Analysis and structure optimization of the temperature and flow fields of the belt dryer with multi-temperature zones

Abstract:Variable temperature during drying can usually be utilized to improve agricultural product quality in recent years. However, it is difficult to control intentionally the temperature distribution inside traditional multi-layer belt dryers. In this study, a novel type of belt dryer was proposed to select proper structural parameters of the air distribution chamber, further evaluating the performance of the new dryer. Four layers of conveyor belts were also added into the air distribution chamber, under which to supply directly hot air for drying products. Firstly, computational fluid dynamics (CFD) was used to optimize the structural parameters of air outlets in a single air distribution chamber, thereby reducing the velocity non-uniformity coefficient (VNUC). The specific structural parameters included the diameter of the outlet orifice, the distance between orifices, and the arrangement of orifices. Secondly, a CFD model was developed for the whole belt dryer with optimized structure parameters of air distribution chambers. Specifically, the drying material was selected as Camellia oleifera seeds, which were granular suitable for belt dryers. A porous media model was also adopted to evaluate the drying effect of seeds on airflow distribution, considering that the material absorbed the heat from surrounding air, according to the drying kinetics of the material. Thirdly, two typical profiles of air temperature were determined for the inlet of each air distribution chamber, including the inlet temperature increasing layer by layer from top to bottom, while the decreasing counterpart. Finally, two evaluation indicators were defined, including the temperature non-uniformity coefficient (TNUC) and temperature stratification deviation (TSD). The TNUC was used to evaluate temperature uniformity adjacent to each conveyor belt, while the TSD indicated to what extent the actual temperature deviated from the set one. The simulation results show that the distance between orifices presented the most significant effect on air distribution. There was no marked effect on the diameter of the outlet orifice, and the arrangement of orifices. Subsequently, the optimal combination of structural parameters was obtained, where the minimum VNUC was achieved concurrently. Additionally, the independent temperature control can be expected to be effectively realized for each layer inside the dryer during the simulation. In particular, the TNUC of each layer from top to bottom were 3.08%, 2.00%, 1.89%, and 1.60% under the decreasing inlet temperature profile, whereas, 2.37%, 2.04%, 2.42%, and 3.31% under the increasing one. Specifically, the TSDs were 4.94% and 3.57% under the above two profiles, respectively. Furthermore, it was also found that the uniformity and deviation of temperature can be further improved by installing the deflector horizontally in the middle of each layer. Consequently, the deflector decreased the TNUC of each layer by 21.1%、13.0%、31.2%、66.3% for the decreasing profiles, and 34.2%、24.0%、29.3%、51.7% for the increasing one, respectively, whereas, the TSDs were reduced by 10.9% and 10.1%, respectively, compared with the original structure. This finding can provide a valuable reference for the multi-layer belt dryers to perform variable temperature drying with independent temperature control for different zones.