The Recent Research of Loop Heat Pipe

*Chunsheng Guo*

## **Abstract**

The loop heat pipe was first studied for the difficult temperature control conditions under aerospace conditions. The loop heat pipe is composed of evaporator, reservoir, capillary wick, vapor/liquid line, and condenser. Different working fluids, different liquid filling amounts, different capillary wicks, different sizes, and different cooling methods will have an important impact on the performance of the loop heat pipe. Therefore, if the loop heat pipe wants to have good heat transfer efficiency, it is imperative to discuss good processing steps and processing techniques. When the loop heat pipe is running, the capillary wick is heated, the liquid in the capillary wick is heated and vaporized, and the gas passes through the vapor line to enter the condenser for condensation. After the condensation, the liquid flows back into the reservoir and the inside of the capillary wick through the liquid line. How to ensure the forward operation of the gas at the evaporator to reduce the reverse leakage heat during this cycle, how to ensure the condenser condensation efficiency is sufficient, etc. are all issues to be considered. This chapter describes in detail the processing and optimization methods for each part, and prepares a loop heat pipe that can work normally.

**Keywords:** nickel-ammonia loop heat pipe, capillary wick, heating power, temperature fluctuation

#### **1. Introduction**

The loop heat pipe is a variant of the ordinary heat pipe, and is a two-phase flow loop type heat pipe, which is a unidirectional heat conduction element similar to a diode. The working medium is mainly composed of a gas phase and a liquid phase in the steam pipe and the liquid pipe respectively. The form of the cycle, complete the transfer of heat. The design idea of the world's first LHP system was proposed by scientists in 1971. Then Soviet scientists Maydanik and Gerasimov successfully developed the world's first LHP system in 1972, after the Soviet Union and the United States. Research institutions are beginning to study loop heat pipes. The loop heat pipe is a high-efficiency loop heat transfer device with two-phase separation heat transfer. Since the structure of the loop heat pipe is more complicated than the ordinary heat pipe, it can adapt to more different working environments. The loop heat pipe has many advantages such as high heat transfer performance, long-distance heat transfer, small heat transfer temperature difference, and flexible installation. Therefore, in recent years, it has been widely used in aerospace heat dissipation, electronic products heat dissipation, anti-gravity ground heat transfer working environment and other fields.

Loop heat pipe was a new kind of two phase flow heat equipment, it used capillary suction force drive the working medium to complete the cycle of working medium inside the heat pipe flow and by using phase change of working medium two-phase flow to transfer heat [1]. Loop heat pipe had many advantages, such as large heat transmission, transmission distance, high heat transfer efficiency, antigravity features was strong, etc. [2]. Loop heat pipe was first applied in aerospace field, with the constant improvement of the loop heat pipe technology, it gradually be applied to every other civilian areas, especially in electronic cooling field [3].

The most important characteristic of loop heat pipe was its resistance to gravity characteristic. Baumann [4] made a theoretical analysis first for the influence of loop heat pipe resistance to gravity, but fails to provide the experimental data to support. Zhang [5] studied the loop heat pipe in start-up and heat transfer performance under antigravity work condition, found the loop heat pipe started up under the condition of antigravity liquid reflux need to overcome additional pressure drop loss gravity bringing, and in outside loop resistance increased obviously. When vapor trough exist vapor, start-up time and temperature increase, loop heat pipe appeared complex compound startup phenomenon. Anti-gravity work increased the working temperature of the loop heat pipe system, reduced the automatic temperature control range, and leaded to vapor produced in the evaporator was more likely to overheat and overall system thermal resistance increased at the same time. In order to match the cooling parts, the evaporators were often made into plates [1]. Research team of Chinese Academy of Sciences institute of physics [6] carried out the establishment of the numerical model of the plate LHP evaporator and the heat transfer model, then they analyzed the heat transfer mechanism inside the evaporator. Mitomi [7] designed three kinds of length loop heat pipe with the length were 2, 5, 10 m. Loop heat pipe working medium was ethanol and capillary core was made by polytetrafluoroethylene porous material. They analyzed the normal working performance and carried out numerical simulation, and found that three pipes could work under the same starting power. South China University of Technology Developers [8] had studied the influence of heat leakage, the initiation characteristics and optimized the evaporator, reservoir and condenser. Beijing Aerospace University and Institute of Space Studied [9, 10] studied the whole loop heat pipe system, including the evaporator and capillary core structure optimization effect on the performance of the loop heat pipe, the influence of reservoir auxiliary cooling and the evaporator auxiliary heat on loop heat pipe performance, performance test of loop heat pipe in microgravity environment, the influence of quantity of quality filling on the performance of the loop heat pipe. In practical application, there could be multiple sources of heat and multiple sources. The number of evaporators, condensers and accumulators is not fixed. Jentung Ku [11] tests the multi-evaporator loop heat pipe with 50 W power in a vacuum environment, and the performance is stable.

#### **2. Capillary core preparation**

Capillary core is an important component in loop heat pipe. Porosity, permeability, pore size distribution and capillary suction ability are the key parameters to reflect the performance of capillary core. In order to improve the ability of antigravity operation and long-distance operation of the loop heat pipe, it is necessary for the capillary core to have the characteristics of high permeability and high capillary suction ability. As the core device in the loop heat pipe, the capillary core transmits heat in the evaporator and provides enough capillary force to drive the working fluid cycle. At the same time, the vapor should be transferred to the vapor pipe in time to prevent the phenomenon of suction. Wolf indicates that LHP has gravity heat pipe

**27**

**Figure 1.**

*The Recent Research of Loop Heat Pipe*

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

the properties of capillary core was obtained.

and then put into drying. The box was dried.

*Sintering temperature curve of nickel based capillary core.*

**2.1 Preparation process of capillary core**

was shown in **Figure 1**.

and capillary pump heat pipe. It has the advantages of anti-gravity, long distance operation, no external power source, high stability, passive energy transportation and so on. Capillary core, as the core component of loop heat pipe, provides the necessary power for the forward operation of heat pipe. At present, the main types of capillary core are: grooved capillary core, metal mesh capillary core, ceramic capillary core and metal powder sintered capillary core. At present, most metal powder sintered capillary cores are made of copper and nickel. These capillary cores are widely used in loop

heat pipes because of their good thermal properties and liquid compatibility. On the basis of previous studies, double pore capillaries were prepared by molten salt pore making technique. Compared with the addition of volatile poremaking agent, NaCl, with 99.5% purity and carbonyl nickel powder as capillary core material has the advantages of easy removal and uniform void distribution. The main parameters of capillary core, such as porosity, permeability, thermal conductivity, pore distribution and capillary suction ability, were measured according to the ratio of pore-forming agent and the pressure of cold pressing molding. The effect of the ratio of pore-making agent and the pressure of cold pressing on

Nickel powder was selected as raw material and NaCl as pore-making agent to prepare double pore capillary core. The main steps were powder ratio, cold pressing, sintering and cleaning. The specific steps were as follows: the preparation process

1.Powder ratio: In this paper, 2 μm nickel powder with 99.5% purity of NaCl was selected as the material to prepare the double aperture capillary core. Firstly, the NaCl particles were ground by ball mill (the positive and negative rotation time was 45 min, the interval time was 5 min, the total milling time was 6 h), and the total milling time was 6 h, the positive and negative rotation time was 45 min, the interval time was 5 min, and the total milling time was 6 h. The particle size of NaCl was mainly distributed in 200–400 mesh after ball milling. There were very few NaCl particles below 400 mesh. The NaCl powder with diameter of 48 μm (300–400 mesh) was screened by vibrating screen, and then the nickel powder and NaCl powder were mixed evenly by ball mill,

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

*Recent Advances in Heat Pipes*

cooling field [3].

Loop heat pipe was a new kind of two phase flow heat equipment, it used capillary suction force drive the working medium to complete the cycle of working medium inside the heat pipe flow and by using phase change of working medium two-phase flow to transfer heat [1]. Loop heat pipe had many advantages, such as large heat transmission, transmission distance, high heat transfer efficiency, antigravity features was strong, etc. [2]. Loop heat pipe was first applied in aerospace field, with the constant improvement of the loop heat pipe technology, it gradually be applied to every other civilian areas, especially in electronic

The most important characteristic of loop heat pipe was its resistance to gravity characteristic. Baumann [4] made a theoretical analysis first for the influence of loop heat pipe resistance to gravity, but fails to provide the experimental data to support. Zhang [5] studied the loop heat pipe in start-up and heat transfer performance under antigravity work condition, found the loop heat pipe started up under the condition of antigravity liquid reflux need to overcome additional pressure drop loss gravity bringing, and in outside loop resistance increased obviously. When vapor trough exist vapor, start-up time and temperature increase, loop heat pipe appeared complex compound startup phenomenon. Anti-gravity work increased the working temperature of the loop heat pipe system, reduced the automatic temperature control range, and leaded to vapor produced in the evaporator was more likely to overheat and overall system thermal resistance increased at the same time. In order to match the cooling parts, the evaporators were often made into plates [1]. Research team of Chinese Academy of Sciences institute of physics [6] carried out the establishment of the numerical model of the plate LHP evaporator and the heat transfer model, then they analyzed the heat transfer mechanism inside the evaporator. Mitomi [7] designed three kinds of length loop heat pipe with the length were 2, 5, 10 m. Loop heat pipe working medium was ethanol and capillary core was made by polytetrafluoroethylene porous material. They analyzed the normal working performance and carried out numerical simulation, and found that three pipes could work under the same starting power. South China University of Technology Developers [8] had studied the influence of heat leakage, the initiation characteristics and optimized the evaporator, reservoir and condenser. Beijing Aerospace University and Institute of Space Studied [9, 10] studied the whole loop heat pipe system, including the evaporator and capillary core structure optimization effect on the performance of the loop heat pipe, the influence of reservoir auxiliary cooling and the evaporator auxiliary heat on loop heat pipe performance, performance test of loop heat pipe in microgravity environment, the influence of quantity of quality filling on the performance of the loop heat pipe. In practical application, there could be multiple sources of heat and multiple sources. The number of evaporators, condensers and accumulators is not fixed. Jentung Ku [11] tests the multi-evaporator loop heat pipe

with 50 W power in a vacuum environment, and the performance is stable.

Capillary core is an important component in loop heat pipe. Porosity, permeability, pore size distribution and capillary suction ability are the key parameters to reflect the performance of capillary core. In order to improve the ability of antigravity operation and long-distance operation of the loop heat pipe, it is necessary for the capillary core to have the characteristics of high permeability and high capillary suction ability. As the core device in the loop heat pipe, the capillary core transmits heat in the evaporator and provides enough capillary force to drive the working fluid cycle. At the same time, the vapor should be transferred to the vapor pipe in time to prevent the phenomenon of suction. Wolf indicates that LHP has gravity heat pipe

**2. Capillary core preparation**

**26**

and capillary pump heat pipe. It has the advantages of anti-gravity, long distance operation, no external power source, high stability, passive energy transportation and so on. Capillary core, as the core component of loop heat pipe, provides the necessary power for the forward operation of heat pipe. At present, the main types of capillary core are: grooved capillary core, metal mesh capillary core, ceramic capillary core and metal powder sintered capillary core. At present, most metal powder sintered capillary cores are made of copper and nickel. These capillary cores are widely used in loop heat pipes because of their good thermal properties and liquid compatibility.

On the basis of previous studies, double pore capillaries were prepared by molten salt pore making technique. Compared with the addition of volatile poremaking agent, NaCl, with 99.5% purity and carbonyl nickel powder as capillary core material has the advantages of easy removal and uniform void distribution. The main parameters of capillary core, such as porosity, permeability, thermal conductivity, pore distribution and capillary suction ability, were measured according to the ratio of pore-forming agent and the pressure of cold pressing molding. The effect of the ratio of pore-making agent and the pressure of cold pressing on the properties of capillary core was obtained.

### **2.1 Preparation process of capillary core**

Nickel powder was selected as raw material and NaCl as pore-making agent to prepare double pore capillary core. The main steps were powder ratio, cold pressing, sintering and cleaning. The specific steps were as follows: the preparation process was shown in **Figure 1**.

1.Powder ratio: In this paper, 2 μm nickel powder with 99.5% purity of NaCl was selected as the material to prepare the double aperture capillary core. Firstly, the NaCl particles were ground by ball mill (the positive and negative rotation time was 45 min, the interval time was 5 min, the total milling time was 6 h), and the total milling time was 6 h, the positive and negative rotation time was 45 min, the interval time was 5 min, and the total milling time was 6 h. The particle size of NaCl was mainly distributed in 200–400 mesh after ball milling. There were very few NaCl particles below 400 mesh. The NaCl powder with diameter of 48 μm (300–400 mesh) was screened by vibrating screen, and then the nickel powder and NaCl powder were mixed evenly by ball mill, and then put into drying. The box was dried.

**Figure 1.** *Sintering temperature curve of nickel based capillary core.*


#### **2.2 Capillary core parameter testing**

#### *2.2.1 Porosity and permeability*

Porosity is the most direct index of porous structure of capillary core. The internal pores of capillary core are divided into connected pores, semi-connected pores, closed pores. The porosity of capillary core prepared by salt solution technique is mainly affected by the proportion and size of pore-forming agent.

Archimedes drainage method for porosity measurement, capillary core wet weight *mwet*, drying thoroughly weighing capillary core dry weight *mdry*, and measured the outer diameter of cylindrical capillary core r and length L, densities of deionized water. Porosity *ε*

$$
\varepsilon = \frac{m\_{\text{net}} - m\_{\text{dry}}}{\text{V} \cdot \rho} \times 100\text{\%} \tag{1}
$$

According to the gas resistance test table shown in **Figure 4**, the experimental device uses compressed air as the air source, and the compressed air in the air compressor enters from the left side air compressor joint. In order to ensure the accuracy of the experiment and improve the service time of the platform, a filter and a steady pressure tank are connected after the air enters the pipeline. The gas flow through the experimental section is controlled by the mass flow-meter and the mass flow controller after the air passes through the unidirectional valve after the steady pressure. The capillary core is installed in the experiment section, and the pressure difference between the two sides of the capillary core is detected by pressure differential transmitter. At the end of the experiment, the gas was emptied into the air through the buffer tank.

The flow in this experiment is the flow inside the tube, the maximum flow rate is 30 L/min, the length of stainless steel tube is 300 mm, the inner diameter of stainless steel pipe is 20 mm, and the Reynolds number is

$$Re = \frac{\rho v d}{\mu} \tag{2}$$

**29**

**Figure 4.**

*Schematic diagram of gas resistance test table.*

**Figure 3.**

**Figure 2.**

*microscope.*

*The Recent Research of Loop Heat Pipe*

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

ability K can be measured experimentally by formula (3).

*qv* = \_\_\_\_\_

*qv* is the flow rate in the experiment (m3

*A* is the cross-sectional area of the sample (m2

*Flow chart of capillary core preparation.*

According to the results of many experiments by French scientist Darcy, perme-

*KA*Δ*P*

*50 kN cold pressure 20% NaCl mass fraction capillary core surface 1200 and 600 times scanning electron* 

*<sup>H</sup>* (3)

/s), *K* is the permeability of the sample (m2

), *H* is the length of the sample (m), Δ*P* is

),

The calculated Reynolds number is 176.9. The experimental fluid flow is laminar flow, which accords with the applicable condition of Darcy formula. The length of stainless steel tube is 15 times the length of capillary core. The working fluid in the pipe can be developed fully and the inlet effect can be reduced effectively. The thickness of stainless steel tube wall 1 mm is much smaller than the diameter of capillary core. This experimental device can accurately measure the permeability of porous media K. A number of empty tube experiments were carried out before the experiment to eliminate the pressure drop caused by friction on the pipe wall.

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

According to the results of many experiments by French scientist Darcy, permeability K can be measured experimentally by formula (3).

$$q\_v = \frac{KA\Delta P}{\mu H} \tag{3}$$

*qv* is the flow rate in the experiment (m3 /s), *K* is the permeability of the sample (m2 ), *A* is the cross-sectional area of the sample (m2 ), *H* is the length of the sample (m), Δ*P* is

**Figure 2.**

*Recent Advances in Heat Pipes*

shown in **Figure 2**.

*2.2.1 Porosity and permeability*

of deionized water. Porosity *ε*

**2.2 Capillary core parameter testing**

2.Cold pressing molding. The powder was compacted by a press (the target pres-

3.The capillary core is sintered. The vacuum hot pressing sintering furnace selected in the experiment is ZT-40-20Y, combined with multiple sintering experiments and the temperature curve drawn in the previous literatures is

4.Ultrasonic cleaning. After sintering, the NaCl particles in the capillary core need to be dissolved by ultrasonic cleaning to form a void and obtain a dual

Porosity is the most direct index of porous structure of capillary core. The internal pores of capillary core are divided into connected pores, semi-connected pores, closed pores. The porosity of capillary core prepared by salt solution technique is

Archimedes drainage method for porosity measurement, capillary core wet weight *mwet*, drying thoroughly weighing capillary core dry weight *mdry*, and measured the outer diameter of cylindrical capillary core r and length L, densities

*<sup>ε</sup>* <sup>=</sup> *mwet* <sup>−</sup> *mdry* \_\_\_\_\_\_\_\_\_ <sup>V</sup> · <sup>ρ</sup> <sup>×</sup> 100% (1)

According to the gas resistance test table shown in **Figure 4**, the experimental device uses compressed air as the air source, and the compressed air in the air compressor enters from the left side air compressor joint. In order to ensure the accuracy of the experiment and improve the service time of the platform, a filter and a steady pressure tank are connected after the air enters the pipeline. The gas flow through the experimental section is controlled by the mass flow-meter and the mass flow controller after the air passes through the unidirectional valve after the steady pressure. The capillary core is installed in the experiment section, and the pressure difference between the two sides of the capillary core is detected by pressure differential transmitter. At the end of

The flow in this experiment is the flow inside the tube, the maximum flow rate is 30 L/min, the length of stainless steel tube is 300 mm, the inner diameter of stain-

The calculated Reynolds number is 176.9. The experimental fluid flow is laminar

flow, which accords with the applicable condition of Darcy formula. The length of stainless steel tube is 15 times the length of capillary core. The working fluid in the pipe can be developed fully and the inlet effect can be reduced effectively. The thickness of stainless steel tube wall 1 mm is much smaller than the diameter of capillary core. This experimental device can accurately measure the permeability of porous media K. A number of empty tube experiments were carried out before the experiment to eliminate the pressure drop caused by friction on the pipe wall.

*<sup>μ</sup>* (2)

the experiment, the gas was emptied into the air through the buffer tank.

less steel pipe is 20 mm, and the Reynolds number is

*Re*= *vd* \_\_\_\_

sure was 30, 40, 50, 60 KN, and the booster speed was 200 N/s).

pore structure. The SEM of biporous wick is shown in **Figure 3**.

mainly affected by the proportion and size of pore-forming agent.

**28**

*Flow chart of capillary core preparation.*

**Figure 3.**

*50 kN cold pressure 20% NaCl mass fraction capillary core surface 1200 and 600 times scanning electron microscope.*

**Figure 4.** *Schematic diagram of gas resistance test table.*

#### *Recent Advances in Heat Pipes*

the pressure difference at the two ends of the sample (Pa), *μ* is experimental fluid viscosity (Pa s). The schematic diagram of the experimental device is shown in **Figure 4**. The principle is that the air compressor acts as a gas source to provide the experimental fluid for the whole experimental device, and the capillary core is put into the experimental section by controlling the flow rate of the experimental fluid by the mass flowmeter (*qv*). According to formula (3), the permeability of capillary core is calculated by observing the pressure difference between the two ends of the experimental section by the pressure differential transmitter.

According to the experimental part, **Figure 5** shows the porous wicks porosity and permeability curves of different NaCl proportion.

It can be seen from the figure that as the proportion of NaCl increases, the porosity and permeability increase gradually. The reasons of this phenomenon are:


**Figure 6** is porous wick porosity and permeability curve for 20% NaCl wt of different cold forming pressures. It can be seen from the figure that as the cold forming pressure increases, the porosity remains basically unchanged and the permeability gradually decreases. The reasons are:


#### *2.2.2 Capillary suction experiment*

The relationship between capillary force and permeability is complex, the relationship between capillary force and permeability is negative, and the increase of permeability is bound to decrease. The most direct method for observing capillary force is to observe the liquid level rising velocity and suction mass velocity in porous media. In this paper, the suction ability of capillary core is determined by observing the suction quality of capillary core.

The principle of the experiment is that the bottom of the capillary core is in contact with the liquid surface by controlling the lifting platform to ensure that only

**31**

**Figure 6.**

**Figure 5.**

*The Recent Research of Loop Heat Pipe*

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

the bottom of the capillary core exists the phenomenon of suction. The electronic

As shown in **Figure 7**, the size of the capillary suction specimen is 100 mm in length and 20 mm in diameter. The suction fluid is deionized water, the bottom of the capillary core is in rigid contact with the deionized water surface through the motion control platform, and the quality of the working fluid inside the beaker is measured by the electronic analytical balance (accuracy is 0.0001 g). The reduced mass is the quality of capillary suction. In order to control the rising velocity of liquid level accurately, the minimum rising speed of motion control platform is 0.01 mm/s. In order to ensure the measuring accuracy and the total quality of suction fluid to reduce the influence of the inside wall of beaker on the suction process, the diameter of beaker is 40 mm. The height is 160 mm. **Figure 8** shows the physical model of capillary aspiration. The suction curve of different PFA agent ratio and

**Figure 11** (the number below the pore-forming agent ratio is the total number of pores in the range of 0–30 μm in the electron micrograph) is 80 times magnification of the surface of the porous wicks, and the surface pore size distribution is measured by image pro plus software, 10, 20, 30, 40%, respectively. It can be seen that most of the pore diameters are distributed at 2–4 μm (about 30%). It can be seen that the total pore size decreases in the range of 2–30 μm with the increase of the proportion of pore-forming agent, The reason is that the proportion of the

analysis balance records the data and draws the suction curve.

*Porosity and permeability curves of different cold pressing pressure.*

*Porosity and permeability curves of different NaCl proportion.*

different cold-forming pressure are shown in **Figure 9** and **Figure 10**.

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

*Recent Advances in Heat Pipes*

differential transmitter.

enon are:

the pressure difference at the two ends of the sample (Pa), *μ* is experimental fluid viscosity (Pa s). The schematic diagram of the experimental device is shown in **Figure 4**. The principle is that the air compressor acts as a gas source to provide the experimental fluid for the whole experimental device, and the capillary core is put into the experimental section by controlling the flow rate of the experimental fluid by the mass flowmeter (*qv*). According to formula (3), the permeability of capillary core is calculated by observing the pressure difference between the two ends of the experimental section by the pressure

According to the experimental part, **Figure 5** shows the porous wicks porosity

1.As the proportion of NaCl increases, the volume of NaCl particles increases, and the total powder mass is equal during cold press forming, so the porosity

2.During the cold pressing and sintering process, as the temperature rises, the gap between the nickel powder particles gradually decreases, and the gap between the nickel powder and the NaCl particles remains unchanged, so the proportion of the pore former increases, and the porous wick shrinks. The

3.The particle size of NaCl particles is significantly larger than the particle size of nickel powder. After cleaning and desalting, the original NaCl particles occupy the pores, the flow resistance of the working medium in the pores

**Figure 6** is porous wick porosity and permeability curve for 20% NaCl wt of different cold forming pressures. It can be seen from the figure that as the cold forming pressure increases, the porosity remains basically unchanged and the permeability

1.As the cold forming pressure increases, the pore size between the nickel powder particles decreases, and the working fluid flow resistance becomes larger,

2.The proportion of total NaCl in the porous wick is the same. The pores occupied by NaCl particles account for the main part of the biporous structure. The proportion of pores formed between the nickel powder particles during

The relationship between capillary force and permeability is complex, the relationship between capillary force and permeability is negative, and the increase of permeability is bound to decrease. The most direct method for observing capillary force is to observe the liquid level rising velocity and suction mass velocity in porous media. In this paper, the suction ability of capillary core is determined by observing

The principle of the experiment is that the bottom of the capillary core is in contact with the liquid surface by controlling the lifting platform to ensure that only

sintering is small, so the porosity remains basically unchanged.

It can be seen from the figure that as the proportion of NaCl increases, the porosity and permeability increase gradually. The reasons of this phenom-

and permeability curves of different NaCl proportion.

increases as the proportion of NaCl increases.

smaller the degree, the larger the porosity.

decreases, and the permeability increases.

gradually decreases. The reasons are:

so the permeability decreases.

*2.2.2 Capillary suction experiment*

the suction quality of capillary core.

**30**

**Figure 5.** *Porosity and permeability curves of different NaCl proportion.*

**Figure 6.** *Porosity and permeability curves of different cold pressing pressure.*

the bottom of the capillary core exists the phenomenon of suction. The electronic analysis balance records the data and draws the suction curve.

As shown in **Figure 7**, the size of the capillary suction specimen is 100 mm in length and 20 mm in diameter. The suction fluid is deionized water, the bottom of the capillary core is in rigid contact with the deionized water surface through the motion control platform, and the quality of the working fluid inside the beaker is measured by the electronic analytical balance (accuracy is 0.0001 g). The reduced mass is the quality of capillary suction. In order to control the rising velocity of liquid level accurately, the minimum rising speed of motion control platform is 0.01 mm/s. In order to ensure the measuring accuracy and the total quality of suction fluid to reduce the influence of the inside wall of beaker on the suction process, the diameter of beaker is 40 mm. The height is 160 mm. **Figure 8** shows the physical model of capillary aspiration. The suction curve of different PFA agent ratio and different cold-forming pressure are shown in **Figure 9** and **Figure 10**.

**Figure 11** (the number below the pore-forming agent ratio is the total number of pores in the range of 0–30 μm in the electron micrograph) is 80 times magnification of the surface of the porous wicks, and the surface pore size distribution is measured by image pro plus software, 10, 20, 30, 40%, respectively. It can be seen that most of the pore diameters are distributed at 2–4 μm (about 30%). It can be seen that the total pore size decreases in the range of 2–30 μm with the increase of the proportion of pore-forming agent, The reason is that the proportion of the

**Figure 7.** *Schematic diagram of capillary suction platform.*

**Figure 8.** *Physical model of capillary aspiration.*

**Figure 9.** *The suction curve of different NaCl ratio.*

pore-forming agent is increased, and the number of pore-forming agent particles in the same section is increased, and the pores of each size are adhered to each other, resulting in a decrease of the number of total pores.

As is shown in **Figure 10**, the capillary suction mass of the porous wick is proportional to the porosity, and the experimental results are in line with the derivation conclusion.

The capillary suction speed of the porous wick is proportional to the porosity and the average pore diameter. It can be seen from the figure that the porous wicks (40% NaCl wt) have the fastest capillary suction speed and the porous wicks (10% NaCl wt) have the slowest capillary suction speed. Porous wicks (20% NaCl wt) with a small diameter of pores more than porous wicks (30% NaCl wt), so the former has a higher suction speed than the latter.

**33**

**Figure 11.**

*The Recent Research of Loop Heat Pipe*

**Figure 10.**

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

*The suction curve of different wicks with different cold pressure.*

As is shown in different cold forming pressures, suction speed porous wicks (30 kN) > porous wicks (40 kN) > porous wicks (50 kN) > porous wicks (60 kN), the suction quality is basically consistent but there are small differences, porous wick (30 kN) > porous wick (40 kN) > porous wick (50 kN) > porous wick (60 kN). It can be seen that as the cold pressure increases, the porosity and permeability

both decrease. When the length of the porous wick is controlled, the internal structure of the porous wick pressed at 30 kN pressure is loose, the permeability is large, the suction resistance is small, so the suction speed is faster, the permeability decreases gradually with the increase of pressure, the suction speed decreases. As is shown in **Figure 12** small pores which produce larger capillary force decrease

*Pore diameter distribution (a), (b), (c), and (d) are 10, 20, 30, and 40%, respectively.*

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

*Recent Advances in Heat Pipes*

**32**

tion conclusion.

*The suction curve of different NaCl ratio.*

*Physical model of capillary aspiration.*

*Schematic diagram of capillary suction platform.*

**Figure 9.**

**Figure 8.**

**Figure 7.**

pore-forming agent is increased, and the number of pore-forming agent particles in the same section is increased, and the pores of each size are adhered to each other,

As is shown in **Figure 10**, the capillary suction mass of the porous wick is proportional to the porosity, and the experimental results are in line with the deriva-

The capillary suction speed of the porous wick is proportional to the porosity and the average pore diameter. It can be seen from the figure that the porous wicks (40% NaCl wt) have the fastest capillary suction speed and the porous wicks (10% NaCl wt) have the slowest capillary suction speed. Porous wicks (20% NaCl wt) with a small diameter of pores more than porous wicks (30% NaCl wt), so the

resulting in a decrease of the number of total pores.

former has a higher suction speed than the latter.

**Figure 10.** *The suction curve of different wicks with different cold pressure.*

**Figure 11.** *Pore diameter distribution (a), (b), (c), and (d) are 10, 20, 30, and 40%, respectively.*

As is shown in different cold forming pressures, suction speed porous wicks (30 kN) > porous wicks (40 kN) > porous wicks (50 kN) > porous wicks (60 kN), the suction quality is basically consistent but there are small differences, porous wick (30 kN) > porous wick (40 kN) > porous wick (50 kN) > porous wick (60 kN).

It can be seen that as the cold pressure increases, the porosity and permeability both decrease. When the length of the porous wick is controlled, the internal structure of the porous wick pressed at 30 kN pressure is loose, the permeability is large, the suction resistance is small, so the suction speed is faster, the permeability decreases gradually with the increase of pressure, the suction speed decreases. As is shown in **Figure 12** small pores which produce larger capillary force decrease

**Figure 12.** *Pore diameter distribution (a), (b), (c), and (d) are 30, 40, 50, and 60 kN, respectively.*

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