The pool boiling heat transfer enhancement from experiments with binary mixtures and porous heating covers

Abstract

Porous heating covers and positive binary mixture composition reduce the bubbles’ size and their coalescence, while increasing the liquid supply towards the heating surface, causing an increase of the convective heat transfer coefficient for pool boiling. The convective heat transfer coefficient h was calculated for boiling of pure water and three aqueous mixtures with 12%, 16%, and 20% weight of ethanol on seven different porous coverings on the heating element. Maximum h values were 50 kW/m 2 K for pure water, and 65 kW/m 2 K for ethanol–water mixtures on the smooth surface of the heating element; using the porous coverings, 180 kW/m 2 K for boiling of water and 220 kW/m 2 K for boiling of ethanol mixtures were obtained. The maximum values of h correspond to the composition of 16% ethanol (on each porous covering), to pore-surface-densities of 45 pores per square inch and a covering thickness of 5 mm. Greater thicknesses decreased the values of h for the same number of pores per square inch expressing an internal resistance for vapor release. For limited vapor-release conditions, the bubbles grew with no increase in the h values, while for resistance-free vapor escape the bubbles radii increased almost linearly. A 3 2 factorial design generated a response surface and its analysis of variance, which confirmed that the most important factors, defining the values of h , are the porous covering construction and its pore-surface density.