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• The modified SGS-TDF model was proposed and implemented in Euler/Euler LES for bubbly flow. • LES coupled with the modified SGS-TDF model can simulate and mimic bubble oscillation. • κ−5/3 and κ−3 scaling laws were recovered in Euler/Euler LES modelled turbulent kinetic energy spectrum. • A correlation of vorticity indicator Q and the local bubble volume fraction was proposed. It has now been recognised and generally accepted that turbulent dispersion may be modelled using the time average of the fluctuating part of the interphase momentum, employing the drag the Favre averaged drag model for turbulent dispersion in Eulerian multi-phase flows. As the turbulent eddies in the surrounding of bubbles interact strongly with the bubbles in bubbly flow, the bubble trajectories and bubble oscillation take place accordingly as the consequence of continuous deformation of the bubble surfaces. When using large eddy simulation for modelling bubbly flow, the sub-grid scale (SGS) filtered velocity fluctuations of liquid phase can be interpreted as many small eddies that may act on the surface of bubbles, consequently giving rise to bubble shape variations and the dispersion of bubbles. This study employs Euler/Euler large-eddy simulation (LES) modelling to demonstrate that the turbulent dispersion force model can be used to effectively indicate the influence of turbulent eddies on bubble dynamics, in particular the bubble cluster oscillations, which leads to remarkable improvements in the prediction of bubble lateral dispersion behaviour. The use of spatial filtering to model the SGS bubble dispersion is proposed with a modification on SGS eddy viscosity to reflect turbulent dispersion due to bubble induced turbulence. The results of the time-averaged LES modelled bubble velocities and bubble volume fraction profiles are in good agreement with the experimental data while the turbulent kinetic energy spectrum obtained at different locations on the centreline of the bubble column still exhibits the conventional −5/3 scaling for shear induced turbulence and a −3 scaling for bubble induced turbulence. [ABSTRACT FROM AUTHOR]

• Considering the effects of bubble induced turbulence in the bubble breakage model. • κ −3 scaling has been identified on bubble induced turbulence kinetic energy spectrum. • Mean turbulent velocity and number density of turbulent eddies have been modified. • Influence of characteristic length scale on daughter bubble PDF is illustrated. • Effect of eddy-bubble contact on mass transfer in the bubble column is demonstrated. A quantitative description of the interaction between the turbulence eddies and bubble groups is crucial for the prediction of the bubble size distribution in bubble columns when adopting the population balance model (PBM) to estimate the heat and mass transfer across the interfaces between bubbles and carrier liquid. The majority of breakage kernels that are currently adopted focus on the shear turbulence in the liquid phase and model the kinetic energy contained in the arrival eddies that hit the bubbles by using the classical single-phase turbulence Kolmogorov −5/3 scaling law. In bubble columns, eddies that collide with bubbles and cause the bubble breakage are mainly generated by bubble-induced turbulence (BIT). The present work focuses on the influence of κ −3 scaling of the bubble-induced turbulence energy spectrum on the bubble breakage due to the eddy-bubble collision in bubble columns. A modified breakage model accounting for the bubble-induced turbulence is proposed. The proposed breakage model considers the mean turbulent velocity of eddies under the influence of bubble-induced turbulence and the characteristic wavenumber/length scale that corresponds to BIT. A comparison of the simulation results with the experimental data clearly demonstrate that the modified breakage model appropriately describes the bubble breakage events occurring under the influence of bubble-induced turbulence in bubble columns. It is also shown that the interaction of bubbles with the bubble-induced turbulence eddies has an impact on prediction of the mass transfer in bubble column flows. [ABSTRACT FROM AUTHOR]

Bubble fluidisation in the fuel reactor adopted in chemical looping combustion (CLC) has a siganificant impact on the operation efficiency. Although a variety of numerical modellings of fluid dynamic process in the fuel reactors have been conducted, studies on predicting the fluidised bubble behaviours in the cylindrical fuel reactor where the effect of the heterogeneous reaction is also considered are still lacking. In this paper, the use of correlations of fluid dynamic parameters to characterise the fluidised bubble formation in the fuel reactor was proposed. A correlation parameter relating the time-dependent fluidised bubbles to the local eddies was introduced by correlating the local gas velocity fluctuation with the pressure fluctuation. The existence of a strong correlation between the concentration of gaseous reactants or products and local vortices was also demonstrated. Three-dimensional multiphase CFD model coupled with the heterogenoeous reaction kinetics was employed to study the details of CLC process in the fuel reactor. The results clearly indicated that the approach used in the present work can effectively monitor the formation of fluidised bubble in the dense fluidised bed during the heterogeneous reaction and may be used in the CLC as an indicator for monitoring the reduction rate as the locally embedded large eddies are strongly associated with the fluidised bubble occurrance. [ABSTRACT FROM AUTHOR]

• E-E LES modelling on three CO 2 chemisorption cases in bubble columns was conducted. • Modified SGS turbulent dispersion (TDF) and added mass stress (AMS) models proposed. • Impacts of the SGS-TDF and SGS-AMS terms on the interfacial mass transfer were assessed. • Scaling laws of turbulent kinetic energy and species concentration spectra were discussed. It has been generally recognised that the turbulent dispersion force plays an important role in interphase momentum transfer. However, the effect of the added mass stress on the momentum and mass transfer in bubble column bubbly flow has not been addressed appropriately. As the turbulent eddies in the surroundings of bubbles interact strongly with the rising bubbles in bubble column bubbly flow, such interactions will bring out the change of interfacial areas between the bubbles and carrier fluid, consequently leading to changes in the interfacial mass transfer. When employing large eddy simulation for modelling bubbly flow coupled with the chemisorption process, the SGS filtered velocity fluctuations of liquid phase can be interpreted as the turbulent eddies that continuously hit the surfaces of bubbles, causing bubble deformation and the variation of bubble interfacial areas, which give rise to the turbulent dispersion and added mass stress forces. The present study will demonstrate through Euler/Euler large-eddy simulations (LES) modelling that by considering the turbulent dispersion force (SGS-TDF) and added mass stress (SGS-AMS), the bubble dynamics and mass transfer under the chemisorption conditions can be better indicated, which leads to remarkable improvements in the prediction of bubble lateral dispersion and the interfacial mass transfer. The turbulent dispersion and added mass stress related to the spatially filtering were proposed with a modification on SGS eddy viscosity to reflect turbulent dispersion due to bubble-induced turbulence. A comprehensive assessment of the effects of these additional filtered stress terms on the time-averaged velocity and bubble volume fraction profiles, flow patterns, mass transfer and the pH variation during CO 2 chemisorption and the turbulent kinetic energy and species concentration spectra was conducted. [ABSTRACT FROM AUTHOR]

Using an empirical drag coefficient model to investigate the laws of bubble rising in non-Newtonian fluids is important for calculating the safe cycle time in avoiding typhoon, tripping operation in offshore drilling engineering. Herein, the drag coefficient for a single bubble rising steadily in a static non-Newtonian fluid within a wellbore was experimentally investigated using a vertical, cylindrical wellbore mimic. The effects of viscosity, density and surface tension of the fluids, as well as bubble diameters on the drag coefficient were studied. The experimental results indicate that the drag coefficient increases with the increase of the fluid viscosity, surface tension and bubble diameters, while the solution density has little effect on the drag coefficient. Meanwhile, the relationship between Reynolds number (Re) and drag coefficient and that between Re and Eötvös numbers ( Eo ) were analyzed, respectively. The result suggests that surface motion leads to a transition of drag coefficient. A new correctional model for the drag coefficient of single bubbles rising in static non-Newtonian fluids in a wellbore was obtained, which showed a good agreement with the experimental data. [ABSTRACT FROM AUTHOR]

Chemical looping combustion (CLC) as a potential CO 2 capture technology has been considered as a promising and likely alternative to traditional combustion technology to mitigate the CO 2 emission due to its prosecution of CO 2 sequestration at a low cost. Although a number of studies on the hydrodynamic behaviours of the CLC process in fuel reactor have been documented in the open literature, there have been rare studies on the correlation between the bubble formation and the local particulate volume fraction. This paper aims to investigate the CLC process in a fuel reactor using the CFD modelling, coupled with the heterogeneous reactions, the hydrodynamics and reaction kinetics occurring in the fuel reactor. A parameter correlating the occurrence of bubble and dynamic properties is proposed. The parameter may be acted as an indicator of time-dependent bubble evolution with a potential to be adopted in the CLC for controlling the bubbling phenomena since the occurrence of the bubbles at specific positions is highly correlated with the local large eddies embedded in the flow. The results obtained clearly indicate that the CFD model developed in the current study reasonably forecasts the hydrodynamic behaviours and important phenomena observed in the fuel reactor. [ABSTRACT FROM AUTHOR]

Abstract: Injecting superheated water vapour bubbles into a sub-cooled shear flow will induce heat and mass transfer in the form of condensation. Of particular interest is finding the characteristic role that the large-scale vortex structures play on bubble condensation. Because the heat transfer and condensation take place mainly due to convection, the bubble is expected to collapse at an earlier stage because of the trapping by large-scale vortices. The aim of this paper is to investigate two-way thermal and momentum interaction between bubbles and the large-scale coherent structures in a plane turbulent shear layer, focusing on the effect of bubble condensation on large-scale vortex structures embedded in plane, free shear layers. The parameters such as initial bubble temperature, bubble injection location, and carrier fluid temperature have been chosen to examine their effects on the condensation and dispersion of bubbles. Because superheated vapour bubbles immersed within a sub-cooled shear flow field experience heat transfer, the gas phase will certainly condense into liquid if these bubbles remain surrounded long enough by this same sub-cooled liquid. Accordingly, bubble condensation is evident within the shear layer and bubble dispersion is influenced by the large-scale vortex structures. The main driving force behind complete bubble condensation was the difference in temperature between the bubble vapour and the carrier fluid. It was revealed that the effect of the large-scale vortex structures is that the condensed bubbles acquire a larger dispersion than bubbles that were not subject to thermal coupling. [Copyright &y& Elsevier]

In the study of meso-scale structures of multi-phase flow in bubble columns, accurate modelling of the interaction between the turbulence eddies and particle/bubble groups is crucial for capturing the heat and mass transfer occurring between the bubbles and surrounding carrier fluid. This work focuses on the influence of bubble shape variations on bubble breakage due to the eddy collision with the bubbles in bubble column flows. An improved breakage model accounting for the variation of bubble shapes was proposed. The improved breakage model coupled with the widely adopted isotropic, homogeneous turbulence kinetic energy spectrums, that are currently available from the open literature, takes into account the different energy requirements in forming the daughter bubbles, i.e. the increase of in surface energy and the pressure head difference of the bubble and its surrounding turbulent eddies. The simulation results compared with experimental data have clearly demonstrated that the improved model effectively describes the various shapes of bubble breakage events, which may consequently have a strong impact on the interfacial area estimation that is crucial for calculation of the transfer rates of mass and heat transfer in the bubble columns. [ABSTRACT FROM AUTHOR]