Motivation and results of a long-term research on pool boiling heat transfer in low gravity

Abstract

This paper summarises the main results of a long-term research, begun more than ten years ago, about the influence of gravity and electric fields on heat transfer. After a somehow detailed analysis of the impact of heat transfer on modern society and of the motivation of low gravity research on this subject, the authors describe their own research work on the effects of gravitational and electric forces on single-phase convection and pool boiling. This research has the twofold aim to investigate the basic mechanisms of convective heat transfer, without the masking effect of gravity, and to identify methods to make free convection possible also in the absence of buoyancy, for space applications. It has been experimentally shown that the application of an external electric field generally enhances the heat exchange between a heated wire and a liquid pool. The single-phase heat transfer coefficient is improved, the nucleate boiling region is extended to higher heat fluxes, by increasing the critical heat flux, CHF, as well as the heat transfer rate in film boiling is augmented. The convective heat exchange is an increasing function of gravity, therefore heat transfer generally deteriorates in low gravity. The application of a sufficiently intense electric field restores the same value of heat transfer coefficient and critical heat flux measured on earth, thus demonstrating the progressive overwhelming of the electrical force on the buoyancy one.