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Numerical Solution

The control-volume-based code FLUENT was used to carry out the two-dimensional (2D) computational simulations in this study. The governing equations were then spatially discretized using a second-order upwind scheme. To avoid solution divergence, small time steps on the order of 1 x 10-4 were chosen, except for energy and species balance for which convergence was set to occur when the residuals fell below 1 x 10-6. To obtain numerical solutions independent of the grid size, 2550 grids were used. Figure 5.1 shows the calculated air temperature exiting from the fluidized bed dryer as a function of time.

Figure 5.2 shows the average water content in the particles in the bed as a function of time. This figure clearly shows that, during the constant rate period, the moisture content is almost linearly decreasing at a higher rate toward the critical moisture content (0.012 kgw/kgs or water content of 1.2%). After the moisture content of the particles in the bed reached the critical moisture content level, the remaining moisture inside the particles was removed by the diffusion process (falling rate period) at a lower rate. About 70 % of the initial moisture was on the surface of the particles and in large surface pores (removed during the constant drying rate period). The remaining 20 % of the moisture was in the porous structure of the particles (mainly removed during the falling drying rate period). The rest of the 10 % moisture remained as the equilibrium moisture content.

 
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