Explosive vaporization of superheated liquids by boiling fronts

Up to 2 l of metastable propane, butane, refrigerant R-134a and water were released from glass receptacles without nucleation sites and expanded to atmospheric pressure over a range of initial superheats created by the sudden depressurization. Above a certain superheat threshold, vaporization occurred only in a thin surface zone of intense boiling and liquid fragmentation. This boiling front traveled from the free surface into the bulk of the superheated liquid and ejected a high-velocity vapor/liquid stream. For pipe sizes in the range from 14 to 80 mm, no significant influence of the cross-sectional area on the front velocity was noted. The complex interaction of vaporization and fragmentation of the superheated liquid at the boiling front appeared as a self-amplifying process, as also noted by others. Below a certain superheat threshold, there was no front propagation, and the vaporization could not carry away surplus liquid. The two-phase flow created at the boiling front had a velocity significantly lower than that expected from isentropic phase change. The pressure created by the acceleration of the two-phase mixture reduces the superheat in the liquid and attenuates the phenomena, as experiments with orifices have shown. An extensive non-dimensional analysis of the data was conducted. The threshold for boiling front creation and the front velocity were correlated in terms of the relevant thermophysical properties and the superheat; the data of other investigators agreed well with the proposed new correlations.