Heat Transfer and Convective Structure of Evaporating Films under Pressure-Modulated Conditions

Abstract

This work examines the fluid mechanical and heat transfer characteristics of evaporating films under cyclical superheat conditions. This research was motivated by the need to further understand the instability drivers in films undergoing unsteady and cyclical evaporation. The superheat was controlled modulating the system pressure. An isolated test cell allowed the films to evaporate into their own vapor without non-condensable present. A non-intrusive thickness measurement technique was used to yield dynamic heat flux measurements. A double pass schlieren system was employed to capture convective structures. System temperature and pressure measurements completed the diagnostics. The primary conclusions are briefly summarized as follows: * The evolution of thermal profile within evaporating films has a strong impact on the development of convective structure and heat transfer. In some cases convective structure appears within the film under pressure-modulated conditions even when the evaporation intervals are sufficiently short that conduction is expected to be the only heat transfer mode within the film. * Convective structure appears to persist in many cases even after evaporation is stopped. * Stopping the evaporation for short time intervals appears to have a negligible effect on the temperature profile in the film based on the subsequent evaporation behavior. * Complex, multi-wavelength convective structure behavior can be induced through cyclical superheating of the films. * A modest gain in short-term heat flux is achievable under some pressure-modulated conditions. * Surface instabilities of quasi-steady evaporating films do not lead to an increase in the evaporation rate. * Reduced gravity tests were seriously compromised by unsteady g-levels and g-jitter.