Computational Fluid Dynamics Simulations Of Phase Distribution In Adiabatic Upward Bubbly Flows Using Interfacial Area Transport Equation

Abstract: In analyzing gas-liquid two-phase flows, it is imperative to take into account the bubble-bubble and bubble-eddy interactions, which may result in bubble coalescence and/or disintegration. The bubble coalescence and disintegration will further affect the interfacial structure, which could, to a first-order approximation, be characterized by the interfacial area concentration and void fraction. In this thesis, a one-group interfacial area transport equation (IATE) that was developed in the literature to describe the evolution of the interfacial area concentration in bubbly flows was implemented into a computational fluid dynamics (CFD) code, namely, FLUENT. Previous studies demonstrated that the following three bubble interaction mechanisms were essential in bubby flows of interest: coalescence of bubbles due to bubble random collisions driven by turbulence, coalescence of bubbles due to wake entrainment, and disintegration of bubbles caused by turbulent eddy impact. These three mechanisms have been taken into account in the one-group IATE. The current study focuses on examining the capability of FLUENT with the one-group IATE in predicting the phase distribution in adiabatic bubbly flows. Eulerian multiphase model in FLUENT 6.2.16 is applied, in which two sets of conservation equations are applied to each phase separately but coupled through interfacial transfer terms. The constitutive relations of the interfacial transfers are provided using the interfacial area concentration described through the IATE. CFD simulations of adiabatic upward bubbly flows in a circular pipe have been carried out in this study. In the simulation, the interfacial area concentration is first introduced into FLUENT as a user- defined scalar and then the corresponding IATE is solved for the interfacial area concentration. In addition, the associated modifications to the interfacial drag force model and turbulence model are made to reflect the evolution of the bubble size by replacing the constant bubble diameter in those models with the Sauter mean diameter, which is a function of the void fraction and interfacial area concentration. With comparisons between the simulation results and available experimental data, satisfactory agreement has been achieved, which demonstrates that FLUENT code with the one-group IATE provide valuable simulation tool for bubbly two-phase flows.