Experimental study on transient heat/mass transfer characteristics during static flash of aqueous NaCl solution

Abstract

The instant height/temperature of waterfilm were selected as a pair of characteristic parameters to study the transient heat/mass transfer properties during static film flash of aqueous NaCl solution though experiments with waterfilm concentration ranging between 0 and 0.15 (mass fraction), superheat between 5.1 and 35.0 K, and initial height of waterfilm between 0.10 and 0.20 m. The heat transfer mainly happened at bubble interface within waterfilm. The mass transfer included two steps of vaporization at bubble interface and vapor overflow from top free surface. The transient intensity of heat transfer was measured by volumetric evaporation coefficient. It shared the same experience with volumetric void fraction of quick increase to a peak value and gradual falling back. But its change always lagged behind that of void fraction. The transient intensity of overflow was measured by overflow speed of flash vapor. It was found to increase with rising superheat or initial height of waterfilm, but dropped significantly with increasing waterfilm concentration. On basis of these results, a calculation model for static film flash was set up. The evolution of waterfilm temperature/height could be computed simultaneously from given evolution of flash chamber pressure and other necessary initial conditions.