CFD modelling of a two-phase jet aerator under influence of a crossflow

The coupling effect between the oxygen mass transfer, due to a two-phase jet aerator and the recirculating how it generates when installed in a large vessel, is modelled using a Computational Fluid Dynamic (CFD) approach. A semi-infinite domain surrounding the jet aerator was built. Specific boundary conditions were applied in order to represent a uniform, oxygen free, transverse flow flushing the aerated zone. The motion of the gas phase was not taken into consideration and its influence on the motion of the liquid phase was neglected. The volume in which oxygen transfer occurs was restricted to a subpart of the domain and it was characterized by a uniform mass transfer coefficient K(L)a. The flow rate due to the aerator is 50 m(3) h(-1), that of the transverse current was varied from 700 to 3000 m(3) h(-1) and three values of K(L)a were investigated (360, 1000 and 3600 h(-1)). For each case, the Navier-Stokes equations were solved to compute the velocity fields. Numerical results proved to be in close agreement with published data of 3-D jets in a crossflow. The oxygen source term was then added and the transport equation was time integrated until a steady state was reached. It was demonstrated that the most influential parameter on the amount of oxygen transferred was the transverse flow rate. When low. modifying the K(L)a value has almost no effect, whereas at high how rates a tenfold increase in the K(L)a value only doubles the oxygen transfer value. An ideal reactor-based model was then used to analyse the results. The behaviour of the aerated zone was found to be close to that of a plug flow reactor. Numerical values of the net mass flux of oxygen are compatible with industrial reports (13 kgO(2) h(-1)) and our modelling was able to account for the differences between laboratory and full-scale experiments. (C) 2000 Elsevier Science Ltd. All rights reserved.