Abstract
The purpose of this paper is to form a more general understanding of the effect an electric field has on boiling heat transfer by considering non-boiling electroconvection and boiling bubble dynamics separately. In an attempt to decouple these two heat transfer mechanisms, an electric field was utilized which produced a uniform dielectrophoretic force (DEP force) across a horizontal platinum wire. Boiling curves were produced with FC-72 as the working fluid for variable DEP and buoyancy forces by varying the electric field geometry (with a constant potential of 23 kV), by varying the orientation of the electric field with respect to gravity, and by performing experiments in a drop tower with very small buoyancy forces. In order to elucidate the relative contributions of the individual forces and correlate the data, an effective gravity g′(b,e) was defined which represents the ratio of the total DEP and buoyancy body forces on the vapor bubbles to the constant terrestrial-gravity buoyancy force.
It was concluded that the effect of the DEP force on the bubbles is analogous to reducing or increasing the gravity locally or inducing vapor flow across the heater surface similar to forced-convection. In semis of the relationship between the bubble dynamics and the heat transfer, it was concluded that nucleate boiling heat transfer will be enhanced if the effective gravity acts to hold the vapor bubbles near the heater surface, while at the same time permitting access of the liquid to the surface in order to prevent dryout. However, a large electroconvective effect can dominate and possibly reverse this trend. For the critical heat flux (CHF) it was discovered that for 1 < g′(b,e) < 3 a quarter power dependence is a reasonable engineering approximation for the increase in CHF with effective gravity.
Based on these results and support from the literature, we concluded that the overall heat transfer coefficient for boiling in the presence of an electric field can be considered as the summation of a heat transfer coefficient due to bubble dynamics and a heat transfer coefficient due to electroconvection. Furthermore, the heat transfer coefficient due to the bubble dynamics can be modeled with current theory based on variable gravity results and/or forced convection.
