Comparative study of two loop heat pipes using R134a as the working fluid

In order to promote future commercial applications of loop heat pipe (LHP) especially in the civil fields, environmentally friendly R134a is considered a good candidate working fluid to replace the commonly used anhydrous ammonia. In this work, two sets of LHPs with R134a as the working fluid were designed and fabricated, one with a nickel wick, and the other with a stainless steel wick. Their thermal performance, mainly including the startup, the power increment test, the thermal resistance and the heat transport capability, were experimentally studied, evaluated and compared. Based on the experimental results, it is found that both of the R134a LHPs present excellent startup performance, achieve a maximum heat transfer capacity greater than 100 W, and exhibit good response characteristics to stepwise change of heat load. The R134a LHP with a nickel wick shows good anti-gravity capability; however, the one with a stainless steel wick is quite sensitive to the adverse elevation. The physical mechanisms responsible for the experimental results mentioned above have been analyzed and discussed. This work contributes to a better understanding on the operating performance and characteristics of the R134a LHPs, providing good design guidance and reference for its future applications.