DEPOSITION OF A FINE POWDER IN HORIZONTAL PIPELINES AND BENDS

The deposition of a very fine powder in a horizontal, lean-phase pneumatic conveying conduit containing a 90° bend has been studied experimentally. The total deposition and the deposition pattern were studied as a function of superficial gas velocity, solids loading and bend geometry: one sharp and three smooth bends of different radii of curvature were used. Most deposition was seen in the sharp bend, while in the smooth bends the deposition did not vary much with radius of curvature. The deposition decreased significantly with increasing superficial gas velocity. The experimental results were compared with CFD simulations of the flow field. The observations are consistent with the notion of deposition in regions with low near-wall gas velocity, indicating that models for deposition should be sought in considerations of the deposit stability for this type of system.     

Numerical Study of Particle Deposition in Bends of a Circular Cross-Section-Laminar Flow Regime

Particle deposition in a 90° bend has been studied numerically using a realistic three-dimensional developing flow field. In addition to the Stokes number as the impaction parameter, both the curvature ratio and the Dean number have been found to have considerable effects on the deposition efficiency. At a fixed Stokes number, the deposition efficiency increases with an increasing Dean number and a decreasing curvature ratio. The inlet velocity profile also influences the deposition efficiency. In the case of a parabolic velocity profile, the deposition efficiency is always higher than that of a uniform profile. These increases in deposition efficiency are due to the increase of secondary flow strength and the increased skewness of the axial velocity profile toward the outside of the bend.     

Interrogating the effect of 90° bends on air–silicone oil flows using advanced instrumentation

When gas/liquid mixtures flow around a bend they are subjected to forces additional to those encountered in a straight pipe. The behaviour of the flows at the inlet and outlet of the bend depends on the orientation of the pipes. Air/silicone oil flows around a 90° bend have been investigated using advanced instrumentation: Electrical Capacitance Tomography (ECT), Wire Mesh Sensor Tomography (WMS) and high-speed video. The first two provide time and cross-sectionally resolved data on void fraction. ECT probes were mounted 10 diameters upstream of the bend whilst WMS was positioned either immediately upstream or immediately downstream of the bend. The downstream pipe was maintained horizontal whilst the upstream pipe was mounted either vertically or horizontally. The bend (R/D=2.3) was made of transparent acrylic resin. From an analysis of the output from the tomography equipment, flow patterns were identified using both the reconstructed images as well as the characteristic signatures of Probability Density Function (PDF) plots of the time series of cross-sectionally averaged void fraction as suggested by Costigan and Whalley (1996). The superficial velocities of the air ranged from 0.05 to 4.73 m/s and for the silicone oil from 0.05 to 0.38 m/s. Bubble/spherical cap, slug, unstable slug and churn flows were observed before the bend for the vertical pipe and plug, slug, stratified wavy and annular flows when the pipe was horizontal. Bubble, stratified wavy, slug, semi-annular and annular flows are seen after the bend for the vertical 90° bend whilst for the horizontal 90° bend, the flow patterns remained the same as before the bend. Flow patterns for the vertical and horizontal 90° bends are shown on the diagram of the gas superficial velocity versus liquid superficial velocity. These results are confirmed by the high-speed videos taken around the bend. A previously proposed criterion, to determine stratification after the bend, based on a modified Froude number have been shown to be valid for a liquid different from that tested in the original paper.     

Some aspects on gas-solid flow in a U-bend: Numerical investigation

Pneumatic drying is widely used in many engineering applications. It has been shown in earlier research [Fyhr C. and Rasmuson A., Mathematical model of a pneumatic conveying dryer, AIChE Journal, Vol. 43, pp 2889-2902, 1997.] that the U-bends in the pneumatic conveying dryer system significantly influence drying behavior since they create enhanced slip velocities between suspended material and the drying medium. On one hand, this slip will increase external heat and mass transfer rates, thereby enhancing drying conditions. On the other hand, increasing the number of bends will cause an increase in pressure drop. Use of the suitable mean gas velocity and the suitable bend radius ratio will result in a better design and improved operating conditions.Two-phase CFD calculations, using a Eulerian-Eulerian model and commercial program Fluent 6.0, were employed to calculate the gas and particle flows in a U-bend. Variables studied include: particle diameter, particle density, particle volume fraction, gas velocity and bend radius ratio. The numerical calculations were validated against experimental data from the literature. The density and diameter of particle vary from 600 up to 1000 kg/m³ and from 0.00025 up to 0.001 m, respectively. The gas velocity and particle volume fraction vary from 10 up to 25 m/s and from 0.001 up to 0.01 m³/m³, respectively. The bend radius ratio varies from 3 up to 8 m/m. The slip velocity is affected by all the studied parameters, in particular, particle diameter, gas velocity and bend radius ratio; whereas the total pressure drop is strongly affected by gas velocity and bend radius ratio. A low mean gas velocity will give a lower total pressure drop and longer particle residence time. A small bend radius ratio will produce a faster dispersion of particles, which benefits drying, but on the other hand, will increase the total pressure drop. Thus, optimizing gas velocity and bend radius ratio is important in reducing energy consumption.     

Numerical Investigation of a New Method for Reducing Bends Erosion from Particles Impacts

The present paper intends to introduce a new method for reducing bends erosion from particles impacts: the ribbed bend erosion protection method. Ribs are evenly fixed in the range of 20°-80° on the inner-wall of inside 90?bend and the bend (including ribs) is made of medium carbon steel. Three-dimensional numerical work is performed and the result shows satisfactory agreement with the experimental measurement. Numerical simulation studies the characteristics of axial gas flow along the bend and secondary flow at cross section. Detailed analyses involving the impact velocity and incidence angle of particle-metal (either particle-rib or particle-duct) impact unveil the mechanism of the anti-erosion effect. As a result, predications achieve that the average erosion rate of the ribbed bends is only a third of the bare bend under test conditions and rectangle ribs possess higher anti-erosion effect than square ribs, while the wear distribution pattern remains unchanged after adding ribs onto the bend. All     

Particle Transport at Low Pressure: Deposition in Bends of a Circular Cross-Section

Particle inertial loss in bends of a circular cross section was studied experimentally in a low-pressure environment down to 0.2 Torr. Uniform aerosols across the bend inlets were produced by a new method developed by Sato et al. (2002a). The deposition efficiency was obtained by measuring the mass of deposited ammonium fluorescein particles with a fluorometer. The ranges of the Dean number and the curvature ratio (the ratio of the radius of curvature to the tube radius) were 1.8-2.2 and 3.6-4.7, respectively. The flow Reynolds number varied from 3.4 to 4.8. The results agreed with the general trend shown numerically by Tsai and Pui (1990), i.e., the deposition efficiency increases with increasing Stokes number and Dean number, and with decreasing curvature ratio. However, the experimental Stk 50 --the Stokes number at 50% deposition efficiency--was larger than that predicted by Tsai and Pui's empirical equation. The difference was attributed to the fact that their equation was obtained using the results for Dean numbers ranging from 38 to 869 and thus failed to predict the Stk 50 for a smaller Dean number. A new analytical equation for Stk 50 as a function of the Dean number and the curvature ratio was obtained. The proposed new equation can be used to calculate the Stk 50 for Dean numbers near unity.     

Numerical investigation of bend and torus flows—Part II : Flow simulation in torus reactor

Flow in a torus reactor with straight parts fitted with a marine impeller is investigated. The laser Doppler anemometry (LDA) is first employed to achieve experimental measurements of mean velocity profiles. Next, a numerical resolution of the steady-state flow is performed using a multiple reference frames (MRF) approach to represent the particular flow induced by the marine impeller in the geometry. A comparison of predictions using different turbulence models to LDA measurements is made, and a k-ω model is assessed.The numerical tool is used to investigate in more details the particular flow induced in the torus geometry. Evolution of the axial and rotating motions when moving away from the impeller is especially investigated, showing the complex hydrodynamical interaction between the main rotating swirl motion involved downstream the impeller, and bend curvature effects. Two different flow conditions can be considered in the torus geometry, with a main swirling motion close to the impeller, which freely decays and vanishes when Dean vortices appear in bends. Simulations for two rotation velocities of the impeller and comparison with the study with simple bends (first part) reveal pertinence of the swirl number Sn to describe the change of flow conditions along the reactor axis. When this parameter decreases below a threshold value around 0.2 in a bend entry, centrifugal effects due to bend curvature are more important than the swirl motion, and Dean vortices appear in bend outlet. One main consequence is the axial distance of the swirl motion persistence, which is found to be smaller for the higher impeller rotation velocity, due to the dual effect of the marine impeller that generates simultaneously both axial and rotating motions.     

垂直U型管两相流流型和传热研究(I)——U型管弯管段两相流流型转变

对垂直U型管弯管段空气-水同向流动时气液两相流在不同速度范围内流型间转变进行了试验,试验用内径21.5mm有机玻璃管,弯管半径分别为694、500和320mm.流型采用直接观察法及电导探针法确定.所得数据与Mandbane等人及Weiman等人的水平管转变关系式作了比较.根据试验结果,给出了以折算气体和液体速度V_(SG)及V_(SL)为座标的总流型图,提出了计算流型转变的关联式.     

Flow patterns and gas fractions of air–oil and air–water flow in pipe bends

An experimental study of two-phase flow in a 180° pipe bends with 0.016, 0.022 and 0.03 m and the curvature radii of 0.11, 0.154, 0.21 m, respectively have been carried out. The experiments were conducted under the input superficial phase velocity: air from 0.038 to 5.4 m s⁻¹, water from 0.018 to 0.92 m s⁻¹ and oil from 0.014 to 0.92 m s⁻¹. The conducted research involved the observation of the forming flow patterns and determination of average volumetric in situ gas fraction. On the basis of the results of experimental flow map was created for gas–liquid flow and a method of calculating gas fractions was established.     

大型快速流化床内颗粒浓度和速度分布的实验研究

采用压力巡检仪和光纤测量仪,对直径300 mm的快速流化床反应器内气固两相流动特性进行了研究,考察了操作条件对快速床轴、径向催化剂颗粒浓度、颗粒速度、筛分分布等的影响. 结果表明,当操作气速提高到2.0~2.6 m/s,相应的催化剂循环强度在60~160 kg/(m2×s),床层密度可保持在50~650 kg/m3;催化剂颗粒浓度在径向上呈中心低、边壁高的不均匀分布,轴向上各径向位置在颗粒加速区逐渐降低、在充分发展区趋于稳定、随表观气速增大或催化剂循环强度减小而减小,且径向均匀性变好,在r/R<0.7的中心区域趋于一致;颗粒速度在径向上呈中心高、边壁低的抛物线形分布,且随操作气速增大或催化剂循环强度增大而更加明显.     

LOCAL HYDRODYNAMICS IN AN EXTERNAL LOOP AIRLIFT SLURRY REACTOR WITH AND WITHOUT A RESISTANCE-REGULATING ELEMENT

The hydrodynamic behavior of an external loop airlift slurry reactor (ALSR) with and without a resistance-regulating element was studied with a fiber optic probe and ultrasound Doppler velocimetry. The influences of the superficial gas velocity and solid holdup on the global gas holdup and radial profiles of the suspension circulation velocity in the downer and of gas holdup, bubble size, and bubble rise velocity in the riser were studied. Local measurements allow a better understanding of the flow behavior in the reactor and can be used for CFD modeling and validation. Experimental results show that the resistance-regulating element increases the gas holdup and decreases the suspension circulation velocity, indicating that an optimum design of the flow resistance is needed to obtain the maximum gas-liquid volumetric mass transfer coefficient for a specific superficial gas velocity. A high superficial gas velocity and low solid holdup are favorable for increased uniformity of the radial profile of the gas holdup and bubble rise velocity. Hydrodynamic models that predict the gas holdup and suspension circulation velocity were developed for an ALSR with and without a resistance-regulating element. Good agreement was obtained between measured and predicted values.     

LOCAL HYDRODYNAMICS IN AN EXTERNAL LOOP AIRLIFT SLURRY REACTOR WITH AND WITHOUT A RESISTANCE-REGULATING ELEMENT

The hydrodynamic behavior of an external loop airlift slurry reactor (ALSR) with and without a resistance-regulating element was studied with a fiber optic probe and ultrasound Doppler velocimetry. The influences of the superficial gas velocity and solid holdup on the global gas holdup and radial profiles of the suspension circulation velocity in the downer and of gas holdup, bubble size, and bubble rise velocity in the riser were studied. Local measurements allow a better understanding of the flow behavior in the reactor and can be used for CFD modeling and validation. Experimental results show that the resistance-regulating element increases the gas holdup and decreases the suspension circulation velocity, indicating that an optimum design of the flow resistance is needed to obtain the maximum gas-liquid volumetric mass transfer coefficient for a specific superficial gas velocity. A high superficial gas velocity and low solid holdup are favorable for increased uniformity of the radial profile of the gas holdup and bubble rise velocity. Hydrodynamic models that predict the gas holdup and suspension circulation velocity were developed for an ALSR with and without a resistance-regulating element. Good agreement was obtained between measured and predicted values.     

Gas hold-up in highly viscous pseudoplastic non-newtonian solutions in three phase sparged reactors

Gas hold-up structure was studied in a three phase sparged reactor of 200 mm diameter. Air and aqueous solutions of carboxymethyl cellulose were used as gas and liquid phases. Spherical glass beads formed the solid phase. The superficial gas velocity, rheological properties, particle size and solid loading were varied over a wide range and their effects on gas hold-up structure were studied. A correlation for the estimation of the gas hold-up as a function of superficial gas velocity, apparent liquid viscosity, particle settling velocity and solid loading was established.     

蓄热段长度对流向变换催化燃烧反应器性能影响的模型研究

在建立低浓度挥发性有机化合物VOCs催化燃烧流向变换反应器一维非均相模型的基础上,编写Matlab程序对模型进行求解。分析在不同的表观气速下,反应器蓄热段长度对反应器性能的影响,着重研究大气速条件下反应器的最佳床层结构比例。计算结果表明,不同的蓄热段长度对反应器性能有很大影响。对于直径0.2 m的反应器,小气速条件下,反应器内最高温度随着蓄热段长度的增大而降低,大气速条件下则相反。在表观气速为0.15 m·s^-1和床层结构比例为1.2~2.0条件下,反应器能达到最佳操作状态。     

Modeling Large-Particle Deposition in Bends of Exhuast Ventilation Systems

A new model is presented to describe particle deposition in 90° bends of exhaust ventilation systems. This model accounts for non-Stokes particle motion and for variable deposition patterns as a function of particle Stokes number. Estimates made with the new model and with two models previously published were compared to measurements of deposition in bends with geometries, particle characteristics, and airflow conditions similar to those found in industry—large duct diameters (15.4 and 20.3 cm), large particle sizes (19–140 μm), and turbulent airflow (Re = 203,000 and Re = 368,000). Whereas the two models published previously explain 30% or less of the variability in the data, the new model explains 85%. The mean residual with the new model, 0.6%, is nearer to zero than that of the two other models, 3.6% and 9.8%. The new model is applicable to mists and to solid particles that stick to bend walls.     

气固错流移动颗粒床过滤器除尘效率

研究了气固错流移动颗粒床过滤器除尘效率与表观过滤气速、颗粒层移动速度、过滤气体粉尘含量的关系,并进一步探讨了粉尘在颗粒层内的沉积对除尘效果的影响.结果表明,颗粒层内粉尘沉积量较低时,沉积的粉尘有效地促进了颗粒层除尘效率的提高,但随粉尘沉积量增大,沉积粉尘的二次飞扬变得严重,其促进效应逐渐减小.在考虑了沉积粉尘对除尘效率影响和颗粒层内粉尘沉积不均匀性的基础上,基于捕集单元的收缩管模型,建立了计算移动颗粒层除尘效率和床层内粉尘沉积分布的数学模型.模型计算结果和实验数据比较表明,在操作气速0.1～0.3m&#8226;s^-1范围内,计算值与实验结果吻合较好.据此对除尘效率和床层内粉尘沉积的变化趋势进行了模拟分析.     

两级气液内环流反应器内气含率和循环液速

基于多釜串联可以有效减小返混的原理,通过引入特殊设计的级间构件构建了一种新型的两级内环流反应器。实验研究了级间构件形式、表观气速、表观液速和气液分离器对每一级内气含率和循环液速的影响。实验结果表明,表观气速对反应器二级(上一级)中上升管与下降管气含率之差和循环液速影响较大,而对一级(下一级)的影响较小;各级内上升管和下降管的气含率均随表观液速的增大而减小,但影响程度较小。基于推动力和阻力平衡建立了预测反应器中每一级的气含率和循环液速的流体力学模型,模型预测值与实验结果吻合较好。     

Recirculation and flow structures of gas in downcomer section of a concentric cylindrical airlift reactor

Recirculation and flow structures of gas in the downcomer section of a concentric cylindrical airlift reactor for air-water systems were studied, using an optical probe and the cross-covariance technique. A semiempirical model for predicting the gas recirculation rates in airlift reactors was developed based on the concept of ideal bubble flow. The entrained gas rate and the gas recirculation rate increased as the superficial gas velocity increased. There were nonuniform radial distributions of local gas holdup and air flow rates in the downcomer. Air flow rates in the downcomer depended on the superficial gas velocity and the distance from the top of the draft tube, but not on the superficial liquid velocity for this experimental range.     

磁稳定床轴向液体分散特性

1 INTRODUCTION Magnetically stabilized beds (MSB) have at- tracted many research interests, owing to the unique feature of combination of characteristics for packed bed and conventional fluidized bed, especially in the field of biotechnology processes such as bioseparation or immobilized enzyme catalyzed systems. However, there are few reports about the effects of physical properties of fluids on the axial liquid dispersion coef- ficients in both L-S and G-L-S MSB. Siegell[1] , Goetz …     

三相环流反应器中的局部相含率

将压差法与气-液相间滑移速度相结合,提出了利用压差法测量三相区局部相含率的新方法,将测得的局部固含率进行轴向平均并与由颗粒装填量计算所得的固含率进行比较,证明了此方法的可靠性. 利用所提出的测量方法,考察了三相环流反应器中气含率和固含率随操作条件的变化规律. 结果表明,气含率随表观气速的升高而增大,且随轴向位置的升高而增大;大颗粒具有破碎气泡的作用,能够增大反应器内的气含率;固含率随表观气速的增加而降低,且沿轴向变化较大. 表观气速较高时,固含率沿轴向近似呈S形分布.