Impact of computational domain discretization and gradient limiters on CFD results concerning liquid mixing in a helical pipe

Abstract Liquid mixing is studied in a helical pipe numerically and experimentally to investigate the influence of computational domain discretization and gradient limiters on the unwanted numerical diffusion. Five mesh types with multiple resolutions are examined, including hexahedral, automated polyhedral, extruded polyhedral, automated tetrahedral, and extruded tetrahedral meshes. The mixing efficiency is calculated using the scalar transport technique and compared with data obtained by Laser-induced Fluorescence measurements. Two gradient limiters are considered in the analysis, i.e., Venkatakrishnan and Min-Mod limiters, which are typically used to stabilize the simulation while limiting the numerical diffusion. The results reveal that all extruded meshes show generally a very good agreement with the experiments, while automated meshes involve unavoidable numerical diffusion, even for resolutions up to 90 million cells. The reason for this is that the cells of the automated meshes are hardly aligned with the flow direction, resulting in higher truncation errors. Furthermore, the use of MinMod limiter with extruded meshes effectively minimized the numerical diffusion in most cases. Compared to all other types, the hexahedral mesh is found the least diffusive and most accurate. Comparing the pressure drop, an acceptable accuracy on all meshes was found already at low mesh resolutions with slightly increased errors when using the automatic tetrahedral mesh. Finally, the computational cost is compared among all the considered cases. All extruded meshes are found very cost-efficient since they are mostly aligned with the flow, consuming only about 50% CPU time compared to automated unstructured meshes. Therefore, using the extruded meshes, and in particular the hexahedral mesh as far as possible, with the MinMod limiter is strongly recommended for an accurate prediction of the mixing performance at low cost.