Experimental Study on the Flow Regime Evolution of Stiff Shotcrete Materials in Pneumatic Conveying

Pneumatic conveying of stiff shotcrete material plays a crucial role in mine support; however, the flow regime evolution mechanism of its complex multi-scale particle system during conveying remains insufficiently studied. This study establishes a customized pneumatic conveying experimental platform, integrating a high-speed camera and pressure sensors to systematically investigate the flow regime evolution and characteristics of stiff shotcrete material under different operating parameters. Experimental results indicate that air velocity and water–cement ratio significantly influence flow regime evolution: at low air velocities, the material primarily exhibits dune flow and stratified flow. As the air velocity increases to 475 m3/h, the flow regime gradually transitions to suspended flow. An increase in the water–cement ratio significantly enhances liquid bridge forces between particles, intensifying particle agglomeration and making the bottom-layer flow characteristics more pronounced. Additionally, pipe diameter plays a crucial role in flow regime distribution, with suspended flow dominating in smaller pipes and bottom-layer flow becoming more prominent in larger pipes. In the downstream section of the elbow, the abrupt decrease in airflow velocity causes the flow regime to degrade from suspended flow to bottom-layer flow, leading to significant particle deposition. This study reveals the flow regime evolution patterns of stiff shotcrete material in pneumatic conveying, providing essential experimental evidence and theoretical support for optimizing long-distance pneumatic conveying systems in mine support applications.