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Investigation of Particle and Air Flows in a Mixed-Flow Dryer

Even though the mixed-flow dryer is well established on the commercial market for the drying of grain, maize, and rice, there further potential as well as a need to optimize the dryer apparatus and to improve product quality. Unfavorable designs can cause uneven mass flow and air flow distributions, resulting in locally different drying conditions and, hence, uneven grain drying. The aim of the present article is to evaluate traditional designs of mixed-flow dryers by numerical and experimental investigation of particle and air flows and to discover design deficits. For this purpose, the dryer geometry and different air duct arrangements (horizontal and diagonal) were studied using the discrete element method (DEM) and computational fluid dynamics (CFD). Drying experiments were performed to evaluate the grain moisture and temperature distributions. With regard to particle flow, a typical core flow was detected as in silos with a retarded particle flow at the dryer walls and a fast flow region in the center of the dryer. This was caused by the wall friction effect and the half air ducts fixed at the side walls. With regard to the air flow, dead zones were discovered for the diagonal air duct arrangement. Based on the design deficits identified for the traditional geometry, a new geometry for the mixed-flow dryer that is still under development is discussed.