Pickup, critical and wind threshold velocities of particles

This work presents experimental results on pickup velocity measurements for a variety of particulate solids in gases and in liquids. Based on our previously published experimental results for pickup in gas flow in pipes a three-zone master-curve is defined by establishing simple relationships between modified Reynolds and Archimedes numbers. The zones are distinguished by cohesive forces (van der Waals): Zone I represents negligible cohesion forces, Zone II represents considerable cohesion forces that increase the required pickup velocity of individual particles, and Zone III represents significant cohesion forces that cause pickup of agglomerates. Previously published experiments by others encompassing about 121 measurements for a wide range of particle sizes, shapes and densities picked up by liquids, were added to our master-curve with excellent agreement. The cohesive forces did not affect the critical velocity in case of liquid-particle systems. Therefore, these experiments extend the line fitting the pickup velocity of big dry particles. In most cases, the critical shear velocity (reported for liquid-particle systems) had to be converted to the average pickup velocity. Furthermore, additional 16 measurements of pickup velocities (in air) conducted in big wind tunnels were added to the master-curve with excellent agreement. We can conclude that our simple master-curve is appropriate for threshold velocities defined in three fluid-particle systems with a maximum error of only ± 30%.     

Particle mixing/segregation in gas-solid fluidized bed of ternary mixtures

The particle concentration profiles and minimum fluidizing velocity of ternary mixtures were investigated experimentally. All the experiments were carried out in an 88 mm I.D. transparent acrylic column containing fluidized beds of ternary particles of different sizes, densities and shape. Mixing/segregation patterns were visualized under different operating conditions. The experimental results are compared with empirical relationships for mixing index and take-off velocity. A new definition of the mixing index, including particle minimum fluidizing velocity and density predicted the mixing/segregation behaviour reliably. The proposed correlation of take-off velocity agrees with the experimental results very well.     

提升管循环流化床气,固局部滑落速度的分布

本研究采用光纤激光多普勒测速仪（ＬＤＶ），通过两列信号处理系统，同时测定了气体和颗粒局部速度，研究了低密度循环流化床提升管内局部滑落速度的变化规律。实验结果表明，提高颗粒循环速率，在任一径向位置的气固滑落速度均增加；提高气体表观速度，反而使气固之间局部滑落速度减小。由于固体颗粒在近壁区的团聚行为，在近壁区气、固局部滑落速度出现一个极大值。     

Comparison between two-fluid model simulations and particle image analysis & velocimetry (PIV) results for a two-dimensional gas–solid fluidized bed

This work compares simulation and experimental results of the hydrodynamics of a two-dimensional, bubbling air-fluidized bed. The simulation in this study has been conducted using an Eulerian–Eulerian two-fluid approach based on two different and well-known closure models for the gas–particle interaction: the drag models due to Gidaspow and Syamlal & O'Brien. The experimental results have been obtained by means of Digital Image Analysis (DIA) and Particle Image Velocimetry (PIV) techniques applied on a real bubbling fluidized bed of 0.005 m thickness to ensure its two-dimensional behaviour. Several results have been obtained in this work from both simulation and experiments and mutually compared. Previous studies in literature devoted to the comparison between two-fluid models and experiments are usually focused on bubble behaviour (i.e. bubble velocity and diameter) and dense-phase distribution. However, the present work examines and compares not only the bubble hydrodynamics and dense-phase probability within the bed, but also the time-averaged vertical and horizontal component of the dense-phase velocity, the air throughflow and the instantaneous interaction between bubbles and dense-phase. Besides, quantitative comparison of the time-averaged dense-phase probability as well as the velocity profiles at various distances from the distributor has been undertaken in this study by means of the definition of a discrepancy factor, which accounts for the quadratic difference between simulation and experiments The resulting comparison shows and acceptable resemblance between simulation and experiments for dense-phase probability, and good agreement for bubble diameter and velocity in two-dimensional beds, which is in harmony with other previous studies. However, regarding the time-averaged velocity of the dense-phase, the present study clearly reveals that simulation and experiments only agree qualitatively in the two-dimensional bed tested, the vertical component of the simulated dense-phase velocity being nearly an order of magnitude larger than the one obtained from the PIV experiments. This discrepancy increases with the height above the distributor of the two-dimensional bed, and it is even larger for the horizontal component of the time-averaged dense-phase velocity. In other words, the results presented in this work indicate that the fine agreement commonly encountered between simulated and real beds on bubble hydrodynamics is not a sufficient condition to ensure that the dense-phase velocity obtained with two-fluid models is similar to that from experimental measurements on two-dimensional beds.     

Experimental Study on Flow Behavior in a Gas‐Solid Fluidized Bed for the Methanol‐to‐Olefins Process

A cold model experimental system is established to investigate the flow behavior in a gas‐solid fluidized bed for the methanol‐to‐olefins process catalyzed by SAPO‐34. The system comprises a gas distributor in a F 300 × 5000 mm acrylic column, double fiber optic probe system and a series of cyclones. The experiments are carried out under conditions of atmospheric pressure and room temperature with different superficial velocities (0.3930–0.7860 m s^–1) and different initial bed heights (600–1200 mm). The effects of radial distance, axial distance, superficial gas velocity, and initial bed height on the solid concentration and particle velocity in the bed are discussed. The time‐averaged solid concentration and rising particle velocity profiles under different conditions are obtained. The results show that an increase in the value of r/R or initial bed height results in an increase in the solid concentration but a decrease in the rising particle velocity in the dense phase area, while improvement of the superficial gas velocity has a negative influence on the solid concentration but results in an increase in the rising particle velocity.     

LASER-BASED FLOW MEASUREMENTS OF DILUTE VERTICAL PNEUMATIC TRANSPORT

Dilute vertical pneumatic transport in a vertical lifter was studied using the sophisticated measurement techniques of laser Doppler anemometry (LDA) and phase Doppler anemometry (PDA). The vertical lifter consisted of a lower fluidized silo, an upper receiving tank, and a connecting vertical transport pipe made of clear glass. The experimental study was performed in order to get detailed information of the complex gas-particle flow behavior in a dilute vertical conveying system. Particle diameter, axial particle, and tangential particle velocities, as well as root mean square velocities, were measured simultaneously for different flow conditions. In addition, overall solid mass fluxes were obtained using weighing cells. Smooth and spherical zirconium oxide (ZrO₂) solids were applied with two different particle size distributions. Measurements were performed using different flow rates of air. The air inlet condition was varied in order to study its effect on the flow behavior. The particle diameter measurements show that no axial or radial segregation by size occurs for this transport condition. The results show that the particle velocity is independent of the particle size as well. The axial velocity profiles at different heights are almost identical and flat, which indicates fully developed turbulent pipe flow. The turbulent velocity measurements show that turbulence is mainly caused by the velocity gradients, and not by particle-particle collisions in dilute flow. The solid mass flux measurements show the importance of optimum inlet condition and how this influences the mass flux.     

LASER-BASED FLOW MEASUREMENTS OF DILUTE VERTICAL PNEUMATIC TRANSPORT

Dilute vertical pneumatic transport in a vertical lifter was studied using the sophisticated measurement techniques of laser Doppler anemometry (LDA) and phase Doppler anemometry (PDA). The vertical lifter consisted of a lower fluidized silo, an upper receiving tank, and a connecting vertical transport pipe made of clear glass. The experimental study was performed in order to get detailed information of the complex gas-particle flow behavior in a dilute vertical conveying system. Particle diameter, axial particle, and tangential particle velocities, as well as root mean square velocities, were measured simultaneously for different flow conditions. In addition, overall solid mass fluxes were obtained using weighing cells. Smooth and spherical zirconium oxide (ZrO₂) solids were applied with two different particle size distributions. Measurements were performed using different flow rates of air. The air inlet condition was varied in order to study its effect on the flow behavior. The particle diameter measurements show that no axial or radial segregation by size occurs for this transport condition. The results show that the particle velocity is independent of the particle size as well. The axial velocity profiles at different heights are almost identical and flat, which indicates fully developed turbulent pipe flow. The turbulent velocity measurements show that turbulence is mainly caused by the velocity gradients, and not by particle-particle collisions in dilute flow. The solid mass flux measurements show the importance of optimum inlet condition and how this influences the mass flux.     

LDV measurements of particle velocity distribution in an annular stripper

Particle descent velocities in an annular stripper were measured by a laser Doppler velocimetry (LDV) system. In the radial direction, particle descent velocity was relatively constant in the mid-region of the stripper and increased towards the walls on both sides, exhibiting an anti-U-shaped distribution. Particle descent velocity in the radial mid-region increased with the increase of superficial gas velocity, and the maximum in the outer wall region increased significantly with the increase of solid mass flux. Superficial stripping gas velocity had stronger effect on particle velocity distributions near the stripper gas distributor, and such effect weakened with the increase of the distance from the distributor. Local particle velocity and its radial profiles could be adjusted by changing the superficial stripping gas velocity. Empirical formulas were established to describe the relationships between the local particle velocity and cross-sectional averaged velocity based on the effects of operating conditions and measuring positions. The result showed that the predicted data was in good agreement with the experimental value.     

Hysteresis in cloud heights during solid suspension in stirred tank reactor: Experiments and CFD simulations

Solid suspension in stirred tank reactor is widely used in process industries for catalytic reactions, dissolution of solids, crystallization, and so on. Suspension quality is a key issue in design and operation of stirred reactor and its determination is not straight forward. Cloud height measurements of solid suspension provide a relatively simple way to quantify quality of suspension. In this work, experiments were carried out to quantify variation of cloud heights with impeller speed and particle characteristics. These measurements were carried out using visual observations, image analysis, and ultrasound velocity profiler techniques. The obtained data demonstrated the existence of hysteresis in cloud heights with respect to impeller speed. Apart from possible applications in reducing power required for achieving desired solid suspension quality, the existence of hysteresis also provides a new way to evaluate computational fluid dynamics (CFD) simulations of solid–liquid flows in stirred vessels. An attempt was made to capture observed hysteresis in cloud heights in CFD simulations. The simulated results were compared with the experimental data. The presented models and results (experimental and computational) will be useful for simulating complex solid–liquid flows in stirred reactors. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

宽Re范围内气泡和液滴曳力系数的关联模型

流粒(气泡或液滴)的曳力系数CD和上升/终端速度因有助于准确预测反应器内相含率分布、液相速度分布、流粒停留时间和传质速率而具有重要意义。但现有用于估算流粒CD的关联式大多分段且只在低雷诺数Re区间内有效,并难以同时准确预测不同实验体系和操作条件下的实验结果。针对这些不足,基于实验测量和理论分析,本工作提出了一个能够在整个Re范围内合理描述实验观测到的流粒CD演变规律的新数学关联式,并利用单组实验数据和编写的求解多维无约束线性优化问题的计算机程序确定了模型参数,且将新流粒CD关联式与现有经验模型、实验结果或三维数值模拟结果进行了对比。结果表明,新流粒CD关联式展现出优异的预测能力,能对不同实验体系(空气?水、空气?甘油?水、空气?甘油、甲苯?水、正丁醇?水、正辛醇?水等)、不同操作条件(气泡：0.1相似文献   

液-固流化床中单个变形气泡的上升速度

以气泡在上升运动过程中的受力分析为基础,建立了描述单个变形气泡在液－固流化床中上升速度的理论模。应用该模型分别对球形和球帽形气泡在液－固流化床中的上升速度进行了计算,并将计算民在不同的床内压力,温度与颗粒相体积分数下气泡上升速度的实验测量数据进行了比较。     

并流下行循环流化床中气固速度剖面以及滑落现象的研究

利用幅值鉴频的激光多普勒测速技术,同时测定并流下行循环流化床（CDCFB）中的气体和固体的局部速度,由此可以获得气固间的局部滑落速度。实验证明,包含提升管的并流下行循环流化床明显改善了床层结构的均一性,最大局部滑落速度比单颗粒的滑落速度高出许多倍,而且滑落速度的极值点位置与颗粒浓度有关,说明在并流下行循环流化床及提升管中存在明显的颗粒团聚现象。     

Effect of elevated temperature and silica sand particle size on minimum fluidization velocity in an atmospheric bubbling fluidized bed

The impact of temperature and particle size on minimum fluidizing velocity was studied and analyzed in a small pilot scale of bubbling fluidized bed reactor. This study was devoted to providing some data about fluidization to the literature under high temperature conditions. The experiments were carried out to evaluate the minimum fluidizing velocity over a vast range of temperature levels from 20 °C to 850 °C using silica sand with a particle size of 300–425 μm, 425–500 μm, 500–600 μm, and 600–710 μm. Furthermore, the variation in the minimum fluidized voidage was determined experimentally at the same conditions. The experimental data revealed that the U_mf directly varied with particle size and inversely with temperature, while ε_mf increases slightly with temperature based on the measurements of height at incipient fluidization. However, for all particle sizes used in this test, temperatures above 700 °C has a marginal effect on U_mf. The results were compared with many empirical equations, and it was found that the experimental result is still in an acceptable range of empirical equations used. In which, our findings are not well predicted by the widely accepted correlations reported in the literature. Therefore, a new predicted equation has been developed that also accounts for the affecting of mean particle size in addition to other parameters. A good mean relative deviation of 5.473% between the experimental data and the predicted values was estimated from the correlation of the effective dimensionless group. Furthermore, the experimental work revealed that the minimum fluidizing velocity was not affected by the height of the bed even at high temperature.     

飞灰颗粒与平板表面撞击过程的实验研究

以煤粉锅炉积灰过程为研究背景,在常温常压环境下对飞灰颗粒与平板表面撞击过程进行实验研究,分析飞灰颗粒入射法向速度对颗粒法向反弹速度的影响,以及飞灰颗粒粒径对临界捕集速度的影响。实验结果表明,飞灰颗粒粒径相同时,法向恢复系数随入射法向速度的增大先增大后平缓最后减小,在增大区域具有相当陡的斜率,反映了不同形式的力在不同阶段所占的比重不同;颗粒入射法向速度相同时,法向恢复系数随着颗粒粒径的增大而增大;而临界捕集速度随颗粒粒径的增大而减小。     

EVALUATION OF MINIMUM FLUIDIZING VELOCITY IN GAS FLUIDIZED BED FROM PRESSURE FLUCTUATIONS*

A new experimental method for the evaluation of minimum fluidizing velocity in gas fluidized beds is proposed based on the pressure fluctuations measurements. The minimum fluidizing velocity is determined solely from the measurements in the fluidized bed regime, unlike the traditional method, where experiments in the fixed bed regime are also necessary. Using the proposed method the on-line evaluation and prediction of the minimum fluidizing velocity is possible which is important for the control of industrial fluidized bed reactors.     

EVALUATION OF MINIMUM FLUIDIZING VELOCITY IN GAS FLUIDIZED BED FROM PRESSURE FLUCTUATIONS*

A new experimental method for the evaluation of minimum fluidizing velocity in gas fluidized beds is proposed based on the pressure fluctuations measurements. The minimum fluidizing velocity is determined solely from the measurements in the fluidized bed regime, unlike the traditional method, where experiments in the fixed bed regime are also necessary. Using the proposed method the on-line evaluation and prediction of the minimum fluidizing velocity is possible which is important for the control of industrial fluidized bed reactors.     

基于静电传感器空间滤波效应的颗粒速度测量

颗粒的流动速度是反应气固两相流流动状况的一个重要参数。介绍了静电传感器的空间滤波特性，推导了颗粒流动速度与静电传感器输出信号频率特性之间的关系。在此基础上，提出了一种基于静电传感器空间滤波效应的颗粒平均速度测量方法，并分析了静电传感器结构参数、颗粒截面分布、颗粒速度分布、颗粒尺寸、材料种类以及频率分辨率等因素对颗粒平均速度测量精度的影响。在重力输送颗粒流实验装置上进行了初步的实验研究。理论和实验结果均证明了静电传感器空间滤波颗粒平均速度测量方法的有效性。     

Simulation of particle movement in a pan coating device using discrete element modeling and its comparison with video-imaging experiments

A MATLAB™-based DEM (discrete element method) code was developed to study the particle motion in a pan coating device. The code was developed as a stand-alone program and provides simulation, visualization (GUI interface), and post-processing statistical analysis of particle movement. Results of DEM simulations are compared with those obtained from video-imaging experiments for 9 mm spherical polystyrene balls in a 58 cm diameter pan. The parameters compared are dynamic angle of repose and particle velocities in the x- (axial) and y- (parallel to cascading layer) directions within the spray zone in the cascading layer. The effects of pan loading and pan speed (6, 9, 12 rpm) on particle motion are compared and discussed. Good agreement was obtained between the DEM simulation and experimental results. The dynamic angle was found to increase with increasing pan speed and pan loading. The average cascading velocity was found to increase linearly with pan speed for both DEM and experiments. The velocity distributions both in the x- and y-direction were compared from simulation and experiments and found to be in good agreement. Velocity profiles along the entire top cascading layer of particles were also obtained using DEM. The particles in the cascading layer were found to reach their maximum velocity at positions close to the mid-point of the cascading surface. Comparison of simulated velocity profiles showed good agreement with published scaling laws for rotating drums, and a new correlation for scaling with respect to the pan loading is proposed.     

Incipient motion of individual particles in horizontal particle-fluid systems: A. Experimental analysis

This paper investigates the incipient motion velocity of individual particles in horizontal conveying systems. The first part presents a wide range of experimental measurements and an empirical analysis on incipient motion velocity (a type of pickup velocity) for a variety of particulate solids, sizes and shapes. Results from the literature for incipient motion of individual particles in gases and liquids are taken into account in the final empirical analysis. It was found that all the results for the single particle incipient motion velocity could be presented with a high accuracy by a simple relationship between the Reynolds and the Archimedes numbers. Furthermore, the friction coefficient should be taken into account for large particles by modifying the Archimedes number.The incipient motion velocity was added to a generalized master curve, which included various threshold velocities such as: pickup velocity from a layer of particle in gas and liquid, minimum pressure velocity, boundary saltation velocity, terminal velocity and minimum fluidization velocity. The different threshold velocities are presenting in this master curve through modified Reynolds and Archimedes numbers. The Reynolds number is modified by taking into account the effect of the pipe diameter and the Archimedes number is modified by taking into account various properties that affect each threshold mechanism. The incipient motion velocity was also compared to the boundary saltation velocity (saltation velocity of single particles) and some hysteresis was found. However, this hysteresis is larger for fine powders than for coarse particles.     

Prediction for particle removal efficiency of a reverse jet scrubber

Numerical computation was conducted to predict the collection performance of a reverse jet scrubber for polydisperse particles. The particle size distribution of polydisperse particles was represented by a lognormal function, and the continuous evolution of the particle size distribution in a reverse jet scrubber is taken into account with the first three moment equations. Numerical results were compared with the analytic results using average relative velocity in all zones and experimental results.

In a reverse jet scrubber, the impaction is the main particle collection mechanism because of high relative velocity and short collection time. The particle collection by impaction increases with an increase in particle size, and geometric mean diameter and geometric standard deviation decrease as time goes on. High droplet velocity and gas velocity increase the particle collection efficiency, and the small droplet size also increases the collection efficiency because smaller droplet size provides broader surface area. The packing density is a factor affecting particle collection efficiency in a scrubbing process. The dense packing density also provides large surface area and leads to high collection efficiency.