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Formation of fluidised bubbles in a fluidised bed has significant impact on the design and scale-up of the bubbling fluidised bed. To fully understand bubble motions in the fluidised bed, fluidised bubble dynamics in a gas-solid fluidised bed is investigated using CFD-DEM numerical simulation. To reveal the mechanism of bubble formation in the bubbling fluidised bed, the time autocorrelation approach is adopted to identify bubble occurrence at different bed height positions. A non-dimensional correlation accounting for the effect of local velocity field on the bubble size, which is obtained by modifying the Darton's model, is proposed for predicting the bubble size at given bed height positions. The non-dimensional correlation for predicting the size of the fluidised bubbles at various given bed height positions is obtained by regression based on the CFD-DEM modelling results. The simulation results indicate that small bubbles with high passing frequency dominate in the bottom region of the fluidised bed, while large bubbles are formed in the zone close to the free surface of the fluidised bed. The probability of the appearance of large bubbles increases as the height measured from the bottom of the fluidised bed increases, which may be attributed to the coalescence of small bubbles; this is also reasonably consistent with the results predicted using the Darton's model and experimental observations. It was observed that the sizes of the fluidised bubbles at the given height positions correlate with the local dynamic parameters characterised by the Reynolds number and the dimensionless height ratio. [ABSTRACT FROM AUTHOR]

CO 2 absorption by a confined free film has been investigated using the CFD approach. Particular attention focuses on the relationship between mass transfer and flow behaviour of the confined free film. Simulation results show that the average K L value grows 26.9% for the open window region and 17.4% for the lower-wall region, respectively, in comparison to the values in the upper-wall region, and the average K L value of confined free film along the flow direction is about 19.2% higher than the value of wall-bounded film, which affirms that the use of an open window can significantly enhance mass transfer performance and such effect also has impact on the entire regions. It was revealed that there exist two types of vortices inside the film, inner vortices and interfacial vortices. In addition, a correlation coefficient R Ωc is proposed to provide a quantitative measure to characterise the relationship between the local mass concentration profile and vorticity distribution of confined free film. Simulation shows that the average value of R Ωc is 0.93, indicating that the two parameters are highly correlated. Furthermore, the cause of lower correlation coefficient value is discussed. [ABSTRACT FROM AUTHOR]

A Taylor–Couette (TC) reactor has been considered as a promising technology to produce uniform ultra-fine particles with controlled morphology and size because of its rather narrow shear rate distribution in comparison to a conventional stirred tank. However, small particles in the TC reactor are likely entrapped by the Taylor vortices and concentrated more in the core of the vortices, thus having an adverse or a positive impact on producing the uniformly fine particles. To mitigate this disadvantage, one approach is to adopt an inner cylinder with a variable cross-section through the proper design to obtain a non-constant gap between the inner and outer cylinders in the TC reactor. This study investigates the use of specific profiles of the cross section for the inner cylinder to deform the Taylor vortices, effectively reducing the regions of low velocity and shear rate in the TC reactor. The profiles are composed of a series of curves. The flow patterns and shear rate distributions in the modified TC reactors were investigated using CFD modelling. The sliding mesh method and the Reynolds stress model were employed in the simulation to account for highly turbulent rotating flows. The simulated results compared with those for the classical TC reactor clearly indicate that the regions of low shear rate are effectively reduced in the modified TC reactors because the Taylor vortices occurring in the gap becomes temporal-periodic with respect to the stationary outer cylinder. The obtained shear rate distributions are remarkably narrower than those of the classical TC reactor. It was revealed that the curvature of curves for the cross-section profiles can significantly affect the shear rate distributions, and there exists an optimal profile for the inner cylinder cross-section. [ABSTRACT FROM AUTHOR]

For the simulation of industrial-scale bubble column reactors, while modelling the gas distributor as uniform inlets oversimplifies the inhomogeneity introduced by inlets, the direct simulation of the full geometry of gas distributor or sparger brings about enormous pre-processing work and huge computational cost. A new inlet model is therefore proposed in this paper to simplify the modelling of gas distributor and meanwhile maintain the simulation accuracy. The new inlet model is validated by the comparison of the model prediction with experiments and the CFD simulation incorporating the full geometry of gas distributor for bubble columns of small or large diameters. Comparisons of three different inlet boundary conditions, i.e., the direct simulation of gas distributor, the uniform inlet, and the new inlet model, are made in the simulation of the total gas holdup, the radial profiles of gas holdup at different cross-sections along the column height, and the axial velocity of liquid at various superficial gas velocities. The results indicate that the new inlet model is capable of achieving a good balance between simulation accuracy and computational cost for the CFD simulation of large-scale bubble column reactors. [ABSTRACT FROM AUTHOR]

The entrainment and flow of a thin film of liquid on a vertically rotating disc partially immersed in a liquid bath has been investigated experimentally and modelled numerically. The Volume of Fluid (VOF) Computational Fluid Dynamics (CFD) modelling approach has been employed to characterise the shape and stability of the thin film thickness profile. The thickness of the rotating disc plays a significant role in the thin film profile and this is confirmed through the comparison of simulation with the experimental results. Other factors determining the film thickness were identified as the rotational speed and the fluid viscosity where the film thickness profile increases with the increase of the rotational speed and also the viscosity. A correlation equation to predict the film thickness as a function of angular position, radius, rotating speed, viscosity and surface tension is proposed. The results given in this study specifically report on the thin film thickness variation with the angular direction and the film entrained into the liquid. In both the simulation and experimental results, it is noted that the film thickness stabilises following a rotation of 15° after drag out of the liquid, and remains so until 10° before being dragged back in. [ABSTRACT FROM AUTHOR]

Traditionally, bubble shapes have been assumed to be spherical in breakup models such as the one developed by Luo and Svendsen in 1996. This particular breakup model has been widely accepted and implemented into computational fluid dynamics (CFD) modeling of gas–liquid two-phase flows. However, simulation results from this model usually provide unreliable predictions about the breakage of very small bubbles. The incorporation of bubble shape variation into breakup models has rarely been documented in literature but the bubble shape plays an important role in the interactions with the surrounding eddies, especially when the effects of bubble deformation, distortion, and bubble internal pressure change are considered during the events of eddy-bubble collision. Thus, the assumption of spherical bubbles seems to be no longer appropriate in reflecting this phenomenon. This study proposes and implements a modified bubble breakup model, which accounts for the variation of bubble shapes when solving the population balance equations for CFD simulation of gas–liquid two-phase flows in bubble columns. The key parameters predicted by the modified breakup model have been compared with the ones predicted by the original model. The simulation results of interfacial area and mass transfer coefficient for larger bubbles have been greatly enhanced by the modified breakup model. [ABSTRACT FROM AUTHOR]

• A multiphase CFD model coupling the cavitation model and chemical reactions is developed for the first time. • Influences of acoustic cavitation on "local" micro-mixing process in the CIJR are investigated. • An increase in the power of ultrasound can improve the micro-mixing efficiency. • Ultrasound has larger effects on the meso-mixing process than micro-mixing process at the tip of transducer head. • Meso-mixing process controls the overall process with or without applying ultrasound. Studies on the influences of ultrasound-induced cavitation on the micro-mixing process in a confined impinging jet reactor (CIJR) are still lacking. In the present study, experimental and numerical approaches are employed to investigate the effects of ultrasound intensification on the mixing processes in a CIJR. The numerical results are experimentally validated. The model is also coupled with chemical reactions for the first time. It is found that local eddies generated by the collapse of acoustic microbubbles and "turbulence"-like acoustic streaming can significantly reduce the micro-mixing time. In all cases, the micro-mixing efficiency is found to increase with an increase in the power of ultrasound. A reduction of 15–24% in micro-mixing time can be achieved at the inlet velocity of 0.2m/s when the power of ultrasound increases from 122.5W to 175W. It is also found that the effect of ultrasound intensification on the meso-mixing process is larger than that on the micro-mixing process in the vicinity of transducer head. The ratio of micro-mixing time to meso-mixing time in the case of P ultra =175W can be 88% larger than that in the case of P ultra =0W in the front of the transducer head at Re in =5000. [Display omitted] [ABSTRACT FROM AUTHOR]