This study presents a comparative numerical solution of a conjugate-transient three-dimensional heat and mass transfer problem between a solid desiccant (silica gel) and a humid transient-laminar air stream in ducts with different cross sectional geometries: square, circular, and triangular. The problem is solved by using a finite control-volume method, and validated relative to available experimental data. The effects of the u velocity gradient normal to the wall (∂u/∂y) and the pressure drop (Δp) on the heat and mass transport for the three ducts are investigated. In duct flows, the results show that the average (∂u/∂y) for the triangular duct is 6.6% and 19.6% larger than that in the circular and square ducts, respectively; therefore, the triangular duct provides the largest convective heat and mass transport, and absorbs 11% and 42% more water than the circular and square ducts, respectively. At the same time, the average pressure drop for the triangular duct is 69% and 73.5% larger than the one for the square and circular ducts, respectively, which would result in higher fan power consumption. Using a figure of merit (Wr) that is a ratio of the required fan work to the dehumidification attained, the circular duct was found to be the best and the triangular the worst among the three geometries compared.

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