Multi-field Coupled Hot Air Drying Characteristics Of Multilayer Corn Seeds

There are problems of high energy consumption,low efficiency and serious deterioration of corn in the hot air drying process.Exploring its heat and mass exchange characteristics will not only deepen the research mechanism,but also lay the theoretical foundation for improving the drying process,optimising the drying method and increasing the energy efficiency of drying.The currently available non-conjugate models,while accurately physically modelling corn kernels,do not take into account the influence of the surrounding flow field on their drying characteristics.Whereas the conjugate model explores the effect of the flow field on the heat and mass transfer characteristics of corn,but in terms of a physical model that reduces the material to a simple three-dimensional graph.Both studies have been carried out on individual corn kernels,and the problem of non-uniformity in the drying of multiple materials in practice has not been explored.Based on the above problems,this paper constructs a three-dimensional multilayer corn seed hot air drying conjugate model based on a single corn seed solid model,considering the fluid domain around the material and coupling CFD with a heat and mass transfer model.The model was used to investigate the hot air drying characteristics of multi-layered corn seeds,their drying inhomogeneities and the factors affecting them.Firstly,to summarise the shortcomings of the existing test system,a multi-condition controllable hot air drying test stand was designed and built.The air is heated by spraying step by step to test the process of cooling and dehumidification,equal enthalpy humidification and equal moisture content heating,thus achieving a stable and continuously controllable drying condition.The test bench was used to carry out multi-layer corn seed hot air drying tests and based on the test data to investigate the drying conditions with the lowest energy consumption in terms of drying ratio from the material perspective.The results show that the optimum working conditions for hot air drying of corn are air temperature around 75°C and air speed around 1.50 m/s at a certain relative humidity.Secondly,a three-layer corn seed hot air drying conjugate model was constructed using several corn kernels as the study object.In order to improve the accuracy of the physical model,a three-dimensional solid model of a single corn grain was used as the basis,and three physical models of multi-layered corn grains with different arrangements were developed.The air domain around the material is also modelled,and the interaction between the internal temperature and humidity fields of the corn grain and the velocity,temperature and relative humidity fields of the surrounding air during drying is considered,coupling CFD with heat and mass transfer to construct a conjugate model.The process of hot air drying of multilayer corn seeds was simulated using COMSOL Multiphysics software and the model was validated using experimental results.Then,the overall drying characteristics and drying inhomogeneities of multilayer corn seeds were investigated based on a simulation model.During the first 75 min of the drying process,the temperature gradient and the concentration gradient together facilitate mass transfer.In the mid to late stages of drying,the main driver of moisture diffusion is the concentration gradient.Because of the hot air and material contact time sequence and cold wet air deposition effect,hot air inlet side above the region of the fastest drying materials,the exit side of the slowest drying below.The drying rate of the top layer of material is greater than the bottom layer,and the difference gradually increases along the direction of air flow.To compare the differences in moisture content of corn seeds at different locations,a dimensionless evaluation index,the drying imbalance（ε）,was proposed.In general,εhas a tendency to rise rapidly in the pre-drying period and to fall slowly in the middle and late stages before gradually rising.Finally,the effects of inlet air temperature(T_in,air),inlet air relative humidity(RH_in,air)and air speed(u_in,air)on the overall drying characteristics and drying inhomogeneity of corn kernels were analysed.The higher the T_in,air,the faster the drying rate,and the greater theεat the same moment,the more pronounced the decreasing trend in the medium term.At the same time,the drying rate tends to decrease as RH_in,air increases,withεdecreasing at the same moment and a clear downward trend in the medium term.In addition,the drying rate tends to increase as u_in,air rises,while at the same timeεdecreases and the downward trend increases in the medium term.The average drying rate of corn kernels increased by 76.81%when T_in,air was increased from 45°C to 75°C at 300 min,with an increase of about 0.1507 per 10°C;an increase in RH_in,air from 30%to 60%reduced the average drying rate of corn kernels by 5.70%and reducedεby 0.2441;u_in,air increased from 0.50m/s to 2.00 m/s and the average drying rate of corn kernels increased by 11.71%andεdecreased by 0.4639.