Impact of Bottom Outlet Boundary Conditions on Simulation Accuracy and Convergence in Gas-Solid Cyclone Separators

Using ANSYS Fluent, the operational characteristics of a standard 2D2D Lapple cyclone separator were simulated. This computational fluid dynamics (CFD) study investigated the effect of different bottom outlet boundary conditions on simulation accuracy, convergence behaviour, and efficiency, evaluating particle loadings between 10⁻⁷ to 10⁻⁴ and for inlet velocities corresponding to Reynolds number up to 8.5×10⁴ revealing the critical importance of boundary condition selection. For clean gas simulations, the wall boundary condition provided the most precise results while reducing simulation time by up to 40%, effectively capturing flow characteristics without artificial disturbances. However, under high mass loadings, the wall condition led to significant numerical inaccuracies and failed to achieve the standard convergence threshold of 10⁻⁵ in most cases, demonstrating its limitations for such scenarios. In addition, there was an increase in the estimated pressure drop by more than 15%. In contrast, the pressure-outlet boundary condition maintained stable performance and accurate results under the same conditions. At low mass loadings, both boundary conditions produced consistent predictions for pressure drop and separation efficiency. These results highlight the importance of selecting boundary conditions appropriate to the operating regime to ensure numerical stability and simulation accuracy. Overall, the study contributes to improving cyclone separator modelling practices, providing practical guidance for optimizing industrial design and operational performance.