6. Conclusion

In Figure 32, the influence of wick structure width on heat pipe performance is shown. Wick structure width is an important factor, which influences heat pipe performance. It is seen that the heat pipe performance increases with the wick structure thickness in operating temperature region of �30 to 60�C. The capillary limitation is a main limitation for this region. On the other way, an increase of the wick structure thickness decreases the heat pipe performance in the operating temperature region of 80–130 �C. It may be caused by bubble nucleation in wick structure, when the returning liquid from the condenser section to evaporator section of heat pipe evaporates. In this case, the main limitation is

Figure 32. Dependence of heat pipe performance from wick structure width of the sintered wick heat pipe.

Figure 31. Dependence of heat pipe performance from sphere diameter of the copper powder in sintered wick heat pipe.

boiling limitation.

174 Porosity - Process, Technologies and Applications

The experiments performed with the heat pipes in this work give several conclusions about the influence of porous wick structures on their heat transfer ability, where the porosity and pore size play main role. Experiments' influence of manufacturing technology on the wick structure porosity show that the sintering time and temperature of the metal powders are not influencing the wick structure porosity. Other finding of the influence of manufacturing technology on the porous wick structure by the metal powders sintering is that main influencing factors of the wick structure pore size are grain size, sintering temperature, and not so much sintering time.

The experiments' influence of working fluid amount, kind of wick structure and working fluid on heat transfer ability of loop heat pipe show that the optimal amount of the working fluid in LHP is in range 50–60%. In view of influence of the wick structure on the LHP operation, we can conclude that the porosity and pore size of the wick structure have influence on heat transfer ability, when the LHP with porous structure with 50% porosity has better effect on heat removal from IGBT than with 70% and LHP with porous structure with bigger pore size has better effect on heat removal from IGBT than with smaller pore size. Generally, the smallest pore size could cause the low capillary pressure in sintered wick structures against total pressure in whole LHP system. Experimental influence of working fluid on the LHP heat transfer ability shows that the LHP with working fluid acetone better removes heat from the IGBT at lower heat load in range of 100–300 W and LHP with the working fluid water better works at higher heat loads of LHP up to 450 W.

The experiments' influence of the wick structure and working fluid on heat transfer ability of the heat pipe did not show the best one combination of porous wick structure and working fluid. This experiment shows that the heat pipes with porous wick structure are able to transfer heat in range 100–200 W in the horizontal position. It depends on the wick structure parameters and kind of working fluid, because each heat pipe with various combination of these factors is unique due its different properties.

The mathematical calculation of the heat pipe heat transport limitations shows that the critical limitations influencing heat transfer ability of wick heat pipe are entrainment limitation, capillary limitation, and boiling limitation. These limitations depend on thermophysical properties, wick, and heat pipe parameters. The thermophysical properties of each working fluid are stable in temperature range and they cannot change. Changing the dimensions of wick structure is possible to optimize total heat flux transferred by heat pipe, because capillary pressure made in the wick structure depends mainly on the wick structure permeability. When the wick structure is designed, it is necessary to be careful because the increase in pore dimension increases permeability but decreases capillary pressure which manages the working fluid circulation in heat pipe. Therefore, the capillary limitation is the main heat transport limitation in wick heat pipe.
