Analysis of heating uniformity considering microwave transmission in stacked bulk of granular materials on a turntable in microwave ovens

Heating uniformity inside materials, as a concerned issue of microwave processing, influences essentially the energy utilization of microwave equipment and desired quality of the final product. For the microwave drying of granular materials like germinated brown rice (GBR) on a turntable in microwave ovens, the mode and route of microwave transmission considering sample rotation and the existence of pores inside GBR stacked bulk were investigated, and further to analyze the heating uniformity. The developed physics-based model and simulation strategy can describe microwave drying of GBR stacked bulk. The edge diffraction and uneven surface reflection for the stacked bulk of GBR dominates the transmission of microwaves. Under the fixed magnetron arrangement, same microwave input power and geometry shape of the sample, the absorption of microwave energy inside GBR stacked bulk mainly relies on the changes in dielectric properties of materials other than those of the electric field. The absorption of microwave energy in GBR stacked bulk greatly dominates the uniformity of temperature distribution. Rotation improves the uniformity of temperature distribution along with equatorial other than radial direction for GBR stacked bulk on a turntable under microwave drying. Sensitivity analysis indicated that increasing rotating speed of the sample has little effect on the heating uniformity as beyond a certain value of 5 circles per minute, and a low bed porosity for GBR stacked bulk relatively improve the heating uniformity, but temperature uniformity is relatively not sensitive to the capillary diffusivity. Research results may enhance understanding of the interaction between microwaves and the stacked bulk of granular materials like GBR with porosity.