Flow structure and bubble dynamics in supercritical water fluidized bed and gas fluidized bed: A comparative study

Supercritical water (SCW) fluidized bed is a new reactor concept for hydrogen production from biomass or coal gasification. In this paper, a comparative study on flow structure and bubble dynamics in a supercritical water fluidized bed and a gas fluidized bed was carried out using the discrete element method (DEM). The results show that supercritical water condition reduces the incipient fluidization velocity, changes regime transitions, i.e. a homogeneous fluidization was observed when the superficial velocity is in the range of the minimum fluidization velocity and minimum bubbling velocity even the solids behave as Geldart B powders in the gas fluidized bed. Bubbling fluidization in the supercritical water fluidized bed was formed after superficial velocity exceeds the minimum bubbling velocity, as in the gas fluidized bed. Bubble is one of the most important features in fluidized bed, which is also the emphasis in this paper. Bubble growth was effectively suppressed in the supercritical water fluidized bed, which resulted in a more uniform flow structure. By analyzing a large number of bubbles, bubble dynamic characteristics such as diameter distribution, frequency, rising path and so on, were obtained. It is found that bubble dynamic characteristics in the supercritical water fluidized bed differ a lot from that in the gas fluidized bed, and there is a better fluidization quality induced by the bubble dynamics in the supercritical water fluidized bed.     

Effect of promoters on dynamics of gas-solid fluidized bed Statistical and ANN approaches

In this study, a bubbling fluidized bed column, 99 mm in inside diameter and 960 mm in height, was used to investigate the effect of rod and disc promoters on fluctuation and expansion ratios. Factorial design （statistical approach） and artificial neural network （ANN） models were developed to predict the fluctuation and expansion ratios in this gas-solid fluidized bed with varying gas flow rates, bed heights, particle sizes and densities. The fluctuation and expansion predicted using these statistical and ANN models, for beds with and without promoters, were found to agree well with corresponding experiments. The statistical model was found to be superior to the ANN model due to its ability to take into account both individual and interactive effects. The rod promoters were found to be more effective in reducing bed fluctuation, and in increasing bed expansion at high gas mass velocities.     

An analysis of the chaotic motion of particles of different sizes in a gas fluidized bed

The dynamic behavior of individual particles during the mixing/segregation process of particle mixtures in a gas fluidized bed is analyzed. The analysis is based on the results generated from discrete particle simulation, with the focus on the trajectory of and forces acting on individual particles. Typical particles are selected representing three kinds of particle motion： a flotsam particle which is initially at the bottom part of the bed and finally fluidized at the top part of the bed; a jetsam particle which is initially at the top part of the bed and finally stays in the bottom de-fluidized layer of the bed; and a jetsam particle which is intermittently joining the top fluidized and bottom de-fluidized layers. The results show that the motion of a particle is chaotic at macroscopic or global scale, but can be well explained at a microscopic scale in terms of its interaction forces and contact conditions with other particles, particle-fluid interaction force, and local flow structure. They also highlight the need for establishing a suitable method to link the information generated and modeled at different time and length scales.     

A novel dual-material probe for in situ measurement of particle charge densities in gas–solid fluidized beds

Particle charge density is vitally important for monitoring electrostatic charges and understanding particle charging behavior in fluidized beds. In this paper, a dual-material probe was tested in a gas–solid fluidized bed for measuring the charge density of fluidized particles. The experiments were conducted in a two-dimensional fluidized bed with both single bubble injection and freely bubbling, at various particle charge densities and superficial gas velocities. Uniformly sized glass beads were used to eliminate complicating factors at this early stage of probe development. Peak currents, extracted from dynamic signals, were decoupled to determine charge densities of bed particles, which were found to be qualitatively and quantitatively consistent with charge densities directly measured by Faraday cup from the freely bubbling fluidized bed. The current signals were also decoupled to estimate bubble rise velocities, which were found to be in reasonable agreement with those obtained directly by analyzing video images.     

The characteristics of gas–solid flow and wall heat transfer in a fluidized bed reactor

Numerical study using computational fluid dynamics has been carried out to investigate the heat transfer characteristics of a laboratory fluidized bed reactor. The fluidized bed reactor of vTI (Johann Heinrich von Thünen-Institute)-Institute of Wood Technology and Wood Biology is modeled. For the simulation of multiphase flow and thermal fields, an Eulerian–Eulerian approach is applied. The flow and thermal characteristics of the reactor are fully investigated for the wide range of superficial gas velocities and two different particle diameters. In particular, the contributions of the gas bubble and emulsion phase flows on the wall heat transfer are scrutinized. From the predicted results, it is fully elucidated that particular near-wall bubble motions mainly govern the wall heat transfer.     

A novel dual-material probe for in situ measurement of particle charge densities in gas-solid fluidized beds

Particle charge density is vitally important for monitoring electrostatic charges and understanding particle charging behavior in fluidized beds.In this paper,a dual-material probe was tested in a gas-solid fluidized bed for measuring the charge density of fluidized particles.The experiments were conducted in a two-dimensional fluidized bed with both single bubble injection and freely bubbling,at various particle charge densities and superficial gas velocities.Uniformly sized glass beads were used to eliminate complicating factors at this early stage of probe development.Peak currents,extracted from dynamic signals,were decoupled to determine charge densities of bed particles,which were found to be qualitatively and quantitatively consistent with charge densities directly measured by Faraday cup from the freely bubbling fluidized bed.The current signals were also decoupled to estimate bubble rise velocities,which were found to be in reasonable agreement with those obtained directly by analyzing video images.     

Experimental and computational studies on flow behavior of gas–solid fluidized bed with disparately sized binary particles

This paper presents experimental and computational studies on the flow behavior of a gas-solid fluidized bed with disparately sized binary particle mixtures. The mixing/segregation behavior and segregation efficiency of the small and large particles are investigated experimentally. Particle composition and operating conditions that influence the fluidization behavior of mixing/segregation are examined. Based on the granular kinetics theory, a multi-fluid CFD model has been developed and verified against the experimental results. The simulation results are in reasonable agreement with experimental data. The results showed that the smaller particles are found near the bed surface while the larger particles tend to settle down to the bed bottom in turbulent fluidized bed. However, complete segregation of the binary particles does not occur in the gas velocity range of 0.695-0.904 m/s. Segregation efficiency increases with increasing gas velocity and mean residence time of the binary particles, but decreases with increasing the small particle concentration. The calculated results also show that the small particles move downward in the wall region and upward in the core. Due to the effect of large particles on the movement of small particles, the small particles present a more turbulent velocity profile in the dense phase than that in the dilute phase.     

Effect of particle size on nonlinear wave propagation in a fluidized bed

The effect of particle size (Archimedes number) on the propagation of a kinematic particle concentration wave in a fluidized bed is investigated. The dependence of the characteristic wave velocity on the porosity of the bed (particle concentration) and the Archimedes number (or the Reynolds number for flow past individual particles of the dispersed phase) is determined. The evolution of a nonlinear perturbation of the bed porosity is investigated and the formation of discontinuities in the concentration of the dispersed phase is studied in relation to the particle size (Archimedes number). It is shown, in particular, that, as distinct from a bed of small particles, in a bed of large particles with quadratic interphase interaction only compression discontinuities can be formed. The results obtained can be used to analyze the formation of inhomogeneities (slugs and bubbles) in a fluidized bed in relation to the particle size.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 3, pp. 96–100, May–June, 1987.     

Analyzing dominant particle-flow structures inside a bubbling fluidized bed

The particle phase of a gas-particle flow inside a bubbling fluidized bed is characterized by strong unsteady flow patterns and intense meso-scale fluctuations that give rise to an intense mixing rate. As such, it is important to gain a deeper insight into how particle-flow structures and the associated fluctuating velocity field contribute to the overall bed dynamics. To this end, advanced post-processing methodologies, i.e., the Proper Orthogonal Decomposition (POD) and the swirling strength criterion, are applied to the particle flow fields predicted by a “two-fluid model” of a cylindrical bubbling bed to identify and analyze the dominant spatio-temporal patterns of the particle phase. Three-dimensional POD results indicate that the dominant particle fluctuating velocity patterns are principally aligned in the axial direction, corresponding to particle mixing by the bubble wakes, with significant laterally directed fluctuating velocity vectors at the bed surface, corresponding to mixing caused by the bubbles bursting. The particle velocity gradient tensor is decomposed based on the swirling strength criterion and reveals formation of extended and flat structures that may be considered as a characteristic feature of the particle vortical motions in bubbling beds.     

粗糙颗粒动理学及稠密气固两相流动的数值模拟

考虑颗粒碰撞过程中摩擦作用,给出了粗糙颗粒碰撞动力学.引入颗粒相拟总温来表征颗粒平动和转动脉动能量的特征.基于气体分子运动论,建立颗粒碰撞中平动和旋转共同作用的粗糙颗粒动理学,给出了颗粒相压力和黏度等输运参数计算模型.运用基于颗粒动理学的欧拉-欧拉气固两相流模型,数值模拟了流化床内气体颗粒两相流动特性,分析了颗粒旋转流动对颗粒碰撞能量交换和耗散的影响.模拟得到的流化床内径向颗粒浓度和提升管内颗粒轴向速度与他人实验结果相吻合.模拟结果表明随着颗粒浓度的增加,颗粒相压力和能量耗散逐渐增加,而颗粒拟总温先增加后下降.随着颗粒粗糙度系数的增加,床内平均颗粒相拟总温和能量耗散增加,表明颗粒旋转产生的摩擦将导致颗粒旋转脉动能量的改变,影响床内气体-颗粒两相宏观流动特性.      

Rotation characteristic and granular temperature analysis in a bubbling fluidized bed of binary particles

In this work, a discrete particle model (DPM) was applied to investigate the dynamic characteristics in a gas–solid bubbling fluidized bed of binary solid particles. The solid phase was simulated by the hard-sphere discrete particle model. The large eddy simulation (LES) method was used to simulate the gas phase. To improve the accuracy of the simulation, an improved sub-grid scale (SGS) model in the LES method was also applied. The mutative Smagorinsky constant case was compared with the previously published experimental data. The simulation by the mutative Smagorinsky constant model exhibited better agreement with the experimental data than that by the common invariant Smagorinsky constant model. Various restitution coefficients and different compositions of binary solids were investigated to determine their influences on the rotation characteristics and granular temperatures of the particles. The particle translational and rotational characteristic distributions were related to certain simulation parameters.     

Motion of inhomogeneities of a developed fluidized bed at small reynolds numbers

The instability of a fluidized system in which the particles are uniformly distributed in space [1–3] leads to the development of local inhomogeneities in the internal structure, these taking the form of more or less stable formations of packets of particles [4]. In accordance with the existing ideas based on experimental data [5–8, 13], the particle concentration within a packet may vary in a wide range from very small values (10^–2–10^–3 [8]) for bubbles to the concentration of the unfluidized bed for bunches of particles in a nearly closely packed state. The paper considers the steady disturbed motion of the fluid and solid phases near an ascending or descending packet of particles in a developed fluidized bed. It is assumed that the motion of the solid phase corresponds to a creeping flow of viscous fluid, and the viscosity of the fluidizing agent is taken into account only in the terms that describe the interphase interaction. The velocity fields and pressure distributions of the phases inside and outside a packet are determined. If the particle concentration within a packet tends to zero, the solution describes the slow motion of a bubble in a fluidized bed. The results of the paper are compared with results obtained earlier for the model of ideal fluids [9] and Batchelor's model [10], in which the fluidized bed is treated in a simplified form as a viscous quasihomogeneous continuum.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 4, pp. 57–65, July–August, 1984.     

A new drag model for TFM simulation of gas–solid bubbling fluidized beds with Geldart-B particles

In this work, a new drag model for TFM simulation in gas–solid bubbling fluidized beds was proposed, and a set of equations was derived to determine the meso-scale structural parameters to calculate the drag characteristics of Geldart-B particles under low gas velocities. In the new model, the meso-scale structure was characterized while accounting for the bubble and meso-scale structure effects on the drag coefficient. The Fluent software, incorporating the new drag model, was used to simulate the fluidization behavior. Experiments were performed in a Plexiglas cylindrical fluidized bed consisting of quartz sand as the solid phase and ambient air as the gas phase. Comparisons based on the solids hold-up inside the fluidized bed at different superficial gas velocities, were made between the 2D Cartesian simulations, and the experimental data, showing that the results of the new drag model reached much better agreement with experimental data than those of the Gidaspow drag model did.     

The influence of gas solubility on the stability of a layer of liquid with bubbles and an immobile filling

The instability of a bubbling layer due to the presence of a vertical gradient in the ascent velocity of the bubbles, causing stratification of the layer with respect to density, is considered in [1]. A similar instability mechanism of a fluidized bed is studied in [2]. The stabilizing influence of electrical and magnetic fields on a bubbling layer is shown in [3]. Consideration is given in [4] to the influence of the conditions of supply of the gas on the stability of a bubbling layer with an immobile filling. The present work deals with the stability of the mechanical equilibrium of a horizontal layer of liquid with an immobile filling through which a gas soluble in the liquid is bubbled. It is shown that there exists a critical solubility of the gas at which the mechanical equilibrium is unstable with respect to monotonie perturbations.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 5, pp. 68–74, September–October, 1984.The author would like to thank V. P. Myasnikov and V. V. Dil ' man for their interest in this work, and M. H. Rozenberg for assistance with the programming.     

Investigations in the field of fluidization,carried out at London University College

The main trend in investigations carried out at London University College is an elucidation of the mechanism of the operation of chemical reactors with a fluidized bed. The work involved in these investigations consisted of a detailed study of the corresponding physical processes to permit the development of an adequate model of a reactor. Verification of the model by a study of the chemical reactions taking place in a reactor with a fluidized bed is one of the most important stages of the work. The investigations were limited to the case of fluidization by gases. The nature of isolated unperturbed bubbles has been rather thoroughly studied, which makes it possible to construct a simple model [1, 2] which with subsequent refinenment, can become considerably more complicated. The necesary information with respect to two essential elements of this model is lacking. In the first place, heat transfer between the gas in a cloud, surrounded by bubbles, and the gas in the adjacent parts of the fluidized bed has been little studied. In the second place, for actual cases there is no acceptable method for calculating the mean size of the bubbles, their particle-size distribution, the change in their dimensions with height, as a function of the determining parameters such as the velocity of the gas, the particle size, and the construction of the gas-distributing device. Over the course of the last five years, a group of associates^* carried out investigations aimed at obtaining this information, as well as at testing models, and at analyzing of the process of stratification of the solid phase. A review of the latest results is given below.London. Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 6, pp. 50–60, November–December, 1972.     

Vibration time series analysis of bubbling and turbulent fluidization

A non-intrusive vibration monitoring technique was used to study the hydrodynamics of a gas–solid fluidized bed. Experiments were carried out in a 15 cm diameter fluidized bed using 226, 470 and 700 μm sand particles at various gas velocities, covering both bubbling and turbulent regimes. Auto correlation function, mutual information function, Hurst exponent analysis and power spectral density function were used to analyze the fluidized bed hydrodynamics near the transition point from bubbling to turbulent fluidization regimes. The first pass of the autocorrelation function from one half and the time delay at which it becomes zero, and also the first minimum of the mutual information, occur at a higher time delay in comparison to stochastic systems, and the values of time delays were maximum at the bubbling to turbulent transition gas velocity. The maximum value of Hurst exponent of macro structure occurred at the onset of regime transition from bubbling to turbulent. Further increase in gas velocity after that regime transition velocity causes a decrease in the Hurst exponent of macro structure because of breakage of large bubbles to small ones. The results showed these methods are capable of detecting the regime transition from bubbling to turbulent fluidization conditions using vibration signals.     

High-density circulating fluidized bed gasifier for advanced IGCC/IGFC—Advantages and challenges

Coal-fired Integrated Gasification Combined Cycle （IGCC） and Integrated coal Gasification Fuel-cell Com- bined cycle （IGFC） are being developed as high-efficiency electric power generation technology. However, the highest theoretical gross thermal efficiency of the conventional IGCC]IGFC is still below 52~. In order to obtain higher power generation efficiency, an advanced IGCC （A-IGCC） or advanced IGFC （A-IGFC） sys- tem making use of the exergy recuperation concept by recycling waste heat from gas turbine or fuel cells for steam gasification of coal and biomass was proposed in our laboratory, Corresponding to this system, a novel high-density triple-bed combined circulating fluidized bed （TBCFB） gasifier, composed of a downer pyrolyzer, a bubbling fluidized bed char gasifier, and a riser combustor, was proposed to replace traditional gasifiers such as the entrained flow bed gasifier. The new system is expected to more effectively utilize the waste heat from gas turbines or fuel cells and the heat produced by the combustion of the unreacted char in the riser combustor for pyrolysis and gasification of coal and biomass. In this short review, the advantages and future challenges in the development of high-density TBCFB gasifier are presented and discussed.     

A new drag model for TFM simulation of gas-solid bubbling fluidized beds with Geldart-B particles

In this work, a new drag model for TFM simulation in gas-solid bubbling fluidized beds was proposed, and a set of equations was derived to determine the meso-scale structural parameters to calculate the drag characteristics of Geldart-B particles under low gas velocities. In the new model, the meso-scale structure was characterized while accounting for the bubble and meso-scale structure effects on the drag coefficient. The Fluent software, incorporating the new drag model, was used to simulate the fluidization behavior. Experiments were performed in a Plexiglas cylindrical fluidized bed consisting of quartz sand as the solid phase and ambient air as the gas phase. Comparisons based on the solids hold-up inside the fluidized bed at different superficial gas velocities, were made between the 2D Cartesian simulations, and the experimental data, showing that the results of the new drag model reached much better agreement with exoerimental data than those of the Gidasoow dra~ model did.