Analysis of convective heat and mass transfer coefficients for convective drying of a porous flat plate by conjugate modelling

Abstract

Convective drying of an unsaturated porous flat plate at low Reynolds numbers (10 3) is analysed by means of conjugate modelling of heat and mass transport in the air flow and the porous material. Conjugate modelling does not require knowledge of convective transfer coefficients (CTCs) but allows determining the CTCs a posteriori, hence identifying their spatial and temporal variability, which is the main focus of this study. Comparison is made with porous-material modelling using spatially and/or temporally constant CTCs. Both spatial and temporal variations of the convective boundary conditions are found to have a distinct impact on the drying behaviour: the spatial CTC variation results in two-dimensional drying of the porous material due to leading-edge effects; the temporal CTC variation identifies distinct maxima in the drying rates at the surface right before the surface dries out locally. The CTCs obtained with conjugate modelling remain approximately constant during the constant and decreasing drying rate period (CDRP and DDRP, respectively), but a distinct variation is noticed at the transition of CDRP to DDRP. The heat and mass transfer analogy is found to be valid during the CDRP and to a lesser extent during the DDRP but large discrepancies are found during the transition of CDRP to DDRP. The advantages of conjugate modelling are indicated in this study but the need for detailed modelling of convective boundary conditions is however strongly dependent on the drying behaviour of the porous material and is not always required.