4.4. Determination of loop heat pipe cooling efficiency

Determination of the LHP cooling efficiency was performed on the experimental measuring unit, which is shown in Figure 15. Fixed IGBT on the evaporator of LHP was loaded by electric power. Produced heat by IGBT on the evaporator of LHP was removed by working fluid to the condenser of LHP. The condenser of LHP was made as tube heat exchanger and the cooling circle of heat exchanger was regulated by the thermostat at constant temperature 20C. The gist of the LHP cooling efficiency determination is on measuring IGBT temperature with gradually increasing loaded heat by IGBT in steps 50 W from 100 W till the IGBT reaches permissible temperature 100C. The temperature of the IGBT was measured by thermocouple inserted under IGBT. For better heat transport, thermal conductive paste was applied on the connection between IGBT and aluminum block and between aluminum block and the evaporator.

At first, measurements of influence of the working fluid amount on LHP cooling efficiency were performed. Four amounts 40, 50, 60, 80% of total LHP volume in LHP with working fluid water were investigated. In Figure 16, the influence of working fluid amount in dependencies on LHP cooling efficiency with working fluid water depending on loaded heat is shown. It is seen that the LHP with working fluid volume is 60% and the best operating LHP is in range of 150–350 W.

Next, the measurement of influence of wick structures on LHP ability to remove heat from IGBT was performed. The measurement was performed on LHP with the working fluid of water and amount of 60% total LHP volume. In Figure 17, the results of the influence of the wick structure on LHP cooling efficiency depending on loaded heat are shown. Two wick

Figure 16. Influence of working fluid amount on LHP operation.

seen that the LHP with first structure did not heat remove from IGBT and the temperature of IGBT exceed 100C. The LHP with second wick structure is able to cool the IGBT under temperature 100C until the IGBT input power 450 W. Comparing the temperature curves of the LHP with third wick structure (made of Ni powder 10 μm) and LHP with fourth wick structure (made of Ni powder 20 μm), it is seen that IGBT temperature cooled by LHP with third structure rapidly increases, already, at an input power 150 W. The LHP with fourth wick structure is able

Figure 18. Influence of the working fluid on LHP cooling efficiency with wick structure made from Cu powder 100 μm

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At third, measurement impact of working fluid in LHP with wick structure made from Cu powder 100 μm and 50 μm and amount 60% of total LHP volume with an ability to remove heat from IGBT was performed. Figure 18 shows the result of influence of the working fluid on LHP cooling efficiency depending on loaded heat. This experiment shows that the LHP with working fluid acetone better removes heat from the IGBT at lower heat load in range of 100–300 W.

The next experiments were performed in frame scientific research of porous wick heat pipes, where the ability of heat transfer depending on wick structure and working fluid is investigated. The popularity of porous wick heat pipe and lack of experiments performed with them were the reason to realize experiment which deals with heat pipes with sintered wick structures made from copper powders. This section describes manufacturing process of wick heat pipe, experimental measurement of heat transfer ability of heat pipe, and mathematical calcu-

cool the IGBT under temperature 100C until the IGBT input power 250 W.

At higher heat loads is the better working LHP with the working fluid water.

5. Heat pipe experiments

and 50 μm depending on loaded heat.

lation of heat transport limitation of heat pipes.

Figure 17. Influence of the wick structure on LHP cooling efficiency with working fluid water depending on loaded heat.

structures made form Cu powder with grain size 50 μm and 100 μm and two wick structures made from Ni powder grain size 20 μm and 10 μm were compared.

Comparing the results of dependence of temperature on input power of IGBT cooled by LHP with variants of sintered wick structure, the LHP with nickel wick structure does not show so good properties of heat removal than LHP with copper wick structure. Comparing the temperature curves of the LHP with first wick structure (made of Cu powder 50 μm) and LHP with second wick structure (made of Cu powder 100 μm), it is seen that both LHP have almost the same results at heat load of up to 200 W. At higher input power than 200 W loaded in to IGBT is

Figure 18. Influence of the working fluid on LHP cooling efficiency with wick structure made from Cu powder 100 μm and 50 μm depending on loaded heat.

seen that the LHP with first structure did not heat remove from IGBT and the temperature of IGBT exceed 100C. The LHP with second wick structure is able to cool the IGBT under temperature 100C until the IGBT input power 450 W. Comparing the temperature curves of the LHP with third wick structure (made of Ni powder 10 μm) and LHP with fourth wick structure (made of Ni powder 20 μm), it is seen that IGBT temperature cooled by LHP with third structure rapidly increases, already, at an input power 150 W. The LHP with fourth wick structure is able cool the IGBT under temperature 100C until the IGBT input power 250 W.

At third, measurement impact of working fluid in LHP with wick structure made from Cu powder 100 μm and 50 μm and amount 60% of total LHP volume with an ability to remove heat from IGBT was performed. Figure 18 shows the result of influence of the working fluid on LHP cooling efficiency depending on loaded heat. This experiment shows that the LHP with working fluid acetone better removes heat from the IGBT at lower heat load in range of 100–300 W. At higher heat loads is the better working LHP with the working fluid water.
