小管径-粒径比颗粒无序堆积通道内壁面效应数值研究

采用DEM-CFD方法对小管径-粒径比颗粒无序堆积通道内壁面效应进行了数值研究。针对D/d_p=5.0圆球无序堆积通道构建了光滑壁面和波节壁面两种通道壁面结构,分析了不同壁面结构堆积通道内孔隙率分布、流动和温度场分布及其流动换热性能。结果表明：小管径-粒径比光滑壁面颗粒无序堆积通道内壁面效应显著,壁面附近平均流速明显高于堆积中心区域,而平均温度要低于堆积中心区域,壁面附近0.5d_p区域内通过的流体质量流量比例为46%;波节壁面结构抑制了通道壁面附近漏流,可小幅提高堆积通道的换热能力,但堆积通道内的流动阻力也随之增大,其综合换热性能较光滑壁面堆积通道有所下降。     

Magnetic Resonance Visualisation of Flow and Pore Structure in Packed Beds with Low Aspect Ratio

Different NMR techniques were combined to obtain the structure and velocity information for a systematic investigation of fixed beds with low aspect ratio (tube diameter to particle diamter, d_t/d_p) in the range 1.4 to 32. The structure of the void space was determined for a variety of packed beds of glass beads or regular and irregular porous pellets by magnetic resonance imaging (MRI). Based on the images the radial distribution of the voids within the bed was obtained. Ordering effects were found even for non‐spherical and polydisperse particles, and a maximum of the fluid density near the tube wall was confirmed for all pellet geometries and sizes. By combining MRI with velocity encoding, velocity profiles and distributions of flow velocity components of a single fluid phase through packed beds have been acquired. The radial velocity distribution follows an oscillatory pattern which largely reflects the ordering of the particles, which can be accessed from the density distribution of the interparticle fluid. Maximum velocities of up to four times the average value were found to occur near the tube wall. This wall effect was observed for all but the smallest particles, where the aspect ratio was d_t/d_p = 32. Moreover, a visualisation of flow pattern in the presence of packed particles was achieved by using a tagging technique, and the stationary flow field could be identified for an experimental time of several hours.     

Axial Equilibrium Distance and Positioning of Particles in a Laminar Tube Flow

Particle transport in a laminar tube flow at low Reynolds numbers leads to accumulation of particles at specific equilibrium radii. The equilibrium radius depends on the particle size. Small particles find their equilibrium radius near the wall and large particles near the tube axis. During their radial migration to the equilibrium position, the particles move in axial direction with the flow. In an experimental setup, the axial equilibrium distance is measurement for several tube Reynolds numbers. The axial equilibrium distance is the distance a particle migrates in the flow direction, until it reaches its radial equilibrium position. The results are compared with CFD‐simulations of single particle movement in a laminar tube flow.     

EVALUATION OF POLYMER ADSORPTION-GEL FORMATION AND SLIP IN POLYMER SOLUTION FLOWS

A superposition model for evaluation of the effects of polymer adsorption-gel formation and slip of polymer solutions exhibiting both phenomena has been applied to the capillary flow of aqueous solutions of two molecular weight grades of hydroxyethyl cellulose (Natrosol 250, types G and HR, supplied by Hercules Powder Company). The flow behaviour of the solutions investigated was non-Newtonian. Evaluations are presented of the effective thicknesses of polymer adsorption-gel formation and pure solvent layers, as a function of the wall shear stress, tube radius and polymer concentration, corresponding to the determinations of the effective velocity at the wall.

The results of the analysis indicate the surface characteristics undergo a dramatic change from polymer adsorption-gel formation at the tube surface to the phenomenon characterized by slip in a narrow tube radius interval which has important implications in enhanced oil recovery by polymer solution floods. It also provides an explanation for the contrasting behaviours observed in the flow of aqueous Natrosol solutions through packed beds (Sadowski, 1963) and filter cakes (Kozicki et al., 1972).     

CFD modeling of pressure drop and drag coefficient in fixed and expanded beds

The aim of this study is computational fluid dynamic (CFD) simulation of the single-phase pressure drop in fixed and expanded beds. A fixed bed with a column to particle diameter ratio (D/d_p) of 5 and having 151 particles arranged in 8 layers was taken as a computational geometrical model. In the case of expanded beds, 0.605 voidage bed consisted of 105 particles and 0.783 voidage bed consisted of 55 particles. Simulations were performed in the creeping, transition and turbulent flow regimes, where Reynolds number (d_pV_Lρ_L/μ_L) was varied from 0.1 to 10,000. The deviations from Ergun's equation due to the wall effects, which are important in D/d_p < 10 beds, were well explained by the CFD simulations. Thus, an increase in the pressure drop was observed due to the wall friction in the creeping flow, whereas, in turbulent regime a decrease in the pressure drop was observed due to the channeling near the wall. Energy balance has been established through the CFD predicted values of energy dissipation rates (viscous as well as turbulent).     

圆管中相变微胶囊悬浮液层流状态下的融化特性

采用基于双流体模型开发的计算程序模拟了圆管内潜热型功能热流体在均匀热流密度条件下层流流动时的融化特性,模型中相变材料为正十六烷,壳层材料为尿素-甲醛树脂,载流体为去离子水. 分析了Re数、相变微胶囊颗粒体积浓度、热流密度对融化特性的影响. 结果表明,当潜热型功能热流体流过被加热圆管时,存在非融化、融化和完全融化区域;随Re数增加,管道近壁处和中心处的融化起始和终止点均向出口处移动,融化区域逐渐变长;随颗粒浓度增加,管道中心处的融化起始和终止点均逐渐向出口处移动,而近壁处的融化区域起始和终止点则基本保持不变,融化区域同样得到延伸;壁面热流密度越大,近壁处和中心处融化区域起始和终止点越靠近入口位置,对应的融化区域则越短.     

Analysis of transport processes with granular media using the constricted tube model

Four kinds of transport processes occurring in granular media were studied, using the recently formulated constricted tube model for media characteriza The problems considered were: (a) momentum transfer associated with the flow through the media; (b) convective mass transfer; (c) axial dispersion; and collection of aerosol particles in granular beds. Special emphasis was placed on the effect of the wall geometry of the constricted tube on the relevan relationships obtained from model predictions.     

Particle-resolved simulation of packed beds by non-body conforming locally refined orthogonal hexahedral mesh

The traditional fixed-bed reactor design is usually not suitable for the low tube-to-particle diameter ratios (N=D/d < 8) where the local phenomena of channeling near the wall and backflow in the bed are dominant. The recent "solid particle" meshing method is too complicated for mesh generation, especially for non-spherical particles in large random packed beds, which seriously hinders its development. In this work, a novel high-fidelity mesh model is proposed for simulation of fixed bed reactors by combining the immersed boundary and adaptive meshing methods. This method is suitable for different shapes of particles, which ingeniously avoids handling the complex "contact point" problem. Several packed beds with two different shapes of particles are investigated with this model, and the local flow in the bed is simulated without geometrical simplification. The predicted pressure drop across the fixed bed and heat transfer of the single particle are in good agreement with the corresponding empirical relations. Compared with spherical particles, the packed bed packing with pentaphyllous particles has lower pressure drop and better heat/mass transfer performance, and it shows that this method can be used for the screening of particle shapes in a fixed bed.     

An unsteady potential flow model of bubble-induced particle motion near a horizontal tube in a fluidized bed

An unsteady hydrodynamic model of bubble-induced particle motion near a horizontal tube immersed in a gas-fluidized bed is proposed. According to the model, a two-dimensional bubble rising past the tube is treated as a moving doublet so that particle motion relative to the bubble is analogous to the potential flow of a gas past a circular cylinder. Also, the condition that particles should not penetrate the tube wall is established using the method of images. The particle velocities induced by the rising bubble are used together with a numerical integration scheme to establish particle trajectories near the tube wall. Results of calculations are shown to compare qualitatively with experimental observations by others. Further calculations illustrate the effect of bubble size upon particle motion in the defluidized cap on the leeward side of the tube.

The proposed model could be used as a basis for estimation of emulsion phase residence time or improved interstitial gas velocity estimates required for heat transfer analyses.     

汽-液-固三相流对石墨加热管内壁面磨损的实验研究

利用汽-液-固三相循环流化床蒸发器及动态信号测试采集系统,对石墨加热管内壁面的磨损进行了研究。实验测取了加热管不同轴向位置处振动加速度时间序列,对其进行了标准偏差和峰度分析,评估玻璃珠对壁面的径向冲击强度;通过测定循环流量,研究颗粒对壁面的轴向摩擦冲击程度。同时对磨损产物进行称量、元素分析,结合传感器测试结果共同确定石墨管壁面磨损速率的影响因素。主要结论如下：随着加热蒸汽压力的升高和颗粒体积分率的增加,壁面磨损速率增大;相同加热蒸汽压力和颗粒体积分率下,较小粒径颗粒对壁面的磨损更为严重。峰度分析表明,颗粒对壁面的冲击频率是影响石墨管磨损速率的主要因素。研究结果对将三相流化床换热技术应用于石墨管换热器系统具有一定的指导意义。     

Numerical and experimental study on multiple-spout fluidized beds

In this paper we study the effect of multiple spouts on the bed dynamics in a pseudo-2D triple-spout fluidized bed, employing the discrete particle model (DPM) and non-intrusive measurement techniques such as particle image velocimetry (PIV) and positron emission particle tracking (PEPT). A flow regime map was constructed, revealing new regimes that were not reported so far. The multiple-interacting-spouts regime (C) has been studied in detail for a double- and triple-spout fluidized bed, where the corresponding fluidization regime for a single-spout fluidized bed has been studied as a reference case. The experimental results obtained with PIV and PEPT agree very well for all the three cases, showing the good performance of these techniques. The DPM simulation results slightly deviate from the experiments which is attributed to particle–wall effects that are more dominant in pseudo-2D beds than in 3D systems. The investigated multiple-interacting-spouts regime is a fully new flow regime that does not appear in single-spout fluidized beds. Two flow patterns have been observed, viz. particle circulation in between the spouts near the bottom of the bed, and an apparent single-spout fluidization motion at a higher location upwards in the bed. These findings show that the presence of multiple spouts in a spout fluidized bed highly affect the flow behaviour, which cannot be distinguished by solely investigating single-spout fluidized beds.     

低温省煤器飞灰沉积阻塞机理分析及结构优化

针对热电厂低温省煤器发生飞灰沉积阻塞炉管间隙的情况,本文采用剪切力传递（SST）k-ω湍流模型和离散粒子模型（DPM）分析飞灰颗粒沉积行为,预测省煤器中支撑梁附近的沉积动态,进而对支撑梁上方翅片布置结构进行优化和流场分析。结果表明：省煤器中由于支撑梁和上方翅片的存在,飞灰颗粒会在支撑梁上表面进行沉积。在脱硝条件下,硫酸氢铵增加了飞灰间的黏结力,加剧了黏附沉积,进而积灰不断地向上累积增长并造成堵塞。通过改变支撑梁上方的翅片布置结构,可改变支撑板附近的烟气流动规律。对于烟气中的绝大部分飞灰颗粒（d_p>40μm）,其所受到的惯性作用占据主导地位,与壁面有着较大的撞击率。通过改变炉管上翅片的布置方式,增加飞灰颗粒横向移动的能力,对支撑梁上的沉积堵塞有着较好的改善作用。     

分流型芯管对导叶式旋风管内颗粒逃逸的控制

Experimental and computational fluid dynamics was used in this study to predict the escape particles and evaluate the performance of PSC type cyclone tube with slotted vortex finder.The simulation results showed that the PSC type cyclone tube could remove the particles with a diameter greater than 5 μm.The PSC type cyclone tube increased the grade efficiency of particles with a diameter greater than 2 μm as compared with the Shell type cyclone tube.Short circuit flow occurred around the vortex finder slots and there was almost no short circuit flow under the vortex finder inlet.Most small particles escaped from vortex finder slots of the PSC type cyclone tube.The slotted vortex finder could develop “upwards flow” near the vortex finder inlet outside wall and control the escape particles under the vortex finder inlet.The force analysis of particles near the slotted vortex finder slots showed that gas flow carried the particles with a diameter smaller than 3 μm out the separator.     

Diffusional Particle Loss Upstream of Isokinetic Sampling Inlets

Isokinetic sampling, in which a subsample is extracted from the center of laminar aerosol flow, is routinely used to collect representative particles for analysis. Isokinetic sampling minimizes wall effects, including particle loss due to Brownian diffusion to the tube wall. This particle diffusion is analogous to the heat transfer problem originally posed by Graetz in 1883. Analytical solutions to the Graetz problem have been applied to calculate particle loss averaged over the entire main flow. However, these solutions overestimate diffusional particle loss near the center of the main flow. In the present solution, confluent hypergeometric functions are used to solve analytically for particle concentration as a function of radius. The solution is integrated near the center of the main flow to determine particle loss for isokinetically sampled aerosols. Sampling efficiencies valid down to nanometer-sized particles are presented in terms of dimensionless parameters. Diffusional particle loss for isokinetically sampled aerosol can be 1.8 times less than that from the main flow aerosol. The present results can be used to design isokinetic sampling systems and to assess particle loss in these systems. For 5 nm diameter particles sampled isokinetically from a laminar flow tube (0.318 cm tube radius, 10 m length) into an ultrafine condensation particle counter, sampling efficiency is strongly affected by main flow Reynolds number, Re; sampling efficiency increases from 4.9%at Re=100 to 99%at Re=1500.     

Application of a three-layer modeling approach for solids transport in horizontal and inclined channels

The three-layer model concept developed previously for solid-liquid flow has been adapted to model solids transport in inclined channels. The present model predicts the pressure loss and transport rate of solids in Newtonian and power-law fluid suspensions by assuming stratified flow conditions. Sets of stationary sand bed transport rate tests were performed to verify the predictions of the model. A 70-mm flow loop was constructed to measure the average transport rates and critical flow rates, which are required to initiate the motion of solids bed particles. The tests were carried out by eroding stationary sand beds with water and an aqueous solution of poly anionic cellulose (PAC) in a transparent pipe. Four sand beds with different particle size ranges were used. The average transport rates of the beds were predicted using the model. The model predictions show a satisfactory agreement with experimentally measured results when the grain Reynolds number is between 15 and 400 and the flow rate is sufficiently higher than the critical flow rate. Therefore, with some degree of limitation, the three-layer model can be applicable for predicting the transport rates of stationary solids beds in inclined channels for both Newtonian and power-law fluids.     

Spouting with a porous draft-tube

Particle motion and gas distribution have been measured in 0.3 m spouted beds with porous and solid wall draft-tubes. Particle velocity in the annulus varied with gas rate and bed geometry. Narrow particle residence time distributions were observed under all conditions. Annular gas flow with the porous tubes exhibited a characteristic maximum in the lower part of the bed. A computer model has successfully simulated this phenomenon. The porous tube had a higher annular airflow than the solid tube at low separation distances, and a lower pressure drop for a given annular flow. A composite porous and solid wall tube offers improved performance.     

The role of contact stresses and wall friction on fluidization

Fluidization and defluidization experiments, where we increased the gas superficial velocity in small increments and then decreased it, were performed in tubes of different diameters to probe the role of wall friction on pressure drop and bed height. Such experiments, covering the regimes of packed bed, stable bed expansion and bubbling bed, were carried out for several different particles. The compressive yield strength of the particle assemblies at various volume fractions was determined by measuring the height of fully defluidized beds at various mass loading levels. The systematic effect of the tube diameter on pressure drop and bed height hysteresis could be rationalized in terms of a one-dimensional model that accounted for the effect of wall friction and path-dependent contact stresses in the particle phase. Bubbling seemed to set in when the yield stress in the particle assembly could be overcome by the inherent fluctuations. Our experiments, which focused primarily on gas velocities below the minimum bubbling conditions, did not reveal any dramatic change across the Geldart A-B boundary. This is consistent with the original observation by Geldart (Powder Technol. 7 (1973) 285). The distinct difference between beds of group A and B particles in the gently bubbling regime reported by Cody et al. (Powder Technol. 87 (1996) 211) is thus likely to be due to changes in the dynamics of the bubbles, as we observed no striking difference between these beds at gas velocities below minimum bubbling conditions.     

Two-phase flow hydrodynamic study in micro-packed beds – Effect of bed geometry and particle size

Microscopic visualizations nearby the wall region of micro-fixed beds and hydrodynamic measurements during gas–liquid two-phase flows were carried out with an aim to investigate the effect of particle size and capillary tube shape on the bed pressure drop, flow regime transition, hysteresis and bed transient response to flow-rate step perturbations. Visualizations through inverted microscopy revealed that a decrease in particle size leads to early inception of a high interaction flow regime whereas changing capillary shape from circular to square had no effect on flow regime changeover. The effect of particle size on the wetting pattern hysteresis in square micro-packed beds was also investigated in both imbibition and drainage paths. It was found that wetting pattern hysteresis decreases with a decrease in particle size. Finally, the transient behavior of micro-fixed beds of circular and square geometries packed with particles of two different sizes were studied by monitoring the bed pressure drop variations upon step changes in liquid flow rate at iso-G conditions. Larger particle sizes and square geometry showed shorter transient times as compared to smaller particle sizes and circular geometry.     

Modeling of flow dynamics in laminar aerosol flow tubes

Flow behaviour in laminar aerosol flow tubes was investigated using a Computational Fluid Dynamics (CFD) model. Since these flow tubes are typically operated at low gas velocity, even small temperature gradients produce convection currents strong enough to interfere with laminar flow. This results in recirculation, causing the residence times of aerosol particles to be poorly defined. The situation is exacerbated when temperature inversions are present, or when the flow direction and temperature are changed simultaneously. We analyzed several characteristic flow tube configurations to define the range of experimental conditions that will ensure a laminar flow profile with minimal recirculation. For a laminar flow situation, we evaluated the extent of diffusion-controlled exchange between aerosol and the flow tube wall.     

小直径比固定床壁效应的CFD分析

引言固定床作为反应器、分离器、换热器等单元设备广泛应用于化工、能源、环境等许多领域。因用途的不同固定床的管径-粒径比(D/d)的范围很宽(1～1000的数量级)。大热负荷的填充床设备,如强放热的化学反应器、核反应堆的冷却壁管列等,常采用较小的管径-粒径比,以利于热量通过