Numerical simulation and optimum design on airflow distribution chamber of air-impingement jet dryer

Abstract: The airflow distribution chamber is an important part of the air-impingement jet dryer. Non-uniform distribution of airflow field can cause inconsistence of materials quality, prolong the drying process and consume more energy. Therefore, it is essential to optimize the flow field structure. However, the traditional way of design-manufacture-further improvements requires long transformation time, high costs and the measurement range is usually disappointing. Computational Fluid Dynamics (CFD) can provide detailed information on airflow patterns and ensure convenient design of agricultural equipments. In this paper, the Computational Fluid Dynamics (CFD) was first used to simulate the inner flow of structures similar to airflow distribution chamber based on the differential equation and RNG k-ε turbulence model. The original structure was a cuboid and the inlet boundary was set to velocity-inlet. The speed deviation ratio(E) and the non-uniformity coefficient(M) were chosen as comprehensive evaluation indicators. The velocity and pressure distributions in chamber flow field were obtained and used to analyze the improved designs based on the original structure. They were inclination models, semi-cylindrical models and flat vortex models. The simulation results indicated that two symmetrical reverse vortex zones were formed in original structure, which led to the nozzle exit velocity first decreases and then increases along the height direction. The airflow velocity of round nozzles ranged from 11.9 to 17.7 m/s under the design condition and the minimum value was got in the eighth row. The E and M values of original structure was calculated to be 24.6% and 18.1%, respectively. This illustrates that the original structure was far from perfect. Decreasing the width of chamber bottom could not improve the distribution of airflow field, while the spoiler models were proved feasible. The flat vortex models built a greater effect on the optimization of airflow distribution than semi-cylindrical models. And the E and M values of the flat models dramatically decreased. It maybe because several uniform vortex zones formed between flats, which had a positive impact on airflow distribution. The optimum solution was got under the flat vortex model with 160 mm and 14 mm as the value of the plate-to-plate distance and the height difference, respectively. Moreover, the airflow velocity merely ranged from 13.1 to 15.3 m/s. The distribution trend of simulation results showed little difference compared to the experiment data. The maximum relative error under different conditions changed from 4.2% to 8% showing an increasing trend as mass flow increased. The results provide a reference for the uniformity design of structures similar to the airflow distribution chamber.