Numerical simulation of air-water slug flow in the aeration diffuser pipe of a membrane bioreactor

In the membrane filtration system of the membrane bioreactor, air is used to scour the membrane. Liquid slug is not desirable as it will cause non-uniform aeration flow through all orifices. Computational Fluid Dynamics (CFD) simulations to predict the onset of air-water slug flow in the aeration diffuser pipe have been performed in this paper using the Volume of Fluid (VOF) method. The effects of various geometrical parameters on aeration diffuser pipe are investigated subsequently to predict the critical gas velocities at the onset of slug flow. Through dimensional analysis, the non-dimensional critical gas flux is found to be strongly correlated to the geometrical parameters of aeration diffuser pipe (area ratio of nozzle outlet and aeration pipe inlet). Through the parametric studies conducted in this work, an empirical model was developed and established for future design of aeration systems that can potentially prevent the intermittency of air bubbles caused by slugging.