Experimental and computational study on the bubble behavior in a 3-D fluidized bed

The results from a two-fluid Eulerian–Eulerian three-dimensional (3-D) simulation of a cylindrical bed, filled with Geldart-B particles and fluidized with air in the bubbling regime, are compared with experimental data obtained from pressure and optical probe measurements in a real bed of similar dimensions and operative conditions. The main objectives of this comparison are to test the validity of the simulation results and to characterize the bubble behavior and bed dynamics. The fluidized bed is 0.193 m internal diameter and 0.8 m height, and it is filled with silica sand particles, reaching a settle height of 0.22 m. A frequency domain analysis of absolute and differential pressure signals in both the measured and the simulated cases shows that the same principal phenomena are reproduced with similar distributions of peak frequencies in the power spectral density (PSD) and width of the spectrum. The local dynamic behavior is also studied in the present work by means of the PSD of the simulated particle fraction and the PSD of the measured optical signal, which reveals as well good agreement between both the spectra. This work also presents, for the first time, comparative results of the measured and the simulated bubble size and velocity in a fully 3-D bed configuration. The values of bubble pierced length and velocity retrieved from the experimental optical signals and from the simulated particle fraction compare fairly well in different radial and axial positions. Very similar values are obtained when these bubble parameters are deduced from either simulated pressure signals or simulated particle volume fraction. In addition, applying the maximum entropy method technique, bubble size probability density functions are also calculated. All these results indicate that the two-fluid model is able to reproduce the essential dynamics and interaction between bubbles and dense phase in the 3-D bed studied.     

Insights in distributed secondary gas injection in a bubbling fluidized bed via discrete particle simulations

Experiments have shown that distributed secondary gas injection via a fractal injector in fluidized beds can significantly reduce the bubble size, and may also decrease the bubble fraction. In order to gain insight into the distribution of the gas between the phases and the mechanisms behind these effects simulations of small bubbling fluidized beds with one or two secondary gas injection points were carried out using a discrete particle model. Although the systems are very small, so that wall effects cannot be excluded, the model predicts that the bubble size and bubble fraction both decrease with secondary gas injection, while the gas flow through the dense phase increases. The secondary gas tends to stay in the dense phase, which limits the amount of gas available to form bubbles and is the main contributor to the decrease in the bubble size and fraction. The gas-solid contact improves as a result.     

Novel phenomenological discrete bubble model of freely bubbling dense gas–solid fluidized beds: Application to two‐dimensional beds

A phenomenological discrete bubble model has been developed for freely bubbling dense gas–solid fluidized beds and validated for a pseudo‐two‐dimensional fluidized bed. In this model, bubbles are treated as distinct elements and their trajectories are tracked by integrating Newton's equation of motion. The effect of bubble–bubble interactions was taken into account via a modification of the bubble velocity. The emulsion phase velocity was obtained as a superposition of the motion induced by individual bubbles, taking into account bubble–bubble interaction. This novel model predicts the bubble size evolution and the pattern of emulsion phase circulation satisfactorily. Moreover, the effects of the superficial gas velocity, bubble–bubble interactions, initial bubble diameter, and the bed aspect ratio have been carefully investigated. The simulation results indicate that bubble–bubble interactions have profound influence on both the bubble and emulsion phase characteristics. Furthermore, this novel model may become a valuable tool in the design and optimization of fluidized‐bed reactors. © 2012 American Institute of Chemical Engineers AIChE J, 2012     

基于EMMS模型的气固鼓泡床的模拟及气泡特性的分析

基于EMMS曳力模型,采用双流体的方法对气固鼓泡床内的气固流动特性进行模拟,建立基于图像处理气泡特性的分析方法,重点研究了不同表观气速下气泡在床层内分布特性,包括气泡平均当量直径、气泡速度和气泡球形度的轴向分布,以及气泡的生命周期。研究结果表明,小气泡多集中在床层底部和壁面区域,而大气泡多集中在床层中间区域。随着表观气速的增加,床层高度不断增加,气泡的球形度降低,气泡的大小、出现频率、上升速度以及生命周期均增加;然而,当表观气速增大到一定程度,继续增加气速对气泡的上升速度影响不大。     

Hydrodynamic modelling of dense gas-fluidised beds: comparison and validation of 3D discrete particle and continuum models

A critical comparison of a hard-sphere discrete particle model, a two-fluid model with kinetic theory closure equations and experiments performed in a pseudo-two-dimensional gas-fluidised bed is made. Bubble patterns, time-averaged particle distributions and bed expansion dynamics measured with a nonintrusive digital image analysis technique are compared to simulation results obtained at three different fluidisation velocities. For both CFD models, the simulated flow fields and granular temperature profiles are compared. The effects of grid refinement, particle-wall interaction, long-term particle contacts, particle rotation and gas-particle drag are studied. The mechanical energy balance for the suspended particles is introduced, and the energy household for both CFD models is compared. The most critical comparison between experiments and model results is given by analysis of the bed expansion dynamics. Though both models predict the right fluidisation regime and trends in bubble sizes and bed expansion, the predicted bed expansion dynamics differ significantly from the experimental results. Alternative gas-particle drag models result in significantly different bed dynamics, but the gap between model and experimental results cannot be closed. In comparison with the experimental results, the discrete particle model gives superior resemblance. The main difference between both CFD models is caused by the neglect of particle rotation in the kinetic theory closure equations embedded in the two-fluid model. Energy balance analysis demonstrates that over 80% of the total energy is dissipated by sliding friction. Introduction of an effective restitution coefficient that incorporates the additional dissipation due to frictional interactions significantly improves the agreement between both models.     

Numerical simulation of bubble interactions using an adaptive lattice Boltzmann method

Bubble interactions have significant impact on the shape and motion of bubbles, and therefore the dynamics of bubbles in a swarm may be considerably different from that of an isolated bubble. This research presents a numerical study of bubble interactions using a novel lattice Boltzmann method (LBM). By using the adaptive mesh refinement (AMR) and the multiple-relaxation-time (MRT) algorithm, this technique is able to accurately capture the deformation of the interface, and can remain numerically stable for low Morton number and high Reynolds number flows. The numerical approach is briefly illustrated, and validated with the experimental results of the buoyant rise of an isolated bubble in the literature. Then the method is applied to simulate the interaction between multiple bubbles during their buoyant rise. A pair of bubbles with spherical or ellipsoidal shapes is first simulated under different configurations and rise velocities. Both attractive and repulsive interactions are observed in the simulations depending on the relative position and the Reynolds number. When the Reynolds number is sufficiently high, the bubble–wake interaction is found to be the main interaction mechanism, which results in strong attraction between the ellipsoidal bubbles in vertical or oblique arrangement. Simulations for a group of 14 bubbles are also carried out, and effects of the bubble shape and Reynolds number on the spatial distribution of the bubbles are briefly discussed. In general, a good agreement is found between the current simulation and the experimental and numerical results reported in the literature.     

Improving conversion and selectivity of catalytic reactions in bubbling gas–solid fluidized bed reactors by control of the nonlinear bubble dynamics

In this paper a model is presented that is a dynamic extension of the classic two-phase reactor models used to predict conversion and selectivity of fluidized reactors. The most important part of the model is a dynamic discrete bubble model that can correctly predict bubble sizes and also exhibits chaotic dynamics. This bubble model is based on the discrete bubble models presented by Clift and Grace [AIChE Symp. Ser. 66 (105) (1970) 14; 67 (116) (1971) 23; in: J.F. Davidson, R. Clift, D. Harrison (Eds.), Fluidization, Academic Press, London, 1985, p. 73] and Daw and Halow [AIChE Symp. Ser. 88 (289) (1992) 61]. The latter showed that this type of models can exhibit chaotic behavior. By application of an extended version of Pyragas' control algorithm [K. Pyragas, Phys. Lett. A 170 (1992) 421] the bubble dynamics can be changed from chaotic to periodic in a ‘flow'-regime in which the model otherwise would predict chaotic behavior. Pyragas' control algorithm is used to synchronize a chaotic system with one of its periodic solutions using a feedback control loop. This results in smaller bubbles, thus enhancing mass transfer of the reactant gas in the bubbles to the catalyst particles. The model is used to predict the effect of the changed bubble dynamics on a catalytic reaction of industrial importance, viz. the ammoxidation of propylene to acrilnitril (Sohio process). It is shown that both conversion and selectivity are appreciably enhanced.     

Discrete Bubble Model for Prediction of Bubble Behavior in 3D Fluidized Beds

A discrete bubble model has been developed taking into account multiple bubble‐bubble interactions and a delayed coalescence method. The obtained simulation results were compared with experimental data reported in literature. The simulation results predicted by the developed model indicate clearly that the multiple interactions of bubbles lead to more reasonable results than those predicted by a binary interaction model. In addition, two types of interaction models were applied and predicted results were compared. The frequency of gas bubbles passing through the bed cross section versus bed height follows the same trend as the experimental data.     

Bubble Splitting in a Pseudo‐2D Gas‐Solid Fluidized Bed for Geldart B‐Type Particles

Bubble splitting in 2D gas‐solid freely bubbling fluidized beds is experimentally investigated using digital image analysis. The quantitative results can be applied for the development of a new breakage model for bubbly fluidized beds, especially discrete bubble models. The variation of splitting frequency with bubble diameter, new resulting bubble volumes, positions, and also the assumptions of mass and momentum conservation for bubbles after breakage are studied in detail. Small bubbles are found to be more stable than large ones and nearly all mother bubbles split into two almost equally sized daughter bubbles. The momentum of gas bubbles in the vertical direction remains approximately constant after breakage, whereas that of bubbles in the horizontal direction changes with no clear trend. The effect of fluidizing gas velocity in breakage frequency is also examined.     

BUBBLE FREQUENCY AND DISTRIBUTION IN FLUIDIZED BEDS

The distribution of bubbles and the rate of coalescence of bubbles are important factors in the overall behaviour of fluidized beds. A simple model capable of predicting bubble distribution and bubble-size distribution is most desirable. Stages in the development of such a simple model are presented. Early studies examined coalescence and related bubble properties in two-dimensional beds and led to the formulation of a simple computer model for two-dimensional beds. The work was then extended to three-dimensional beds covering the following cases: (i) a single orifice; (ii) a row of orifices in a line and (iii) orifices arranged in a plane. Reasonable agreement with experimental data was obtained for cases (i) and (iii), there being no data available for (ii). The present program is probably adequate for engineering purposes, so far as the prediction of bubble sizes and bubble distribution is concerned, in fluidized beds without internals, although some qualifications are still necessary.     

BUBBLE FREQUENCY AND DISTRIBUTION IN FLUIDIZED BEDS

The distribution of bubbles and the rate of coalescence of bubbles are important factors in the overall behaviour of fluidized beds. A simple model capable of predicting bubble distribution and bubble-size distribution is most desirable. Stages in the development of such a simple model are presented. Early studies examined coalescence and related bubble properties in two-dimensional beds and led to the formulation of a simple computer model for two-dimensional beds. The work was then extended to three-dimensional beds covering the following cases: (i) a single orifice; (ii) a row of orifices in a line and (iii) orifices arranged in a plane. Reasonable agreement with experimental data was obtained for cases (i) and (iii), there being no data available for (ii). The present program is probably adequate for engineering purposes, so far as the prediction of bubble sizes and bubble distribution is concerned, in fluidized beds without internals, although some qualifications are still necessary.     

DYNAMIC EFFECTS OF BUBBLE MOTION

The dynamic effects of the motion of single bubbles entrained in a liquid are investigated. Extensive original experimental results are presented on the pressures associated with the motion of large, isolated spherical cap air bubbles rising both freely and through constricting orifices in a vertical cylindrical pipe filled with quiescent water. The pressures, measured at the pipe wall, were found to be as high as the dynamic bubble pressure for translational bubble motion, and an order of magnitude larger for oscillatory bubble motion following bubble formation.     

DYNAMIC EFFECTS OF BUBBLE MOTION

The dynamic effects of the motion of single bubbles entrained in a liquid are investigated. Extensive original experimental results are presented on the pressures associated with the motion of large, isolated spherical cap air bubbles rising both freely and through constricting orifices in a vertical cylindrical pipe filled with quiescent water. The pressures, measured at the pipe wall, were found to be as high as the dynamic bubble pressure for translational bubble motion, and an order of magnitude larger for oscillatory bubble motion following bubble formation.     

Numerical Study on Interaction Between Two Bubbles Rising Side by Side in CMC Solution

A numerical simulation was performed to investigate the interaction of two bubbles rising side by side in shear-thinning fluid using volume of fluid (VOF) method coupled with continuous surface force (CSF) method. By considering rheological characteristics of fluid, this approach was able to accurately capture the deformation of bubble interface, and validated by comparing with the experimental results. The rising of bubble pairs with different configurations, including horizontal alignment and oblique alignment, was simulated by the method. The influences of the bubble initial distance and the bubble alignment were studied by analyzing the bubble deformation, rising paths and flow fields surrounding bubbles. The results indicate that within certain initial bubble spacing of S^* 3.3 (S^* S_I/D, S_I initial distance between bubbles, and D bubble diameter), the dynamic interaction between two bubbles aligned horizontally shows repulsive effect that decreases with the increase of initial bubble spacing, but weakens to certain degree by the shear-thinning properties of fluid. However, the interaction between two bubbles aligned obliquely presents a repulsive effect for the small angle involved but an attractive impact for the large one, which is yet strengthened by the rheological characteristics of fluid.     

Bubble dynamics in bubbling fluidized beds of ellipsoidal particles

This article presents a CFD-DEM study on the effect of particle shape on bubble dynamics in bubbling fluidized beds. The particles used are ellipsoids, covering from disk-type to cylinder-type. The phenomena such as bubble coalescence and splitting are successfully generated, and the results are compared with literature, showing a good agreement. The results demonstrate that the bubble forming/rising regions and patterns are influenced significantly by particle shape. Ellipsoidal particles have asymmetrical bubble patterns with two or more circulation vortices while the bubbles for spherical particles form at the bed centerline and rise through the center of the bed. Hence, the vertical mass flux at the bed centerline for spheres is always positive, and ellipsoids have negative or positive vertical mass fluxes. The solid mixing estimated based on the dispersion coefficient revealed poor mixing for ellipsoids. Spherical particles have a larger bubble size and higher bubble rising velocity than ellipsoids.     

A visualized investigation of bubble breakup in a swirl-venturi bubble generator

The dynamics and breakup of bubbles in swirl-venturi bubble generator (SVBG) are explored in this work. The three-dimensional movement process and breakup phenomena of bubbles are captured by one high-speed camera system with two cameras while the distribution of swirling flow field is recorded through Particle Image Velocimetry technology. It is revealed that bubbles have two motion trajectories, which are deeply related to bubble breakup. One trajectory is that mother bubble moves upward in an axial direction of the SVBG to the diverging section, and the other trajectory is that mother bubble rotates obliquely upward to another side-wall along the radial direction. Meanwhile, binary breakup, shear-off-induced breakup, static erosive breakup, and dynamic erosive breakup are observed. For relatively high liquid Reynolds number, vortex flow regions are extended and the bubble size is reduced. Furthermore, it is worth noting that the number of microbubbles increases significantly for intensive swirling flow.     

Modelling of bubble dynamics in a venturi flow with a potential flow method

A simple model for cavitation bubble dynamics has been developed using a combination of boundary element methods and one-dimensional bubble dynamic equations. Each bubble is assumed to be spherical and is modelled using a potential source. The strength of the source is governed by one-dimensional bubble dynamic equations so that the velocity of the growing bubble at the interface between the vapour and liquid is correctly represented. The model has been implemented into computer program to study the growth, collapse, and interaction of cavitating bubbles as they flow through a venturi. Interactions between a bubble starting as a nucleus of gas, the surrounding liquid and the venturi boundaries are described. Although this is a simple model, surprisingly complex interactions can be studied with short computational times and limited computer resources. Thus, insights have been gained which otherwise would have been extremely difficult to obtain. These are described in terms of the bubble history, instantaneous velocity maps and instantaneous stream function contours.     

Simulation of a Bubble Column by Computational Fluid Dynamics and Population Balance Equation Using the Cell Average Method

An axisymmetric computational fluid dynamics (CFD) simulation coupled with a population balance equation (PBE) has been applied in simulating the gas‐liquid flow in a bubble column with an in‐house code. The novel feature of this simulation is the application of the cell average method in a CFD‐PBE coupled model for the first time. The predicted results by this method are compared with those by the traditional fixed pivot method and experimental data. For both methods, the simulated results are in reasonable agreement with the reported experimentally measured values. However, the bubble size distributions determined by the cell average method are slightly better than those found by means of the fixed pivot method, i.e., the latter provides a smaller peak value and a wider bubble size distribution, and the probability density function of large bubbles is higher.