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 (1 900 r/min, 2 100 r/min and 2 300 r/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 0.000 763 N. 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 10.61 N, 7.78 N and 18.76 N at 1 900 r/min, 2 100 r/min and 2 300 r/min, respectively. After 0.2 s, 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 1 900 r/min or 2 100 r/min. The maximum average interaction force between chopped straw and chopped knife shaft wall was 123.73 N and 184.88 N, respectively. The average interaction force at 2 300 r/min after 0.5 s was fluctuated within (175.228±19.08) N, and its fluctuation intensity was less than 1 900 r/min and 2 000 r/min. After 0.8 s, 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 1 900 r/min, 2 100 r/min and 2 300 r/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 1 900 r/min to 2 100 r/min, the average energy of chopped straw was not obvious. However, when it was increased to 2 300 r/min, the average energy of chopped straw was increased significantly, with the maximum increase rate of 68.26% and 63.92%, compared with 1 900 r/min and 2 100 r/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 1 900 r/min, 2 100 r/min and 2 300 r/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 1 900 r/min, 2 100 r/min and 2 300 r/min was (82.35±6.57)%, (76.14±7.18)% and (74.22±5.65)%, respectively. Furthermore, the working power at 1 900 r/min, 2 100 r/min and 2 300 r/min was 8.83 kW, 9.51 kW and 13.16 kW, 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. © 2020, Chinese Society of Agricultural Machinery. All right reserved.
