Destabilization of film boiling

Some recent results are reviewed on the destabilization of film boiling, both under quasi-steady and fast-transient conditions, and with both solid-liquid and liquid-liquid systems. Interest has been generated in this phenomenon in connection with the triggering of vapor explosions and the rewetting of hot dry surfaces. Several theoretical and experimental models are used in calculations: an adsorption theory for the minimum film boiling temperature, data for the critical Weber number for destabilization, a correlation for the calculated minimum vapor film thickness for coalescence, and data on the heat transfer following vapor film collapse on a nickel tube. A three-stage model is presented for destabilization: thinning of the vapor film; partial contact by tongues of liquid and spreading of the contact regions. It is found that the minimum film boiling temperature for UO2/sodium is above the estimated boiling point of UO2, which means that the rapid fragmentation following thereon will be on a time scale too long for sustained propagation of a thermal detonation wave. It is also found that relatively small shock overpressures produce large transient heat fluxes, even at 'contact' temperatures above the critical temperature, and that nearly all large vapor explosions fall into this category.