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The constrained vapor bubble experiment scheduled to fly aboard the International Space Station in the near future promises to give us new insight into the fundamental science of interfacial thermophysics. The evaporating meniscus formed at the corner of the vapor bubble is expected to behave in a significantly different manner in the microgravity environment as compared with the Earth's gravity environment. Since the constrained vapor bubble can also behave as a micro heat pipe, it will additionally help in gaining a technical understanding of the performance of a micro heat pipe in a space environment. Earth-based experiments have been conducted for the past two decades to gain a better knowledge of the rich phenomenon observed in the relatively simple constrained vapor bubble setup. Here, some recent Earth's-gravity-environment-based data obtained on a 30-mm-long constrained vapor bubble have been presented. The data were fitted to a model, and a self-consistent value of the inside heat transfer coefficient was obtained. The external convective and radiative heat transfer coefficients were also determined. These ground-based experiments form a calibration against which the future data from space-based experiments will be compared.

Microgravity experiments on a constrained vapor bubble heat exchanger (CVB) are being developed for the space station. Results are presented of precursory experiments and modeling on microscale transport processes in the evaporator of the vertical constrained vapor bubble in the Earth's environment. A nonisothermal experimental setup was designed and built to study heat transfer and fluid flow in an ethanol/quartz vertical CVB system. Temperature profiles were measured using an in situ Labview® data acquisition system via thermocouples. Film thickness profiles were measured using image analyzing interferometry. A mathematical model was developed to yield the liquid film curvature profile in the evaporator. Experimentally measured curvature profiles were in good agreement with modeling results. A theoretical relation between the curvature and temperature profiles in the Earth's environment was developed. Under microgravity conditions, an analytical expression that reveals an inherent relation between temperature and liquid film curvature profiles was derived for the first time.

The microscopic details of fluid flow and heat transfer in the contact line region of an evaporating curved liquid film were experimentally and theoretically evaluated. The evaporating film thickness profiles were measured optically using null ellipsometry and image analyzing interferometry. These thickness profiles were analyzed using the augmented Young-Laplace equation to obtain the pressure field. Using the liquid pressure field, the evaporative mass flux profile was obtained from a Kelvin -Clapeyron model for the local vapor pressure. A correlation for the local slope (apparent contact angle) at a film thickness of 6 = 20 nm as a function of a dimensionless contact line heat sink was thereby obtained for a group of completely wetting fluids. This change in local slope leads to a decrease in the maximum value of the possible capillary suction at the base of the meniscus. A complementary macroscopic interfacial force balance was also used to describe the effects of viscous losses and interfacial forces on the local values of the apparent contact angle and curvature that are functions of the film thickness and heat flux. These two perspectives give a complete description of an evaporating, nonpolar, completely wetting curved film in the contact line region.

Heat transfer by evaporation from a thin film is studied theoretically. The film thickness decreases with position and approaches an asymptotic value. The film is influenced by long-range iniermolecular forces, in particular van der Waals forces. According to the model, as well as previous models, these forces may partially suppress evaporation, locally, but may draw fluid into the thin film from a bulk pool, generally. The insulation effect of the fluid is included, whereas, capillary and thermocapillary effects are not considered. Analytical expressions are presented for the film slope, curvature, and flow in terms of the film thickness. The film is semi-infinite and steady, therefore, the flow at some position is equal to evaporation from the portion of the film downstream of that position and is proportional to the integral heat sink. Curvature may become quite large for small film thicknesses. An expression for the film thickness as a function of position is presented for the special case of a relatively strong insulation effect. Also presented is an engineering model relating the integral heat sink to a reduction in the Laplace pressure driving force for porous media flow, through a dynamic contact angle.