COMBINED HEAT AND MASS TRANSFER NATURAL CONVECTION BETWEEN PARALLEL PLATES (LAMINAR)

A two-dimensional analysis of steady laminar natural convection in an open ended parallel plate channel is presented for combined heat and mass transfer. Symmetric and asymmetric boundary conditions of uniform wall temperature and concentration (UWT/C) and uniform heat and mass flux (UH/MF) are considered for both aiding and opposing buoyancy due to mass transfer. An original contribution has been made in the following areas.; Closed form solutions for inclined parallel plates are obtained in the limit of fully developed flow. For UWT/C, the nondimensional velocity, temperature and concentration are independent of Prandtl, Schmidt and Grashof numbers. For UH/MF, an analytical solution can be obtained only for Pr = Sc. By assuming a purely parabolic velocity profile, an approximate solution for general Pr and Sc is obtained that reduces to the analytical solution for Pr = Sc. The highest Nusselt and Sherwood numbers occur for asymmetric boundary conditions, opposite of the results for forced convection duct flows.; A numerical analysis of the developing flow regime is presented for the vertical case. A method of enforcing local and global conservation of mass is developed for asymmetric channel flow. The Rayleigh number limit for backflow at the channel exit and the limits for fully developed flow and single plate behavior are established.; The UH/MF fully developed flow parameter is useful for scaling the UH/MF and (in modified form) UWT/C developing flow results as well. At high Rayleigh number, the symmetric parallel plate channel has higher heat and mass transfer than the equivalent problem for free convection from a single plate.; The numerical method is then applied to model a glazed collector/regenerator component of an open-cycle solar absorption refrigeration system. The solution is coupled to an energy and mass balance on a thin film absorbent flow along one wall. The results indicate that the influence of glazing height is small if the flow is between the fully developed and single plate limits. The glazed collector/regenerator water evaporation rate is higher relative to the unglazed case because the reduction in convective and radiative heat losses increase the absorbent temperature and vapor pressure sufficiently to overcome the concomitant reduction in the mass transfer coefficient.