Interfacial instability, convective structure, and heat transfer in liquid films undergoing phase change

This work examines the fluid mechanical and heat transfer characteristics of evaporating and condensing films in a planar geometry and is motivated by a desire to reveal the physics behind liquid films undergoing phase change, especially the connection between the convective structure and the heat transfer through the liquid film. These films play important roles in a variety of terrestrial and space-based engineering applications. Cyclically condensing and evaporating films, condensing films subject to constant subcooling, non-volatile, heated films, evaporating films subject to steady superheat, evaporating films subject to an impulsively imposed superheat, and films evaporating into air were examined.;With the exception of the cyclically varying experiments, all configurations were upward-facing. Except the non-volatile and open-air tests, all experiments took place in absence of non-condensable gases. The degree of superheating or subcooling was controlled by regulating the system pressure. A new, non-intrusive ultrasound technique was developed for the measurement of film thickness. A double-pass schlieren imaging system and pressure and temperature measurements completed the diagnostics. Six working fluids were used (n-pentane, dichloromethane, chloroform, diethyl ether, acetone, and methanol).;The primary conclusions are briefly summarized as follows: (1) The ultrasound thickness measurement system proved to be accurate and precise to +/-10% and +1 microm respectively and was capable of measuring film thicknesses as little as 8microm. (2) In cyclically varying films the heat flux matches well with previous results and the rise in heat flux at the onset of Rayleigh-Taylor instability coincides with a decrease in median film thickness. (3) Quasi-steady evaporating films subject to constant superheat exhibit a progression of convective structures that does not appear to be dependent on the fluid properties or the degree of superheat. The changes in convective structure correlated with changes in measured heat flux. The critical Rayleigh number is 1580+176, consistent with the value predicted for buoyancy driven films. The measured heat flux and the convective instability nature appear to depend essentially on buoyancy effects only. (4) Films of sufficient thickness subject to impulsive superheat display initial behavior that is independent of the thermal boundary conditions of the substrate.