Convective Structure and Heat Transfer in Transient, Evaporating Films

Abstract

The fluid mechanics and heat transfer of evaporating planar films under unsteady conditions were studied experimentally. The initially quiescent films were subjected to superheating by rapidly dropping the pressure, and the films evaporated into their own vapor only. The test fluids were various hydrocarbons with film thicknesses of 1–5 mm. Ultrasound ranging was used to measure changes in the film thickness, from which the heat flux at the liquid surface was determined. Double-pass schlieren imaging was employed to image the convection patterns in the film. Two distinct rises in the heat flux were observed. The initial rise in heat flux was associated with the transient superheating and was not accompanied by convective structure within the film. The second rise in heat flux was associated with a substantial evolution in the convective structure and the associated wavelength. The penetration depth of the convective structures in films undergoing transient evaporation was estimated based upon the wavelength-to-depth ratio versus Rayleigh number trends observed in quasi-steady experiments. The estimated penetration depth was less than the actual film depth, suggesting that the wall boundary condition was not important in transient, evaporating films at the onset of convection.