Explosive vaporization of superheated liquids by boiling fronts

Up to 2 1 of metastable propane, butane, refrigerant R-134a and water were released from glass receptacles without nucleation sites and expanded to at mospheric pressure over a range of initial superheats created by the sudden depressurization. Above a certain superheat threshold, vaporization occurr ed 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 sup erheated liquid and ejected a high-velocity vapor/liquid stream. For pipe s izes in the range from 14 to 80 mm, no significant influence of the cross-s ectional area on the front velocity was noted. The complex interaction of v aporization and fragmentation of the superheated liquid at the boiling fron t appeared as a self-amplifying process, as also noted by others. Below a c ertain superheat threshold, there was no front propagation, and the vaporiz ation could not carry away surplus liquid. The two-phase flow created at th e boiling front had a velocity significantly lower than that expected from isentropic phase change. The pressure created by the acceleration of the tw o-phase mixture reduces the superheat in the liquid and attenuates the phen omena, as experiments with orifices have shown. An extensive non-dimensiona l analysis of the data was conducted. The threshold for boiling front creat ion and the front velocity were correlated in terms of the relevant thermop hysical properties and the superheat; the data of other investigators agree d well with the proposed new correlations. (C) 2001 Elsevier Science Ltd. A ll rights reserved.