Gas-phase unsteadiness and its influence on droplet vaporization in sub- and super-critical environments

This paper aims to investigate quantitatively the influence of gas-phase un steadiness on the droplet vaporization process in sub- and super-critical e nvironments. Two comprehensive models of high-pressure droplet vaporization , including a transient model and another assuming gas-phase quasi-steadine ss, are presented. Both models are first compared with experimental data an d then used to calculate vaporization processes of single droplets of diffe rent initial sizes for environmental conditions in which the ambient pressu re and temperature range from 1-150 atm and 500-2000 K, respectively. The u nsteady effects are quantified by introducing characteristic time scale rat ios. It is shown that strong gas-phase unsteadiness exists during the early period of the vaporization process. The unsteadiness attains a maximum val ue in the gas near the droplet surface and decreases quickly to a nearly st eady value within a short distance from the surface. With increasing ambien t pressure, the unsteadiness increases nearly linearly at low ambient tempe ratures and rapidly at high ambient temperature. Gas-phase unsteadiness als o increases with increasing ambient temperature and is affected even more s trongly by temperature. Compared to the transient model, the quasi-steady m odel predicts a smaller regression rate initially and a larger regression r ate during the later period. The differences between the predicted regressi on rates, and thus between the predicted vaporization processes, are magnif ied with increasing ambient temperatures and/or pressures. The vaporization process predicted using the quasi-steady model reaches the critical mixing state earlier than that predicted using the transient model. These conclus ions also apply for the vaporization processes of single droplets of differ ent initial sizes. (C) 2001 Elsevier Science Ltd. All rights reserved.