Parametric effects on gas–solid two-phase flow and particle acceleration in a low-pressure abrasive air jet with a multi-stage acceleration structure

Low-pressure abrasive air jets offer a promising water-free technique for permeability enhancement in soft coal seams. However, the acceleration efficiency of abrasive particles is fundamentally constrained by insufficient momentum transfer and pronounced energy dissipation under low inlet-pressure conditions. To overcome these limitations, a multi-stage acceleration structure integrating a working nozzle and an ejector nozzle is proposed. A coupled computational fluid dynamics–discrete element method framework is adopted to simulate the gas–solid two-phase flow and to quantify the effects of key parameters, including the working-nozzle inlet pressure P2, throat-diameter ratio β, and relative nozzle position Ln. The results indicate that abrasive particles experience three successive acceleration stages, which effectively extend the acceleration path. The particle kinetic energy reaches a maximum at an optimal working pressure P2 of approximately 3 MPa, while maintaining favorable velocity uniformity. Beyond this optimal condition, jet over-expansion occurs, resulting in enhanced velocity dispersion and reduced energy-utilization efficiency. A throat-diameter ratio β of about 4 improves jet stability, and aligned nozzle positioning ensures the most stable particle acceleration and the highest particle kinetic energy. The regression analysis demonstrates that the working pressure P2 exerts the dominant influence on particle kinetic energy, followed by the relative nozzle position Ln, while the effect of the throat-diameter ratio β is comparatively weak. This study clarifies the gas–solid coupling mechanism in a multi-stage acceleration structure and provides quantitative guidance for the design and operation of efficient water-free abrasive air-jet systems for soft coal seams.