Computer simulations of gas-solid flow in spouted beds using kinetic-frictional stress model of granular flow

A gas-solid two-fluid flow model is presented. The kinetic-frictional constitutive model for dense assemblies of solids is incorporated in the simulations of spouted beds. This model treats the kinetic and frictional stresses of particles additively. The kinetic stress is modeled using the kinetic theory of granular flow, while the friction stress is from the combination of the normal frictional stress model proposed by Johnson et al. (J. Fluid Mech. 210 (1990) 501) and the modified frictional shear viscosity model proposed by Syamlal et al. (MFIX documentation. US Department of Energy, Federal Energy Technology Center, Morgantown, 1993). The body-fitted coordination is used to make the computational grids best fit the shape of conical contour of the base in the spouted beds. The effects of inclined angle of conical base on the distributions of particle velocities and concentrations in the spout, annulus and fountain zones were numerical studied. Calculated particle velocities and concentrations in spouted beds were in agreement with experimental data obtained by He et al. (Can. J. Chem. Eng. 72 (1994a) 229; (1994b) 561) and San Jose et al. (Chem. Eng. Sci. 53 (1998) 3561).     

Heterogeneity analysis of gas-solid flow hydrodynamics in a pilot-scale fluidized bed reactor

Mesoscale drag model is of crucial significance for the reliability and accuracy in coarse-grid EulerianEulerian two-fluid model(TFM) simulations of gas-solid flow hydrodynamics in fluidized bed reactors.Although numerous mesoscale drag models have been reported in the literature,a systematic comparison of their prediction capability from the perspective of heterogeneity analysis is still lacking.In this study,in order to investigate the effect of several typical drag models on the hydrodynamic ...     

CFD modelling of a liquid-solid fluidized bed

A multifluid Eulerian computational fluid dynamics (CFD) model with granular flow extension is used to simulate a liquid-solid fluidized bed. The numerical simulations are evaluated qualitatively by experimental data from the literature and quantitatively by comparison with new experimental data. The effects of mesh size, time step and convergence criteria are investigated. Varying the coefficient of restitution did not alter the results significantly. The Gidaspow drag relationship predicted a higher voidage than the Wen and Yu drag law. Two different liquid distributors (uniform and non-uniform) were simulated and compared, but a better representation of the geometry of the distributor plate did not greatly influence the results. Qualitatively, the simulations show trends similar to experimental trends reported by various authors. The predictions are also compared with new experimental results for 1.13 mm glass spheres at a wide variety of superficial liquid velocities (0.0085-0.110 m/s) and two different temperatures (12 and ) significantly affecting the liquid viscosity. The CFD model predictions are within 5% of the steady-state experimental data and show the correct trend with variation in viscosity.     

振动流化床的设计计算

对振动流化床的基础研究资料进行分析和归纳，结合实际选型经验，提出振动流化床的设计计算方法。     

Nonintrusive characterization of fluidized bed hydrodynamics using vibration signature analysis

There are many techniques to characterize the hydrodynamics of fluidized beds, but new techniques are still needed for more reliable measurement. Bed vibrations were measured by an accelerometer in a gas–solid fluidized bed to characterize the hydrodynamics of the fluidized bed in a nonintrusive manner. Measurements were carried out at different superficial gas velocities and particle sizes. Pressure fluctuations were measured simultaneously. Vibration signals were processed using statistical analysis. For the sake of the evaluation, the vibration technique was used to calculate minimum fluidization velocity. It was shown that minimum fluidization velocity can be determined from the variation of standard deviation, skewness, and kurtosis of vibration signals against superficial gas velocity of the bed. Kurtosis was proved to be a new method of analyzing vibration signals. Results indicate that analyzing the vibration signals can be an effective nonintrusive technique to characterize the hydrodynamics of fluidized beds. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Hydrodynamic studies on three-phase fluidized bed using CFD analysis

Three-phase fluidization refers to fluidization of solid particles by co-current, upward flow of gas and liquid-phases for the purpose of bringing three-phases in contact in a single operation. Due to complications in understanding hydrodynamics of three-phase fluidized bed, CFD analysis is used to predict the hydrodynamics of it. In this study, liquid-phase is water which flows continuously, where as the gas phase is air which is distributed discretely throughout the bed. Ceramic particle of 1 mm diameter, density of 2650 kg/m³ is used as a solid phase. Excellent mixing, heat and mass transfer rates are the unique features of three-phase fluidized bed. The selection of distributor plays an important role in the quality of fluidization [1]. CFD model is created as the realistic representation of actual fluidized bed. The liquid and solid flow is represented by the mixture model. The air is injected from the bottom of the fluidized by means of discrete phase method (DPM). Simulation results are obtained by using porous jump and porous zone model to represent the distributor. It is found that porous zone model is best applicable in the industries, since stability of operating conditions is achieved even with non-uniform air, water flowrates and with different bed heights(100 mm, 200 mm, 300 mm, 400 mm and 500 mm).Simulated Pressure drop values of the fluidized bed have good agreement with the experimental findings. As the gas flowrate increases, the pressure drop in the column is decreases, provided the initial bed height, diameter of the column, and liquid flowrate are constant. This is due to decrease in density of the fluid medium in the bed by means of more gas hold up. The approach of the simulated values to the experimental values can be reduced with better understanding the nature of the fluidized bed.     

Simulation of bubbling fluidized bed of fine particles using CFD

Computational fluid dynamics (CFD) simulation for bubbling fluidized bed of fine particles was carried out. The reliability and accuracy of CFD simulation was investigated by comparison with experimental data. The experimental facility of the fluidized bed was 6 cm in diameter and 70 cm in height and an agitator of pitched-blade turbine type was installed to prevent severe agglomeration of fine particles. Phosphor particles were employed as the bed material. Particle size was 22 μm and particle density was 3,938 kg/m³. CFD simulation was carried by two-fluid module which was composed of viscosity input model and fan model. CFD simulation and experiment were carried out by changing the fluidizing gas velocity and agitation velocity. The results showed that CFD simulation results in this study showed good agreement with experimental data. From results of CFD simulation, it was observed that the agitation prevents agglomeration of fine particles in a fluidized bed.     

CFD based combustion model for sewage sludge gasification in a fluidized bed

Gasification is one potential way to use sewage sludge as renewable energy and solve the environmental problems caused by the huge amount of sewage sludge. In this paper, a three-dimensional Computational Fluid Dynamics (CFD) model has been developed to simulate the sewage sludge gasification process in a fluidized bed. The model describes the complex physical and chemical phenomena in the gasifier including turbulent flow, heat and mass transfer, and chemical reactions. The model is based on the Eulerian-Lagrangian concept using the nonpremixed combustion modeling approach. In terms of the CFD software FLUENT, which represents a powerful tool for gasifier analysis, the simulations provide detailed information on the gas products and temperature distribution in the gasifier. The model sensitivity is analyzed by performing the model in a laboratory-scale fluidized bed in the literature, and the model validation is carried out by comparing with experimental data from the literature. Results show that reasonably good agreement was achieved. Effects of temperature and Equivalence Ratio (ER) on the quality of product syngas (H₂ + CO) are also studied.     

带导流管多层流化床流体力学特性实验研究

多层流化床的应用范围受操作可调性和稳定性等因素限制。在改变传统筛板结构的基础上,研究了传统穿流板多层流化床和导流管多层流化床床层压降随表观气速和进料速率的变化规律,实验结果表明,导流管多层流化床不仅大幅减小了床层压降、提高了床体处理能力和可调范围,而且也改善了物料的流化质量。此外,当料层达到一定高度时,导流管多层流化床还具有喷动流化床的特性,同时导流管还具有溢流物料的作用,进一步加大了气固传质效率和床体处理能力。在流体力学分析的基础上,推导出导流管多层流化床床层压降的关联式,得到了床层压降随进气气速和进料速率的关系,与实验数据基本吻合。     

Hydrodynamic modelling of binary mixture in a gas bubbling fluidized bed using the kinetic theory of granular flow

A multi-fluid Eularian CFD model with closure relationships according to the kinetic theory of granular flow has been applied to study the motions of particles in the gas bubbling fluidized bed with the binary mixtures. The mutual interactions between the gas and particles and the collisions among particles were taken into account. Simulated results shown that the hydrodynamics of gas bubbling fluidized bed related with the distribution of particle sizes and the amount of energy dissipated in particle-particle interaction. In order to obtain realistic bed dynamics from fundamental hydrodynamic models, it is important to correctly take the effect of particle size distribution and energy dissipation due to non-ideal particle-particle interactions into account.     

The characteristics of wave propagation in a vibrating fluidized bed

Based on experiments, this paper describes that the vibration energy imported to a packed bed is through the collision between the vibration plate and the bed, and the vibration energy imported to a fluidized bed is through continuous wave propagation. A new type sensor was used to measure the wave signals in VFB. The wave characteristics are affected by bed properties. By analyzing the wave frequency spectrum, the bubble's behavior in the bed can be followed.     

Circulating fluidized bed hydrodynamics with air staging: an experimental study

The influence of secondary air injection (SA) on the hydrodynamics of circulating fluidized beds was studied in a 0.23-m ID riser. The secondary to primary air ratio, the vertical position, and the mode of injection (radial, tangential, and 45° entrance) are considered to be the key parameters of SA injection. It was found that the amount and location of SA have direct influence on the solids holdup and the segregation patterns in the riser. The SA divided the riser into two different flow zones: a dense turbulent zone below and a relatively dilute bed above the injection port. The mean solids velocity is found to be upwards with a greater magnitude in the center region. It is downwards with a smaller magnitude along the walls, suggesting core-annular flow structures for both above and below the SA injection region.     

气固密相流化床颗粒温度的流体力学模拟与时间序列分析

应用基于颗粒动力学理论的欧拉-欧拉模型,模拟了不同表观气速下的气固密相流化床,测定了冷模流化床中压力脉动沿床高的变化,将CFD模拟与实验得到的床层压力分布及压力脉动频谱图相对照,验证了数值模拟方法的正确性。采用统计分析的手段对模拟得到的颗粒温度时间序列进行研究,结果表明,随着表观气速的增加,颗粒温度增大,颗粒温度时间序列的标准偏差增大,平坦度基本不变。将声能量理论与颗粒温度相结合并比照声能量沿床高的变化趋势,发现颗粒温度、颗粒温度时间序列的标准偏差及平坦度可用于确定颗粒运动“滞留区”的位置。引入颗粒温度谱,并根据脉动能的级串理论将颗粒温度谱划分为含能尺度、惯性尺度和耗散尺度,发现颗粒温度谱在惯性尺度内普遍偏离Kolmogorov-5/3定律而趋向符合Levy-Kolmogorov定律。在Levy-Kolmogorov定律的适用范围内,“滞留区”的颗粒温度谱衰减指数达到最大值,据此提出颗粒温度谱的衰减指数具有表征“滞留区”位置的潜力。     

Comparative study of flow structures in a circulating-turbulent fluidized bed

This paper presented a comparison of flow structures in a newly designed circulating-turbulent fluidized bed (C-TFB), which has a high solids holdup and solids flux, with two commonly used fluidized beds reactors (circulating fluidized bed (CFB) and turbulent fluidized bed (TFB)). Experimental results included instantaneous solids concentration and particle velocity, and local solids flux. Results indicated that the flow behavior in the C-TFB possessed both similarities and differences with the TFB and high-density CFB. Many distinct advantages of the C-TFB flows were found.     

Experimental and computational study of gas–solid fluidized bed hydrodynamics

The hydrodynamics of a two-dimensional gas–solid fluidized bed reactor were studied experimentally and computationally. Computational fluid dynamics (CFD) simulation results from a commercial CFD software package, Fluent, were compared to those obtained by experiments conducted in a fluidized bed containing spherical glass beads of 250– in diameter. A multifluid Eulerian model incorporating the kinetic theory for solid particles was applied in order to simulate the gas–solid flow. Momentum exchange coefficients were calculated using the Syamlal–O’Brien, Gidaspow, and Wen–Yu drag functions. The solid-phase kinetic energy fluctuation was characterized by varying the restitution coefficient values from 0.9 to 0.99. The modeling predictions compared reasonably well with experimental bed expansion ratio measurements and qualitative gas–solid flow patterns. Pressure drops predicted by the simulations were in relatively close agreement with experimental measurements at superficial gas velocities higher than the minimum fluidization velocity, U_mf. Furthermore, the predicted instantaneous and time-average local voidage profiles showed similarities with the experimental results. Further experimental and modeling efforts are required in a comparable time and space resolutions for the validation of CFD models for fluidized bed reactors.     

循环流化床气固流动行为的数值模拟

采用欧拉-欧拉双流体模型,在Fluent数值模拟平台上对循环湍动流化床进行了研究。采用Gidaspow模型分段描述密相湍动和稀相循环输送区气固的相互作用,考察了反应器内压力、颗粒浓度等气固湍动特性参数的变化。计算流体力学(CFD)的模拟结果表明:反应器中压力分布较为均匀;固体颗粒相和气体相间隙成絮状分布,在轴向中心区域颗粒浓度分布较为均匀;边壁区域颗粒浓度变化较大,存在强烈的气固相互作用。     

Investigation of hydrodynamics and mass transfer in a three-phase fluidized bed

On the basis of semiempirical turbulence theory and wave theory of the motion of liquid films, we have composed an engineering method for calculating the energetic, amplitude-frequency, and kinetic characteristics of phase interface, which are used in calculating the processes of absorption and chemosorption.     

The effect of vibrating conditions on the electrostatic charge in a vertical vibrating granular bed

In this study, we carried out experiments to measure the electrostatic charge of a granular matter in a vertical shaker device. The purpose was to quantify the effect of the vibrating conditions on electrostatic charging in the granular matter. In each experimental run, 3 mm glass beads were first discharged to remove any residual charge prior to subsequently studying their electrostatic charging. The accumulative electrostatic charges of the granular materials were measured using a Faraday cage. The findings show that the vibrating conditions play an important role in the saturated electrostatic charge and time constant. The electrostatic charges of granular materials are mainly generated by the contact potential difference mechanism in the vibrating granular system. The results show that the saturated accumulation charge increases as the dimensionless vibrating acceleration increases, and decreases with increasing vibrating frequency. The time constant is small when a higher vibrating frequency is applied in the vibrating granular system. Finally, we demonstrate that the saturated accumulation charge increases linearly with the increase of the dimensionless vibrating velocity regardless of the vibrating frequency.