Bubble dynamics and nucleate pool boiling heat transfer on microporous copper surfaces

Abstract

Nucleate pool boiling experiments were performed on microporous copper surfaces and plain surfaces using saturated HFE-7100 as the working fluid. Quantitative measurements of the bubble dynamics, such as the nucleation site density, bubble diameter at departure, and bubble departure frequency, were obtained using high-speed visualization. The microporous surfaces, with coating thicknesses in the range of 100–700μm, porosity of 55–60%, and cavity sizes in the range of 0.5–5μm, showed a significantly lower boiling incipience temperature, which enhanced the heat transfer coefficient by 50–270% and enhanced the critical heat fluxes by 33–60% when compared to the plain surface. At low heat flux levels, the surface with a thicker microporous coating showed better performance than the thinner one. However, the thinner microporous coating resulted in higher critical heat flux than the thicker surface. The site density, departure diameter, and departure frequency were compared against the predictions using various correlations from the literature. Based on a heat flux partition model, using the measured values of the active site density and bubble departure diameter and frequency, and neglecting the single-phase heat transfer effects of bubble coalescence, the individual modes of heat transfer (evaporative, quenching, and convective) were computed. Reasonably good agreement between the partition model results and the experimental data was obtained. On the plain surfaces, the evaporative and quenching components were approximately equal. On the microporous surfaces, the evaporative component was found to be significantly higher.