Experimental and Theoretical Studies of a Loop Heat Pipe with Biporous Wicks
Date Issued
2009
Date
2009
Author(s)
Yeh, Chien-Chih
Abstract
At high heat fluxes, formerly a monoporous wick in a loop heat pipe (LHP) was easily occupied by the vapor to form a vapor blanket, leading to limit the heat transfer capacity. Besides, the surface area for liquid film evaporation was limited after determining the design of outer vapor grooves on a monoporous wick. For these reasons, biporous wicks are utilized to improve the foregoing problems. Because the evaporative heat transfer of a biporous wick is exceedingly sensitive to the internal volume fractions of liquid and vapor phases, the purpose of this study is to investigate the effects of various pore size distributions of the biporous wicks for a LHP by the experimental design and theoretical analysis. xperiments were performed to control the pore size distributions of the biporous wicks by changing the particle size of pore former (32~48μm and 74~88μm), the pore former content (20vol% and 25vol%), and the sintering temperature (650℃ and 750℃). Furthermore, a statistical approach was carried out to analyze the evaporative heat transfer of the biporous wicks, so as to understand the effects of the parameters more effectively. At the same time, how various pore size distributions of the biporous wicks influence the heat transfer capacity of a LHP was examined. According to the statistical analysis, the effect of the pore former content was significantly associated with the evaporative heat transfer of a biporous wick. This is because the large pores formed by the pore former in a biporous wick can self-regulate the amount of vapor passages. With the increase of the pore former content, this not only decreased the vapor blanket but also extended the surface area for liquid film evaporation. Experimental results also showed that, at the sink temperature of 10℃ and the allowable evaporator temperature of 85℃, the evaporative heat transfer coefficient of the best biporous wick, which reached a maximum value of 68kW/m2.℃, was approximately 5~7 times higher than that of the monoporous wick. In addition, in terms of the biporous wicks applying to a LHP, the maximum heat transfer capacity of the best biporous wick achieved 570W (the heat flux of 29.3W/cm2) and the minimum total thermal resistance was 0.09℃/W. Comparing to a monoporous wick of 400W (the heat flux of 20.5W/cm2) and 0.15℃/W, the biporous wick had the suitable degree for the promotion of heat transfer.n improved LHP steady-state model was developed; meanwhile, the pore size distribution of a wick structure and the phase-change heat transfer were taken into account. The results showed that the comparison between the predicted results and experimental data were within 25% of the mean absolute percentage error (MAPE). The model analysis indicated that the vapor blanket thickness formed was affected by various pore size distributions of the biporous wicks and was one of important reasons to influence the heat transfer capacity of a LHP. Moreover, the results of the model calculation also showed the biporous wicks, which were advantageous to the vapor easily escape from the wick, were affected less by the vapor blanket than the monoporous wick and would improve the heat transfer capacity of a LHP.o conclude, the biporous wicks cannot only improve the heat transfer capability but also be advantageous to simplify and replace the manufacturing technology of outer vapor grooves on a wick structure. For passive cooling of high-power components, it will be an efficient and simple approach.
Subjects
loop heat pipe
monoporous wick
biporous wick
pore size distribution
vapor blanket
Type
thesis
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