(134d) Lattice-Boltzmann Computational Fluid Dynamics (CFD) Simulation of Jet Mixing in Tanks

Jet mixers have been used for a number of years in process industries. Common applications include adding acid or base for pH adjustment, ensuring the tank is well mixed prior to sampling for chemical analysis, and adding material to adjust the composition of the tank contents. In addition, they are widely used at U.S. Department of Energy sites to mix and process waste in ~ 1 million gallon tanks. Because of the size of the vessels and the radioactivity of the material in these tanks, testing at full-scale or pilot-scale is problematic and expensive. An alternative to assist in the design and operation of jet mixers in these large tanks is to use computational fluid dynamics (CFD). The authors will discuss the development of CFD models using the M-Star® lattice-Boltzmann software to simulate jet mixing in tanks.

The authors developed models of mixing in tanks using stationary jet mixers with a single jet, jet mixers with two opposing jets, and rotating jet mixers with two opposing jets. The models included the inlet or pump suction and the recirculation of the inlet stream to the jet nozzle. A refinement zone was added to the model to allow for a small lattice spacing near the jet nozzle discharge, to capture the physics of the jet formed, and a coarser lattice spacing away for the jet nozzle, to reduce the computation time. The jet nozzle diameter and nozzle discharge velocity were varied to challenge the models.

The results of the simulations performed with these models were compared with experimental data in the technical literature. For the stationary jet mixers, the comparisons were of miscible liquid blend times in vessel of diameters between 0.6 and 36 meters. The rotating jet mixer model was used to simulate the mixing of Bingham plastic fluids in a tank. The comparisons were of the effective cleaning radius of the jet mixing, which is a measure of its zone of influence. The effective cleaning radius is analogous to the mixing cavern in an impeller mixed tank. The results of the simulations and the comparisons to experimental data will be discussed.