| Sweet potato vines with high nutritional values are good feeds,but mechanical crushing and returning-to-field of sweet potato vines cause a waste of resources and easily induce the spread of plant diseases and insect pests in sweet potato fields.Moreover,manual cutting,cleaning and transportation of vines are limited by labor intensity and high costs.Hence,mechanization of sweet potato vine crushing and recycling is an urgent problem to be solved before the realization of lightweight and simplified production in the sweet potato planting industry.In this work,given the difficulty in recycling furrow bottom vines by sweet potato vine machine,the low qualified rate of sweet potato vine crushing,and the low recycling rate,the two key technical problems viz.crushing and throwing that restrict sweet potato vine machinery harvesting were studied.A ridge-imitating sweet potato vine recycling machine crushing and throwing technical scheme was proposed,and the key devices viz.the ridge-imitating knife roller device and the throwing device were designed.Then the mechanisms of sweet potato vine crushing and throwing were investigated.The effects of key device structures and working parameters on the flow field characteristics and broken potato vine particle swarm moving rules in the recycling machine were studied by numerical simulations.Multi-objective optimization of the ridge-imitating knife roller device and the throwing device was conducted to obtain the best structural and working parameters.The findings will theoretically and methodologically underlie the development of sweet potato vine efficient crushing and recycling machine.(1)Based on the mechanical test of sweet potato wines,the mechanical characteristic parameters were measured,and the discrete element simulation parameters of broken sweet potato wines were obtained by the combination of direct measurement and virtual calibration.Relationship between water contents of main vine stems and harvesting time from three varieties of sweet potatoes were tested,which showed the water contents declined and shear strength was strengthened with the prolonging of harvesting time.The intrinsic parameters of broken sweet potato vines were obtained by direct measurement method.The collision recovery coefficient,static friction coefficient and rolling friction coefficient between broken sweet potato vines were calibrated by Plackett-Burman test,steepest climbing test and Box-Behnken optimization test,which provided a theoretical basis for the design of mechanical crushing and recovery mechanism of sweet potato vines and the discrete element analysis of broken sweet potato vines.(2)The crushing and throwing technical scheme of a ridge-imitating sweet potato vine recycling machine was proposed,and the key devices were designed.Then dynamic models of sweet potato vine crushing and throwing were built.The ridge-imitating knife roller has Y-shaped flail knives equipped with baffles in the middle and edges outside.The flail knives were arranged in the spiral ridge-imitating mode,and can cut and crush the sweet potato vines at the top,sides and bottom of each ridge.The sweet potato vine crushing process was dynamically analyzed to build a crushing process dynamic model.The main influence factors on the crushing quality and stubble height of sweet potato vines were determined to be knife roller rotation speed,number and arrangement way of flail knives,ground clearance,and machine advancing speed.The broken vine throwing and recycling process was dynamically analyzed to build a broken vine throwing and recycling dynamic model.The main influence factors on the throwing of broken sweet potato vines were identified to be flail knife structure,knife roller rotating speed,fan rotating speed,fan blade number,blade inclination angle,and pipe structure.(3)The effects of key device parameters on the internal flow fields of the recycling machine were studied.A computational fluid dynamics(CFD)model for the recycling machine was established,and its internal flow field pressure and airflow were characterized.It was found the airflow speed at the inlet of the recycling machine was slow and stable,forming a negative pressure zone.The gas flow rate gradually rose from the center of the knife roller shaft to the tips of flail knives,and slowly increased outwards the fan shaft,and the airflow generally moved upwards along the recycling machine walls,which were all favorable for the transportation of sweet potato vines.The effects of parameters of the ridge-imitating knife roller device and the throwing device on the airflow properties in the recycling machine were investigated.It was found the flail knife double-helix arrangement,and faster knife roller rotating speed can enlarge the area and speed vector of air flow in the crushing chamber.As the fan rotating speed was accelerated,the average flow rate in the recycling machine was gradually increased,and the wind speed was large on the walls of the transportation pipes.Increasing fan blade number and blade inclination angle made the air flow distribution more uniform in the chamber,but the flow rates dropped.Simulation comparative analysis showed the upper wall three-segment radian combination,single curvature radius contracting transition,and appropriate fan inlet opening in the transportation curved pipes made the flow fields in the transportation pipes more favorable for delivery of sweet potato vines.(4)The motion of broken vine particle swarms was analyzed,and the key devices were sent to multi-objective optimization.The throwing process of broken vine particle swarms was numerically simulated by gas-solid coupling to determine the moving rules of broken vine particle swarms.Simulation comparison demonstrated that the flail knife double-helix symmetrical arrangement,faster knife roller rotation,appropriate fan rotating speed,blade number,and blade inclination all contributed to the throwing and collection of broken sweet potato vine particles.A throwing device multi-objective optimization method integrating gas-solid coupling and response surface methodology was proposed.Three-factor three-level center combined simulation optimization tests were performed with the factors of fan rotating speed,blade number and blade inclination,and with the objectives of specific power consumption of the throwing device,vine recycling rate,and throwing speed.A throwing device multi-objective optimization prediction model was built,and validated by a bench test.When the fan rotating speed,blade number and blade inclination angle were 880r/min,3 and7°respectively,the specific power consumption of the throwing device,vine recycling rate,and throwing speed were 718 m~2/s~2,92.79% and 5.96m/s respectively.Comparative tests showed after the throwing device was optimized,the specific power consumption decreasing amplitude,vine recycling rate,and throwing speed were raised to 15.83%,by 2.9% and by 5.49% respectively.The combination of ridge-imitating knife roller structure optimized parameters was acquired on basis of the vine crushing and throwing test bed.When the optimal parameter combination of the throwing device was adopted(double-helix arrangement of flail knives,ground clearance of 30mm),three-factor and five-level orthogonal rotation combination tests were conducted,with the factors of knife roller rotating speed,vine delivering speed,and flail knife number,and with the test indices of crushing qualified rate,recycling rate,and specific power consumption of knife rollers.With the optimal parameter combination(knife roller rotating speed at 2100r/min,vine delivering speed at 0.54m/s,20 flail knives),the crushing qualified rate,recycling rate,and specific power consumption of knife rollers were 92.14%,93.01% and 2891.2m~2/s~2 respectively.(5)Field test.With crushing qualified rate,recycling rate,stubble height,potato injured rate,and soil rate as the test indices,field tests were conducted to study the field effects on key device working parameters of the ridge-imitating sweet potato vine recycling machine.The structural parameters of the throwing device and the ridge-imitating knife roller structure were optimized.Five testing indices were affected by the ground clearance and knife roller rotating speed,and the recycling rate and soil content were impacted by the fan rotating speed.Thus,center combined tests were conducted with knife roller rotating speed,fan rotating speed,and ground clearance as the testing factors.The regression model between influencing factors and evaluation indexes was obtained by using the central combination test method.It was concluded that with the reduction of knife roller speed and ground clearance,the crushing qualified rate of sweet potato wines increased,the stubble height decreased,but the potato injured rate increased.The recycling rate was increased rapidly first and then slowly with the increase of knife roller rotating speed,increased slowly with the decrease of ground clearance and increased with the increase of fan speed.With the increased of ground clearance,appropriately reduced of the fan speed and the knife roller rotating speed,the soil content was reduced.When the knife roller rotating speed,ground clearance,and fan rotating speed were 2000r/min,19mm and 800r/min respectively,the crushing qualified rate,recycling rate,stubble height,potato injured rate,and soil content were 91.48%,90.77%,49.7mm,0.28% and 4.8% respectively. |
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