Numerical investigation of fluid flow in oscillation baffled columns

Abstract

This research aims at investigating the fluid mixing in oscillatory baffled columns (OBC), by utilising ANSYS CFX 15 under single and two-phase (gas-liquid) flow conditions. The thesis consists of two major parts, and in the first part, the primary focus was to find a suitable model to describe fluid flow in an OBC. Several models were considered, and the results validated using literature results. The preferred turbulence model for OBCs was found to be the shear stress transport (SST) model. The effect of operational conditions on mixing needs to be evaluated for improving OBC performance. Presently, the evaluation of mixing was mainly done by the calculation of velocity ratio, which was questionable. Hence, a variable, mass concentration was introduced successfully to assess the mixing associated with oscillating flow patterns. This parameter was then applied to a pipe flow where velocity ratio method had been unsuccessful. Using the preferred turbulence model, the effects of operational variables and simultaneous oscillations of the flow and baffles were investigated for a single phase flow. Simulations showed that the overall turbulence increased for two-oscillation sources, but the power consumption increased. In the second part, the numerical modelling was carried out for gas-liquid flows in an OBC using computational fluid dynamics, related to which gaps of knowledge can be found in literature, particularly in relation to the effects of amplitude variation and aeration rate at a constant frequency. The gas hold-up, magnitude difference between the maximum and minimum velocities, as well as the mean bubble velocity were obtained, which showed good agreement with experimental results. The fine bubble generation was also attempted. Effects of three major variables, oscillation frequency and amplitude and aeration rate were investigated, and the results compared with literature results. At low aeration rate, a single-hole sparger produced very small bubbles accompanied by very large bubbles. At a high oscillation frequency and amplitude, small bubbles were accompanied by a large tail of air, which was smaller in volume compared to the total bubble volume. The results showed that the developed numerical models can produce reliable results in single and two-phase (gas-liquid) systems.