Numerical Simulation Study On Gas-Solid Two-Phase Flow And Heat Transfer Characteristics In Pneumatic Drying Pipe

Pneumatic drying is one type of convective heat transfer drying. In the pneumatic drying process, the materials are dispersed in the heated air to achieve both functions of transporting and drying. As the materials to be dried are uniformly suspended in the air, which leads to large contact area between the two phases, enhancing the heat and mass transfer process. This advantage makes the pneumatic drying widely used in industry and agriculture.The pneumatic drying method has been used to dry rapeseed cake. In this thesis, the physical properties were firstly studied, the density, particle size distribution, specific heat capacity and thermal conductivity were measured. The drying curve and drying rate curve were plotted. Meanwhile, the relationship between moisture content and specific heat capacity and thermal conductivity were analyzed.The Computational Fluid Dynamics (CFD) software package Fluent was used to carry out numerical researches into the flow and heat transfer characteristics about the drying process of rapeseed cake particles in two typical drying pipes:vertical pipe and U-type pipe, and the Eulerian model was used. In the terms of flow characteristics, adopting the pressure drop as the index, in the terms of heat transfer characteristics, adopting the heat transfer coefficient as the other index. The effects of different working conditions on both indexes were analyzed.When other parameters were set the same, for the vertical pipe, different working conditions were analyzed- including the effects of different gas temperature, gas velocity, solid loading ratio and particle diameter on the pressure drop. Meanwhile, the effects of these parameters on the heat transfer coefficient were also analyzed. For the U-type pipe, the effects of different gas temperature, gas velocity, solid loading ratio, particle diameter and bend radius ratio on the pressure drop were analyzed, and the effects of these parameters on the heat transfer coefficient in the U-type pipe were also analyzed. The results show:1. For the vertical pipe, the total pressure drop increases with the increase of gas velocity and solid loading ratio, it decreases with the increase of particle diameter, gas temperature has no impact on the pressure drop; the heat transfer coefficient increases with the decrease of particle diameter, it also increases with the increase of gas velocity, the solid loading ratio and gas temperature have no impact on the heat transfer coefficient. 2. For the U-type pipe, the total pressure drop increases with the increase of gas velocity and solid loading ratio, it decreases with the increase of particle diameter and bend radius ratio, gas temperature has no impact on the pressure drop; In the bent part, the heat transfer coefficient increases with the decrease of solid loading ratio and particle diameter, and it increases with the increase of gas velocity. The gas temperature has no impact on the heat transfer coefficient, and bend radius ratio has little impact on it.The CFD method is used to make numerical simualtion about pneumatic drying, and the results obtained can provide theoretical foundation for optimization of the structural design and operating parameters for pneumatic dryer.