Sang W. Joo

Yeungnam University

Stephen H. Davis and S. George Bankoff

Northwestern University

Abstract

When a pool of liquid is heated from below beyond a critical temperature, a complex sequence of boiling phenomena occur as the superheat is increased. The last stage of this sequence is the film boiling, where the liquid is no longer in contact with the heated bottom, but is separated by a continuous film of the vapor. When the superheat is reduced from the film-boiling state, the vapor film can disintegrate into isolated vapor bubbles, resulting in a transition-boiling (also called partial film boiling) state. There is still considerable interest in the accurate assessment of the critical heat flux and other physics associated with this evolution.

Experimental data reported so far indicate that near the lower temperature end of the film boiling intermittent contact between the liquid and the heated bottom occurs. Stability analysis of the vapor-liquid interface in film boiling also shows a spontaneous incipient rupture process of the vapor film. The liquid, upon touching the bottom, may either spread (toward transition boiling) or vaporize (toward film boiling). Dynamics of the interface between the liquid and the vapor near the onset of film boiling is the subject of the present investigation.

We develop two simple hydrodynamic models to analyze the evolution between transition and film boiling. One is based on inviscid theory for the liquid phase, and is used to predict the motion of a liquid column penetrating the vapor film and touching a heated bottom. The other applies a lubrication theory to thin vapor drops on a heated bottom surrounded by liquid. These models enable us to predict the motion of the contact line and thus the conditions for the transition between two boiling states .