The mechanism of nucleate boiling in pure liquids and in binary mixtures—part IV: Surface boiling

Abstract

A favourable effect of the composition in binary mixtures with a more volatile component on the peak flux in surface boiling of subcooled liquids with free convection has been predicted by the “relaxation microlayer” theory, cf. Parts I-III. The theoretical values are in quantitative agreement with the author's experimental data on the systems water-methylethylketone and water-l-butanol. Expressions, for the peak flux and for the maximum bubble radius, frequency, growth and condensation rates, and lifetime have been derived in dependence on the wire superheating and on the degree of sub-cooling. An increase in the subsequent condensation rate in certain binary mixtures is predicted. At present, no experimental data on bubbles in subcooled mixtures are available, but the theoretical predictions for pure liquids are in reasonably good agreement with some preliminary data by the author and with data on water by Ellion for free convection and by Gunther for forced circulation. The occurrence of a minimal critical expansion ratio for supersaturated vapour mixtures in presence of air in Wilson cloud chambers is explained by, and the numerical value of the minimum ratio can be calculated from, the new theory. The corresponding liquid concentration in the droplets is shown to coincide with the concentration of maximal slowing down of bubble growth and maximum peak flux density in superheated liquid mixtures. This is in good agreement with data by Flood on water-ethanol vapour and by Froemke, Bloomquist and Anderson on water-ethanol. Finally, a note is added on the ultrasonic velocity and absorption in binary liquid mixtures.