| Grain drying is a unit operation of spontaneous water removal under the complex condition of multiple disturbance factors,and is the key link of post-harvest handling post and quality assurance.The heat and mass transfer and conversion of drying system have essential changes due to the influences of drying conditions,physical parameters,flow pattern,drying process,heating mode,mechanical structure,inertia characteristics and process characteristic parameters,which restricts the development of adaptive control technology and effective utilization of energy in drying system.As a result,the problems of high energy consumption and low efficiency still exist in grain drying.Therefore,this paper focuses on the goal of energy-saving and efficient drying,and takes the development of drying analytical theory and reasonable matching of drying system energy as the main task.Combined with dimensional analysis,the analytical models of drying system were established based on thermodynamics theory and conservation law of energy and mass.And starting from the energy structure,the subjective and objective energies were analyzed theoretically.Using the research method of theoretical analysis and experiment,the dynamic change law of process characteristic parameters was explored,and the change characteristics of energy was revealed based on the specific grain drying system.Meanwhile,the energy efficiency of drying system was evaluated.The creative conclusions of this study are as follows:1.Based on the thermodynamics theory and the conservation law of energy and mass,and the introduction of dimensionless thickness,dimensionless time and mass ratio of air volume grain,the analytical solutions and change law of the states of grain and drying medium were proposed in three kinds of steady-state drying processes,namely,stationary-bed,concurrent-flow and countercurrent.The characteristics of three drying processes were compared,and the results of which show that the best way to meet the energy matching requirements of grain drying is countercurrent drying.2.Based on rheological properties of solid materials and theories of poroelasticity and Agricultural materials,the experimental equipment for analyzing dynamic change of porosity in flowing grain layer was constructed.Moreover,the intersectant relations between the porosity and many factors,namely,material flow pattern,structure size and physical parameters,had been studied in depth.The results show that the porosity increased with the increase of paddy velocity,and was significantly larger than the porosity of the static paddy layer;the volume of paddy grain and interparticle void decreased with the decrease of moisture content,whereas porosity of the grain was significantly increased,and the increased porosity of the paddy grain was larger than the reduced interparticle void.Thus,the porosity increased with moisture content decrease;porosity of the flowing paddy layer tended to decrease with thickness of the paddy layer increase.3.The interactions between porosity and velocity,moisture content and thickness of paddy layer were revealed.Thickness of the paddy layer,paddy velocity and moisture content significantly(p < 0.01)affected the porosity,but moisture content had the most significant(p < 0.0001)effect on the porosity.The interaction of these factors did not have a significant effect on the porosity.4.A function of porosity of flowing paddy layer was developed.Statistical analysis showed that this fitted function had good agreement and reliability.Based on Ergun equation and experimental apparatus for measuring the airflow resistance,the verify test results show that the average relative difference between the practical measured values and the predicted values of pressure drop was 5.33%.It can be seen that the feasibility of the function of the porosity was further validated.Thus,the proposed function may be reliably used to predict the porosity of a flowing paddy layer.5.Based on the drying theory,thermodynamics knowledge and psychrometric chart of air,and starting from the energy structure of drying system,the theoretical analysis results of the subjective energy were analyzed by experiments.During the multistage counter-flow drying process,the heat loss of exhaust in drying chamber accounted for 0.17%～0.93%of the total thermal energy consumed in dryer,and its change trend was gradually increasing.The ratio of heat loss of paddy warming to total thermal energy consumed in dryer was ranged from 0.09% to 0.75%,and the heat loss rate of paddy warming was gradually decreased in this range,while the heat loss of paddy warming was the largest in the first high-temperature drying chamber.Other heat loss rate ranged from 0.074% to0.65%,and its value showed a gradual increase in high and low temperature drying segments.Heat loss rate of evaporated water vapor warming was in the range of 0.004%-0.013%,which deduced that heat loss rate of evaporated water vapor warming was the minimum in each drying chamber,and the change trend of this heat loss first decreased and then increased.Compared to the other three heat losses,heat loss of evaporated water vapor warming was relatively small.6.In the initial period of drying,low-temperature and low-humidity drying medium should be used for paddy drying because heat loss rate for paddy warming is the largest in first high temperature chamber.In this way,the difference of partial pressure of water vapor can be increased to realize the purpose of quickly removing part of water in paddy,also the heat loss for paddy warming can be reduced.Other heat loss in the high-temperature drying segment accounted for a larger proportion.Hence,in order to reduce this heat loss and improve the utilization rate of thermal energy of drying system effectively,thermal insulation should be done to the wall of high-temperature drying segment during the process of mechanical structure designing.The heat loss of exhaust was one of the main energy losses in the process of paddy drying.Thus,a device of waste heat recovery should be designed in paddy drying system.So that waste heat in exhaust air discharged from drying chamber can be fully utilized,and energy consumption cost can be reduced.7.Under different drying conditions,the extents of these four heat losses(heat loss of exhaust gas,heat losses of paddy and evaporated water vapour warming,and other heat loss)were different.Therefore,in the process of energy consumption optimization of paddy drying,scientific and reasonable optimization measures should be presented combined with the analyses of heat loss characteristics of different parts in drying chamber,so as to fundamentally optimize energy consumption and improve utilization efficiency of energy.8.Based on theoretical analysis and cyclic drying experiment,the results of this study show that the energy consumed in drying system included subjective energy and objective energy,and the energy loss degree of drying chamber increased with the decrease of moisture content.With the decrease of moisture content,the water removal power and objective energy of corn was decreased,while the moisture binding energy in corn grain was increased gradually.At the same time,the unscientific nature of the traditional evaluation method of thermal efficiency,and the essential difference between it and exergy efficiency were revealed,which shows that exergy efficiency was a scientific method to evaluate the energy efficiency of grain drying system. |
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