Uniform Drying Of Grain Based On Flow Segregation And Granular Convection

Uniform drying is the key to saving drying energy consumption and realizing the deep-bed drying.In this work,a method for deep-bed grain drying based on flow segregation and granular convection is proposed.Water diffusion behavior of grains during drying,flow segregation of accumulated mixed grains and granular convection under vibration excitation are combined to break the thin layer limitation of existing drying technology and realize the uniform drying of deep bed.To obtain the quantitative contribution of water binding forms,water content,and temperature to the water diffusion,the migration and diffusion characteristics of different water components in grains were investigated through LF-NMR and MRI.The results show that for the fresh wheat particles with an initial moisture content of about 0.4(d.b.),at the early drying stage,various bound water are largely converted into free water,and the conversion amount of bound water was higher than the water evaporation amount of particles.The internal water migration rate is less than the external water evaporation rate.In the middle and late drying stages,the bound water with low binding energy is preferentially converted,resulting in the continuous increase of the average binding energy of various bound water,the conversion amount of bound water is gradually less than the water evaporation amount of particles.The internal water migration rate gradually became the main control factor of the drying rate.Results are helpful to the selection of drying temperature and drying methods.For particles with large size difference,to solve the problem of ununiform drying caused by different size,a moving granular bed that can automatically separate the mixture of particles with different size was designed,and achieving the size segregation of fresh grains before entering the drying equipment in this work.The effects of operating parameters and particle properties on size segregation were studied.Results show that increasing the free slope length is beneficial to improve the degree of segregation,while an increase of mass fraction of large particles at the inlet weaken the degree of segregation.The influence of flow rate,density,and surface roughness of particles within a certain range on the degree of segregation can be ignored.The differences in heating temperature,heating time,and moisture content of gas in deep bed are the main reasons of ununiform drying.By introducing vibration into the deep-bed grain,the combination of vibration and airflow enhances the granular convection and improves the uniformity of drying conditions.Considering the shape characteristics of most grains,the granular convection in an ellipsoidal granular system under the action of vibration and airflow was experimentally studied.Results showed that increasing frequency and amplitude can both increase the convection area and strengthen the convection intensity.The wheat particles were more likely to enter the global convection state under the action of the airflow in the opposite direction of gravity,while the maximum convection intensity of wheat particles under the airflow in the opposite direction of gravity was approximately 30%-35%of the value measured under the airflow along the direction of gravity.Numerical results showed that convection consists of collision and deceleration processes.The granular system forms longitudinal displacement differences in the deceleration process,and completes the lateral migration in the collision process.A method to predict the critical vibration parameters of granular convection was presented and the physical mechanism of convection patterns transformation was explained.Results are helpful to realize the artificial control of convection patterns.Finally,the effects of airflow direction,airflow velocity,tempering and especially vibration on the drying uniformity in deep-bed wheat drying were quantitatively analyzed.Compared with the drying of alternating flow without vibration,the degree of ununiform drying is reduced 72.0%when vibration and alternating flow are combined.