2D Numerical Simulations of Bubble Flow in Straight Pipes

Abstract

The main purpose of water aeration is to maintain healthy levels of dissolved oxygen (DO) concentration. Water aeration involves the injection of air or air bubbles into water treatment reservoir commonly through pipes. Fine bubble has higher mass transfer when its diameter gets smaller and smaller bubbles are more capable of enhancing DO concentration level. Two-phase flow consisting of air and water inside horizontal pipe with small diameter is capable of transferring fine bubbles into a body of water and its mechanism should be clearly understood for better system designing. Nevertheless, there are only a few studies that deal with the relationship between mathematical characteristics of two-phase flow inside horizontal pipe and DO concentration level. The main objective of this study is to perform 2-dimensional two-phase simulations inside horizontal pipe using the computational fluid dynamics (CFD) OpenFOAM (Open source Field Operation And Manipulation) tools to examine the effect of pipe wall shear stress on bubble size, which is the major factor effecting DO concentration level. Under different initial conditions, two-phase numerical simulations using Reynolds-averaged Navier-Stokes (RANS) combined with Eulerian-Eulerian method were performed to compute the axial Sauter Mean Diameter (SMD) of bubbles, water velocity, and wall shear stress within a 13.4 m long horizontal pipe with 50.3 mm inner diameter. The coalescence and breakage of bubbles caused by random collisions were considered during the simulations to predict the values of axial SMD. The water velocity and SMD were validated against the experimental data of Kocamustafaogullari and Wang (1991) and the relative errors ranged from 4% to 15% and 8% to 30%, respectively. Two additional experimental results obtained by Yin et al. (2012) and Water Supply Engineering Laboratory (WSEL) at SNU were gathered. These experiments deal with two-phase horizontal pipe flow under different configurations and DO concentration level. Their results were compared with the results obtained by Kocamustafaogullari and Wang (1991) and the aforementioned numerical analysis to determine the effect of pipe wall shear stress on bubble diameter and DO concentration level. As a result, the increase in pipe wall shear stress decreases bubble size and increases DO concentration level. By comparing the results and making links between them, it was concluded that the pipe wall shear stress plays a key role in breaking up the bubbles.