Experimental investigation of a capillary-assisted high performance evaporator

Abstract

A particular architecture for a miniaturized evaporator is proposed in which liquid is spread on the heat source using a thin capillary structure connected to two manifolds. The vapor produced by the heat source is extracted from the device using a separate outlet. A prototype has been designed, manufactured and integrated in a mechanically pumped loop in order to evaluate the evaporator efficiency in terms of heat transfer coefficient and induced pressure drop. In this paper, a capillary structure consisting in parallel rectangular micro-grooves is investigated with HFE7000 used as the working fluid. Results show that the evaporator is able to extract heat flux of the order of 10 W.cm-2 with heat transfer coefficients close to 104 W.m-2.K-1 while inducing pressure losses below a few hundred Pascal. Direct visualizations of the flow in the capillary structure allowed a precise characterization of the evaporator functioning mode, in particular the existence of two distinct two-phase flow regimes depending on the flooding of the capillary structure: nucleate boiling and liquid films evaporation. In order to predict the occurrence of these operating modes, a hydraulic model of the evaporator assembly has been developed and validated thanks to dedicated experimental campaigns. In particular, this model is able to predict the separation between the nucleate boiling and evaporation mode and highlights the fact that the capillary limit is not the main restricting factor in terms of the device maximum manageable heat flux.