Effect of Flow-Duct Geometry on Solid Desiccant Dehumidification

Abstract

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 (Ap) 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 that for the square and circular ducts, respectively, which would result in higher fan power consumption. Using a figure of merit (W r ) 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 duct was found to be the worst among the three geometries compared. The results obtained from this study can also be useful for designing desiccant ducts, as they give guidance about duct length optimization and duct cross-sectional shape considering both dehumidification effectiveness and flow pressure drop.