Author(s): Massimo Raboni; Emanuelatorti; Stefano Sibilla
Linked Author(s):
Keywords: Turbulent multiphase jets; Dissolved oxygen; Mixing
Abstract: The analysis of the dynamics of mixing jet systems is very important in chemical, pharmaceutical and environmental engineering because it can help to understand, improve and accelerate processes. In particular, 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 bubbly and dissolved oxygen (DO) 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 ADV probe (Acoustic Doppler Velocimeter), allowing us to validate a finite-volume RANS numerical model of the flow, where turbulent effects are taken into account by a standard k-ε turbulence model. The obtained numerical velocity field is used as input for a Lagrangian numerical model of the transport-diffusion of oxygen inside the water, leading to an estimate of the time evolution of oxygen concentration; diffusion effects are computed by a random walk model based on the turbulent kinetic energy field. An evaluation of oxygen bubble diameter distribution inside the multiphase jet (needed both to simulate the bubble dynamics and to evaluate the dissolution rate of oxygen) is obtained by postprocessing of high-resolution digital images of the multiphase flow. The obtained numerical results are compared with experimental measurements (performed by a DO probe) of dissolved oxygen concentration inside the lab-scale model during a transient. The developed numerical technique will be useful to analyse and improve the behaviour of jet systems for oxygenation and mixing inn activated sludge plants.
Year: 2009