Experimental Study of Meniscus Thin-Film Evaporation in a Horizontal-Tube, Falling-Film Heat Exchanger, Focusing on Dynamic Contact Angle Variation of an Advancing Meniscus
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Meniscus dynamics were studied with respect to thin-film evaporation, specifically focusing on a horizontal-tube, falling-film heat exchanger. Experiments were carried out investigating the effects of solution subcooling, evaporator wall superheat, and porous-layer coating on the portions of sensible and evaporation heat transfer in a falling-film system. The porous layer was found to enhance both sensible and evaporation heat transfer by exploiting the capillary pumping of the porous matrix, which worked to passively spread the solution fluid across the entire tube surface. The thin film created in the many menisci in the porous layer worked to further enhance evaporation due to the lowering thermal resistance as the meniscus approaches the triple-phase contact line. This phenomenon was found to be more pronounced in evaporation dominant conditions, i.e. low solution subcooling, high wall superheat, and low solution flow rate. Geometric and fluidic properties also have a strong influence on meniscus formation, affecting parameters such as permeability, porosity, effective pore radius, and capillary pumping power. Dynamic contact angle behavior, which determines the component of capillary force in the flow direction, was investigated through a capillary rise experiment. Close-up photographs of the meniscus profile were captured at various rise heights and the contact angle was found through specific image processing techniques. The instantaneous contact angle was also calculated explicitly via a pressure balance about the liquid-vapor interface. Finally, a dimensionless analysis was performed, showing how and why certain forces dominate capillary rise and contact angle formation for a given set of experimental conditions. This analysis was then extended to past research as a means showing how different parameters such as pore radius and working fluid can affect the overall influence of inertial, viscous, gravitational, and capillary forces on meniscus formation and dynamic contact angle variation.