Variation of Heat and Mass Transfer Coefficients During Drying of Granular Beds

During convective drying of initially fully saturated granular beds, the solid matrix is gradually exposed to the ambient air, resulting in heat transfer to both the liquid and solid. In an attempt to examine the heat and mass transfer processes occurring on the surface and to examine the influence of particle size and Bond number on the drying rate, experiments are performed in which granular beds constructed of spherical particles (which range from d = 0.2 mm to d = 25.4 mm) are convectively dried. For beds constructed of very small particles (d = 0.2 mm, Bo = 0.0035) the surface areas of the liquid and solid are difficult to estimate due to the random arrangement of the particles. The experimental results confirm existing knowledge that the evaporation rate is nearly constant during the funicular state of drying. For beds constructed of large particles (d = 25.4 mm, Bo = 21.7) an estimate of the surface areas of the liquid and solid reveals that the surface areas and the evaporation rate are highly dependent on surface saturation, contact angle, and surface tension. The results indicate that heat transfer from the solid to the liquid is significant and that the heat and mass transfer processes are not analogous.