Microwave drying of softwood in an oversized waveguide: Theory and experiment

A comparison between experimental and theoretical results for the combined microwave and convective drying of softwood is presented. The microwave applicator used for the experiments was an oversized waveguide, and the results for both sapwood and heartwood were analyzed. To elucidate on the physics of the process at a fundamental level, a complete model is proposed, which considers the intricate link that transpires between the heat- and mass-transfer phenomena and the power distribution throughout the sample during drying. The resulting model, which uses a comprehensive 2-D set of equations to describe the drying process, together with a complete 3-D solution of the Maxwell equations within the waveguide in the time domain, can be used to investigate many aspects of dielectric drying. This research deals with the spatial variation of the power density within the material at various drying times and the effect of the anisotropy of the transfer properties on the shape and evolution of the power distribution. Most importantly, it focuses on the prediction of the location of hot spots and thermal runaway within the sample from the viewpoint of product quality. Strengths and weaknesses of the model are highlighted.