Fluidization of molten salt fluid-particles using low density ratio kinetic theory of granular flow

To deduce and analyze the hydrodynamics of molten salt fluid and particles, computational simulations were performed using a low density ratio kinetic theory of granular flow with two-fluid model in a fluidized bed. Two types of transition fluidization of molten salt fluid-particle mixtures were found in the fluidized bed. One represented the coexistence of wave-like flow at the bottom regime and large scale turbulent regime with chunk-like flow at the bed upper. The other characterized the coexistence of particulate fluidization near the bottom regime and particle aggregations at the upper part along bed height. The molten salt fluid-particle mixtures transited from particulate fluidization to transition state with increasing molten salt fluid temperatures, inlet fluid velocities and particle diameters and densities. The computed expansion heights and fluid volume fractions agreed with measured data in a water-particles fluidized bed.     

Experimental study of fluidization characteristics of Geldart-D particles in pressurized bubbling fluidized bed

The pressurized bubbling fluidized bed shows great advantage in retreating municipal solid waste because it could effectively capture CO₂ and enhance the reaction rate of the process of combustion and gasification. In the present work, fluidization characteristics of Geldart-D particles at elevated pressure were experimentally investigated, such as flow pattern, pressure drop, minimum fluidization gas velocity. At the same fluidization gas velocity, as elevating operating pressure, the fluidization of Geldart-D particles became more intense, the bubbles got larger, the standard deviation and the power density of dominant frequency of the pressure drop signal increased. While, under the same fluidization number, as increasing operating pressure, the fluidization of Geldart-D particles became smoother, the bubble size decreased, both the standard deviation and the power density of dominant frequency of the pressure drop signal decreased. It seems that, under elevated pressure, the fluidization behavior of Geldart-D particles would transition to that of Geldart-B particles. Finally, the minimum fluidization velocity of the Geldart-D particles was found decreased with the increase of the operating pressure. A new correlation for the prediction of the minimum fluidization velocity of Geldart-D particles at elevated pressure was also formulated based on the present experimental results.     

超细颗粒外场流态化研究进展

综述了外场作用下流化床流化超细颗粒的基础研究进展,着重介绍了磁场、声场和振动场对超细颗粒流化行为的影响,总结了磁场中颗粒的受力分析和理论模型;并对现有研究成果作了简要评述。     

The hydrodynamics of liquid-solid and gas-liquid-solid inverse fluidized beds with bioparticles

The hydrodynamic characteristics, such as minimum fluidization velocity (U_lmf for liquid-solid (LS) system and U_g,if for gas-liquid-solid (GLS) system) and bed expansion ratio (BER), of liquid-solid and gas-liquid-solid inverse fluidized beds (LSIFB and GLSIFB) with bare particles and particles with biofilm were investigated. In the LSIFB system, U_lmf and BER of the bare particles were independent of the solids loading. For bioparticles, the increase of the biofilm thickness reduced U_lmf and increased BER, suggesting that the fluidizability increases with the presence of the biofilm. In the GLSIFB system, the initial fluidization gas velocity (U_g,if) and the complete fluidization gas velocity (U_g,cf) both increased with increasing particle diameter and decreasing particle density under fixed superficial liquid velocities. Biofilm attachment led to a decrease of both U_g,if and U_g,cf, and an increase of bed expansion, again suggesting increased fluidizability with the presence of biofilm.     

二元气-固流化系统临界流化速度的研究

在直径为4cm的有机玻璃流化床中,对由不同尺寸和质量分数的铁矿石和煤颗粒组成的二元气-固流态化系统进行了流化特征的实验研究,得到了该系统的流化特性曲线,给出了临界流化速度和二元系统混合颗粒平均直径之间的关系,用该关系式对临界流化速度进行了预测,并且将预测值和实验值进行了比较。结果表明,当煤颗粒的质量分数为10%时,临界流化速度umf与颗粒直径的平方dm2的关系为umf=0.13dm2+1.2;当煤颗粒的质量分数为20%时,两者的关系变为umf=0.12dm2+1.2,预测值与实验值误差在10%以内。     

Time and speed of fruit drying on batch fluid-beds

Drying of particles (pieces) in a fluidized bed affords better quality of end products, especially for better product structure and its shorter reconstitution time. Fluid-bed drying of different fruit particles has been investigated. Starting water content varies from grape berries 81.5% and peach 87.7% to apricot 86.9%. The temperature of fluidization varies from 70°C to 100°C at air velocities from 0.98 ms⁻¹ to 5.2 ms⁻¹. The product should be dried fruit with 10% to 14% of water content with good rehydration quality that varies from 8 to 20. Experimental data have been measured, relevant values have been calculated and a mathematical model introduced. The results have shown that drying of fruit in a fluidized bed produces dry fruit particles of improved quality in a much shorter time, with improved rehydration times and qualities, than in continuous belt-driers that are generally used.     

A molecular dynamics approach in simulations of fluid-solid particles flow

In the paper a discrete system of particles carried by fluid is considered in a planar motion. The volumetric density of particles is assumed to be small enough such that they can be treated within the framework of a molecular dynamics model. The fluid is then considered as a carrier of particles. The Landau-Lifshitz concept of turbulence is used to describe the fluctuating part of fluid velocity. This approach is applied to simulate different regimes (laminar and turbulent) and various states of particle motion (moving bed, heterogeneous flow, and homogeneous flow) using only two parameters, which have to be determined experimentally. These two parameters, found for a particular pipe and for a particular velocity from a simple experiment, then can be used for other pipe diameters and different velocities. The computer simulations performed for the flow of particles in pipes at different flow velocities and different pipe diameters agree favorably with experimental observations of the type of flow and critical velocities identifying transitions from one type to another. Received: 8 January 1999     

Investigation on dynamic movement of cylindrical biomass particles in a fast fluidized bed

The processing of biomass particles is complex in a fluidized bed due to their heterogeneous characteristics. To further understand biomass particles, the dynamic movement of cylindrical biomass particles were investigated in a fast fluidized bed. Cylindrical particles were studied via the impulse momentum theorem and the Discrete Element Method (DEM). Meanwhile, the contact action, the drag force and other conventional forces were considered. To validate the present method, the predicted orientation and the minimum fluidization velocities of cylindrical particles were compared with the relative results, and validate the present approach. Then the characteristics of biomass particle flow dynamics were analyzed in terms of the particle concentration, the orientation distribution and the residence time distribution (RTD). It was found that most particles present as the horizontal or nearly horizontal states (0°) during the fluidization, and the percentage can reach and exceed 16%. High concentration occurs near the wall due to the back flow. The lower fluidization velocity corresponds to a wider RTD. The particles with the same size leave the riser with various residence times. The applied method and the obtained results provide helpful consults to study the cylindrical and other non-spherical biomass particles in an extensive way.     

Increase in processing flue gas flow rate while maintaining the fluidization state and filtration performance in a low-temperature continuous regeneration filter using a fluidized bed

To increase the processing gas flow rate in a fluidized bed filter, the effects of superficial velocity and fluidization state on PM filtration and combustion were examined by experiments using large bed particles (710 μm). The fluidization state at 710 μm was measured by image analysis and recurrence plot, and the superficial velocities as experimental conditions were determined to obtain almost the same fluidization state and filtration efficiency as those for small bed particles (420 μm) in previous studies. The BET-surface area of 710 μm is slightly larger than that of 420 μm, and the amount of potassium catalyst doped on large bed particles is comparable to that at 420 μm. The gas phase velocity is increased by increasing the processing gas flow rate, and the contact probability between PM and oxidizer increases. The PM combustion reaction is significantly promoted owing to the effects of the potassium catalyst and the increase in the gas phase velocity, and the minimum continuous regeneration temperature is 30 °C lower than that at 420 μm. As a result, fluidized bed filters using large bed particles can be operated in continuous regeneration mode at a bed temperature of 320 °C while maintaining a filtration efficiency of 100%.     

双层连续管固态流化开采水合物的内管分析

为研究双层连续管内管举升水合物的井筒多相流动规律，建立了采用双层连续管固态流化开采天然气水合物的井筒多相流动模型，采用有限差分法对模型进行求解。基于以上模型，对南海某实例井采用双层连续管固态流化开采天然气水合物的井筒多相流动行为进行了模拟。研究结果表明，随着流体上升，双层连续管内管温度和压力逐渐降低；在海底 1 600 米至 700 米时，固相含量以及液相含量相对稳定，含气率为 0；当流体上升至 700 多米的高度时，固相水合物开始逐渐分解。当钻井液密度与排量过高或者过低，都会使得井底压力脱离安全窗口，威胁到井控安全。因此，合理地选用钻井参数，对确保井控安全尤为重要。     

Fluidization of snow

Fluidization experiments were performed on snow of various particle diameters and types by placing in a transparent tube through which was passed an upward air flow at various velocities and temperatures. Stable fluidization could be generated only for snow of relatively large particles of simple shape at lower temperatures. Newly fallen snow of dendritic crystals and snow of particle diameters less than about 0.3 mm could not be fluidized uniformly even at temperatures as low as −30°C. The onset of fluidization had to be initiated by an outside mechanical shock unless the temperature was very low or the snow particles large and spherical. This implies that some nuclei of fluidization must be formed within limited areas of snow before the whole of the snow is fluidized.The minimum air velocity to cause fluidization increased as the square of particle diameter, in accordance with the Kozeny-Carman equation. The overall behavior of fluidized snow is very similar to that of a liquid: the pressure within the fluidized snow equals the weight of snow in suspension above a unit area. Buoyancy was found to exist, and the volume increased linearly with air velocity, just as the volume expansion of a liquid with temperature. It was also found that the momentum and energy transfer in fluidized snow was very effective as in liquids: the apparent viscosity of fluidized snow, of particle diameter 1.25 mm, was roughly equal to that of water at 25°C and the heat transfer was three or four times larger than that in an air flow containing no ice particles.     

Photophoretic velocimetry for colloid characterization and separation in a cross-flow setup

We introduce photophoretic velocimetry as a new technique for characterization of particulate matter on the basis of optical particle properties. Complementary to well-established techniques, we could show that, by measuring the photophoretic velocity of the single particles, it is possible to distinguish particles of different sizes as well as particles of one size but different refractive indices. The difference in photophoretic migration of particles can be applied to the separation of particles. Polystyrene, melamine, and SiO2 microparticles (0.3-10 mum) suspended in purified water were used as test samples for validation of a cross-flow setup. The particles were pushed perpendicular to a uniform, pulsation-free fluid flow by a focused He-Ne laser (lambda = 633 nm, P = 47 mW, I(max) = 14.0 kW cm(-2)) providing a well-defined Gaussian-shaped flux distribution. The migration behavior was observed by means of a video camera system, and the velocities and displacements were calculated by using an adapted particle imaging velocimetry code as an approach to automatic characterization. The photophoretic displacement depends on both flow conditions and particle properties and can be applied for separation means.     

Mixing characteristics of binary mixtures in a spout-fluid bed

Mixing characteristics of binary mixtures in a flat-bottom cylindrical spout-fluid bed using glass beads and air are reported in this work. Experiments were carried out to investigate the mixing characteristics for binary mixtures at three flow conditions, i.e., only spouting, only fluidization and spout-fluidization. The experiments were performed at different gas velocities, diameter ratios of binary mixtures and three different bed arrangements. Mixing index was determined for fluidized bed and static bed conditions. It was found that, in all cases, lowest-diameter ratio mixture gave good mixing index values. For all flow conditions, mixing index for large–small bed arrangement was increasing with time, whereas for small–large bed arrangement, the mixing index deteriorated with time. However, in both cases, the mixing index reached almost a constant value. For well-mixed bed arrangement and spout-fluidization flow condition, segregation and re-mixing were observed.     

Aerodynamic Characteristics of Popcorn Ash Particles

Popcorn ash particles are fragments of sintered coal fly ash masses that resemble popcorn in low apparent density. They can travel with the flow in the furnace and settle on key places such as catalyst surfaces. Computational fluid dynamics (CFD) models are often used in the design process to prevent the carryover and settling of these particles on catalysts. Particle size, density, and drag coefficient are the most important aerodynamic parameters needed in CFD modeling of particle flow. The objective of this study was to experimentally determine particle size, shape, apparent density, and drag characteristics for popcorn ash particles from a coal-fired power plant. Particle size and shape were characterized by digital photography in three orthogonal directions and by computer image analysis. Particle apparent density was determined by volume and mass measurements. Particle terminal velocities in three directions were measured in water and each particle was also weighed in air and in water. The experimental data were analyzed and models were developed for equivalent sphere and equivalent ellipsoid with apparent density and drag coefficient distributions. The method developed in this study can be used to characterize the aerodynamic properties of popcorn-like particles.     

Typical Fluidization Characteristics for Geldart's Classification Groups

This article describes a comprehensive experimental analysis that defines typical fluidization characteristic curve for Geldart's classification groups. Geldart defined four types of materials which differ by the cohesion forces between particles. An experimental apparatus containing fluidized beds of four pipe diameters and fully controlled by LabVIEW was used to perform the fluidization tests. All tests were performed automatically by gradually increasing the air velocity and measuring the pressure drop over the bed. For each test, the fluidization curve was recorded and the minimum fluidization, bubbling, and slugging velocities were defined. It was found that the fluidization curve of material define accurately the Geldart's group to which the material belongs. In addition, was reviewed the reason for those materials and under which conditions the pressure drop increases in the slugging state. Finally, the influence of height to bed diameter H/D ratio on the shape of characterization curve was found. The present study has significant interest for researchers and designers since it enables to predict the fluidization characteristics of two-phase (fluid-solids) flows.     

Motion of a viscous fluid with suspended particles in a curved tube

In this paper we have discussed the motion of a viscous fluid with suspended particles through a curved tube of small curvature ratio. The system is treated as two separate interacting continua. Solutions for axial and secondary velocities are obtained in the form of asymptotic expansions in powers of Dean Number. The streamline pattern for the particulate phase reveals many interesting features. The influence of the particulate continium on the fluid is described by the parameter τ which depends on the density ratio of the two continua. The concentration distribution of the particles in a given cross section is determined. It is noticed that the particles move closer to the wall for certain values of the concentration and the density ratio.     

Study on the flow behavior of irregular plastic particles in a spout-fluid bed with a draft tube

This work presents an experimental investigation on the hydrodynamic performance of a draft tube spout-fluid bed with irregular particles. Nonmetal particles from waste printed circuit boards (NPCBs) were used as a spouting solid, and polypropylene (PP) particles were selected as an assistant to fluidization particles. The flow pattern, minimum spouting velocity (U_ms), and minimum spout-fluidization velocity (U_msf) were investigated under different operating conditions. The irregular cohesive particles from NPCBs showed poor flowability and channeling, which restrained stable spouting in the spout-fluid bed. The quality of fluidization and spouting improved when greater than 40?wt.% PP particles were added into the NPCB/PP mixtures. The mechanism was that the PP particles accelerated the movement of NPCB particles. Meanwhile, lower density differences between NPCB and PP particles decreased the segregation of the mixtures. The minimum spouting velocity decreased with an increase in fluidization gas velocity and the ratio of NPCB particle in the NPCB/PP mixtures. Two flow patterns, unstable spouting and unstable spouting fluidization, were observed over a large range of gas velocity. The ranges of gas velocity in these two flow patterns enlarged with the increase in mass fraction of NPCB particles within the NPCB/PP mixtures.     

细颗粒添加组分流态化研究进展

介绍了细颗粒聚团流态化的类型;从添加组分的种类、添加量以及粒径大小3个方面,综述了近些年来国内外研究者对细颗粒添加组分流态化的研究进展,进一步阐述了细颗粒中添加磁性大颗粒后在外加磁场作用下的流化性能的研究情况,重点指出了细颗粒添加组分流态化研究中存在的问题和发展方向,提出应开展细颗粒聚团与其添加颗粒之间的相互作用机理的研究,以得出定量的关系,从而为工业应用提供理论依据。     

Pressure Drop and Gas Holdup Studies in a Spout-Fluid Bed

Spout-fluid beds are used for a variety of processes involving particulate solids. They are employed where the particle agglomeration, dead zones, and sticking of particles to the vessel are the common problems in conventional spouted beds. Applications involved are granulation, coating, drying, combustion, and gasification. In this study, experimental studies have been carried out in a cylindrical Perspex column (0.094 m internal diameter and 1.217 m height) using glass beads and air. The effects of initial bed loading, spout velocity, and background (fluidization) velocity on pressure drop and gas holdup have been investigated. It is found that the minimum spout-fluidizing velocity increases with increase in initial bed loading. The pressure drop and gas holdup increase with increasing bed loading. In spout-fluid bed condition, at a constant spout velocity, as the background gas velocity increases, the gas holdup increases, and it is found to be high for smaller bed loading and is low for larger bed loading at higher velocities. The fountain height increases as spouting velocity increases and it decreases with initial bed loading. The total velocity required to fluidize the particles in spout fluidization is lower in comparison to spouted beds and fluidized beds.