Simulation of a stirred-tank reactor flow and effect of free surface modelling

The aim of the present paper is to validate OpenFoam solvers for simulating flows in stirred tank reactors (STR). The influence of the free surface motion on the flow in the vicinity of the impeller is assessed, confronting VOF with single-phase computations carried out with a symmetry boundary condition at the top of the domain. In order to evaluate the capability of OpenFoam to resolve such flows, the numerical results are validated with experimental data from literature, obtained with Particle Image Velocimetry (PIV). The test case chosen uses water as a fluid, and features a six blade, 45 deg pitched axial turbine, in a stirred vessel with 4 baffles. Steady computations are performed with the simpleFoam (MRF) solver, transient ones use pimpleDyMFoam, while the two-phase simulations with VOF use interDyMFoam. The sensitivity to the mesh is studied using four different grids, and based on the dimensionless power and flow numbers. The velocity field in the vicinity of the impeller shows that the use of a symmetry condition at the top of the domain dos not influence strongly the recirculation at the bottom of the tank in this type of configuration. This approximation allows reducing the typical CPU times of one order of magnitude. Without baffles (typical STR design for viscous fluids), the free surface features a depression at the center, and the flow topology is more affected by the deformation of the latter. In this case, an iterative correction of the free surface height as a function of the pressure on the top boundary may help reducing the computational times.