Numerical simulation of the gas-solid flow in three-dimensional pneumatic conveying bends

The laden gas-solid flow in a pneumatic conveying bend is featured with intensive gas-solid, particle-particle, and particle-wall interactions, which are however difficult to quantify experimentally. In this work, these interactions are obtained by use of a three-dimensional combined continuum and discrete model. The model is achieved by combining our code for discrete element method for solid phase with the commercial software package Fluent for computational fluid dynamics for gas phase. The applicability of the approach is first qualitatively verified by comparing the simulated results with the observations in the literature in terms of typical flow features in bends such as roping, particle segregation, particle velocity reduction, particle recirculation, and pressure fluctuation. The gas-solid, particle-particle, and particle-wall interaction forces are then analyzed to understand their role in governing the complicated flow. It is found that the intensive gas-particle interaction at the outer wall makes the peak of the axial velocity shift from the outer wall to the inner wall of a pipe. Correspondingly, the so-called secondary flow is suppressed in the outer wall region but enhanced along the side wall. The spatial distribution of particle-wall interaction is obtained and shown to correspond to the wearing pattern in a bend. This distribution is also found in the particle-particle interaction close to the bend wall. Not only gas-solid interaction but also particle-particle interaction contributes to the dispersion of a rope. Finally, simulations are also conducted to investigate the effects of inlet conditions such as gas and solid flow rates on these interaction forces.