Hydraulic conveying characteristics of coarse particles in an annular jet pump: A semi-resolved CFD-DEM study

Hydraulic cavity formation in downward boreholes is an effective method for enhancing coal seam permeability. In conventional practice, insufficient conveying capacity for cavity-forming particles can cause particle retention and borehole blockage, impairing cavity formation and operational continuity. A simultaneous hydraulic cavity formation and particle conveying technology using dual-channel drill pipes integrated with an annular jet pump (AJP) offers a promising solution. However, the solid–liquid coupling mechanisms governing particle conveying in the AJP and the effects of key operating parameters on particle conveying performance remain unclear. Accordingly, a semi-resolved CFD-DEM approach was employed to investigate the flow-field distribution and particle conveying behavior. Mean residence time (MRT), residence-time long-tail index ( I res ), and specific energy consumption ( E p ) were adopted to evaluate the effects of primary pressure, particle mass flow rate, and borehole inclination angle. Results show that localized particle accumulation occurs at geometric transitions, resulting in a spatially nonuniform dense–dilute distribution along the AJP axis. Particle conveying performance improves with increasing primary pressure and particle mass flow rate, but degrades slightly as borehole inclination angle increases. Within the investigated ranges, primary pressure has the greatest effect on particle conveying performance, followed by particle mass flow rate, whereas borehole inclination angle has a comparatively minor effect. A primary pressure of 5 MPa and a particle mass flow rate of 4 kg/s provide favorable conveying performance. These findings provide guidance for the safe and efficient hydraulic conveying of coarse particles in downward boreholes and for engineering parameter selection.