| The microscopic details of fluid flow, heat and mass transfer in a curved thin film in a loop Constrained Vapor Bubble (CVB) were experimentally and theoretically studied. A novel experimental setup with an in situ LabView data acquisition system was designed and built to study the microscale transport processes of evaporating and condensing pentane and ethanol films. Two novel optical techniques were developed to measure the thin film thickness profile for studies of interfacial phenomena in the three phase contact line region: (1) Double Wavelength Interferometry was developed to measure not only the non-uniform film thickness in the transition region, but also the equilibrium uniform thickness in the thin film region. The dispersion constant and the disjoining pressure were obtained for the pentane/quartz system; (2) An optical technique to obtain simultaneously curvatures and apparent contact angles for both a curved thin film and a microdrop was developed.; Microscale transport processes in the evaporator and condenser regions for both stable and oscillating thin films were studied. A mathematical model was developed to predict curvature profiles for a stable film in the evaporator. For microgravity conditions, an analytical expression, which reveals an inherent relation between temperature and curvature profiles, was derived. The stable film becomes unstable when the heat input is increased to a certain level. The onset of instability, apparent contact angle and interfacial curvature vary with axial location and heat input. Different phenomena associated with the instabilities of pentane and ethanol films were compared. The oscillating phenomena were described using a macroscopic interfacial force balance that relates viscous losses to interfacial forces and apparent contact angles.; The slow growth characteristics of a condensing ethanol sessile drop in the condenser section were also studied using interference microscopy. A Kelvin-Clapeyron model of interfacial mass flux was used to obtain the interfacial temperature difference. The results demonstrated that the curvature, apparent contact angle, interfacial subcooling, interfacial mass flux, spreading velocity, and adsorption are coupled at the moving contact line. Nusselt correlations were derived for the region of film condensation and the flow from the drop to the meniscus at the moment of merging. |
