Influence of Bubble-Bubble Interactions on the Macroscale Circulation Patterns in a Bubbling Gas-Solid Fluidized Bed

The macro-scale circulation patterns in the emulsion phase of a gas-solid fluidized bed in the bubbling regime have been studied with a 3D Discrete Bubble Model. It has been shown that bubble-bubble interactions strongly influence the extent of the solids circulation and the bubble size distribution

Experimental study on solids mixing and bubble behavior in a pseudo-2D, freely bubbling, gas-solid fluidized bed using PIV and DIA

The hydrodynamics of a freely bubbling, gas-solid fluidized bed has been investigated experimentally with non-invasive measuring techniques in a pseudo-2D column filled with glass beads of 400-600 μm fluidized with air. Particle Image Velocimetry (PIV) combined with Digital Image Analysis (DIA) has been used to determine simultaneously the emulsion phase circulation patterns, bubble hold-up and bubble size and velocity distributions. The combination of DIA with PIV allows correcting for the influence of particle raining through the roof of the bubbles on the time-averaged emulsion phase velocity profiles. The time-averaged emulsion phase circulation patterns have been measured as a function of fluidization velocity. Moreover, with DIA the average bubble diameter and bubble velocity as a function of height and fluidization velocity have been determined and found to correspond reasonably well with literature correlations. The experimental data provides a basis for development and validation of CFD models to describe the solids-mixing in gas-solid fluidized beds

Modelling of large-scale dense gas–solid bubbling fluidised beds using a novel discrete bubble model

In order to model the complex hydrodynamic phenomena prevailing in industrial scale gas–solid bubbling fluidised bed reactors and especially the macro-scale emulsion phase circulation patterns induced by bubble–bubble interactions and bubble coalescence, a discrete bubble model (DBM) has been developed. In the DBM, the (larger) bubbles are modelled as discrete elements and are tracked individually during their rise through the emulsion phase, which is considered as a continuum. The DBM, originally developed for the description of gas–liquid flows, has been adapted to cope with bubbles with a diameter larger than the size of an Eulerian cell, which is required in view of the large bubble size distribution at higher gas flow rates. Moreover, a new drag model for a single bubble rising in a fluidised bed derived from empirical correlations has been implemented, as well as a simple model to account for bubble coalescence and break-up. The strong advantage of the DBM compared to other models previously reported in the literature for the description of large-scale fluidised beds is that it fully accounts for the two-way coupling between the bubbles and the emulsion phase, which enables direct computation of the emulsion phase velocity profiles. Comparison of the results of simulations ignoring bubble coalescence and simulations taking bubble coalescence properly into account demonstrated the significant effect of bubble coalescence on the large-scale circulation patterns prevailing in bubbling fluidised beds. The simulation results for the lateral profiles of the visible bubble flow rate have been compared qualitatively with experimental results reported by Werther [1974. Influence of the bed diameter on the hydrodynamics of gas fluidized beds. A.I.Ch.E. Symposium Series 70(141), 53–62]. The effect of the superficial gas velocity on the velocity and porosity profiles has been studied. In general, it can be concluded that the DBM is able to capture the salient features of the hydrodynamics of bubbling fluidised beds. However, further research is required to improve the closure equations for the bubble behaviour, bubble–bubble interactions and bubble coalescence and break-up to enable a complete quantitative description

Experimental study on solids circulation patterns and bubble behavior using particle imagevelocimetry combined with digital image analysis

The hydrodynamics, viz. the solids circulation patterns and\ud bubble behavior, of a freely bubbling gas-solid fluidized bed\ud has been investigated experimentally using Particle Image\ud Velocimetry (PIV) combined with Digital Image Analysis\ud (DIA). Coupling of these non-invasive measuring techniques\ud allows us to obtain information on both the bubble behavior\ud and emulsion phase circulation patterns simultaneously, in\ud order to study in detail their intricate interaction. In\ud particular, the combination of DIA with PIV allows correcting\ud for the influence of particle raining through the roof of the\ud bubbles on the time-averaged emulsion phase velocity\ud profiles. Because of the required visual access, this technique\ud can only be applied for pseudo-2D fluidized beds.\ud The bubble rise velocity as a function of the equivalent\ud bubble diameter and the average bubble diameter as a\ud function of the position above the distributor were\ud determined with DIA and compared with literature\ud correlations. Subsequently, the importance was demonstrated\ud of filtering the instantaneous emulsion phase velocity profiles\ud obtained with PIV for particle raining, using DIA, to obtain\ud the time-averaged emulsion phase velocity profiles. The timeaveraged\ud solids circulation patterns have been studied as a\ud function of the superficial gas velocity and bed aspect rati

Experimental and computational investigation on the macroscopic circulation patterns in a bubbling gas-solid fluidized bed

The hydrodynamics of a freely bubbling, pseudo 2-D fluidized bed has been investigated experimentally for different bed aspect ratios at different superficial gas velocities by using Particle Image Velocimetry (PIV) combined with Digital Image Analysis (DIA). Coupling of both non-invasive measuring techniques allows us to obtain information on both the bubble behavior and emulsion phase circulation patterns simultaneously. In particular, the combination of DIA with PIV allows to correct for the influence of particle raining through the roof of the bubbles on the time-averaged emulsion phase velocity profiles

Approximation for the enhancement factor applicable to reversible reactions of finite rate in chemically loaded solutions.

A new explicit relation is proposed for the prediction of the enhancement factor for reversible reactions of finite rate in chemically loaded solutions which also allows for unequal diffusivities. The relation for the enhancement factor is not based on an approximation of the absorption process, but is derived from a similarity which can be observed between the results of the approximation for an irreversible (1,1) order reaction given by, for example, DeCoursey (surface renewal model), and the exact numerical results. The present relation combines the solution of DeCoursey (1974 Chem. Engng Sci. 29, 1867¿1872) for irreversible finite rate reactions, and the solution of Secor and Beutler (film model, 1967 A.I.Ch.E. J. 13, 365¿373) for instantaneous reversible reactions. The diffusivity ratios in the solution of Secor and Beutler (1967) were replaced by the roots of these ratios in order to adapt the enhancement factors to the penetration theory. In general, this adaptation of the solution of Secor and Beutler gave reasonably good results, however, for some situations with unequal diffusivities deviations up to 20% were found. The results of the present approximation were for various reactions compared to the numerical enhancement factors obtained for the model based on the Higbie penetration theory. Generally, the agreement was reasonably good. Only 26 of 2187 preselected simulations (1.18%) had a deviation which was larger than 20%, while the average deviation of all simulations was 3.3%. The deviations increased for solutions with a substantial chemical loading in combination with unequal diffusivities of the components. For reactions with a kinetic order unequal to unity, the Ha number had to be multiplied by a factor, ¿¿, so that Ea = ¿¿H aA in the regime 2 < HaA Ea,¿. This factor agreed well with the factor given by Hikita and Asai (1964, Int. Chem. Engng 4, 332¿340) in their dimensionless numbe

Investigation of the influence of bubble-bubble interactions on the hydrodynamics of bubbling gas-solid fluidised beds using the discrete bubble model

To investigate the hydrodynamic behaviour of industrial scale bubbling fluidised bed reactors, a 3D Discrete Bubble Model (DBM) has been developed. In the DBM, an Euler-Lagrange model, the bubbles are treated as discrete elements and the bubble trajectories are tracked individually, while the emulsion phase is considered as a continuum and is described with the continuity and Navier-Stokes equations. The main advantage of the DBM is that it fully accounts for the two-way coupling, allowing computation of the prevailing macroscopic circulation patterns in large scale gas-fluidised beds. In this paper, we have examined the effect of bubble-bubble (wake) interactions on the macro-scale velocity profiles using the DBM. It has been found that the extent of the macroscopic circulation is significantly increased by the bubble-bubble interaction force

Electronic Properties of Ultra-Thin Aluminum Nanowires

We have carried out first principles electronic structure and total energy calculations for a series of ultrathin aluminum nanowires, based on structures obtained by relaxing the model wires of Gulseren et al. The number of conducting channels is followed as the wires radius is increased. The results suggest that pentagonal wires should be detectable, as the only ones who can yield a channel number between 8 and 10.Comment: 9 pages + 3 figures, to appear on Surface Scienc