Film boiling collapse in solid spheres immersed in a sub-cooled liquid

Abstract

The present study examines the thermal reaction phenomenon associated with the penetration of a hot metal solid sphere into a cold liquid reservoir. The effect of various parameters on the sudden collapse of the vapor film around the sphere is studied. A pre-heated metal sphere, made of aluminum, copper, or steel having different dimensions and a variable initial temperature, has been immersed in a varied degree of sub-cooled water. The evolution of the time-dependent temperature of the solid body has been recorded. Simultaneously, the film collapsing process has been recorded with a video camera. It is observed that above a certain sub-cooling degree, film collapsing noise (vapor explosion) and higher heat transfer rates are involved. The temperature, at which the film collapses, the quenching temperature, seems to be dependent on the sub-cooling degree, on the sphere material, and on the sphere size. We found that the steam explosion is followed by an apparent “golf-ball” like boiling period. This occurs between the transition and the film boiling regimes. During the “golf-ball” boiling period, the sphere surface appears to be covered by a large number of concaved cells having a characteristic dimension of about 3 mm. In order to explore the possible effect of the hot surface curvature of the immersed body, generalization of the pool boiling hydrodynamic theory for MHF conditions has been developed for arbitrarily curved heater geometries. The MHF is related to the mass rate of the vapor leaving the vapor film under these conditions. An account for the excess pressure acting on the base of the bubble arises from the excess pressure of the vapor film, due its curvature, has been added. The applicability of this model is presented for a horizontal plate, horizontal cylinders, and spheres. The model seems to fit well available experimental data for the different systems.