Numerical modelling of gas-liquid flow in stirred tanks

A numerical modelling method is presented for gas-liquid flow in mechanically stirred tanks. In this method, the tank is represented by a mesh which explicitly includes the impeller geometry, with impeller motion treated by the multiple frames of reference or sliding mesh methods. Gas-liquid flow is modelled using an ensemble-averaged form of the Eulerian two-fluid equations. This leads to an interfacial force term containing a turbulent dispersion force term, for which a closure expression is applied following the method proposed by Bel F'dhila and Simonin (Proceedings of the Sixth Workshop on Two-Phase Flow Prediction, Erlangen, Germany, pp. 264-273). An important consideration is the increase in drag coefficient on bubbles due to interaction between bubbles and turbulence. By analysing literature data, a new correlation is proposed to account for this increase in drag. Bubble size is also predicted through a bubble number density equation, which accounts for break-up and coalescence phenomena. However, the rapid coalescence near impeller blades is modelled through an algebraic equation, as a function of gas volume fraction, allowing prediction of gas cavities. The modelling method has been applied to a baffled tank stirred by a Rushton turbine, and simulation results are compared with the experimental measurements of Barigou and Greaves (Chem. Eng. Sci. 47(8) (t992) 2009; Trans. 1. Chern. E. 74 (1996) 397) for bubble size and gas volume fraction. In comparison with this data, reasonable agreement is obtained over a range of operating conditions, and a significant improvement is demonstrated using the proposed correlation for drag in turbulent flow. The modelling method has also been applied to a tank stirred by a Lightnin A315 impeller. Again, improved results are obtained using the proposed correlation for drag in turbulent flow. and predicted gas holdup is in good agreement with experimental data. (c) 2005 Elsevier Ltd. All rights reserved.