The near-wall treatment for solid particle erosion calculations with CFD under gas and liquid flow conditions in elbows

Abstract

Erosion caused by solid particles is a complex process involving many parameters. Computational Fluid Dynamics (CFD), in combination with more reliable correlations that are provided in recent years, has shown acceptable performance in erosion calculations for many cases. However, there is an inherent limitation in many CFD-based particle tracking models that treat particles as a point-mass with no volume which can result in significant errors in erosion calculations if an appropriate grid spacing in the near-wall region is not utilized. In wall-bounded flows, such as in pipes, significant gradients are present in the near-wall region and it can lead to noticeable errors in predictions when particles are treated as points in the Lagrangian description of the particle motion. In this study, a general approach is proposed to minimize such errors, by accounting for the particle size near the wall and identifying impacts and rebounding particles when the center-point is one radius away from the wall. This approach is compared to another guideline that was developed that restricts the first-layer thickness (FLT) on the wall to be the same as the particle size. Single-phase air and water flows in a 76.2 mm elbow with particle sizes of 300 µm, 150 µm, 75 µm, 50 µm, and 25 µm are examined and global and local differences are presented. It is found that erosion prediction for particles smaller than 150 µm could be challenging if no special treatment is applied. The proposed approach for rebounding particles at their radius provides reasonable trends for solid particles entrained in both gas and liquid flows. Furthermore, the non-physical erosion hot spots that could be observed for small particles trapped in viscous sub-layer regions, and causing many impacts is investigated by applying various restrictions on particle impacts. Through the comparisons, the responsible mechanisms for the non-physical impacts are presented. Finally, the effect of the secondary impacts of a particle, in contrast to one impact per particle, is examined which suggests that in both gas and liquid cases, the erosion is affected by the secondary impacts and in the liquid flows even more secondary impacts are needed to obtain a realistic erosion prediction.