Transient boiling heat transfer from initial steady state caused by rapid depressurization

Abstract

Transient boiling on single horizontal cylinders in water caused by rapid depressurization from initial natural convection or partial nucleate boiling was investigated. For an initial heat flux and a pressure reduction period respectively larger and shorter than certain threshold values, a transition to film boiling was observed. In a rapid transient due to depressurization from initial natural convection, the transient boiling heat transfer process followed a hypothetical nucleate boiling curve at the higher superheat side of the successive steady state nucleate boiling curves corresponding to each transient pressure. The maximum heat flux value was far lower than the steady state maximum heat flux at the corresponding pressure. The effects of various parameters, namely initial heat flux, pressure reduction rate, initial pressure and cylinder diameter, on the transient heat transfer processes (nucleate boiling heat transfer, maximum heat flux, transition to film boiling or not, film boiling heat transfer) were examined. A mechanism based on heterogeneous spontaneous nucleation from the originally flooded cavities on the solid surface was suggested for the transition from non-boiling to film boiling under rapid depressurization.