An Eulerian–Lagrangian method for the simulation of the oxygen concentration dissolved by a two-phase turbulent jet system

Multiphase jet systems are used in environmental engineering to mix and provide oxygen in activated sludge plants for the aerobic digestion of degradable substrates. Here a laboratory experiment, based on a lab-scale model where mixing is obtained by a turbulent multiphase jet of tap water and pure oxygen, is used to validate a multiscale numerical model of the oxygen concentration evolution based on a “one-way coupling” approach. The velocity field inside the lab-model, measured by means of a 3D Acoustic Doppler Velocimeter, allows to check the data provided by a finite-volume RANS numerical model of the flow, where turbulent effects are taken into account by a RNG k–ε turbulence model. The obtained numerical velocity field is then used as input for an Eulerian–Lagrangian numerical model for the dissolved oxygen balance equation, where diffusion effects are computed by a random walk model based on the turbulent kinetic energy field. The bubble size distribution of the jet, determined by a photographic technique, is taken into account in the numerical model in order to calculate the mass transfer from the bubbles to the liquid. Several mass transfer models are implemented and tested in order to perform a sensitivity analysis. The obtained numerical results are compared with experimental measurements of dissolved oxygen concentration inside the lab-scale model during a transient. Results show that the developed numerical technique yields an accurate reproduction of the measured oxygen concentration in time and can be applied to analyse the behaviour of jet systems for oxygenation and mixing in activated sludge plants