Direct numerical simulation of turbulent particle dispersion in an unbaffled stirred-tank reactor

Turbulent dispersion of inertial particles in a flat-bottom stirred-tank reactor equipped with an eight-blade Rushton impeller is investigated using accurate numerical techniques (Verzicco et al., 2004, Flow in an impeller-stirred tank using an immersed-boundary method. A.I.Ch.E. Journal, 50(6), 1109–1118.). Direct Numerical Simulation of the turbulent flow field in the vessel is obtained using a second-order finite-difference scheme coded in a cylindrical reference frame, and an immersed-boundary approach is used to simulate the motion of the impeller. The flow scales are resolved explicitly down to the Kolmogorov scale. To give a comprehensive picture of the turbulence structure in the vessel, angle-resolved averages of turbulent kinetic energy, turbulent energy dissipation rate and Kolmogorov time-scales are evaluated in vertical planes aligned with the blade and mid-way between two blades. The dispersion of heavy particles of different diameter is then investigated by Lagrangian tracking. The particle-to-fluid mass loading ratio is low enough to assume one-way coupling momentum transfer between continuous and dispersed phase. Three sets of particles, characterized by different response time, are investigated and, for each set, two equal, randomly distributed swarms are initially released above and below the impeller, which is placed mid-way between top and bottom of the tank. Statistics calculated after 3 impeller revolutions are used to evaluate the evolution of particle dispersion in the flow and to quantify their preferential accumulation into specific regions of the tank.