The effects of superheat and surface roughness on boiling coefficients

Abstract

The effects of liquid superheat and surface roughness on boiling coefficients were investigated in a series of experiments in which water, acetone, n‐hexane, carbon tetrachloride, and carbon disulfide were boiled on a flat plate. In addition to the usual thermal measurements, the number of active boiling centers was determined, whenever possible, by visual means, and a quantitative measure of surface roughness was made. It was found that the number of active boiling centers on the plate increased with increasing surface roughness and that the calculated boiling coefficients were proportional to the one‐third power of the number of bubble columns rising from the heated surface. An equation has been derived relating boiling coefficients to fluid properties and the number of active boiling centers on a surface. A second equation, based on the theory of thermal fluctuations, has been proposed to relate the number of active boiling centers to the independent variables of surface‐roughness and temperature‐difference driving force. The limited data available have been found to follow this proposed relation. The results of this work suggest a quantitative method of relating the boiling coefficient to the character of the surface which may explain the discrepancies observed in the slopes of boiling curves reported in the literature and in the actual values reported for the boiling coefficients measured on different surfaces.