Film boiling of subcooled liquids. Part II: Steady regimes of subcooled liquids film boiling

As it was shown in the first part of the present study [1], during film boiling of highly subcooled water the particular regime of boiling occurred revealed first in the works [2,3], where the term “micro-bubble boiling” was introduced for this regime. In order to understand general regularities of the cooling process in film boiling it is reasonable to conduct investigations with different subcooled liquids. The present paper presents new systematic data on cooling the metal spheres in subcooled isopropanol and in perfluorohexane (FC-72) at very high subcoolings, up to 160 K. About thirty experimental runs with isopropanol and eight runs with perfluorohexane clearly showed that at any subcooling value the micro-bubble boiling regime with high intensity of heat transfer was not observed. Stable film boiling occurred to be the dominant cooling regime; heat transfer intensity in this regime was approximately two-three times greater than in saturated film boiling at the sphere surface. An approximate model of heat transfer in stable film boiling of subcooled liquids has been derived; it accounted for an influence of liquid movement at the vapor/liquid interface due to natural convection effect. The predicting equation obtained, with two empirical numerical factors, manifests good agreement with the experimental data of the present study including the new ones on subcooled ethanol pool boiling and with the available results of other researchers. The experiments showed that film boiling of subcooled liquids is sensible to sphere surface characteristics; some surface defects, even visually undetectable can promote more intensive local heat transfer intensity.