Loop Heat Pipe (LHP) performance strongly depends on the performance of a wick that is porous media inserted in an evaporator. In this paper, the visualization results of thermo-fluid behavior on the surface of the wick with microscopic infrared thermography were reported. In this study, 2 different samples that simulated a part of wick in the evaporator were used. The wicks were made by different two materials: polytetrafluoroethylene (PTFE) and stainless steel (SUS). The pore radii of PTFE wick and SUS wick are 1.2 μm and 22.5 μm. The difference of thermo-fluid behavior that was caused by the difference of material was investigated. These two materials include 4 different properties: pore radius, thermal conductivity, permeability and porosity. In order to investigate the effect of the thermal conductivity on wick’s operating mode, the phase diagram on the q- keff plane was made. Based on the temperature line profiles, two operating modes: mode of heat conduction and mode of convection were observed. The effective thermal conductivity of the porous media has strong effect on the operating modes. In addition, the difference of heat leak through the wick that was caused by the difference of the material was discussed.
Recently, high efficiency heat transport devices are expected due to increase of power dissipated in electronic components. Therefore loop heat pipe (LHP) gets much attention as advanced thermal control devices. LHP is a two-phase heat transfer device using capillary action in a microscale porous structure that is called a wick inserted in the evaporator. LHP has several advantages i.e., high heat transport capability with no electrical power, operability against gravity and flexible transport lines. These advantages make a growing awareness of LHP as a future heat transport technology. The LHP is composed of the evaporator, a vapor line, a condenser, a liquid line and a compensation chamber (CC) as shown in
The purpose of this study is to reveal thermo-fluid behavior in the wick based on microscale infrared observation. In this paper, the observation results of 2 different samples that simulate a part of wick in the evaporator as shown in
graphs are taken by microscopic infrared thermography (Advanced Thermo TVS-500EX and TVM-7025U) that has spatial resolution of 18μm, measurement wavelength of 8 - 14 μm. The detecting element is an uncooled microbolometer that measures the change of resistance caused by change of bolometer element temperature and converts it into voltage. The time resolution is 0.017 sec. The temperature measurement resolution is 0.05˚C. The measurement accuracies are ±2˚C in case that measured temperature is less than 100˚C and ±2% in case that measured temperature is over 100˚C. The thermographs are recorded at 1 frame per second. The experiment was conducted under the atmospheric condition (Tamb = 24˚C). The vapor groove temperature (Tv) and liquid reservoir temperature (Tr) were measured by T-type thermocouples that have measurement accuracy of ±0.5˚C.
The sample simulates a part of the wick in the evaporator.
where, Qapply is applied heat.
On the other hand, in case of SUS wick, phase-change started at 6.0 W/cm2 and maximum heat flux reached 52 W/cm2. After phase-change occured, Tcnt was substantially constant at 78˚C - 80˚C. The vapor pocket was not formed at every heat flux. When heat flux of 54 W/cm2 was applied, Th and Tcnt rose quickly and reached the limited temperature. The difference of thermo-fluid behavior was discussed as follow.
In case of PTFE wick, the microscopic gap between the heating plate and the wick was observed with micro-
Name | PTFE | SUS |
---|---|---|
Cross section | ||
Material | Polytetrafluoroethylene | SS316L |
Pore radius (μm) | 1.2 | 22.5 |
Porosity | 0.34 | 0.71 |
Permeability (m2) | 2.0 × 10−14 | 1.0 × 10−11 |
Bulk thermal conductivity (W/mK) | 0.25 | 16.3 |
Width of groove (mm) | 0.5 | |
Number of groove | 14 | |
Depth of groove (mm) | 1.0 | |
Contact area between heating plate and wick (mm2) | 75 | |
Fin lateral area (mm2) | 280 |
PTFE | Cross Section | 3.0 W/cm2 | 4.0 W/cm2 | 5.0 W/cm2 | 6.0 W/cm2 | 7.0 W/cm2 | 8.0 W/cm2 | |
---|---|---|---|---|---|---|---|---|
SUS | Cross Section | 2.0 W/cm2 | 4.0 W/cm2 | 6.0 W/cm2 | 8.0 W/cm2 | 10 W/cm2 | 12 W/cm2 | |
22 W/cm2 | 26 W/cm2 | 30 W/cm2 | 36 W/cm2 | 40 W/cm2 | 46 W/cm2 | 52 W/cm2 | ||
scope. It is considered that liquid that is attached to the heating plate evaporate when Th reaches 78.3˚C. Therefore the evaporation occurs at contact surface between the wick and heating plate at low heat flux. It is considered that there is thin vapor region at the contact surface in case of low heat flux. In addition, the nucleate boiling does not occur at the contact surface because of microscopic gap. As the heat flux is increased, the mass flow increases and the pressure loss in the wick and contact surface become larger. When it reaches the maximum capillary pressure at the top of fins, the vapor pocket is formed in the wick.
In case of SUS wick, the liquid bridges formed in the vapor grooves as shown in
This phenomenon stopped when the heat flux is 18 W/cm2. This is because the evaporation increased with increase of heat flux. When the heat flux is 18 - 52 W/cm2, the nucleate boiling was observed at the contact surface between the wick and heating plate.
0.0 sec | 1.0 sec | 2.0 sec | 3.0 sec | 4.0 sec |
---|---|---|---|---|
In case of SUS wick, the profile line is not smooth. This is caused by the difference of emissivity between pore part and bulk part of porous media. The emissivity of bulk part is lower than that of pore part. That makes temperature profile lines notched. However there is a tendency which is similar to the trend of PTFE wick temperature distribution.
The profile lines are liner at 6.0 - 12 W/cm2 and the base part temperature increases. On the other hand, the temperature distribution becomes nonlinear at 14 - 52 W/cm2. In case of SUS wick, wick’s operating modes are: 1) Mode of heat conduction at 6.0 - 12 W/cm2 and 2) Mode of convection at 14 - 52 W/cm2. Equation (2) explains the balance of energy in the wick.
where, Qev is evaporation heat; Qleak is heat leak; Qsens is sensible heat from liquid reservoir to the contact surface; Qloss is heat loss. Qev is explained by Equation (3).
where,
where, keff is effective thermal conductivity; Awick is wick cross sectional area; Tbase1 and Tbase2 are measured temperature and temperature distribution between Tbase1 and Tbase2 is liner. L is the distance between measuring point of Tbase1 and Tbase2. Qsens is explained by Equation (5).
where, cp is isobaric specific heat. Based on Equations (3)-(5), Equation (2) is expressed by Equation (6).
There are competing thermodynamic processes; evaporation, heat conduction through the wick and capillary- driven convection. In case of mode of heat conduction,
Two wick’s operating modes are mapped into the phase-diagram on the q-keff plane as shown in
It is clear that the wick’s operating mode changes to the mode of convection at smaller heat flux in case of PTFE wick that has smaller effective thermal conductivity. On the other hand, in case of SUS wick, it changes to mode of convection at 14 W/cm2 that is larger than that of PTFE. When the heat flux is higher and the effective thermal conductivity is smaller, the effect of convection becomes dominant. This tendency has good agreement with the calculation results that were reported by C. Ren [
The observations of the vapor-liquid phase change phenomenon with microscopic infrared thermography were conducted. The conclusions are as follow.
1) In case of PTFE wick, the vapor pocket formation was observed at 5.0 - 8.0 W/cm2. On the other hand, in case of SUS wick, the vapor pocket was not formed at every heat flux. The nucleate boiling at the contact surface was observed in case of SUS wick. It is considered that it makes contact surface temperature constant at 78˚C - 80˚C.
2) In case of both PTFE wick and SUS wick, wick’s operating mode change from the mode of heat conduction to mode of convection was observed. The mode change of SUS wick occurs at higher heat flux than that of PTFE wick because its high thermal conductivity makes the effect of heat conduction larger. It is clear that the effective thermal conductivity has the effect on the wick’s operating modes.
3) The maximum Qleak/Qapply of PTFE wick and SUS wick become 4.28% and 40.7%. At the point of heat leak reduction, PTFE wick is greater than SUS wick.
These works gave fruitful information about thermo-fluid behavior in the wick. Based on them, the detailed numerical analysis of thermal hydraulics in the wick will be conducted as a future work.
This research was partially supported by JST Presto.
Kimihide Odagiri,Masahito Nishikawara,Hosei Nagano, (2016) Microscale Infrared Observation of Liquid-Vapor Phase Change Process on the Surface of Porous Media for Loop Heat Pipe. Journal of Electronics Cooling and Thermal Control,06,33-41. doi: 10.4236/jectc.2016.62003