- Title
- Influence of turbulent dispersion on transition in multiphase flows
- Creator
- Karim, Ifsana
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2019
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Bubble columns, where the liquid is the continuous phase and the gas phase is dispersed in the form of bubbles, are widely utilised in many industrial applications. When designing these bubble columns, it is important to be able to predict the conditions that mark the transition from homogeneous (bubbly) to heterogeneous (churn-turbulent) flow. The transition is a function of many variables, including liquid and gas superficial velocities, bubble diameter, liquid and gas physical properties. Linear stability analysis (LSA) has been successfully applied by many researchers to determine the gas volume fraction at which the instability takes place; and correspondingly a one-dimensional stability factor, f1, has been proposed. Briefly, the LSA analysis utilises the velocity fluctuations of both the dispersed and continuous phases associated with the specific energy dissipation rate of the two-phase mixture. Typically, the specific energy dissipation rate is correlated with the density of the bed. The previous analysis, however, does not consider the energy input which associated with the turbulence intensity (velocity fluctuations) of the incoming liquid stream. Usually, this component can be ignored in sparged bubble columns because its magnitude is relatively small and it also decays in the axial direction. In plunging liquid jet bubble columns, the liquid is introduced as a high-speed jet that entrains gas which is then broken into fine bubbles in the Mixing Zone. The bubbly mixture then passes into the Two-Phase Flow Zone where instabilities can be generated. The Mixing Zone is a region of high energy dissipation resulting in relatively large liquid velocity fluctuations, which can directly influence the instability of the Two-Phase Flow Zone. In this study, the existing linear stability analysis is modified to include the influence of inlet liquid velocity fluctuations on the stability parameter, f1. The modified theory is applied to the previous experimental work for a plunging liquid jet bubble column to determine the critical gas volume fraction at which transition takes place in the Two Phase Flow Zone. In order to apply the model, drift-flux analysis has been used to obtain bubble diameter as a function of gas and liquid superficial velocities, and computational fluid dynamics has been utilised to quantify the velocity fluctuations of the liquid exiting the Mixing Zone. Solid-liquid fluidised beds (SLFB) can operate in either homogeneous or heterogeneous regime characterised by respectively the uniform and non-uniform distribution of solid and liquid phases. Different industrial applications and hydrometallurgical operations are involved in the homogeneous system such as crystallisation, ion exchange, adsorptive, chromatographic separations etc. At some critical functional condition, regime transition could occur in the reactor. To ensure that the desirable regime prevails, it is desirable to sensibly control the interaction between the discrete and the continuous phase. The aim of the present work is therefore to investigate transition behaviour. Firstly, experiments are carried out on a solid-liquid fluidised bed comprising of glass beads with 8 mm diameter, 2520 kg/m3 density and water as the fluidising medium. A drift-flux analysis is presented which efficiently incorporated two-dimensional velocity and volume fraction profile across the flow area. The one-dimensional linear stability analysis is applied to determine the critical liquid superficial velocity and volume fraction at which the transition takes place in the system. In addition, the results are compared with the visually experiential transition.
- Subject
- bubble columns; linear stability analysis; velocity fluctuations; Two Phase Flow Zone; solid-liquid fluidised beds
- Identifier
- http://hdl.handle.net/1959.13/1397821
- Identifier
- uon:34359
- Rights
- Copyright 2019 Ifsana Karim
- Language
- eng
- Full Text
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| View Details Download | ATTACHMENT01 | Thesis | 6 MB | Adobe Acrobat PDF | View Details Download | ||
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