Simulation Study on the Influence of Swirl Structure on the Performance of Static Low-Concentration Coalbed Methane Mixer

The utilization of low-concentration coalbed gas as an energy source in coal mines has garnered significant attention due to the intensifying global warming trend. Achieving a uniform gas mixture is essential for the effectiveness of various gas utilization technologies. In this study, we conducted a series of numerical studies on the pressure loss and mixing uniformity of a swirl-type static low-concentration coalbed methane (SLCCM) mixer with spiral blades as turbulence elements. The results indicate that installing spiral blades at the low-concentration gas inlet can significantly improve gas mixing uniformity, while installing spiral blades at the air inlet has a relatively small impact on gas mixing uniformity. When spiral blades are installed at both the low-concentration gas and air inlets, and the spiral direction of the two sets of blades is the same, the pressure loss of the mixer is smaller than that of the mixer with opposite spiral directions. Moreover, the mixing uniformity of the former is better than that of the latter when the length of the mixing zone is shorter. However, as the length of the mixing zone increases, the mixing uniformity of the mixer with opposite spiral directions of the two sets of blades will be significantly improved and better than that of the mixer with the same spiral direction. When a set of spirals with opposite directions to the blades at the low-concentration gas inlet is added at the forefront of the mixing zone, the mixing uniformity at the outlet of the mixer can be greatly improved, but it will increase the mixing pressure loss of the system. When the two spirals are in the same direction, the impact on the system pressure loss and mixing uniformity is relatively small. The numerical simulation results show that by setting a set of spiral blades with the same rotation directions at the inlet of low-concentration gas and at the forefront of the mixing zone (i.e., b2-type structure), the spatial non-uniformity of the mixing is about 5.24%, and the system pressure loss is less than 400 Pa, which is significantly better than the requirement of less than 15% spatial non-uniformity for engineering requirements. Moreover, with the increase in the length of the mixing zone, the spatial non-uniformity still tends to decrease.