Population balance modelling of gas-liquid bubbly flow: capturing coalescence and breakup processes

Deju, L 2014, Population balance modelling of gas-liquid bubbly flow: capturing coalescence and breakup processes, Doctor of Philosophy (PhD), Aerospace, mechanical and manufacturing engineering, RMIT University.


Document type: Thesis
Collection: Theses

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Title Population balance modelling of gas-liquid bubbly flow: capturing coalescence and breakup processes
Author(s) Deju, L
Year 2014
Abstract Computational Fluid Dynamics (CFD) analysis is increasingly being used for the study of complex hydraulic and thermal behaviour in nuclear and chemical engineering research. The bubble column reactors provide an effective mean for heat and mass transfer or even chemical reactions with relatively low maintenance and operation cost. The scale-up and scale-down design of bubble column have acquired great importance from various industries due to its versatile application. To obtain a rational design and better operation, the hydrodynamics of the embedded gas-liquid flow must be understood. Bubbles within the bulk liquid flow undergo deformation, coalescence, breakage and condensation subject to local flow conditions and heat and mass transfer processes. To account for the coalescence and breakup phenomenon of gas-liquid bubbly flows, the population balance modelling (PBM) has been used along with continuity and momentum equations within the two fluid modelling frameworks. A comprehensive population balance model validation study has been done for assessing DQMOM (Direct Quadrature Method of Moments) in simulating gas-liquid flow with wide range of bubble sizes and strong bubble interactions, furthermore the relative merits and capabilities of applying DQMOM has also been studied in comparison to the ABND (Average Bubble Number Density) and MUSIG (MUltiple SIze Group) models under the same gas-liquid flow. Specific attention is directed towards evaluating the performance of DQMOM, ABND model and MUSIG (homogeneous and inhomogeneous) model in capturing the transition from wall peak to core peak radial void fraction distribution especially in large pipe flow, corresponding to the prevalence of lift forces acting on the small- and large-sized bubbles. Numerical results are validated against gas-liquid flow experiments published in literature.

The evolution of bubble size and its associated bubble migration due to the lift forces is well described by the inhomogeneous MUSIG approach. The assessment for performance of different population balance approaches reveals that behaviour of breakup and coalescence kernels has dominant effect on solution method of PBM. Hence, the research work is focused to gain more insight on the applicability of existing models in capturing the bubble coalescence and breakage phenomenon in a large bubble column comparable to practical industrial systems. In order to account this subject some widely adopted bubble coalescence and breakage kernels assessment were done in simulating the local hydrodynamic variables (e.g. void fraction and bubble size distribution) in large bubble column. A total of six coalescence and breakage kernels were considered. Numerical results were validated using the experimental data for the large-scale bubble column with inner diameter of 195.3 mm. The physical mechanism of each kernel and its coupling effects with the two-fluid model via interfacial forces has been investigated.
Degree Doctor of Philosophy (PhD)
Institution RMIT University
School, Department or Centre Aerospace, mechanical and manufacturing engineering
Keyword(s) Population balance
gas-liquid flow
coalescence
breakage
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Created: Fri, 28 Nov 2014, 09:40:14 EST by Denise Paciocco
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