| Grain drying is a process which heat and mass exchange spontaneously among materials and air.Drying system behave complicatedly because of fluctuations of heat supply,air flow,material flow,environmental state and transformation of physical properties.It is difficult to measure the system states accurately and comprehensively by sensors,therefore,it is a effective method to predict the behavior of drying system by mathematical models.However,the existing models based on phenomenological theory are not good enough,which restricts the development of dynamic control technology and equipment of grain drying,and leads to the problems of low automation,high energy consumption and poor quality.In this study,a 3D mathematical model was developed for deep-bed grain drying system based on balance of heat and mass,it was solved by finite difference method with first order upwind difference scheme.A algorithm was programmed on matlab and the solves of grain drying for different processes,structures and conditions were presented.Based on the modern control theory,the internal model controler(IMC)of drying processes was developed,and the on-line detection technologies of moisture content were improved by combining with theoretical calculation and production practice,the research work and innovation of this paper are as follows:1.11 dimensionless numbers of drying process were figured out based on the П theorem.By introducing dimensionless time τ,dimensionless thickness ε and air-grain quantity ratio π,the dimensionless solutions of stationary-bed,concurrent-flow,couter-flow and cross-flow drying process were presented.The results showed that the moisture ratio of stationary drying process varies with time and position,and the drying rate increased and then decreased along air flow.The moisture ratio through the concurrent-flow drying bed decreased quickly and then slowly along grain flow,the moisture ratio through counter-flow drying bed decreased slowly and then quickly along the grain flow,and the average moisture ratio through cross-flow drying bed decreased steadily along grain flow.Compared with three typical flow drying processes,in the counter-flow drying bed,high moisture rice met low temperature air,and low moisture rice met high temperature air,that accorded with demand of drying energy2.Based on the theory of heat and mass transfer,the transient model of three-dimensional deep bed drying was developed by Euler space description method,and the first order upwind difference scheme was used to solve the PDE,and the solution algorithm was programmed on matlab.The error analysis results showed that the smaller the difference step size was,the more the numerical solution tended to be the analytical solution.When the difference step size was 0.5% of the total size,the root mean square error of numerical solution of stationary drying was less than 0.501%,the root mean square error of numerical solution of concurrent-flow drying was less than 0.012%,the root mean square error of counter-flow drying was less than 0.022%,and the root mean square error of cross-flow drying was less than 0.011%.The convergence of the numerical solution was verified.Based on the transient model,the multi-dimensional deep-bed drying under variable conditions were analyzed.The results showed that In concurrent-flow drying bed,the air cooled gradually and absorbed moisture when passed through drying layer,and then reached equilibrium with grain,the state of outlet air would not change any more when the layer thickness increased to a certain extent.In counter-flow drying bed,the air cooled gradually and absorbed moisture,and tended to saturation when passed through drying layer.When temperature of air reduced below the dew point,the moisture of air would be precipitated into grain.The analytical results of three-dimensional problems showed that the flow state of air and grain had a significant impact on drying quality.In the area that air velocity was faster,the air temperature was higher,the moisture content was lower,and the relative drying rate is higher.In flow bed drying,the moisture content of discharge grain was higher in the area that air velocity was faster.It meant that the nonuniform distribution of drying would be aggravated because of unreasonable flow design,and the uniformity of drying can be effectively improved by optimizing the structural parameters of drying bed.3.The multi-stage counter-flow dryer(5HNH-15)of rice was analyzed.The calculation results showed that when the operation condition was constant,moisture content alternated between descent-phase and plateau-phase with position in drying bed,grain temperature alternated between descent-phase and ascent-phase with position in drying bed.The model was validated by performing experiments in a multistage counter-flow rice dryer(5HNH-15).The maximum deviation of the predicted value of moisture content from the measured value at the fixed point was 0.41%d.b.,and the root mean square error was 0.20%d.b.the maximum deviation of the predicted value of grain temperature from the measured value was 2.3℃,and the root mean square error was 1.1℃,the model predictions of air humidity and temperature at outlet vs.drying time were observed to be close to the measured values in the drying experiments.4.The circulation counter-flow rice dryer(5HNH-15)was analyzed.The numerical simulation results showed that in counter-flow drying stages,moisture content decreased and grain temperature rise along the grain-flow,humidity increased and air temperature decreased along the air-flow;high moisture grain versus low temperature air,low moisture grain versus high temperature air,which accorded with demand of drying energy.When the operation condition was constant,moisture content alternated between descent-phase and plateau-phase with position in drying bed,grain temperature alternated between descent-phase and ascent-phase with position in drying bed;the maximum moisture point moved periodically in the first drying stage,and the minimum one was at the outlet of second drying stage constantly.Coefficient of variation(c.v.)of moisture distribution can be used to measure the uniformity of dried products,statistical results showed the c.v.of moisture distribution changed with time periodically,ranging from 0.006 to 0.059,and was minimum when drying system went through compete cycling time.At different operation conditions,the higher the inlet air temperature and initial moisture content,the higher the average drying rate,however,the effect of grain flow velocity on average drying rate was not significant.The model was validated by performing experiments in a circulation counter-flow rice dryer(5HP-20).The predictions of outlet moisture content and grain temperature vs.drying time were observed to be close to the measured values in the drying experiments,the root mean square error between predicted and measured values of moisture content and grain temperature were 0.99%d.b,and 0.49℃,respectively.The model could be applied to analyze and predict the circulation drying process.5.The state space expression was developed and applied to analysis the flow bed drying system.The results showed that the system was asymptotic stable at the steady point.The system was not completely controllable as the input was constrained by objective conditions.The transient response analysis showed that the moisture content of discharge grain charactered with pure time-delay,and the delay time was related to the grain flow velocity,the smaller the flow velocity was,the larger the delay time was.According to the characteristics of the flow drying system,a controller which based on internal model control(IMC)technology was developed.The simulation results showed that the IMC suppressed the disturbance of drying conditions effectively,but model mismatch would reduce the performance of the controller,which was reflected in the increase of overshoot and the increase of adjustment time,due to error feedback correction,static error free could still be realized.The controller(IMC)was validated by performing experiments in a multi-stage counter-flow dryer(5HNH-15),the results showed that the correlation coefficient between manual control and internal model control was 0.767.6.The design equations of sensor was developed based on the material characteristics,design index and circuit detection constraints,and the graphic solution was figured out.Design parameters of plate spacing 80 mm,plate width 80 mm,plate height 77 mm,vessel wall thickness 2 mm,total height 157 mm were obtained.The verification test results showed that the range of sensor capacitance was 3.37 ～ 7.52 pf corresponde to the moisture content of rice ranging 15.03 ～33.36% w.b.,which met the design requirements and proved that the method was feasible;the on-line detection data were in good agreement with those measured by oven method,the average moisture content error of on-line capacitance detection was less than ±0.4% w.b.The flexible reset unit had solved the problem of clamping when the plate was closing,and runs stably during the test.7.Based on the series-parallel resistance model,the calculation method of apparent resistance of grain during rolling process was established.The analysis results showed that the grain resistance decreased and then rised,and the resistance-moisture detection method was propoed.The measurement range of detection circuit was 10 k～100 M,and the accuracy was 1%.The success rate of single grain sampling by vibration sampling unit for rice was 100%.The verification test results showed that the average moisture content error of on-line resistance detection was less than ±0.3% w.b. |
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