The Influence of Surface Conditions in Nucleate Boiling

Abstract

<p>A theoretical and experimental study of the influence of surface conditions in nucleate boiling is presented. The surface conditions are represented by the density and distribution of the active nucleation sites as well as the size distribution of the cavities which constitute the nucleation sites.</p> <p>One of the important boiling parameters known to be a function of the nucleation cavity size is the frequency of bubble departure. A theoretical model is formulated to predict the bubble frequency as a function of the nucleation cavity radius as well as the surface superheat and liquid subcooling in which the time variations of the surface temperature throughout the bubble cycle are incorporated. Parametric study of this model shows that the frequency of bubble departure decreases with decrease of surface superheat and increase of liquid subcooling, a trend which agrees with the published data. It is also shown that smaller nucleation cavities are able to emit vapour bubbles with higher frequency than that corresponding to larger cavities. Experimental results obtained by boiling water and isopropyl alcohol on a single copper surface having two different surface finishes showed good agreement with the theoretical model.</p> <p>In addition, the concept of the bubble flux density is introduced. The bubble flux density is defined as the rate of bubble emission per unit area of the boiling surface and a method of evaluating it as a function of bubble frequency and active site distribution is proposed. A uniform correlation between the boiling heat flux and the bubble flux density is found to exist for a particular solid-liquid combination irrespective of the surface finish within the region of isolated bubbles.</p>