Abstract
Visualization experiments are conducted to explore the condensation process in sintered copper wicks of operating flat-plate heat pipes. Two working fluids, water and methanol, and two wick types, mesh and powder wicks, are investigated. The effect of non-condensable gas has been carefully controlled at an insignificant level. The steady-state condenser resistances R c and the evaporator resistances R e are measured at incremented heat loads Q. For water, cyclic dropwise condensation is observed on exposed wick tops due to the finite water/copper contact angle. For methanol, the condenser is submerged in the liquid as a result of zero contact angle and ineffective liquid delivery out of the condenser. This leads to quiescent methanol condensation with higher R c s. For all the tests, R c s (in Kcm 2 /W) are several times larger than R e s. The main reasons are the thicker liquid layer in the condenser wick and the large thermal resistance associated with the condensed liquid layer, with the latter dominating. Considerable under-prediction of R c may exist in current theoretical models which simply applies the conduction-limit model without considering the thermal resistance from the condensed-liquid layer. © 2013 Elsevier Ltd. All rights reserved.