Loop heat pipes (LHPs) are used in many branches of industry, mainly for cooling of electrical elements and systems. The loop heat pipe is a vapour-liquid phase-change device that transfers heat from evaporator to condenser. One of the most important parts of the LHP is the porous wick structure. The wick structure provides capillary force to circulate the working fluid. To achieve good thermal performance of LHP, capillary wicks with high permeability and porosity and fine pore radius are expected. The aim of this work was to develop porous structures from copper and nickel powder with different grain sizes. For experiment copper powder with grain size of 50 and 100
The trend development of electronic components is miniaturization of the dimension. It leads to an increase of waste heat. This heat often leads to lower performance and failure of electronic components in case of insufficient cooling. In order to maintain appropriate working conditions, waste heat must be removed. One possibility to remove waste heat is to use loop heat pipe (LHP). LHPs are two-phase heat transfer devices that utilize the evaporation and condensation of a working fluid to remove heat and the capillary forces developed in fine porous wicks to circulate the fluid. LHP consists of an evaporator with wick, a condenser, a compensation chamber, and liquid and vapor line (Figure
Schematic diagram of LHP [
In order for the loop to continue to function, the wick in the evaporator must develop a capillary pressure to overcome the total pressure drop in the loop. One of the advantages of a capillary loop is that the meniscus in the evaporator wick will automatically adjust its radius of curvature such that the resulting capillary pressure is equal to the total system pressure drop. The total pressure drop in the system is the sum of frictional pressure drops in the evaporator grooves, the vapour line, the condenser, the liquid line, and the evaporator wick, plus any static pressure drop due to gravity:
The capillary pressure rise that the wick can develop is given by
Further increase of the heat load will lead to vapour penetration through the wick and system depressed. Thus, under normal operation, the following condition must be satisfied at all times [
Williams and Harris [
Holley and Faghri [
Typically, the rate-of-rise test requires observing the liquid front as it rises in a dry wick partially immersed in a liquid pool. As the precise location of this front can be difficult to detect, the authors devised a method using mass uptake rather than the meniscus front to determine the rate of rise of liquid in the wick. By analyzing the climbing meniscus, the authors developed a series of equations which could be used to numerically reduce the mass uptake data to yield permeability and pore size results.
Several relationships for permeability can be found; the most common is the Blake-Kozeny equation [
Ren and Wu [
Zhao and Liao [
Iverson et al. [
The majority of heat load is used in vaporization on the outer surface of wick [
Ku [
In steady state operation, the heat leak to the compensation chamber must be offset by the liquid returning from the condenser; (
Chuang [
Chuang derived the following expressions for the axial and radial heat leak, respectively:
To achieve good thermal performance, capillary wicks with high permeability and porosity and fine pore radius are expected. These parameters depend mainly on the manufacturing process. The most frequently used wicks are made of sintered metal, like nickel, copper, titanium, stainless steel, or polymers (polyethylene, polypropylene, and PTFE) [
Tap powder sintering technique using a graphite matrix is used by Reimbrechta et al. [
In [
According to the above-mentioned experiences with sintered structures for LHP we decide to make wick structures from nickel and copper powder. At first we do analysis of several sintered structures depending on grain size, sintering temperature and sintering time on porosity, pore size, and strength. In electric furnace etalons from copper powders with grain sizes of 50 and 100
The porosity of a wick structure describes the fraction of void space in the material, where the void may contain working fluid [
The results of porosity measuring are shown in Tables
Porosity of sintered structures from copper powder with grain size of 50
Grain size ( |
50 | 50 | 50 | 50 |
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Sintering temperature (°C) | 800 | 800 | 950 | 950 |
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Sintering time (min) | 30 | 90 | 30 | 90 |
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Porosity (%) | 55 | 54 | 52 | 50 |
Porosity of sintered structures from copper powder with grain size of 100
Grain size ( |
100 | 100 | 100 | 100 |
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Sintering temperature (°C) | 800 | 800 | 950 | 950 |
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Sintering time (min) | 30 | 90 | 30 | 90 |
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Porosity (%) | 58 | 56 | 55 | 52 |
Porosity of sintered structures from nickel powder with grain size of 10
Grain size ( |
10 | 10 |
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Sintering temperature (°C) | 600 | 600 |
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Sintering time (min) | 30 | 90 |
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Porosity (%) | 69 | 67 |
Porosity of sintered structures from nickel powder with grain size of 25
Grain size ( |
25 | 25 |
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Sintering temperature (°C) | 600 | 600 |
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Sintering time (min) | 30 | 90 |
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Porosity (%) | 72 | 70 |
Investigation of etalons sintered structures by microscopic analysis shown, how influent sintered temperature and time pore size and ratio grain size to pore size of each structure. In Figure
Microscopic pictures of the sintered structures from copper powders. (a) Grain size of 50
In Figure
Microscopic pictures of the sintered structures from nickel powders. (a) Grain size of 10
From results of porosity measurement and microscopic analysis for wick structure of LHP two copper etalons and two nickel etalons were chosen. The first structure was made of copper grain size of 50
Copper powder sintered wick structure.
Nickel powder sintered wick structure.
All parts of LHP (evaporator, compensation chamber, vapor, and liquid line) were made from copper pipes. As a working fluid distilled water was used. In the evaporator sintered wick structure from copper powder was inserted. To avoid heat loss (it is also called heat leak) into the compensation chamber a brass flange with rubber seal was inserted between the evaporator and the compensation chamber. In Figure
Main design parameters of the LHP.
Evaporator | |
Total length (mm) | 130 |
Active length (mm) | 86 |
Outer/inner diameter (mm) | 28/26 |
Material | Copper |
Saddle | |
Size (length/height/width) | 118/89/40 |
Material | Alumina |
Sintered copper powder | |
Number of vapor grooves | 6 |
Porosity (%) | 52–55 |
Outer/inner diameter (mm) | 26/8 |
Sintered nickel powder | |
Number of vapor grooves | 6 |
Porosity (%) | 67–70 |
Outer/inner diameter (mm) | 26/8 |
Compensation chamber | |
Outer/inner diameter (mm) | 35/33 |
Length (mm) | 110 |
Charge mass | |
Distilled water | 60% |
Vapor line | |
Length (mm) | 670 |
Outer/inner diameter (mm) | 6/4 |
Liquid line | |
Length (mm) | 820 |
Outer/inner diameter (mm) | 6/4 |
Condenser | |
Length (mm) | 420 |
Outer/inner diameter (mm) | 6/4 |
Model of design LHP: 1: compensation chamber, 2: rubber seal, 3: evaporator, 4: vapor line, 5: condenser, 6: filling valve, and 7: liquid line.
The LHP with sintered wick structure was proposed to test cooling of IGBT. On the evaporator of LHP the aluminum block with fixed insulated gate bipolar transistor (IGBT) was mounted. For better heat transport thermal conductive paste was applied on the connection between IGBT and aluminum block and between aluminum block and the evaporator [
Schematic diagram of measuring device: 1: PC, 2: logger, 3: IGBT, 4: power supply voltage and current, 5: thermocouple, and 6: thermostat.
The temperature of the IGBT was measured by thermocouple inserted under IGBT. The maximum permissible temperature of IGBT is 100°C. Transistor was connected to DC power of source and it was gradually loaded by DC. Like this was performed measurement impact of four kinds wick structures in LHP to heat remove from IGBT. The results from measurement of IGBT cooling by LHP with copper wick structures are shown in Figures
Dependence of temperature on input power of IGBT cooled by LHP with first variant of wick structure.
Dependence of temperature on input power of IGBT cooled by LHP with second variant of wick structure.
Dependence of temperature on input power of IGBT cooled by LHP with third variant of wick structure.
Dependence of temperature on input power of IGBT cooled by LHP with fourth variant of wick structure.
Comparing results of dependence of temperature on input power of IGBT cooled by LHP with various variants of sintered wick structure, the LHP with nickel wick structure did not show better properties of heat removal than LHP with copper wick structure. Comparing dependencies of temperature course on input power of IGBT cooled by LHP with first and second wick structure it is seen that at load of up to 200 W has both LHP almost the same results. At higher input power than 200 W loaded into IGBT it is seen that the LHP with first structure did not remove heat from IGBT and the temperature of IGBT exceeded 100°C. The LHP with second wick structure is able to cool the IGBT under temperature of 100°C until the IGBT input power of 450 W. Comparing dependencies of temperature course on input power of IGBT cooled by LHP with third and fourth wick structure it is seen that evaporator temperature of LHP with third structure at input power of 100 W gradually increases with time and is stabilized at temperature 92°C. The LHP with fourth wick structure is able to cool the IGBT under temperature of 100°C until the IGBT input power of 250 W.
This experiment was performed in framing scientific research of porous structures suitable for LHP and finding ability of heat removal produced by IGBT. We lead off previous research works about LHP in which materials specification suitable for porous structure was preferred. We choose copper and nickel powder with two various granularities. At first etalons were manufactured from each material sintered at various temperatures and times. It was observed that temperature is the main influencing factor on wick structure porosity and pore size is depending on powder grain size. After them for each material one wick structure was manufactured with best characteristics of porosity and pore size and used in LHP for IGBT cooling. The knowledge gained from the IGBT cooling by LHP has given us the information necessary to know how much heat flux is LHP able to remove from heat source. This piece of information will be in the future useful in the design of cooling devices working with the LHP. In the future we would like to focus deeper on analysis of the physical characteristics (e.g., thermal conduction and capillary pressure) of manufactured wick structures and on research of construction design LHP able to remove heat by natural convection to the surroundings.
According to microscopic analysis of sintered structures, which clarifies their shape and profile, we can conclude that the main influencing factors of pore size are grain size, sintering temperature, and not so much sintering time. The measurement comparison of dependency IGBT temperature from input power cooled by LHP with copper or nickel wick structure can conclude, however, that in both cases the structures had the same porosity and better effect on heat removal from IGBT that had porous structure with bigger pore size. Generally the smallest pore size could cause the low capillary pressure in sintered wick structures against total pressure in whole LHP system.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This paper was created within the solution of project APVV-0577-10 and sponsored in the frame of program OPV-Podpora kvality vzdelávania a rozvoj ľudských zdrojov v oblasti technického výskumu a vývoja v priestore modernej vedomostnej spoločnosti ITMS 26110230117.