Design and experiment of the internal and external opposed double screw burying roller of rice straw returning to farmland

Burying uniformity in soil is often required during rice straw returning. In this study, a burying roller was designed with an internal and external opposed double-screw using the straw crushing-rotary tillage and burying returning machine. A single-factor discrete element simulation (two-factors and ten-levels) was conducted to analyze the influence of the straw length (6-15 cm) and the screw angle (30°-75°) of the external screw belt in the burying roller on the coefficient of variation of the straw mass distribution. The parameters of the burying roller were optimized on the patterns after simulation. The field experiments were conducted to verify the performance of the device. The simulation results showed that the coefficient of variation of the straw mass distribution initially decreased and then increased as the screw angle increased at the constant straw length. The quadratic function was used to fit the relationship between the coefficient of variation of the straw mass distribution and the screw angle under different straw lengths. The curves of 10 straw lengths were determined for the minimum coefficient of variation of the straw mass distribution in the range of 55°-62°. Once the screw angle was constant, the coefficient of variation of the straw mass distribution was linearly related to the straw length. Specifically, the coefficient of variation of the straw mass distribution decreased by 3.255 percentage points for every 1 cm reduction in the straw length. The straw length was positively correlated with the resistance encountered in the soil. Shorter straw also encountered less resistance in the soil. It was more easily buried in the deeper soil layer, thereby reducing the coefficient of variation of the straw mass distribution. When the screw angle of the screw blade was 55°, and the straw length was 6 cm, the coefficient of variation of the straw mass distribution reached its minimum of 27.59 percentage points, and the straw burying uniformity was the highest, which was the optimal working parameter on the simulation. And the spatial distribution of the straw in the soil showed a decreasing distribution along the depth of the soil. The proportion of straw mass in the upper soil layer (>0-50 mm) was greater than that in the middle soil layer (50-100 mm). In turn, there was more than that in the lower soil layer (100-150 mm). The reason was that the straw was initially distributed on the soil surface, and then moved to the depth of the soil under the action of the burying roller. The greater the downward movement was, the greater the displacement and the stronger the resistance encountered, and the less the straw distribution was. In the two factors of the screw angle and straw length, the percentage of the straw mass in the upper soil layer (>0-50 mm) and the lower soil layer (100-150 mm) shared the opposite trends, while the percentage of straw mass in the middle soil layer (50-100 mm) fluctuated slightly. Since the increasing straw mass in the lower soil layer was mainly from the upper soil layer, the straw mass in the upper soil layer improved the uniformity of the straw mass distribution. Field experiments showed that the percentage of straw mass in the soil layers at the depth of >0-50 mm, 50-100 mm and 100-150 mm were 40.91%, 34.58% and 24.51%, respectively, while the coefficient of variation of the straw mass distribution was 25.35 percentage points, when the screw angle of the external screw belt was 55° and the straw length was 6 cm. Compared with the simulation, the percentage deviations of the straw mass in the different soil layers and the coefficient of variation of the straw mass distribution were 0.04-2.24 percentage points, with the relative errors of 0.2%-8.8%. The simulation was basically consistent with the experiment, indicating the effectiveness of the parameter optimization on the burying roller and the better uniformity of the rice straw returning and high seedbed quality after the machine operation. The findings can provide sound support to improve the straw returning quality for the straw crushing-rotary tillage and burying returning machine.