Investigation on the Influence of the Condenser Length and Its Coupling Effect With Elevation on the Performance of Loop Heat Pipe

Abstract

The effects of condenser length and evaporator/condenser elevation on the performance of a loop heat pipe (LHP) in a gravity field were investigated, respectively, as well as their coupling effect. The refrigerant R40 was selected, and the condenser length was altered by changing the number of fins connected to the water-cooled plates. The experiments were conducted at three evaporator/condenser elevations: zero elevation, favorable elevation, and adverse elevation. The experimental results indicated that the evaporator temperatures of LHP with three different condenser lengths are very close when the LHP operates in variable conductance mode (VCM). However, an excessively long condenser will increase the LHP thermal resistance in VCM. In constant conductance mode (CCM), the longer the condenser, the lower the evaporator temperature. As the heat load increases, the LHP first exhibits gravity-driven mode and then capillarity-gravity-driven mode at a favorable elevation. The positive coupling effect on reducing the evaporator temperature by extending the condenser length and running at a favorable elevation manifests that the influence of condenser length is very small in gravity-driven mode, while in capillarity-gravity-driven mode, the effect of operating at a favorable elevation gets weakened, and the positive effects of the two factors cannot be simply calculated by the linear superposition principle. Additionally, reverse flow and flash evaporation were also observed and analyzed in depth. These results and conclusions can be used to guide the design of the condenser and the installation of LHP for ground applications.