【作者】 金辉；

【导师】 朱明；

【作者基本信息】 华中农业大学 ， 农业机械化工程， 2010， 硕士

【摘要】 气流干燥是对流传热干燥方式的一种。在此过程中,被干燥物料分散在气体中,使得气固两相接触面积较大,传热与传质过程得到强化,气力输送和干燥两种功能被同时实现。此种优点使得气流干燥在工农业上得到了广泛应用。对油菜籽饼粕的干燥采用气流干燥法。本文首先对油菜籽饼粕的物性进行了研究,测定了油菜籽饼粕的密度、粒度分布、比热容和导热系数,绘制了油菜籽饼粕在恒温常压下的干燥曲线与干燥速率曲线图,分析了比热容和导热系数随含水率变化的关系。应用计算流体力学软件Fluent中的欧拉模型,对油菜籽饼粕颗粒在两种典型的干燥管—竖直管和U形管中的流动和传热特性进行了数值模拟研究,得到了气固两相在两种类型管中的与流动和传热有关的物理量的场。在流动方面,以压力降作为指标；在传热特性方面,以传热系数作为指标,分析了不同工况下两个指标的差异。在其它变量参数设置相同的情况下,针对竖直管,分析了进气温度、进气速度、固气比和颗粒粒径等因素对压降的影响,同时分析了这些因素对竖直管内传热系数的影响；针对U形管,分析了进气温度、进气速度、固气比、颗粒粒径和弯曲半径比等因素对压降的影响,同时分析了这些因素对U形管内传热系数的影响。结果表明：(1)在竖直管中,压降随进气速度和固气比的增大而增大,随颗粒粒径的增大而减小,进气温度的变化对压降基本无影响；传热系数随颗粒粒径的减小和进气速度的增大而增大,而固气比和进气温度的变化则对传热系数基本无影响。(2)在U形管中,压降也随进气速度和固气比的增大而增大,随颗粒粒径的增大而减小,单位长度压降随弯曲半径比的增大而减小,进气温度的变化对压降基本无影响；在U形管弯曲部分处,传热系数随固气比和颗粒粒径的减小而增大,随进气速度的增大而增大,进气温度的变化对传热系数基本无影响,弯曲半径比的变化对传热系数影响不大。本文运用CFD方法对气流干燥进行数值模拟,所得结论可为气流干燥器的结构和操作参数的优化提供理论依据。更多还原

【Abstract】 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.更多还原