Regulation mechanism of wall roughness on micron-sized particle deposition in pneumatic conveying bend pipes

The multi-scale characteristics of wall roughness are the main factors inducing particle deposition in pneumatic conveying bend pipes. However, existing simplified models cannot effectively characterize the complex features of rough walls. To elucidate the mechanism by which wall roughness regulates the deposition behavior of particles of different sizes, this study constructs a 90° bend pipe model with multi-scale random roughness characteristics based on the three-dimensional Weierstrass–Mandelbrot [three-dimensional (3D) W–M] function. By integrating the Reynolds stress model, the discrete phase model, and a critical deposition velocity model, the deposition characteristics of 1–100 μm sepiolite particles were systematically investigated. The results indicate that near-wall recirculation vortices and secondary flows are the main factors driving differences in deposition hotspots for particles of varying sizes. The particle-size screening mechanism is identified as the main reason for the enhanced deposition of particles with Stokes number (St) < 1 on rough walls. Significant differences in deposition behavior among particles of different sizes arise from the competition between adhesion-collision mechanisms. This study systematically reveals the physical mechanisms by which wall roughness regulates multi-size particle deposition, providing a theoretical basis for optimizing pneumatic conveying pipe design and reducing system energy consumption.