Numerical Simulation And Structure Optimization Of Four-way Ventilation Mixed-flow Drying Section For Rice

Rice is one of the main foodstuffs in China.The annual rice production has exceeded 200 million tons for 11 consecutive years.To ensure yield and quality of grain and prevent the rice from mold,sprouting and some defects caused by environmental and other factors.Rice should be dried in time after harvest.Hot air drying method is the main way to dry rice in which heated air passes through the grain layers to reduce the moisture.At this stage,the one-way ventilation method is adopted by many tower-type hot air dryers.However,it leads to the phenomenon that rice cannot be dried evenly and produces local over-dried or under-dried rice.The quality and market value of rice would be severely affected by this drawback.In this paper,the four-way ventilation mixed-flow drying process of rice will be based.In this paper,the rice flow characteristics,flow field and other distributions in the drying section are analyzed by using EDEM and Fluent software.The structure of the ventilated angular tube in the drying section is optimized to improve the drying uniformity and quality of rice.Finally,the drying parameters are optimized to meet the requirements of uniformity for rice drying based on bench tests and analysis.The main research contents are listed as follows:(1)A mathematical model of coupled heat and moisture transfer within the rice layers is established based on the theory of four-way ventilation mixed-flow drying of rice,CFD as well as porous media principle.Comparative analysis was carried out by deep drying test and numerical simulation to verify the reliability of the mathematical model and simulation analysis method.The theoretical foundation is established for simulation analysis of the flow field,rice temperature and humidity when the rice is static in the drying section.(2)To study the flow characteristics of rice,the discrete element model of rice grain is established based on EDEM software.Simulations and experiment results showed that main characteristics when rice grain flows can be adequately predicted by the simulation study approach.The flow velocity of rice in the drying section is different and shows a spatial S-shaped motion trajectory.Rice has a large active area.The rice flow velocity is highest at the center region between adjacent angular tubes.This conclusion can provide a reference for the structural optimization of the drying section and numerical simulation analysis,etc.(3)Based on Fluent software,the optimal reducer angle of the double-side inlet angular tube was determined.The flow field,as well as temperature and humidity distribution of rice before and after optimized in the drying section at rest,is simulated and compared.The simulation results showed that the best reducer angle β of the double-side air inlet angle pipe was0.85 ° which meets the requirement of uniform air distribution in the drying section.Moreover,the maximum stress value and the total deformation of the angular tube are within the allowable range of the material.The temperature and humidity distribution in the flow field and rice showed obvious improvement after the optimized angle-shaped tube was applied.The test results of the air velocity field in the drying section indicated that the distribution of the air velocity field in the drying section was consistent with the numerical simulation results.After the double-sided air inlet reducer angle-shaped pipe was used in the drying section,the air velocity non-uniformity coefficients below the inlet angle pipe and exhaust angle pipe were reduced by about 10.25% and 3.70%,respectively.By adopting the double-side air inlet reducer angle-shaped pipe in the drying section can the uniformity of rice drying be promoted.(4)A four-way ventilation mixed-flow drying test bench was constructed.In order to study the effect of each factor on the drying characteristics of rice and to determine the parameter range.Single-factor test was applied to study and analyze the hot air temperature,the air velocity of the angular tube inlet and the initial moisture content.Quadratic rotation-orthogonal combination experiments and analyses were performed to study the hot air temperature,the air velocity at the inlet of the angular tube and the initial moisture content.The optimal combination of operating parameters obtained by Design-Expert 8.0.6 software are listed as follows: optimal hot air temperature,the inlet air velocity of the angular tube and the initial moisture content were calculated as 43℃,4.1 m/s and 18.2%.The test was conducted according to the optimized combination of parameters.The drying rate is 0.0186%/min,the value crackle-added ratio is1.7%,and the taste value is 80.33 points.The moisture non-uniformity of rice after drying was0.7%.By applying the parameters to the drying section can the rice be dried evenly which proves the practical application value of the drying section.