Abstract
The evaporation process in the wick of an operating flat-plate heat pipe was studied for three different working fluids having widely different figures of merit: deionized water, methanol and acetone. A sintered two-layer 100 + 200 mesh copper wick was adopted. Uniform heating was applied to the copper base plate near one end, and cooling with a water jacket at the other end. The evaporator resistance was determined using the temperature difference between the copper plate and the vapor respectively under and above the evaporator center. While the steady-state evaporation process was visualized through the upper glass plate of the heat pipe, the evaporator resistances were measured simultaneously. The maximum heat loads for water are far greater than those of methanol and acetone, with their values correlating well with their figures of merit. The minimum evaporator resistances for the three working fluids differ slightly in spite of the large differences in their properties. The differences can be mainly attributed to the different average thicknesses of the evaporating liquid layers, according to visualization. While nucleate boiling was not observed for water, very weak and localized nucleate boiling was observed for methanol near the maximum heat loads, beyond which local dryout began. For acetone, slow, periodic up-and-down motion of the liquid layer as a result of nucleate boiling was observed near the maximum heat loads over a significant portion of the evaporator. Beyond the maximum heat loads, nucleate boiling was again suppressed. © 2011 Elsevier Masson SAS. All rights reserved.