When a falling liquid film flows from one horizontal tube to another below it, its mode (droplets, jets, or a sheet) has a significant impact on the hydrodynamics and heat transfer. There is no complete model or empirical basis for predicting the falling-film mode, its transitions, or effects. The proposed research is directed at understanding which physical parameters govern the mode and its transitions, quantifying the coupling between mode and heat transfer, and providing a complete basis for predicting mode behavior. In this experimental program, a liquid film falls between two heated or cooled specimen tubes in a thermally controlled environment. Measurements of film location, thickness, and velocity, along with measurements of the surrounding vapor temperature, pressure, velocity, and heat transfer, characterize the falling film and its influence. Experiments are conducted with different working media, tube diameters and spacings, and in quiescent and forced flows, with varying heat fluxes, liquid mass flow rates, and thermodynamic conditions. Candidate parameters are examined for their influence on transitions and hysteresis. Coupling between the mode and heat transfer are studied, to develop a basis for evaluating the mode and its effects.
