The mixing of wastewater sludge inside a stirred anaerobic digestion tank with a height and diameter of 15.7 m is simulated using Large-Eddy-Simulation and the finite-volume multigrid coupled solver MGLET. The mixing propeller is modelled as an actuator disc and line that adds a constant momentum to the right hand side of the momentum equations to generate an axial downward flow of approximately 20 m^3 s^-1 through the propeller swept area. Anaerobic digestion sludge is treated as a single-phase fluid with Newtonian rheological behaviour. Two distinct dynamic viscosites of 0.1 kg m^-1 s^-1 and 5.0 kg m^-1 s^-1 corresponding to a low and high Reynolds number are inspected. Multiple simulations were performed on the base grid differentiating between actuator disc and line at low Reynolds number in a baffled tank, actuator line at low Reynolds number in a baffled and unbaffled tank as well as actuator line at low and high Reynolds number in a baffled tank. The conducted simulations emphasize the strong dependence of the simulation results on the propeller model, fluid viscosity as well as grid resolution. The actuator disc potentially underestimates whereas the actuator line model potentially overestimates mixing efficiency. The beneficial effect of baffles on the global flow field inside stirred tanks that is described in literature could not be confirmed. Comparing the actuator disc and line simulation both conducted at the low Reynolds number shows significant differences in the flow field that could better be explained using a Reynolds number based on volume averaged total energy instead of average flow rate through the area bounded by the actuator. For the grid convergence study the grid resolution of the actuator line and high Reynolds number case was refined two times in the vicinity of the actuator and between the actuator and the tank bottom. Grid convergence can be shown for the total energy and velocity magnitude. Due to time limitations of this work the simulations are not validated against measured data.     «

The mixing of wastewater sludge inside a stirred anaerobic digestion tank with a height and diameter of 15.7 m is simulated using Large-Eddy-Simulation and the finite-volume multigrid coupled solver MGLET. The mixing propeller is modelled as an actuator disc and line that adds a constant momentum to the right hand side of the momentum equations to generate an axial downward flow of approximately 20 m^3 s^-1 through the propeller swept area. Anaerobic digestion sludge is treated as a single-phase...     »