CFD modelling of gas-liquid flow in a stirred tank

Computational fluid dynamics (CFD) is becoming an increasingly useful tool in the analysis and design of mechanically-agitated vessels. However, most simulations reported in the literature consider only single-phase flow, whereas multiphase mixtures are very common, and in such cases simulation methods require further development. This paper reports on progress in developing CFD simulations of gas-liquid mixing in a stirred tank. The model is three-dimensional and the impeller region is explicitly included in the simulation. Fluid flow is calculated with a turbulent two-fluid model using a finite-volume method. A Multiple Frames of Reference method is used to account for the relative movement of impeller and baffles, which provides an efficient steady-state calculation method. Several alternative treatments of the multiphase equations are possible, including various expressions for drag and dispersion forces, and a number of these are being tested. Variation in bubble size due to coalescence and break-up is also modelled. Compared with experimental data, the simulation results show the expected liquid velocities and correct patterns of gas distribution and bubble size throughout the vessel. Work is ongoing to improve quantitative agreement with experimental measurements of gas holdup and bubble size, by further development of the models for interfacial forces and turbulent dispersion, and improved prediction of bubble breakup and coalescence rates.