be condensed completely into liquid through the condenser when the loop heat pipe is running, and is lower than the saturation temperature under the current pressure. The temperature curve and thermal resistance of different heating power are shown

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

*Recent Advances in Heat Pipes*

in the outlet of evaporator.

of water of the condenser inlet, *waterout* is the temperature of water of the condenser outlet*, l line 1* is the temperature of the liquid line near condenser, *l line 2* is the temperature of the liquid line near chamber, *CC 1* is the temperature of chamber, *CC 2* is another temperature of chamber, *steamout* is the temperature of vapor

The startup of LHP can be divided into the following four processes: (1) after the evaporator is heated, the heat is transferred to the working fluid and vaporized. Because of the barrier of the capillary core sintering structure, the vapor can only enter into the vapor pipe through the vapor trough; (2) the vapor enters the condenser to cool through the vapor pipe, (3) after condensing into the liquid in the condenser, the liquid working fluid is pumped back into the liquid storage room because of the capillary force of the capillary core; (4) the working fluid of the reflux flows back into the evaporator through the permeability of the capillary core. In this paper, it is found that the minimum starting power of the loop heat pipe is 5 W. **Figure 23** shows the starting heat transfer characteristics of the loop heat pipe at 5 W startup. It can be seen from the diagram that the loop heat pipe operates steadily after a period of time (3500 s), and the temperature of each part remains constant. In the early stage of the experiment, the evaporator and vapor line begin to rise steadily, the internal working fluid of the loop heat pipe evaporator absorbs the external heat, the internal working fluid reaches the saturation temperature and begins to vaporize, and the vapor enters the vapor pipeline, which leads the internal temperature of the pipeline to increase. Vapor passes through the vapor line and enters the condenser, so the inlet temperature of the condenser jumps. Through the condenser enough gas refrigerants are condensed into the liquid line, so the liquid tube is filled with liquid. The line temperature is low. After that, the liquid working fluid is reflowing back to the liquid accumulator and evaporator under the action of the capillary core to complete the forward circulation of the loop heat pipe. At the initial stage of starting the loop heat pipe, there was a small fluctuation in the temperature at the evaporator. The temperature first decreased and then increased. This was caused by the liquid working fluid began to condensate and reflux, and it also marked the formal start

of the loop heat pipe. The starting time of the loop heat pipe is 1000 s.

The threshold value for the initial start-up of the loop heat pipe is 5 W. When the

heating power of the loop heat pipe evaporator is increased, the temperature of each part of the loop heat pipe is different. In order to further understand the heat transfer performance of the loop heat pipe prepared in this paper, the heating power of the loop evaporator is gradually increased, and the relationship between the temperature of each part of the loop heat pipe and the heating power is discussed. In this paper, the heat transfer experiment of loop heat pipe with different heating power has been carried out from 5 to 1 W interval. The experimental scheme is carried out according to the above experimental scheme. The external cold source temperature is −5°C when the heat transfer is heated by the transverse power heating method. The temperature curve of heating power 8W and 10W with −5°C heat

With the increase of evaporator power, the loop heat pipe can continue to operate normally, and its operation law is similar to that of each part of the heat pipe heated at 5 W. The temperature inside the evaporator and vapor line rises first and then remains stable. The inlet temperature of the condenser is also stable after rising sharply at the beginning, and the temperature difference between the inlet of the condenser and the inlet of the condenser has been maintained, indicating that the loop heat pipe has been in a positive operating state. The experimental results

**4.2 Study on heat transfer characteristics of loop heat pipe**

sink is as shown in **Figures 24** and **25**.

**42**

show that the liquid line temperature of the loop heat pipe is approximately the same under different heating power, which indicates that the working fluid can be condensed completely into liquid through the condenser when the loop heat pipe is running, and is lower than the saturation temperature under the current pressure. The temperature curve and thermal resistance of different heating power are shown in **Figures 26** and **27**.

**Figure 23.** *5 W heating power loop heat pipe temperature curve.*

**Figure 24.** *8 W heating power loop heat pipe temperature curve (−5°C heat sink).*

**Figure 25.** *10 W heating power loop heat pipe temperature curve (−5°C heat sink).*

**Figure 26.** *Curve of temperature change with heating power.*

**Figure 27.** *Relationship between heating power and thermal resistance.*

The temperature variation of loop heat pipe is different with different heating power. As is shown that the higher the heating power, the higher the temperature of the evaporator, the higher the internal pressure, and the faster the temperature of the evaporator increases with the increase of heating power, so the heating power of the loop heat pipe should not be too large. Prevent the internal pressure from exceeding the pressure limit of the loop heat pipe. With the increase of the evaporator temperature, the temperature of the vapor pipeline entering the gaseous medium also becomes higher, which leads to the continuous increase of the vapor pipeline temperature to the inlet temperature of the condenser. In this paper, the low temperature alcohol is used as the external heat sink in the experiment, and the loop heat pipe is condensed. The working fluid in the condenser can be condensed completely, so even though the external heating conditions are different, the outlet temperature of the condenser remains basically unchanged at about 0°C. The experimental results show that the temperature of the loop heat pipe liquid accumulator increases rapidly with the increase of heating power, which is due to the fact that the evaporator tube wall and the liquid storage tube wall are made from the same stainless steel jacket. The heat from the outer wall of the evaporator is transferred to the outer wall of the liquid storage device by heat conduction, which results in the increase of the temperature of the external wall of the liquid storage device. The increase of external wall temperature may lead to excessive internal temperature and high pressure, which may hinder the forward operation of the loop heat pipe. So in subsequent studies, How to reduce the heat leakage from evaporator to liquid accumulator will be an important and difficult problem.

**45**

**Figure 28.**

*Temperature curve of heating power loop heat pipe (−10°C heat sink).*

*The Recent Research of Loop Heat Pipe*

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

*Rtotal* = \_\_\_\_\_\_\_

the evaporator and the average temperature of the condenser.

*Tev* − *Tcool Q load*

The heat transfer efficiency of loop heat pipe is usually measured by its overall thermal resistance. The total thermal resistance (*Rtotal*) of the loop is expressed as follows: the difference between the average temperature of the evaporator and the average temperature of the condenser is used to compare the heating power of the upper loop heat pipe with the difference between the average temperature of

> = *Tev in* + *Tev*

The experimental results show that the thermal resistance of the loop heat pipe becomes smaller and the overall heat transfer performance of the loop heat pipe becomes more and more excellent when the heating power is increasing, that is to say, the thermal conductivity of the loop heat pipe is getting better and better. However, the heat resistance of the loop heat pipe has a minimum value. The heat transfer efficiency of the loop heat pipe is the highest when the loop heat pipe works under this condition, but at the same time the temperature inside the evaporator is also very high, and the heat transfer limit of the loop heat pipe reaches its heat transfer limit. The experimental results show that when the external condensation temperature is insufficient, the heat absorbed by the loop heat pipe evaporator cannot be transferred out, which leads to the high pressure inside the loop heat pipe. The capillary force provided by the capillary core cannot overcome the resistance in the loop heat pipe normally and the suction of the working fluid leads to the failure of the loop heat pipe operation. However, when the condenser and the external heat exchange is sufficient, the evaporator can transfer the heat completely, and the different undercooling degree has no great influence on the steady state of the loop heat pipe. As is shown in curves of heat sink temperature of −10°C at −15°C under heating power of 10 W of the loop heat pipe. It can be seen that the temperature distribution of loop heat pipe evaporator, liquid accumulator, vapor line and liquid line are basically the same at different external heat sink temperatures. The temperature at the outlet of the condenser is affected by the external heat sink of the condenser. The lower the external heat sink temperature, the lower the temperature of the condenser, and the difference of the temperature at the outlet of the condenser is exactly the difference of the external heat sink. The external heat sink temperature also affects the time required for the loop heat pipe to reach stable operation. It can be seen from the diagram that the lower the external heat sink

*out* − *Tcool*

\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ 2*<sup>Q</sup> load*

*in* − *Tcool out*

(4)

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

*Recent Advances in Heat Pipes*

**44**

**Figure 27.**

**Figure 26.**

*Relationship between heating power and thermal resistance.*

*Curve of temperature change with heating power.*

The temperature variation of loop heat pipe is different with different heating power. As is shown that the higher the heating power, the higher the temperature of the evaporator, the higher the internal pressure, and the faster the temperature of the evaporator increases with the increase of heating power, so the heating power of the loop heat pipe should not be too large. Prevent the internal pressure from exceeding the pressure limit of the loop heat pipe. With the increase of the evaporator temperature, the temperature of the vapor pipeline entering the gaseous medium also becomes higher, which leads to the continuous increase of the vapor pipeline temperature to the inlet temperature of the condenser. In this paper, the low temperature alcohol is used as the external heat sink in the experiment, and the loop heat pipe is condensed. The working fluid in the condenser can be condensed completely, so even though the external heating conditions are different, the outlet temperature of the condenser remains basically unchanged at about 0°C. The experimental results show that the temperature of the loop heat pipe liquid accumulator increases rapidly with the increase of heating power, which is due to the fact that the evaporator tube wall and the liquid storage tube wall are made from the same stainless steel jacket. The heat from the outer wall of the evaporator is transferred to the outer wall of the liquid storage device by heat conduction, which results in the increase of the temperature of the external wall of the liquid storage device. The increase of external wall temperature may lead to excessive internal temperature and high pressure, which may hinder the forward operation of the loop heat pipe. So in subsequent studies, How to reduce the heat leakage from evaporator

to liquid accumulator will be an important and difficult problem.

The heat transfer efficiency of loop heat pipe is usually measured by its overall thermal resistance. The total thermal resistance (*Rtotal*) of the loop is expressed as follows: the difference between the average temperature of the evaporator and the average temperature of the condenser is used to compare the heating power of the upper loop heat pipe with the difference between the average temperature of the evaporator and the average temperature of the condenser.

\_{\textit{\\_}}}
\text{evaporator and the average temperature of the condenser.}

$$R\_{\text{total}} = \frac{T\_{\text{ev}} - T\_{\text{coal}}}{Q\_{load}} = \frac{T\_{\text{ev}}^{\text{dis}} + T\_{\text{ev}}^{\text{out}} - T\_{\text{coal}}^{\text{in}} - T\_{\text{coal}}^{\text{out}}}{2 \, Q\_{load}}\tag{4}$$

The experimental results show that the thermal resistance of the loop heat pipe becomes smaller and the overall heat transfer performance of the loop heat pipe becomes more and more excellent when the heating power is increasing, that is to say, the thermal conductivity of the loop heat pipe is getting better and better. However, the heat resistance of the loop heat pipe has a minimum value. The heat transfer efficiency of the loop heat pipe is the highest when the loop heat pipe works under this condition, but at the same time the temperature inside the evaporator is also very high, and the heat transfer limit of the loop heat pipe reaches its heat transfer limit.

The experimental results show that when the external condensation temperature is insufficient, the heat absorbed by the loop heat pipe evaporator cannot be transferred out, which leads to the high pressure inside the loop heat pipe. The capillary force provided by the capillary core cannot overcome the resistance in the loop heat pipe normally and the suction of the working fluid leads to the failure of the loop heat pipe operation. However, when the condenser and the external heat exchange is sufficient, the evaporator can transfer the heat completely, and the different undercooling degree has no great influence on the steady state of the loop heat pipe. As is shown in curves of heat sink temperature of −10°C at −15°C under heating power of 10 W of the loop heat pipe. It can be seen that the temperature distribution of loop heat pipe evaporator, liquid accumulator, vapor line and liquid line are basically the same at different external heat sink temperatures. The temperature at the outlet of the condenser is affected by the external heat sink of the condenser. The lower the external heat sink temperature, the lower the temperature of the condenser, and the difference of the temperature at the outlet of the condenser is exactly the difference of the external heat sink. The external heat sink temperature also affects the time required for the loop heat pipe to reach stable operation. It can be seen from the diagram that the lower the external heat sink

**Figure 28.** *Temperature curve of heating power loop heat pipe (−10°C heat sink).*

**Figure 29.** *Temperature curve of heating power loop heat pipe (−15°C heat sink).*

temperature, the longer the loop heat pipe operation will be stable. The temperature curve of different heat sink (–10°C and –15°C) are shown in **Figures 28** and **29**.
