5.6. Results of the mathematical model

5.5. Verification of mathematical model

172 Porosity - Process, Technologies and Applications

The mathematical model was created according to above equations of limitations and input heat pipe parameters. Result of mathematical model is graphic dependencies of heat transport limitations on heat pipe working temperature. Mathematical model results of heat transport limitations of specific types of heat pipes were compare with results from measurement of heat pipe performance at temperatures 50�C and 70�C. In Figure 28, graphic comparison results of heat transport limitations determining total performance of heat pipe from mathematical model with measured performance of ethanol wick heat pipe with sintered wick structure and sphere diameter of copper powder 0.1 mm are shown. Dotted line creates boundary of heat pipe performance by capillary limitation and dashed line is boiling limitation. The full line is measured results of heat pipe thermal performance at temperature 50�C and 70�C. Figure 29

Figure 28. Verification of mathematical model by measuring of heat pipe performance (ethanol wick heat pipe with sintered wick structure and sphere diameter of copper powder 0.1 mm and axial orientation of heat pipe ψ 180�).

Figure 29. Verification of mathematical model by measuring of heat pipe performance (water wick heat pipe with

sintered wick structure and sphere diameter of copper powder 0.63 mm axial orientation of heat pipe ψ 180�).

Results of the heat pipe calculation are some interesting graphs of the maximal heat flux transferred by heat pipe depending on the wick structure parameters. It could be used in design optimization of the heat pipe wick structure. The curves present an area of maximal heat flux transferred by heat pipe depending on operating temperature.

Next graphic dependencies of heat pipe performance are created from mathematical model for ethanol wick heat pipe with sintered wick structure and various porosity, sphere diameter of copper powder, and wick structure width. In Figure 30, the influence of porosity on heat pipe performance is shown. Porosity of wick structure can change by adding some additives to sintered technology. There is clearly seen a rise in heat pipe performance with increasing porosity of wick structure. Heat pipe with the higher permeability of the wick structure can transfer higher heat flux. But with increasing permeability of wick structure, entrainment of liquid flow to evaporator by vapor flow can occur. This may cause dry out of heat pipe evaporation section and decrease total heat pipe performance.

In Figure 31, the influence of sphere diameter copper powder on sintered wick structure is shown. Using the bigger sphere diameter of copper powder to sintered technology, higher porosity wick structure is created. It can be said that increasing porosity is directly proportional to sphere dimension of copper powder, and to make more porosity wick structures, adding additives to sintered technology is not needed. In this case, an increase of heat pipe performance with used bigger sphere dimension of copper powder is seen.

Figure 30. Dependence of heat pipe performance from wick structure porosity of the sintered wick heat pipe.

6. Conclusion

not so much sintering time.

factors is unique due its different properties.

limitation in wick heat pipe.

450 W.

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

Porous Structures in Heat Pipes

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http://dx.doi.org/10.5772/intechopen.71763

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

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

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

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

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 boiling limitation.

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