引用本文:胡众欢, 杨明金, 杨卓然, 杨玲.基于多物理场耦合的热风干燥模型及其验证[J].西南大学学报(自然科学版),2020,42(2):118~128
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基于多物理场耦合的热风干燥模型及其验证
胡众欢, 杨明金, 杨卓然, 杨玲
西南大学 工程技术学院/丘陵山区农业装备重庆市重点实验室, 重庆 400715
摘要:
热风干燥是多物理场耦合的过程,存在热风外环境和物料内部湿热迁移共同作用.在质量和能量守恒定律的基础上应用达西定律、菲克定律、傅里叶导热定律,分别构建了热风干燥过程中物料外部与内部的流场、温度场、质量场的控制方程及模型,描述了热风干燥过程中整个干燥室内的湿热传递规律.针对油菜籽热风干燥过程,基于COMSOL Multiphysics对干燥模型进行求解并进行了油菜籽热风干燥实验,以验证模型的有效性.结果表明:物料干基含水率的模型求解结果与真实实验结果最大相对误差为13.3%;在干燥过程中物料存在干区、湿区、蒸发区之分,干区与湿区被蒸发区分开,且蒸发区逐渐由物料外部向物料内部迁移;干燥过程中干燥室内水蒸气浓度先增大后减小,且干燥室中心区域水蒸气浓度比干燥室边缘区域高;物料平均温度在干燥初期迅速上升,中期上升速度逐渐减小,后期趋于平稳且接近热风温度;干燥室边缘区域风速比中心区域风速大,热风流场在极短的时间内达到稳态,其中心区域风速接近为0.
关键词:  热风干燥  传热传质  多物理场耦合  多孔介质
DOI:10.13718/j.cnki.xdzk.2020.02.015
分类号:
基金项目:国家自然科学基金项目(31301575).
A Multiphysics Coupling-Based Mathematical Model of Hot-Air Drying and Its Verification
HU Zhong-huan, YANG Ming-jin, YANG Zhuo-ran, YANG Ling
School of Engineering and Technology/Chongqing Key Laboratory of Agricultural Equipment for Hilly and Mountainous Regions, Southwest University, Chongqing 400715, China
Abstract:
The mildew of grain during storage constitutes a major part of grain loss. The industry usually reduces the moisture content of the grain by drying, which reduces the loss caused by mildew. Among many drying methods, hot air drying has long occupied a large market share due to its advantages of simple operation, low cost, and low equipment requirements. Therefore, it is extremely necessary to optimize the hot air drying machines. During the optimization process, engineers often try to understand the distribution of physical fields such as temperature, wind speed and humidity in the drying machines. However, it is uneconomical to directly measure these physical fields from the drying machines, and there are great difficulties to measure them. Thus, based on the previous studies, this paper considers the influence of heat and mass transfer in the hot air drying process, and incorporates the fluid dynamics equations into the model framework, considering conservations of mass, energy and momentum. An analysis is carried out to establish mathematical equations based on the conservations, which fully describes the entire hot air drying process. In the text, four parts are used to introduce the whole mathematical model. (1) The hot air flow field adopts the fluid dynamics equation as the governing equation, which describes the transfer law of air outside and inside the material. (2) The temperature field is based on the law of conservation of energy, ignoring some minor thermal phenomena, and a heat exchange governing equation is constructed. (3) The gas phase transfer model of the moisture is based on the law of conservation of mass, and the phase change factor of moisture is introduced into the governing equation by means of source term. (4) The liquid phase transfer model of moisture is also based on the law of conservation of mass, whose governing equations have different signs on the source term than the gas phase transfer model. An experiment with rapeseed was carried out to verify the numerical simulation results of the model. The results showed that the maximum relative error between the model numerical simulation results and the real experimental results was 13. 3%, which indicated that the model could satisfactorily describe the hot air drying process. In addition, the numerical simulation results showed that a dry zone, a wet zone and an evaporation zone existed in the drying material during the drying process, the dry zone and the wet zone were separated by the evaporation zone, and the evaporation zone gradually migrated from the outside to the inside of the material. During the drying process, the concentration of water vapor in the drying chamber first increased and then decreased, and the concentration of water vapor was higher in the central area of the drying chamber than in the edge of the drying chamber. The average temperature of the material rose rapidly at the beginning of the experiment, and tended to be stable in the middle and late periods, which indicated that the material had a preheating time during the drying process, and the hot air temperature could be appropriately reduced in the middle and late drying period to reduce the energy consumption. The hot air flow field in the dry chamber reached a steady state in a very short time, which indicated that the steady solution of the hot air flow field could be directly used to calculate heat and mass transfer, for purpose of simulation cost saving.
Key words:  hot air drying  heat and mass transfer  multiphysics coupling  porous medium
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