## 4 Simulation Results Analysis  
Numerical simulations were conducted for the four structures without spiral blades in the mixing zone. The streamlines of air and gas inside the mixer, as well as the methane mass fraction distribution along the YZ plane and at the outlet, are shown in Figures 5 and 6. Analysis of the images shows that when only the air inlet had a spiral blade structure, due to the small air flow rate, the rotation of air had a relatively small disturbance on the overall gas flow, which was beneficial for axial flow and reduces pressure loss. However, the relative motion between air and gas was small, resulting in poor mixing, the uneven distribution of gas at the outlet, and a tendency for gas to accumulate at the pipe edges. Under the structure where only gas rotates, the central air was effectively driven to mix, and the relative motion between air and gas was significant, resulting in better mixing and less gas accumulation at the edges, with relatively higher pressure loss compared to the structure where only air rotates. When both air and gas rotated in the same direction, the overall rotation effect was significant, but the relative motion between the two was small, which slightly reduced pressure loss, but had little effect on the mixing of air and gas, with significant gas accumulation at the pipe edges. When both air and gas rotated in opposite directions, the rotation effect was significant, and the relative motion between the two was significant, increasing the number of collisions between the two gases, which was beneficial for mixing. However, the pressure loss increased slightly. As shown in Figure 6c, the gas accumulation at the pipe walls was significantly reduced, and the overall mixing effect was excellent. The total pressure at various cross-sections of the four structures without spiral blades in the mixing zone was area-weighted averaged, and the total pressure variation along the cross-section is shown in Figure 7. The zero point on the horizontal axis in the figure is the outlet position of the spiral structure. As shown in Figure 7, when only air rotated, the pressure loss was the lowest, much lower than the other three structures, and the pressure loss along the cross-section area did not change much. The pressure loss trend of the structure where air and gas rotated in opposite directions was similar to that of the structure where only gas rotated, but the overall pressure loss of the structure where only gas rotated was slightly lower than that of the structure where air and gas rotated in  
Figure 5: Streamline diagram air and gas in mixer. (**a**) Air rotation, (**b**) gas rotation, (**c**) air–gas counter-rotating, and (**d**) air–gas co-rotating. opposite directions, indicating that air rotation had little effect on the overall pressure loss. At 0.81 m from the outlet of the spiral structure, the pressure loss of the air and gas counter-rotating structure was higher than that of the air and gas co-rotating structure. Beyond this position, the pressure loss of the air and gas co-rotating structure was greater than that of the air and gas counter-rotating structure. According to the analysis of Figure 5c,d, this is because the co-rotating airflow near the outlet was close to stable and the pressure level gradually decreased. The flow field of the counter-rotating airflow still had a strong relative motion, which kept the pressure at a certain level, so the pressure drop was smaller than that of the co-rotating airflow. In addition, the pressure cloud diagrams of the air and gas counter-rotating structure and the air and gas co-rotating structure are shown in Figure 8. Compared with the air and gas co-rotating structure, the pressure on the outlet cross section of the air and gas counter-rotating structure was higher. Figure 6: Gas distribution map along the mixer’s axial plane and outlet surface. (**a**) Air rotation, (**b**) gas rotation, (**c**) air-gas counter-rotating, and (**d**) air-gas co-rotating.