Numerical simulation of capillary evaporator with microgap in a loop heat pipe

Abstract

A mathematical model to investigate the heat transfer characteristics of a capillary evaporator in a loop heat pipe with a microgap between the case and the wick was developed, and the gap effect on the evaporator heat-transfer coefficient is discussed. The model was validated by comparison with experimental results obtained for a polytetrafluoroethylene–ethanol loop heat pipe. Calculated and experimental results agreed well below a 100 μm gap distance, with both approaches indicating the existence of an optimal gap between 0 and 50 μm. We clarify why the heat-transfer coefficient had a local maximum against the gap distance. The effect of the gap is determined for a change in heat flux to the evaporator, wick shape, working fluid, and thermal conductivity of the wick. The simulation showed that an optimal gap exists on the evaporator, except for the stainless steel wick. The microgap evaporator was workable for a wick made from a low thermal conductivity material. When a microgap is introduced in the loop heat pipe, the evaporator heat-transfer coefficient can be enhanced, hysteresis of the liquid–vapor interface in a porous structure can be prevented, and the evaporator thermal performance can be predicted easily. Therefore, the microgap is an effective approach for thermal performance enhancement.