Investigating the fluidization of disk-like particles in a fluidized bed using CFD-DEM simulation

Fluidization of monodispersed disk-like particles with different aspect ratios in the fluidized bed is simulated by CFD-DEM, with disk-like particles being modeled by the super-ellipsoids. The relatively comprehensive investigations are performed in order to understand the fluidization behaviors of disk-like particles and to evaluate how the aspect ratio influences the fluidization. The results obtained demonstrate that disk-like particles with a larger aspect ratio possess stronger particle movement and more apparent fluidization. Comparisons between spherical particles and disk-like particles elucidate their differences in the fluidization behavior. Particle orientation is also investigated in this paper due to its important influence on the fluidization. Particles possess different preferred orientations in the static bed and in the fluidization state, and a reduced aspect ratio can drive particles to be in the preferred orientation. The existence of the walls will prompt particles to align their cross sections to be parallel to the plane of the walls.     

An investigation of the hydrodynamic similarity of single-spout fluidized beds using CFD-DEM simulations

The applicability of the hydrodynamic similarity criteria (scaling law) introduced by Glicksman (1988) was investigated using fully coupled Computational Fluid Dynamics and Discrete Element Method (CFD-DEM) simulations for single-spout fluidized beds. Four test cases were performed to investigate the scaling law in a pseudo-2D spouted-fluidized bed. In addition, the applicability of Glicksman’s scaling law for simulating 3D fluidized beds was studied. In all simulations, characteristic dimensionless groups, i.e. the Reynolds number (Re), Froude number (Fr), particle-to-fluid density, bed initial height to particle diameter and bed width to particle diameter were kept constant for the both base and scaled cases. Comparing the time averaged particle velocities, gas velocities and volume fractions between the base and scaled cases indicated a very good overall hydrodynamic similarity for all test cases. A minor discrepancy observed between the simulation results of the base and scaled cases was explained by a force analysis.An advantage of the scaling approach, i.e., reducing computational time, was also presented in the last four test cases, including a large-scale simulation, showing that this approach can be considered as a promising way to simulate large-scale spouted-fluidized beds.     

Effect of flow pulsation on fluidization degree of gas-solid fluidized beds by using coupled CFD-DEM

In this paper, the effect of inlet flow type on fluidization of a gas-solid fluidized bed was studied by using numerical simulations. Gas-solid fluidized beds are widely used in processes such as heating, cooling, drying, granulation, mixing, segregating and coating. To simulate the gas-particle flows, the unresolved surface CFD‐DEM was used considering Eulerian–Lagrangian approach. The fluid phase was modeled by computational fluid dynamics (CFD) while the solid phase was solved by discrete element method (DEM), and the coupling between gas and solid phases was considered to be four-way. The uniform and pulsed flows were injected through three nozzles located at the bottom of a rectangular bed. Three types of pulsed flow were considered: sinusoidal, rectangular and relocating. The fluidized bed behavior was discussed in terms of minimum fluidization velocity (MFV), pressure drop, bubble formation, bed expansion, particles velocity and, gas-solid interaction and particle contact forces. The results of different simulations indicated that the minimum fluidization velocity of the beds fluidized by pulsed flows was decreased by up to 33%. The influence of the pulsation amplitude on the minimum fluidization velocity was more significant than that of the pulsation frequency. The bed expansion and particles average velocity were increased by the pulsed flows, while the pressure drop and interaction force were decreased. As the pulsation frequency increased, the pressure drop and gas-solid interaction force increased, although size of the bubbles and bed expansion decreased. It was also observed that in large vibration frequencies, the bubbles became more regular. In the sinusoidal flow, the velocity and contact force between the particles were initially increased by frequency and in larger frequencies they were decreased.     

CFD-DEM study of the effects of solid properties and aeration conditions on heat transfer in fluidized bed

The solid properties are of significant influence on the thermal performance of the fluidized bed. In order to provide valuable information for the application of this equipment, a numerical study is carried to clarify the effects of solid properties on the heat transfer characteristics in a lab-scale fluidized bed by means of the CFD-DEM method. Specially, two aeration conditions, i.e. the same superficial velocity and the same fluidization number, are considered. The results show that the violent convective mechanism at bed bottom plays a significant role in the heating of the bed material. The entrainment of rising bubbles and hence solid mixing are the key factors to get better temperature uniformity of the bed during the heating process. With the decrease of particle density and size, the internal circulation of solid phase is strengthened under the same superficial velocity, while slightly weakened under the same fluidization number. Obvious resemblance can be captured between solid mixing and temperature uniformity, and the enhanced solid mixing usually leads to homogeneous temperature distribution of the bed. It can be found that the heating rate decreases with increasing solid density regardless of aeration setup. In addition, it is positively related to particle diameter under the same fluidization number, while keep unchanged under the same superficial velocity. Furthermore, enhanced solid mixing and better temperature uniformity can be captured with increasing solid heat capacity, which confirms that gas temperature shows considerable effect on gas-solid flow.     

Particle dynamics in a multi-staged fluidized bed: Particle transport behavior on micro-scale by discrete particle modelling

This work studies the particle exchange rates in horizontal fluidized beds equipped with different weir designs between compartments. These particle exchange rates provide information on the axial dispersion of the solid material within the process. For this purpose discrete particle modelling (DPM) was used to determine the particle exchange on microscopic level. This method uses a coupled CFD-DEM approach to observe particle dynamics in a fluid field. The model was validated against exchange rates in a lab-scale setup as determined by Particle Tracking Velocimetry (PTV) with very good quantitative agreement, showing the suitability of the method for the evaluation of weir designs. Simulations were performed for different weir designs and under variation of the hold-up mass, the feed rate and gas velocity to predict their transport behavior in a pilot-scale 3D horizontal fluidized bed. The results indicate that the solids transport behavior is strongly dependent on the used weir design and the main driving force for the particle transport that can be influenced by the process conditions. The installation of weirs between two compartments induces a transport resistance, while the base type without the installation of a weir between the two chambers represents the fastest possibility for mixing the particles of a two-compartment system. It has been observed that the general trend shows higher particle recirculation rates for the overflow weir and base configuration (no weir), whereas the underflow and sideflow weir applications improve the solids transport through the horizontal fluidized bed.     

Influence of particle size on the exit effect of a full-scale rolling circulating fluidized bed

This paper investigated the influence of particle size on the exit effect of a full-scale rolling circulating fluidized bed (CFB) by using the Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) method. The gas–solid two-phase flow of the full-scale rolling CFB was compared with that of a simplified rolling CFB. Thus, the exit effect of the full-scale rolling CFB was clarified. In the air phase, a peak of air axial velocity v_ya was observed when the full-scale rolling CFB reached the maximum angular displacement. The particle phase possessed back mixing and radial exchange phenomena at the top and bottom of the full-scale rolling CFB, respectively. However, those phenomena were not obvious at the top and bottom of the simplified rolling CFB. The mechanism of the above-mentioned exit effect was then clarified by analyzing the forces acting on the particles under different particle sizes. Finally, the increases in particle size lead to the intensification of the peak of v_ya, particle back mixing, and radial exchange phenomena. Therefore, the intensity of the exit effect of the gas–solid two-phase flow increased as the particle size increased_. The results suggested that the small particles had the potential to promote the purification rate of the full-scale rolling CFB on account of its small exit effect.     

Gas flow influences on bubbling and flow characteristics of fluidized bed with immersed tube under electrostatic effects

Industrial bubbling fluidized beds are used to fluidize particles. When particles are fluidized, electrostatic effects will cause the particles to form obvious agglomerates, thus reducing fluidization performance. For better fluidization performance, internal component immersed tubes are usually placed in fluidized bed to limit the bubble size and reduce particle agglomerates. Meanwhile, pulsed gas flow can increase particle disturbance, which is also an effective method to reduce particle agglomerates. In this paper, the CFD-DEM model under electrostatic effects is constructed to research the bubbling and flow characteristics in fluidized beds. Firstly, particle mixing qualities with and without the immersed tube are compared. Then, the effects of different superficial gas velocities are investigated with an immersed tube. Finally, different frequencies are applied to study the energy loss and flow characteristics around the immersed tube. The results show that the addition of the immersed tube can reduce bubble size to facilitate particle mixing. Due to the obstruction of the immersed tube, the bubbles are generated near the wall. As the superficial gas velocity increases, the larger bubbles are generated. Moreover, the electrostatic force applied to the particles varies periodically with the frequency of incoming pulsed gas flow, with fluctuations maximal at 2.5 Hz.     

Experimental and CFD-DEM numerical evaluation of flow and heat transfer characteristics in mixed pulsed fluidized beds

Pulsed fluidized beds can make gas-solid mix and contact more uniform, therefore obviously improving heat transfer efficiency. The mixed pulsed fluidized bed, whose total gas flow is composed of stable gas flow and pulsed gas flow, is proposed in this research. Firstly, the experimental device for drying particles in a mixed pulsed fluidized bed is established. Pressure signals with different frequencies and gas flow ratios are collected, and flow pattern diagrams are obtained through a high-speed camera. Secondly, the CFD-DEM parallel numerical simulation method is constructed to research the mixed pulsed fluidized bed performance. Particle mixing, motion and heat transfer characteristics under different pulse frequencies and flow ratios are studied. Results show that particles in the mixed pulsed fluidized bed exhibit regular periodic motion, thereby promoting the mixing effect of particles. Moreover the bubble nucleation point moves to the bottom of the bed with the increasing pulse frequency. When the total gas velocity is relatively low, particle mixing effect can be enhanced by increasing the proportion of pulsed gas. However, when the velocity is relatively high, particle mixing effect will be enhanced by decreasing the proportion.     

CFD-DEM study on the mixing characteristics of binary particle systems in a fluidized bed of refuse-derived fuel

The fluidization of quartz in the fluidized bed has great influence on the combustion and gasification of refuse-derived fuel (RDF). The combined computational fluid dynamics (CFD) and discrete element method (DEM) approach was used to explore the gas-solid hydrodynamics and mixing characteristics in a three-dimensional fluidized bed. All numerical analyses were performed referring to the experiments (Goldschmidt, Beetstra, and Kuipers 2004 Goldschmidt, M. J. V., R. Beetstra, and J. A. M. Kuipers. 2004. Hydrodynamic modelling of dense gas-fluidised beds: Comparison and validation of 3D discrete particle and continuum models. Powder Technology 142 (1):23–47. doi:10.1016/j.powtec.2004.02.020[Crossref], [Web of Science ®] , [Google Scholar]). The simulation results indicated that the quartz volume fraction agrees well with the experimental data. Furthermore, the cylinder-shaped RDF particles can mix well with the quartz particles as they were added from upside. For binary systems, it is necessary to investigate solid flow characteristics as well as pressure drops and examine the influence of superficial gas velocity on the solid mixing. Two main parameters are discussed: mixing degree and the time required to reach the steady state. It is also found that inlet gas velocity and particle properties (particle density ratio, shape and size) are significant factors on particle mixing in a fluidized bed.     

Optimization of the impeller of sand-ejecting fire extinguisher based on CFD-DEM simulations and Kriging model

To explore the optimal impeller of fire extinguishing equipment within a larger range, the Latin hypercube sampling and Kriging method was adopted to establish the response surface with two input factors (the blade number and the angle of inclination). The study found that a longer scattering distance can obtain a better fire-extinguishing effect, and the air volume flow rate at the scattering outlet has no obvious relationship with the scattering effect. The predicted optimal result is the impeller with 10 blades and 15.13° forward. However, compared with previous research results, this only brings a slight scattering distance improvement. Therefore, increasing the number of blades did not lead to a significant increase in scattering distance. This study will provide effective references for further optimization research of the sand-ejecting fire extinguisher and guide its optimization direction to other parameters of the impeller.     

CFD-DEM study of segregation and mixing characteristics under a bi-disperse solid-liquid fluidised bed

In this work, a recent smoothed CFD-DEM model is employed to study the segregation and mixing behaviours in a bi-disperse solid–liquid fluidised bed (SLFB) with different superficial liquid velocities and wide size ratios. Here, the poly-disperse drag model is applied, and its validity and advantage are confirmed. Besides, the qualitative and quantitative characteristics are systematically studied using a variety of indices, while the intermixing behaviour in the transition zone is investigated. The results show that segregation degree is well quantified by the mixing index, and the circulation motion of particles and liquid is confirmed in the transition zone. The more drastic motions can be observed in the transition zone under lower superficial velocities or size ratios. Furthermore, the force analysis in terms of the drag force and collision force indicates that the combined contribution of hydrodynamics and collision is the main underlying mechanism of segregation behaviour in the bi-disperse SLFB.     

Simulated pulsed flow of gas and particles in a horizontal oppose-pulsed gas jets of bubbling fluidized bed

Flow behavior of gas and particles with a horizontal oppose-pulsed gas jets are simulated by means of a three dimensional Computational Fluid Dynamics (CFD) model with the kinetic theory of granular flow in a gas-particles bubbling fluidized bed. The effects of amplitudes and frequencies on the hydrodynamics of gas and particles are analyzed. The simulation results are presented in terms of phase velocity vector plot, volume fraction of phases, granular temperature, power spectrum and Reynolds stresses in the bed. Results show that the impingement caused by the oppose-pulsed gas jets oscillates with the variation of pulsed gas velocity. The impingement zone with the high solid volume fraction reciprocates from the left side to the right side through the bed center with the variation of pulsed jet gas velocities. The lateral velocity and gas turbulent kinetic energy, granular temperature and Reynolds stresses of gas and particles are larger near the pulsed gas jets than that at the center of the bed. The large dispersion coefficients of particles using the horizontal oppose-pulsed gas jets enhance the mixing of particles in gas-solid fluidized bed.     

Characteristics of a single jet injected into an incipiently fluidized bed: A magnetic resonance imaging study

Rapid magnetic resonance imaging (MRI) was used to characterize properties of a single central gas jet injected into a 3D gas fluidized bed under incipient fluidization conditions. Snapshots of both particle concentration and particle velocity are provided. The average jet height, oscillations in jet height and the size of bubbles breaking off from the jet increased with increasing jet velocity. The frequency of bubble breakoff from the jet decreased with increasing jet velocity. The jet height measurements are compared with various correlations in the literature, and the quantitative data provided here can be compared directly with that from numerical simulations and theoretical predictions for validation purposes.     

Three-dimensional simulation of a plasma jet with transverse particle and carrier gas injection

In the present paper, three-dimensional modeling results are presented concerning the turbulent thermal plasma jet with transversely injected carrier gas and metal particles at atmospheric pressure. The standard K− model is employed for the numerical simulation of the turbulent plasma flow in coupling with the variable-property continuity, momentum and energy equations. For predicting the motion of the injected particles in the turbulent flow field, an improved particle stochastic-trajectory model is adopted in the calculation. The heating histories of the injected particles are also calculated in their moving processes. The modeling results show that including the effect of carrier gas on jet and particle behavior is very important. The plasma jet is deflected from its geometrical axis due to the transverse injection of carrier gas, and the particle trajectories are also appreciably changed by the carrier gas injection. The particles disperse around their average trajectories in the turbulent flow field.     

Agglomerate behavior in a recirculating fluidized bed with sheds: Effect of sheds

A Radioactive Particle Tracking (RPT) technique was used to study the effects of the internal baffles in the stripping section of the Fluid Coker?, called sheds, have on the behavior of wet agglomerates that are formed when residual oil is injected into the Coker. Vapor emitted by reacting wet agglomerates below the sheds rises and causes shed fouling. The release of vapor from agglomerates can be estimated by combining the RPT results with a coking reaction model. The study found that the sheds reduce the time agglomerates spend in the shed zone, which in turn reduces the amount of organic vapor that reaches the sheds, but at the same time increase the wetness of the agglomerates that exit to the recirculation line, which results in the loss of valuable liquid. The research also found that the best type of shed, from the point of view of agglomerate motion, is the mesh-shed. Finally, experimental data indicate that reducing the cross sectional area of the sheds from 50% to 30% increases the time that the agglomerates spend above the shed zone, and thus reduces the flow of vapor emitted below the sheds.     

Experimental analysis of overtaking behavior in disks falling in a low-density particle bed

Cooperative behavior displayed by five steel disks falling in a low-density particle bed involves the formation of upward and downward convex configurations, which resembles the flying pattern of a flock of birds. In this study, we focused on overtaking behavior in two falling disks, which causes the cooperative behavior, and we investigated the effects of differences in the disk release time and the initial disk separation distance on the falling behavior of the disks experimentally. Expanded polystyrene (EPS) particles (diameter 5.08 mm, mass 1.45 mg) were used as the bed particles and steel disks (diameter 25.4 mm, thickness 5.22 mm, mass 20.2 g) were used as the falling disks. We released one to five disks with various disk release time differences (0–0.154 s) and initial separation distances (0–100 mm). We recorded the disk falling behavior in the particle bed with a high-speed video camera (500 fps) and analyzed the behavior with image analysis software. Five-disk cooperative behavior similar to that reported in the literature occurred in our experimental setup. In the two-disk experiments, we observed overtaking behavior for an initial separation distance of 10 mm and release time difference of ≤ 0.076 s, and for an initial separation distance of ≤ 60 mm and release time difference of 0.02–0.03 s. The overtaking behavior arose from the decrease in the falling velocity of the first disk released. The EPS particle packing fraction in the area above the disk one disk diameter wide and a quarter of the disk diameter high determined the disk falling velocity. This mechanism was explained by the displacement behavior of EPS particles around the disks as the disks fell.     

CFD simulation of electrostatic effect on gas interchange,vortex and heat transfer in the gas-solid fluidized bed

Vortex and electrostatic charges in the gas-solid fluidized bed have a significant influence on its transport abilities and hydrodynamics. In this work, the electrostatic model coupled with energy model has been applied to reveal the electrostatic effect on hydrodynamics, vorticity and local heat transfer coefficients based on the kinetic theory of granular flow. The results indicate that particle vortices change the gas and solid phase interaction around the bubble and enhance the local heat transfer coefficients. Gas interchange decreases by 6.5% compared to Davidson model at the jet velocity of 10?m·s^?1 and 13% of 5?m·s^?1. After adding electrostatic charges, bubble diameter decreases with the increasing specific charges. Furthermore, vorticity at the initial stage of bubble formation is larger and the particle vortex diffuses to a large extent. The simulation results can be applied to modify and estimate the overall heat transfer coefficient of the fluidized bed reactor and provide the basis for studying the effect of electrostatic effect on heat transfer.     

Effect of particle shape on raceway size and pressure drop in a blast furnace: Experimental,numerical and theoretical analyses

Experiments and simulations are conducted to explore the changing laws of blast furnace (BF) raceway morphology and pressure drop with cylindrical particles. Experimental data show that there are five typical stages for the pressure drop during the raceway formation. The closer the aspect ratio (Ar) of the particle to 1, the bigger the raceway size and the wider the particle moving band will be. When the raceway is in stable stage, the pressure drop can be ascribed to the cooperative action of the bed height, inlet gas velocity and Ar. Numerical results reveal that the formation of large raceway for sphere-like particles is due to the small drag and contact forces. The contact forces in the prolate particle system are very large and thus result in a small raceway. Finally, the influence of particle shape is employed to improve a raceway size predictive correlation which can increase the average calculational accuracy by 3.4%.