MODELING TURBULENT GAS-LIQUID BUBBLY FLOW IN A VERTICAL PIPE
Date
2019-11-25
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Journal ISSN
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Type
Thesis
Degree Level
Doctoral
Abstract
Bubbly gas-liquid turbulent flow occurs in various industrial applications, for example oil and gas production, petrochemical plants, nuclear reactors, etc. The analysis of bubbly gas-liquid turbulent flow remains a challenging task due to complexities such as the dispersed gas phase effects on the continuous liquid phase turbulence, interphase momentum exchange, and redistribution of the gas volume fraction due to bubble coalescence and breakup. The focus of this thesis is to develop a computational model to address these challenges. The model developed in this thesis uses a state-of-the-art two-fluid method, which minimizes computational resources and is based on the Reynolds-Averaged Navier-Stokes (RANS) equations. The predictions obtained for bubbly upward flow in a vertical pipe were validated against the available experimental data.
The first part of this thesis, chapter 2, documents a one-dimensional Eulerian-Eulerian two-fluid model for mono-disperse bubbly gas-liquid flow. The main challenge is the prediction of the gas volume fraction profile, based on the radial force balance of the non-drag forces for the gas phase. The shape of the volume fraction profile across the pipe changes depending on the bubble size. The volume fraction profile exhibits a peak value near the wall and at the centre line of the pipe for smaller and larger bubbles, respectively, which is consistent with experimental measurements. For the model tested, the turbulence kinetic energy was observed to increase for larger size bubbles compared to the smaller size bubbles.
The second part of the thesis, chapter 3, reports a thorough investigation of the effect of bubbles on the liquid phase turbulence, referred to as turbulence modulation. The presence of bubbles in the flow can either enhance or attenuate the liquid phase turbulence. For the same flow conditions, the effect of the turbulence modulation shows both enhancement and suppression for the turbulence kinetic energy in different locations in the pipe. A budget analysis of the turbulence transport equations was used to provide insight on the relative importance of the turbulence modulation and to identify the region where it plays a significant role. The turbulence modulation was often found to have an insignificant effect on the prediction for the mean flow variables.
The third part of the thesis, chapter 4, describes a numerical study of poly-disperse gas-liquid flow, which contains bubbles of different diameter. For a poly-disperse distribution of gas bubbles, the model must account for the consequences of bubbles either breaking up or coalescing with each other. To explore their effect, an inhomogeneous multiple size group (iMUSIG) approach with a bubble coalescence and breakup model was implemented. The developed model was shown to correctly redistribute the gas volume fraction among the bubble groups based on the coalescence and breakup processes. The turbulence modulation for the poly-disperse flow was found to be larger than for the mono-disperse case, which indicates one additional effect of a poly-disperse distribution of gas bubbles.
Overall, this thesis research implemented a two-fluid model that is able to capture important features of bubbly gas-liquid flow for both mono-disperse and poly-disperse cases. Some significant features of the model are: the use of a radial force balance for the gas volume fraction evaluation; a turbulence modulation contribution based on source terms in the turbulence transport equations; and incorporating the effect of coalescence and breakup processes and the resultant exchange of gas volume fraction among different bubble groups. As such, the thesis documents an improved predictive model for RANS simulations of bubbly gas-liquid flow in industrial applications.
Description
Keywords
Computational fluid dynamics, Reynolds-Averaged Navier-Stokes modeling, Two-fluid model, gas-liquid flow, gas volume fraction, low Reynolds number k-ε model, turbulence modulation, poly-disperse bubbly flow, multiple bubble size group, coalescence and breakup.
Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
Mechanical Engineering
Program
Mechanical Engineering