气体射流冲击干燥机气流分配室流场模拟与结构优化

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

为了改善气体射流冲击干燥机气流分配室流场结构,提高喷管速度分布均匀性,以气体运动微分方程和RNGk-ε湍流模型为基础,利用计算流体动力学软件Fluent对气流分配室内气流流场进行了三维数值模拟,得到了热气流在气流分配室内的流动特征,并对原模型结构进行改进,提出了3类优化方法,同时将最优模型的预测值与试验数据进行了比较。计算结果表明,气流分配室原始结构的速度矢量在气流腔室内部形成2个左右对称的反向涡流区,对应喷管出口速度分布沿高度方向呈先减小后增加的趋势,设计工况下速度偏差比和速度不均匀系数分别达到24.6%和18.1%;减小分配室下端宽度这一常见思路并不能改善气流分配室的速度分布均匀性,而扰流模型则被证明是可行的;平板扰流模型的效果优于半圆柱扰流模型,其最佳结构参数为平板间距为160mm且第一块平板较喷管轴线高14mm,速度偏差比降为7.7%,而速度不均匀系数仅为4.7%,数值模拟结果与试验数据最大偏差不超过8%。该文的研究思路对类似于干燥机气流分配室结构的均匀性设计提供了参考。

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-εturbulence 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.