| As an important part of the combine harvester, Cleaning unit’s performance directly affects the whole machine characteristic. Agricultural materials cleaning process is the combined effect of gas-solid two-phase flow field, their own characteristics of agricultural materials group and vibration screen surface. There are many factors affecting the cleaning performance. It is of great significance to study air-and-screen cleaning device, to analyze air flow distribution of cleaning shoe, to explore the motion law of agricultural materials on the screen surface, which will not only provide theoretical basis for the design and optimization of the existing typical cleaning unit, but also give theoretical inspiration to looking for a new way of the cleaning. Therefore, in this paper, computational fluid dynamics, discrete particle dynamics and gas-solid two-phase flow theory were comprehensively applied to study on working mechanism of air-and-screen cleaning unit. The major research work is as follows:Composition of rice cleaning mixture was analyzed. The threshing mixture consists of grain, light impurities, short stems and long grass. The main compositions influencing the performance were short stems and light impurities. Three-dimension geometry size, shape characteristics and mechanical parameters of the rice grain such as density, friction coefficient and recovery coefficient were measured; as a result, a 3-D rice grain model was established. Suspended speed of the main components of the mixture was measured by using self-made DFPF-25 materials suspended speed measuring device. Suspended speed of small branch with grain closes to full grain suspended speed. The range of suspended speeds of short stem overlaps with that of grain. To separate overlapping parts of the grain and short stem by air cleaning is difficult. These data provide the basis for the bench test and numerical simulation. The three dimensional models of the whole flow was established according to the mechanism of the air-and-screen cleaning unit. In order to study the distribution of air flow field in air-and-screen cleaning device, meshing was completed by using the Computational Fluid Dynamics (CFD) theory and method, Reynolds continual equation, N-S equations and standard k-εturbulence model were applied to form closed equations. The air flow field simulation was carried out on the conditions of different fan outlet angles, different fan outlet speeds and different distances from top to surface of the screen. The three parameters’ effects on the air flow field were analyzed and the experimental verification was carried out. The results show that angle of fan outlet and distances from top to surface of screen have greater effect on air flow field in air-and-screen cleaning device. Air speed at fan outlet has less effect on air flow field. And a good agreement was found between measured value and simulated value. This demonstrates that numerical simulation of air flow field in air-and-screen cleaning mechanism using CFD software Fluent is correct. And this provides a new method for studying on air-and-screen cleaning unit.Contact-impact models of grain-grain and grain-wall were established based on distinct element method (DEM). Numerical simulations of the single particle and material group were carried out using particle discrete element software (EDEM). The results show that throwing index is determinant factor affecting particle motion. Simulation results and theoretical analysis are consistent. As other conditions kept unchanged, with the amplitude and frequency increased, the longitudinal moving speed of the material and efficiency of the material passing the screen increased, and cleaning loss increased as well. At 25°to 45°range, with vibration direction angle increased, longitudinal moving speed of the material increased, the highest velocity occurred at 45°; when the angle was more than 45°, longitudinal moving speed of the material gradually decreased; efficiency of the material passing the screen was highest at 35°and the cleaning loss was almost 0. Based on comprehensive analysis of simulation, results can be drawn that without wind factor at the condition that screen surface dip of 4°, amplitude of 30mm, frequency of 5Hz, vibrating direction angle of 35°, the cleaning result is best, in which case throwing index was 1.56.Using CFD-DEM coupling technique, the grain-grain and grain-wall impact was simulated in a Hertz-Mindlin model, the movement particles moving on the screen of the air-and-screen cleaning device was described by using Eulerian model which considers coupling impact. Simulation results showed that:(a)Due to the small density of short stem, the fan outlet speed influenced the stem’s longitudinal moving speed on the screen more, while the density was big enough, the influence was much less; the fan outlet speed influenced the vertical velocity slightly; as speed increased, the faster move of materials enhance the processing capacity of the air-screen cleaning device, however, the grain loss increased. (b)The grain distributed mainly in the range of 97.5-390mm to the front of the screen, as the fan outlet speed increased, the distribution of the grain moved back and there are few grain more than 487.5 mm to the screen. As a result, the screen could be shortened if there were few impurities in materials. (c) Comparison of numerical simulation results with experiments has demonstrated adequate agreement. It showed that numerical simulation of material motion on vibrating screen of air-and-screen cleaning device based on CFD-DEM is feasible. The results provide a basis for improving the design on air-and-screen cleaning device of combineOrthogonal design (L6(44)) of four factors was established, including angle of centrifugal fan, centrifugal fan speed, vibration screen amplitude and frequency of vibrating screen, four levels of every factor. Arranged the tests based on the orthogonal test table, improved clean rate of the grains and reducing loss rate were regarded as detecting index, the optimum method of air-and-screen cleaning unit was assessed. As test results for variance analysis, results indicate that:within test range, centrifugal fan speed, vibration screen amplitude and frequency of vibrating screen had significant greatly effect on clean rate, angle of centrifugal fan evident effect on clean rate; centrifugal fan speed had significant greatly effect on loss rate, vibration screen amplitude had significant greatly effect on loss rate, but angle of centrifugal and fan frequency of vibrating screen had no significant effect on loss rate; effect clean rate and loss rate of factors primary and secondary order is:centrifugal fan speed\vibrating screen amplitude\frequency of vibrating screen\angle of centrifugal fan.By means of regression analysis, applying DPS software, mathematical models of clean rate Y, losing rate S, centrifugal fan angle x1, centrifugal rotating speed x2, vibration screen amplitude x3 and vibration screen frequency x4 were established. We applied multi-objective optimization based on the genetic algorithm. The Pareto optimal solution was obtained and verified. The results provided reference basis for practical design and utilization of the air-and screen cleaning unit, so as to give consideration to both clean rate and loss rate. |
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