Experimental Study on the Loop Heat Pipe with a Bidispersed Wick Structure

Loop heat pipe (LHP) is an effective phase-change cooling device which can achieve long-distance transport, low thermal resistance, and high heat transfer capacity. Improving the heat transfer capacity of LHP by a wick structure will be an important topic. The purpose of this study is to investigate the bidispersed wicks’ heat transfer performance of different cluster sizes and to compare their heat transfer characteristics with a monoporous wick on a LHP. The experiment used the granulation mixing method to create clusters and formed connective pores between clusters. The bonding nickel powder produces smaller pores inside the cluster to make a bidispersed wick.To discuss the manufacturing parameters’ effect on bidispersed wicks, we select binder composed of polypropylene(PP), paraffin wax(PW), stearic acid(SA), as well as multi-component binder composition in this experiment,. Three binder contents were choosed as the following: 50vol%, 60vol%, 70vol%. And the bonding elements PP, PW, SA were choosed in the ratios of 40:55:5, 50:45:5. The bidispersed wicks were debinded and sintered with the sintering curve under the fixed temperature of 700 ℃. The results presented that bidispersed wicks formed better shape with the binder content of 60vol% and the ratio between the binder elements PP, PW, SA of 40:55:5. Experimental results revealed that, at the sink temperature of 10℃ and the maximum allowable evaporator temperature of 85℃, the maximum heat transfer capacity of the best bidispersed wick achieved 550W and the minimum total system thermal resistance was 0.13℃/W. Comparing to a monoporous wick for 400W and 0.19℃/W, overall performance has significantly improved. In addition, the experimental results also showed that the maximum amount of heat transfer can be enhanced and the total system thermal resistance can be reduced by reducing the cluster size, but too small cluster sizes(53~62μm) would reduce the heat transfer performance contrarily. This results may be explained that closer accumulation causeing by too small cluster sizes reduced permeability, which impacted the evacuation of the vapor. Finally, by comparing the heat transfer characteristics can be seen that the heat transfer coefficient in the evaporator of the best bidispersed wick reached to a maximum value of 23kW/m2•℃, which was approximately 3 times higher than that of the monoporous wick. The significant enhancement of the heat transfer coefficient of bidispersed wicks can be explained that the wick had increased the surface area available for the thin film evaporation with increasing heat flux. Therefore, the heat transfer coefficient reaches a maximum value. The future work will consider more parameters such as sintering temperature, nickel particle size etc. , the loop heat pipe heat transfer performance shall be upgraded with greater potential.