Bubbles In Fluidized Bed Research Articles

Prediction of Particle Dynamics in Bubbling Fluidized Beds with Inertial Number-Based Drag and Solid Stress Models

Prediction of Particle Dynamics in Bubbling Fluidized Beds with Inertial Number-Based Drag and Solid Stress Models

A sub-grid gas–solid interaction model for coarse-grained CFD–DEM simulations

Computational fluid dynamics has been coupled with coarse-grained discrete element method (CFD–CGDEM) to study industrial-scale gas–solid fluidized beds. However, solving the fluid flow at a grid size of several coarse-grained particles (CGPs) cannot accurately predict the gas–solid interaction, and thus reasonable correction model is required. This study develops a sub-grid model to correct the gas–solid interaction and thereby improve the accuracy of coarse-grid CFD–CGDEM simulation. First, the fluid grids with the size of several CGPs are divided into the auxiliary three-dimensional sub-grids. Then, the CGPs are mapped onto the sub-grids by the weight function to reconstruct the local solid velocities and solid fractions of the sub-grids. After that, the simplified mass and momentum conservation equations are solved to resolve the local gas velocities of sub-grids. Finally, the drag on each CGP is corrected based on the local gas-phase hydrodynamics of sub-grids. Results show that the sub-grid model resolving the local gas velocity in all the three directions works better than that just in the main stream direction. The coarse-grid CFD–CGDEM simulations with this sub-grid model agree well with the experiment for the bubbling, turbulent and rapid fluidized beds. Besides, by solving the simplified conservation equations with graphics processing units, the coarse-grid CFD–CGDEM coupled with the sub-grid model achieves 674 times speedup compared to the fine-grid simulation, demonstrating a much more efficient method to study industrial-scale gas–solid fluidized beds.

Revealing instantaneous gas-particle segregation in bubbling and turbulent fluidized beds through fiber optic sensing

Bubbling and turbulent fluidized beds (BFB and TFB) are widely used in the chemical and energy industries, owing to their enhanced heat transfer and improved operation flexibility. In BFB and TFB, the gas-particle suspension is featured with a dilute phase (bubbles) and a dense phase. The gas exchanges between the bubbles and the dense phase often through an intermediate region, further enhancing the gas-particle contact. However, few comprehensive studies have been conducted on the intermediate region and the dense phase. In this work, the instantaneous solids holdup was obtained in BFB and TFB through optical fiber sensing. Subsequently, through the use of statistical signal analysis, solids holdup thresholds were proposed to discriminate between the bubbles, the intermediate region (named as ‘mid-phase’) and the dense phase. Finally, gas-particle segregation details were further revealed through the quantification of the different phases to advance the development of future modelling, design and scale-up processes.

Solids Mixing in Marinized Bubbling Fluidized Beds: Gas Distribution Study

Marinized bubbling fluidized beds hold promise for reducing CO2, NOX, and SOX in ship exhaust gases, but their use at sea is hampered by a limited understanding of the influence of sea waves on hydrodynamics, heat transfer, and efficiency. To address this gap, this study used direct visualization techniques to investigate the solids mixing and hydrodynamics of bubbling fluidized beds under different gas distribution patterns in pseudo-2D vertical, inclined, and rolling beds. Digital Image Analysis (DIA) was used to determine the Lacey mixing index and the local void fraction, while Particle Image Velocimetry (PIV) was used to capture the particle velocity fields. Additionally, the effects of tilt angles and oscillation parameters of the nonvertical beds were compared with the conventional straight bubbling fluidized bed unit. The uniform inlet distribution of the fluidizing agent was compared with a variety of convex feed distributions at the bed entrance to correct for the negative effects of static/dynamic deviations from bed verticality necessary for optimal operation of bubbling fluidized bed reactors in marine environments.

Assessment of Irregular Biomass Particles Fluidization in Bubbling Fluidized Beds

Biomass as a clean and renewable source of energy has immense potential to aid in solving the energy crisis in the world. In order to accurately predict the fluidization behavior of biomass particles using the Eulerian–Eulerian approach and the kinetic theory for granular flows (KTGF), employing appropriate models that adapt to irregularly shaped particles and can precisely predict the interaction between particles is crucial. In this study, the effects of varying radial distribution functions (RDF), frictional viscosity models (FVM), angles of internal friction (ϕ), and stress blending functions (SBF) on the performance of two-fluid models (TFM) were investigated. Simulation predictions were compared and validated with the previous experiments in the literature on Geldart B biomass particles of walnut shells. When applying sphericity to account for size irregularities of biomass particles, the results of this study demonstrated that predictions of both the Ma–Ahmadi and the Carnahan–Starling RDFs along with the Schaeffer FVM agree with experimental data. More specifically, the bubbling behavior prediction slightly favored the use of the Ma–Ahmadi RDF for biomass particles. The results also revealed the importance of using FVM regardless of the initial void fraction. The use of the Schaeffer FVM became more important as time proceeded and particle bulk density decreased. With the change of ϕ and the application of SBF, no significant differences in the time-averaged results were observed. However, when ϕ ranges were between 30 and 40, the predictions of bubbling behavior became more greatly aligned with experimental results.

Heat transfer within dynamically structured bubbling fluidized beds subject to vibration: A two‐fluid modeling study

AbstractBubbling fluidized beds are often used to achieve a uniform particle temperature distribution in industrial processes involving gas and particles. However, the chaotic bubble dynamics pose significant challenges in scale‐up. Recent work (Guo et al., 2021, PNAS 118, e2108647118) has shown that using vibration can structure the bubbling pattern to a highly predictable manner with the characteristic bubble properties independent of system width, opening opportunities to address key issues associated with conventional bubbling fluidized beds. Herein, using two‐fluid modeling simulations, we studied heat transfer characteristics within the dynamically structured bubbling fluidized bed and compared to unstructured bubbling fluidized beds and packed beds. Simulations show that the structured bubbling fluidized bed can achieve the most uniform particle temperature distribution because it can achieve the best particle mixing while maintaining a global heat transfer coefficient similar to that of a freely bubbling fluidized bed.

Numerical investigation for the suitable choice of bubble diameter correlation for EMMS/bubbling drag model

Mesoscale bubbles exist inherently in bubbling fluidized beds and hence should be considered in the constitutive modeling of the drag force. The energy minimization multiscale bubbling (EMMS/bubbling) drag model takes the effects of mesoscale structures ( i.e ., bubbles) into the modeling of drag coefficient and thus improves the coarse-grid simulation of bubbling and turbulent fluidized beds. However, its dependence on the bubble diameter correlation has not been thoroughly investigated. The hydrodynamic disparity between homogeneous and heterogeneous fluidization is accounted for by the heterogeneity index, H d , which can be affected by choice of bubble diameter correlation. How this choice of bubble diameter correlation influences the model prediction calls for further fundamental research. This article incorporated seven different bubble diameter correlations into EMMS/bubbling drag model and studied their effects on H d . The performance of these correlations has been compared with the correlation used previously by EMMS/bubbling drag model. We found that some of the correlations predicted lower H d by order of a magnitude than the correlation used by the original EMMS/bubbling drag. Based on such analysis, we proposed a modification in the EMMS drag model for bubbling and turbulent fluidized beds. A computational fluid dynamics (CFD) simulation using two-fluid model with the modified EMMS/bubbling drag model was performed for two bubbling and one turbulent fluidized beds. Voidage distribution, time averaged solid concentration and axial solid concentration profiles were studied and compared with the previous version of the EMMS/bubbling drag model and experimental data. We found that the right choice of bubble diameter correlations can significantly improve the results for CFD simulations.

Hydrodynamics-reaction-coupled simulations in a low-scale batch FCC regenerator: Comparison between an annular and a free-bubbling fluidized beds

Although annular fluidized beds (AFB) has been widely studied, a direct comparative evaluation of its reactive performance with bubbling fluidized beds (BFB) under similar operating conditions is still lacking. In this study, a comparative simulation using Euler–Lagrange MP-PIC method is performed under varying superficial gas velocities to assess their chemical reaction performance in relation to coke combustion rate, flue gas and temperature profiles in both reactors. The macroscale hydrodynamics of transport phenomenon, pressure fluctuation, solids holdup, slip velocity, and radial nonuniformity index (RNI) are also evaluated in detail. The results show, compared to BFB, the AFB shows big hydrodynamic differences, as indicated by the stronger cross-flow of solids, higher pressure fluctuation magnitudes, nonuniform lateral solids distribution, higher slip velocity and RNIs. However, reaction performance in relation to coke burning pattern and rates, flue gas distribution and temperature profile in the two beds are close. Under the constant bed inventory and air flowrate criteria, bigger changes in hydrodynamics are needed to affect the reaction performance significantly in the two small reactors due to the batch operation mode and the slow coke burning rate.

Comparison of flow dynamics and reactor performance in gas-solids circulating turbulent fluidized bed with other fluidized beds across regimes

The hydrodynamics and the reactor performance in the recently proposed gas-solids circulating turbulent fluidized bed (CTFB) were compared with that in the low-velocity conventional fluidized beds and the high-density circulating fluidized bed (HDCFB) riser. The average solids holdup distributions, fluctuations, and probability density distributions of the instantaneous solids holdup signals indicate that the CTFB either more resembles a TFB if the Gs/Ug ratio is large or more resembles an HDCFB if the Gs/Ug ratio is small. The distributions of the bubble occurrence, equivalent bubble size, and bubble phase fraction were extracted from the optical fiber signals of the local solids holdup using the wavelet analysis method. The CTFB was found to have a greater number of smaller bubbles than the bubbling and turbulent fluidized beds and relatively fewer and larger bubbles than that in an HDCFB. The effect of flow dynamics and bubble behaviors on the reaction performance in different types of fluidized beds are compared by analytical calculation based on the K-L reactor model. A superior reactor performance can be achieved using the CTFB reactor for a higher effective reaction rate, higher mass transfer rates, and a lower degree of solids backmixing.

Investigation on gas–solid flow regimes in a novel multistage fluidized bed

Abstract Gas–solid flow regime in a novel multistage circulating fluidized bed is investigated in this study. Pressure fluctuations are first sampled from gas–solid flow systems and then are analyzed through frequency and time-frequency domain methods including power spectrum and Hilbert–Huang transform. According to the flow characteristics obtained from pressure fluctuations, it is found that the gas–solid motions in the multistage circulating fluidized bed exhibit two dominant motion peaks in low and high frequencies. Moreover, gas-cluster motions become intensive for the multistage circulating fluidized bed in comparison with the fast bed. Unlike the traditional methods, the fuzzy C-means clustering method is introduced to objectively identify flow regime in the multistage circulating fluidized bed on the basis of the flow characteristics extracted from bubbling, turbulent, fast, and multistage fluidized beds. The identification accuracy of fuzzy C-means clustering method is first verified. The identification results show that the flow regime in the multistage circulating fluidized bed is in the scope of fast flow regime under examined conditions. Moreover, the results indicate that the consistency of flow regime between two enlarged sections exists. In addition, the transition onset of fast flow regime in the multistage circulating fluidized bed is higher than that in the fast bed.

A scale-independent modeling method for filtered drag in fluidized gas-particle flows

A scale-independent modeling method for filtered drag force is proposed for coarse-grid simulation of fluidized gas-solid flows based on the fine-grid data generated using kinetic theory-based two-fluid method in periodic sedimentation systems. In the modeling process, the filtered solid volume fraction at a larger scale whose side length is three times of the filter size is introduced as a new marker. New developed closures show an excellent performance in a priori test, and exhibit a fairly good agreement with the fine-grid data and the experimental data in a posteriori test for three typical fluidized regimes including bubbling, turbulent and fast fluidized beds. Furthermore, the available correction methods to consider the effect of the material properties via adding a multiplicative factor are tested. The results show that these correction methods are not suitable for the considered cases.

Effect of Drag Models on the Numerical Simulations of Bubbling and Turbulent Fluidized Beds

AbstractGranular option enabled two‐fluid model coupled with different homogeneous and heterogeneous drag correlations, i.e., Gidaspow, Syamlal and O'Brien, Beetstra, Brucato, Gibilaro, energy minimization multiscale method (EMMS), and Tenneti, are employed for the prediction of bubbling and turbulent fluidized‐bed characteristics. Numerically predicted results are validated against the literature data in terms of bubble shape and diameter, axial and radial variation of time‐averaged void fraction, pressure drop, and bed expansion. The Gidaspow, Syamlal and O'Brien drag model‐predicted mean void fraction and bubble shapes are close to the experiments in bubbling and turbulent fluidized beds with both the 2D and 3D geometries. All drag models overpredicted the pressure drop at low superficial gas velocities in a turbulent fluidized bed.

Effect of particle shape on bubble dynamics in bubbling fluidized bed

Particle shape can significantly affect the bubble dynamics of bubbling fluidized beds (BFB). In this paper, findings obtained from simulations using CFD-DEM are summarized to discuss the effect of particle shape on bubble dynamics and bubble properties such as bubble size, shape and velocity at a single orifice and uniform fluidized bed. Particles with aspect ratios at 0.5 (oblate), 1 (spherical) and 2 (prolate) are employed to represent disc-like, spherical and rod-like particles, respectively. Both single jet and uniform fluidized bed simulations demonstrate that the bubble forming/rising regions, bubble coalescence locations, and bubble splitting phenomena are significantly influenced by particle shape. The CFD-DEM results for bubble size and bubble velocity show good agreement with literature correlations.

Scale-up effects of the flow structure in bubbling and turbulent fluidized beds

In bubbling and turbulent fluidized beds, gas bubbles are dispersed in the emulsion phase with the bubble cloud as the transition, offering a vigorous interaction between gas and particles. Despite numerous studies in the literature, many difficulties were encountered for modeling and scale-up. Hence, it is imperative to further study the flow details and scale-up effects. In this work, instantaneous solids holdup was captured in bubbling and turbulent fluidized beds using an optical fiber probe. The degree of local segregation was quantified with the intermittency index. Then, according to the signal details, phases, namely bubble phase, bubble cloud and emulsion phase, can be easily identified and characterized in perspectives of solids holdup and time fraction. Finally, the influences of the column diameter and static bed height were systematically analyzed to study scale-up effects.

Experimental investigation of bubble behavior in gas-solid fluidized bed

In this work, to investigate the source of pressure fluctuations, behavior of a single bubble in a two-dimensional gas–solid fluidized bed was studied. Pressure sensors located at different heights of the bed measured presure fluctuations, and simultaneously a high speed camera was used to pursue all steps from formation to eruption of bubbles. Two types of particles were applied with different sizes and densities. Experiments showed that the maximum amplitude of formation was independent of the bubble diameter. But, it depended on density of particles, velocity of injection and the distance from bed surface. When injection stopped, there was a minimum in pressure profile related to the higher dense phase voidage for a higher superficial gas velocity after injection. Also, the maximum pressure fluctuation of bubble eruptions was related to the bubble diameter, density and size of particles. It was concluded that pressure fluctuations of formation, passing and eruption of bubbles in fluidized beds are originated due to changes in dense phase voidage, bed voidage and movement of particles during bubble eruption.

Investigating the bubble dynamics in fluidized bed by CFD-DEM

Fluidized beds are of great importance in various industries and presence of bubbles significantly influence their efficiency. In order to gain a more accurate vision on the dynamics of a single bubble in fluidized beds, formation, rising, and eruption of a single bubble in a rectangular fluidized bed were studied by CFD-DEM and validated by experimental data. Single bubble in a 2-D rectangular fluidized bed at different stages, from forming to erupting, was modeled by the CFD-DEM technique. Investigating the pressure fluctuations and bubble dynamics were the main focus of this study. Pressure fluctuations were measured using a pressure transducer and a high-speed digital camera was used to record the whole bubbling process, from bubble formation on the distributor to its eruption at the surface. Validation was performed by comparing experimental and simulated bubble diameter and pressure fluctuations. CFD-DEM results such as particles and gas velocity vectors, bed voidage and pressure contours revealed new information on characteristics of pressure fluctuations which was not possible to gain by experimental studies. In the formation stage, due to the compression wave from injection, particles accelerate and bed reaches to a packed state. This causes a pressure peak in the pressure. At the beginning of bubble formation on the distributor, pressure decreases and when the bubble detaches from the distributor, pressure reaches a minimum. While bubble is rising through the bed, pressure increases again, however, it remains below zero because of the lower voidage behind the bubble in comparison to pre-injection state and the greater velocity of the gas above and below the bubble. By bubble eruption, another small compression wave is seen in the pressure.

Spherical cap bubbles in fluidized beds and bubble columns: A new formula for added mass

A simple approximate formula was found on heuristic basis for the added mass of both rigid and deformable cap bodies. Its prediction agrees with the published CFD data. It can be used as a closure for numerical studies.

Modified MFIX code to simulate hydrodynamics of gas-solids bubbling fluidized beds: A model of coupled kinetic theory of granular flow and discrete element method

The coefficient of restitution (CoR) of monodisperse particles used in the kinetic theory of granular flow (KTGF) is predicted using a coupled Euler solid phase-Euler gas phase-Lagrangian discrete particles (CEEL) approach in a bubbling fluidized bed (BFB). In the CEEL model, the CoR is obtained from dynamic information of Lagrangian discrete particles by means of discrete element method (DEM), and used to predict Euler solid phase properties in KTGF. The flow behavior of Euler solid phase, Euler gas phase and Lagrangian discrete particles is predicted in a gas and monodisperse particles BFB. The distributions of velocities and volume fractions using CEEL model are compared to numerical simulations using Euler-Euler two-fluid model (TFM) and Euler gas phase-Lagrangian discrete element method (CFD-DEM) in a BFB. The predicted distributions of granular temperature of Euler solid phase using KTGF are compared to Lagrangian discrete particles granular temperatures by means of DEM. The predicted distributions of velocities of Euler solid phase and volume fractions of Euler gas phase using CEEL model agree with experimental results.

CFD simulations of tapered bubbling/turbulent fluidized beds with/without gas distributor based on the structure-based drag model

The hydrodynamic characteristics of tapered bubbling/turbulent fluidized beds with/without gas distributor are simulated based on the structure-based drag model. Besides the only parameter of gas voidage (εg) contained in the original drag model, the heterogeneous drag index (Hd) for the tapered fluidized bed (TFB) also includes the parameter of gas velocity (ug) for the axial velocity gradient along bed height. Both the simulations of tapered bubbling and turbulent fluidized beds achieve more accurate predictions than the traditional drag models. There exist dilute center and dense annular regions with solids cycle flow structure in the TFB. The simulation also gives reasonable prediction for TFB without gas distributor, the variation of gas voidage profile reveals gas converging toward bed center with axial location rising.

Wake volume of injected bubbles in fluidized beds: A magnetic resonance imaging velocimetry study

Rapid magnetic resonance imaging (MRI) was used to determine the volume of wakes of bubbles injected into incipiently fluidized beds. MRI was used to generate 2D maps of particle velocity surrounding bubbles with a temporal resolution of 18 ms. The bubble rise velocity, ub, was determined from the change in bubble position in successive maps, and pixels of particles which had an upward velocity greater than or equal to 0.8 ub were considered to be in the “wake” if they were below the bubble and in the “crown” if they were above the bubble. Results show that (i) the wake volume is much larger than the crown volume and (ii) the ratio of the wake volume to bubble volume increases as the bubble rises in the bed but decreases with increasing bubble volume. The influence of two bubbles interacting on bubble wake volume is also investigated. Comparison with geometric theory for wake volume exposes flaws in this theory for fully developed bubbles, bubbles near the distributor and interacting bubbles.