Effect of Cell Size on Particle-Eddy Interaction and Erosion Predictions Using Commercial CFD Software (FLUENT)

Some commercial CFD codes have the ability to predict erosion. For low concentration flows, a one-way coupling approach is used. The flow solution is determined then thousands of particle trajectories are determined using a Lagrangian approach, and the particle impact information is used in an erosion equation to predict the erosion. It is necessary practice to insure grid independence for the flow solution; however, discussion of the effect of grid size on particle tracking is not common. In this paper, the effect of cell size on particle tracking and erosion modeling is studied using a commercial computational fluid dynamics (CFD) code, FLUENT. Two surprising issues were found during detailed investigation of CFD results. First, eddy size is limited by cell size in FLUENT, which affects particle-eddy interaction in the discrete phase model and consequently erosion modeling. Limiting the eddy size based on the cell size can have a huge effect on the particle trajectories in geometries like sudden contraction/expansions since eddies play an important role in particle behavior in these geometries. This is particularly true for very small particles and when liquid is the carrier fluid. Second, the particle impact behavior seems unrealistic. Particles tend to stay near the wall and impact the wall over and over in a small area. During each impact series, the particle’s impact velocities do not reduce with successive impacts as expected. This promotes simulated erosion rates that are high. In addition to the CFD investigation, an experimental facility was designed and built to develop erosion equations for small particles (average size of 25 μm) to enable more appropriate comparisons of erosion data with predicted erosion.