A Numerical Investigation of the Natural Convection Heat and Mass Transfer From Uniformly Heated Falling Films in Vertical Channels

In the design of solar collector/regenerators for use in open-cycle absorption refrigeration (OCAR), the problem of predicting evaporation rates and solution temperatures is of paramount importance in determining overall cycle performance. This transport of heat and mass is dominated by natural convection with buoyant forces primarily generated as a result of film heating by the solar flux, but aided by the evaporation of water (the lighter species) into the rising moist air stream. In order to better understand the mechanism of these combined buoyant interactions, the governing equations for natural convection flow in a vertical channel bounded by a heated falling film (simulating a glazed collector/regenerator) were solved using several different finite difference techniques. Under the assumptions appropriate to solar collector/regenerator falling films, the gas-side problem may be uncoupled from the falling film side, with the interfacial conditions treated as unknown boundary conditions for the gas-side flow which are iteratively determined from the film mass and energy balances. The numerical results were validated against existing experimental and numerical results for simplified boundary conditions. The appropriate nondimensionalization for the falling film boundary condition was established, ostensibly for the first time, and a parametric study for an air-water vapor mixture was conducted. Curve fits to the numerical results were determined for engineering design applications.