Investigations of liquid flashing and evaporation due to sudden depressurization

Abstract The thermal-hydraulic response of a quiescent liquid layer to sudden depressurization was examined. Freon-11 at room temperature was used as a test liquid. The degrees of depressurization were such that the liquid bulk became superheated and flashing occurred. Two sets of experiments were conducted. In the first, a 20.3 cm Mach-Zehnder interferometer, high-speed photography, and associated equipment were used to record a succession of interferograms of the flashing liquid. These showed a heterogeneous mixture of superheated, saturated, and subcooled liquid. The degree of subcooling corresponded to the saturation temperature at approximately twice the pressure drop. Initial conditions varied from 97.2 to 101.0 kPa pressure and 19.3 to 21.1°C temperature, and depressurizations varied from 11.4 to 27.0 kPa. Liquid superheats up to 5.7° C and subcooling as much as 9.2°C below saturation were noted. The second set of experiments were conducted to determine mass transfer under flashing conditions. These showed mass transfer rates during flashing to be 10–12 times thosedue to evaporation alone. This is due to the agitation of the liquid as well as to the presence of entrained drops of subcooled liquid in the departing vapor. An empirical ‘flashing factor’, F, defined as the ratio of the mass transfer found in flashing and in evaporation was determined and given for the conditions of the experiments by F = 27.5−0.527 T 0 −0.129ΔP , where T0 is the initial temperature (°C) and ΔP the pressure drop imposed (kPa).