Abstract:Corn straw retention of which need chopped quality and spreading uniformity to fit performance standard, is an important method to overcome waste and open burning of corn straw. However, because of lacking mechanical and motion characters of chopped straw in chopped charmer, which lead to difficultly know the kinematic and dynamic characters of chopped straw in chopped charmer, the development of straw spreading uniformity device was constraint. The computational fluid dynamics technology (CFD)-discrete element method (DEM) coupling simulation was used to conduct motion and force analysis of chopped corn straw at different chopped shaft rotation velocities (1900r/min, 2100r/min and 2300r/min). The simulation results conducted that most chopped straws flowed out from the top of the chopped charmer after multiple collisions, and small part of chopped straw flowed out from the bottom of the chopped charmer and rotates around the chopping knife shaft. In the chopped charmer, the air flow velocity in the area of the chopping knife shaft and the exit of the chopped charmer was larger than the surroundings. Simultaneously, vortex region was generated near chopped knife shaft area and the front wall of chopped charmer. The average interaction force between air and chopped straw was small, and the maximum average interaction force was 0.000763N. Fluctuation range of average interaction force between air and chopped straw and average interaction force between chopped straw were increased with the increase of rotation velocity of chopped knife shaft. This was the reason of collision increasing between chopped straw. Consequently, the energy of chopped straw in the motion was decreased. The maximum average interaction force between chopped straw was 10.61N, 7.78N and 18.76N at 1900r/min, 2100r/min and 2300r/min, respectively. After 0.2s, the violent fluctuation of average interaction force between chopped straw and chopped knife shaft wall was occurred, especially at the rotation velocity of chopped knife shaft of 1900r/min or 2100r/min. The maximum average interaction force between chopped straw and chopped knife shaft wall was 123.73N and 184.88N, respectively. The average interaction force at 2300r/min after 0.5s was fluctuated within (175.228±19.08)N, and its fluctuation intensity was less than 1900r/min and 2000r/min. After 0.8s, the average interaction force between chopped straw and chopped knife shaft wall was fluctuated within (112.36±8.32)N, (101.15±8.02)N and (107.25±4.97)N at 1900r/min, 2100r/min and 2300r/min, respectively. The average energy of chopped straw in chopped charmer showed a trend of increasing fluctuations. Increasing the speed of chopped knife shaft from 1900r/min to 2100r/min, the average energy of chopped straw was not obvious. However, when it was increased to 2300r/min, the average energy of chopped straw was increased significantly, with the maximum increase rate of 68.26% and 63.92%, compared with 1900r/min and 2100r/min, respectively. In addition, average pitch angle of chopped straw was fluctuated within 89°. Importantly, the change of air flow field and chopped straw motion charmer would directly impact the chopped straw uniform distribution. The simulation results based on CFD-DEM coupling simulation conducted that chopped straw uniform distribution at 1900r/min, 2100r/min and 2300r/min was (85.40±4.77)%, (78.52±5.56)% and (75.17±5.32)%, respectively. The field test results conducted that chopped straw uniform distribution at 1900r/min, 2100r/min and 2300r/min was (82.35±6.57)%, (76.14±7.18)% and (74.22±5.65)%, respectively. Furthermore, the working power at 1900r/min, 2100r/min and 2300r/min was 8.83kW,9.51kW and 13.16kW, respectively. The field test and simulation results all conducted that when the length of chopped straw met the requirement, increasing rotation velocity of chopped shaft was not conducive to improving the uniformity distribution and correctness of CFD-DEM simulation results were confirmed. Importantly, the working power dissipation would be sharply increased with the increase of rotation velocity of chopped knife shaft when corn straw chopping qualified rate satisfied with the standard. The research would provide the reference to design and optimization of corn straw chopped and spread machine.
