DYNAMIC SIMULATION OF DISPERSED GAS-LIQUID 2-PHASE FLOW USING A DISCRETE BUBBLE MODEL

In this paper a detailed hydrodynamic model for gas-liquid two-phase f low will be presented. The model is based on a mixed Eulerian-Lagrangi an approach and describes the time-dependent two-dimensional motion of small, spherical gas bubbles in a bubble column operating in the homo geneous regime. The motion of these bubbles is calculated from aforce balance for each individual bubble, accounting for all relevant forces acting on them. Contributions from liquid-phase pressure gradient, dr ag, virtual mass, liquid-phase vorticity and gravity are considered, w hereas direct bubble-bubble interactions are accounted for via an inte raction model resembling the collision model developed by Hoomans et a l. (1996) to model gas-fluidized beds. The liquid-phase hydrodynamics are described using the volume-averaged, unsteady, Navier-Stokes equat ions. A preliminary model validation has been performed by comparing t he computational results with experimental observations published prev iously in literature by various authors. The model is shown to predict correctly the motion of a bubble plume in a pseudo-two-dimensional bu bble column operated at different superficial gas velocities, provided that a detailed description of the bubble dynamics is incorporated in the model. The effect of bubble column aspect ratio on the hydrodynam ic behaviour of the column has also been investigated. Our model predi cts the effect of aspect ratio on the flow structure in the bubble col umn. The importance of the various forces acting on;the bubbles will a lso be discussed and it will be shown that the added mass force and th e lift force cannot be neglected in bubble column simulation. Finally, the model has been used to study the start-up behaviour of a two-dime nsional bubble column. It will be shown that the history of the gas-li quid two-phase flow significantly affects the flow structure ultimatel y obtained in a bubble column. This finding has, to our knowledge, not been reported before in literature. (C) 1997 Elsevier Science Ltd.