Evolution of Convective Structure and Heat Transfer in Evaporating Films Under Cyclic Superheating Conditions

Abstract

The convective structure and heat transfer characteristics of evaporating films under conditions of cyclical superheating were examined experimentally. An isolated closed test cell allowed the films to evaporate into their own vapor without noncondensable gases present. The level of superheat was controlled by modulating the system pressure. A double-pass schlieren system was employed to capture convective structures, and a nonintrusive, ultrasonic thickness measurement technique was used to yield dynamic heat flux measurements at the film surface. The evolution of the superheat level has a strong impact on the development of convective structures and heat transfer. In some cases, convective structures appear within the film under pressure-modulated conditions even when the evaporation intervals are sufficiently short that conduction is the dominant heat transfer mode. Convective structures persist in many cases after evaporation is stopped. Stopping the evaporation for short time intervals has a negligible effect on the convective structure. Complex, multiwavelength convective structure behavior can be induced through cyclical superheating of the films.