(281g) RANS and LES Studies of Turbulent Flow and Heat Transfer for a Single Sphere, with Applications to High-Re Flow in Packed Tubes

Reynolds-Averaged Navier-Stokes (RANS) modeling and large eddy simulation (LES) of flow around a sphere at subcritical turbulent Re (400 - 20,000) is described. Comparisons are made to standard correlations for the drag coefficient (C_D), separation angle, reattachment length and local heat transfer Nusselt number (Nu_loc), with good agreement found at the lower Re for both approaches. At higher Re the RANS methods cannot reproduce the trends in C_D as eddy-shedding and wake dynamics become more important, but good agreement is obtained using LES, at the cost of a very fine mesh and integrating over a long enough time period for steady averages to be collected. Reasonable results are obtained from the steady k-ω SST method, provided a fine enough mesh is used to resolve the boundary layer, for quantities that are not strongly linked to form drag, such as Nu.

The fine meshes required for the best quality results prohibit extension to assemblies of particles, such as fixed beds, at this level of detail. Results are reported here of mesh coarsening studies, to determine the meshing strategy to retain the desired features of the flow, while reducing mesh size to a practical level. For example, mesh refinement to resolve eddies in the wake of the sphere, which is essential for good prediction of C_D for the single particle study, is not necessary for particle beds in which wake formation is suppressed.