Numerical and experimental analysis of a confined turbulent multiphase jet

Multiphase jet systems are used in environmental engineering to mix and provide oxygen in activated sludge plants for the aerobic digestion of sludge. In this frame, the possibility to forecast flow dead zones and by-passes by studying the reactor velocity field assumes strong importance. In this paper we present an experimental and numerical analysis of the velocity field and of the dissolved oxygen concentration in a lab-scale model of an activated sludge plant where mixing is obtained by a turbulent multiphase jet of water and pure oxygen. The velocity field inside the model is measured by means of a 3D Acoustic Doppler Velocimeter, allowing us to validate a finite-volume RANS numerical model of the flow, where turbulence effects are taken into account by a RNG k-ε turbulence model. The obtained numerical velocity field is used as input for a Eulerian-Lagrangian numerical model of the transport-diffusion of oxygen inside the water, leading to an estimate of the time evolution of oxygen concentration; dispersion effects on bubble motion are computed by a random walk model based on the turbulent kinetic energy field. The oxygen bubble diameter distribution inside the multiphase jet needed as input by the numerical model is obtained by post-processing of high-resolution digital images of the multiphase flow. The obtained numerical results are compared with experimental measurements of dissolved oxygen concentration inside the lab-scale model during a transient, validating a numerical technique useful to analyse the behaviour of jet systems for oxygenation and mixing in activated sludge plants.