Macroinstability uncovered in a Rushton turbine stirred tank by means of LES

Low-frequency mean flow variations, identified experimentally in various stirred-tank geometries, are studied by means of large eddy simulations in a Rushton turbine stirred tank. The focus is on flow structures, and the spectral characteristics of the velocity components. The lattice-Boltzmann Navier-Stokes solver, and the Smagorinsky subgrid-scale model, are adopted for solving the stirred tank flow, and boundary conditions are imposed by means of an adaptive force-field technique. Simulations performed at Reynolds numbers 20,000 and 30,000 on grid sizes of 1203, 1803, and 2403 grid nodes confirm the experimentally found flow variations at various monitoring points in the bulk flow. The period of the flow variations found in the bulk of the tank corresponds to approximately 250 blade passage periods. A simulation performed at a Reynolds number of 12,500 showed pronounced flow variations with a timescale of about 65 blade passage periods, which is consistent with experimental observations. Transient flow field results in the bulk flow of the tank uncover a whirlpool type of precessing vortex, of which the impact on the kinetic energy of the velocity fluctuations is further analyzed. © 2004 American Institute of Chemical Engineers AIChE J, 50: 2383–2393, 2004