Physical Aspects of Foaming in Steam Generation

Abstract

Vaporization of pure liquids is reviewed as a problem of heat transfer and the rate of vapor bubble creation. Applying an idea of Gibbs, the influence upon the activation energy of nucleus formation exerted by the contact angle at the vapor/liquid/solid phase boundaries as well as by the shape and size of submerged solids is quantitatively expressed by the reduction in the volume requirement of the nucleus. Possible effects of solutes in superheated liquids upon steam bubble creation and growth are discussed. The stabilization of foam in boiling electrolyte solutions is explained by the thermoelectric potential differences originating from the temperature gradient around growing bubbles. The effect of foam inhibiting agents (e.g., castor oil) is interpreted as a change in the rate of bubble creation caused by surface conditioning of heating elements and suspended particles.