Considerations for Modeling Particle Entrainment into the Wake of a Circular Cylinder

The objective of this work is to evaluate the performance of the steady state Reynolds Averaged Navier-Stokes (RANS) computational fluid dynamics (CFD) models for estimating concentration of low Stokes number aerosols (Stk = O(10^?4)) in the wake of a bluff body. These simulations are compared with experimental data. In the simulations and experiments, particles are released upstream of the body and convected downstream, where some are entrained into the wake. The air velocity is computed using a steady state renormalized group k ～ ? model. Lagrangian particle trajectory simulations are performed in conjunction with each airflow model to calculate concentrations. The experiments are performed in an aerosol wind tunnel in which phase Doppler velocimetry measurements are obtained for the velocity field and aerosol concentration.

The RANS model yields a wake concentration deficit that extends downstream past x/D = 10, while the experiments produce elevated concentrations immediately downstream of the near wake. It is postulated that the concentration peak is at least in part attributed to particle interaction with the boundary layer by the following mechanism. Particles are transported into the boundary layer by turbulent diffusion, turbophoresis, and/or inertial forces. Particles then separate from the cylinder with the airflow and travel in a sheath around the periphery of the near wake to converge at the downstream edge of the near wake. Underestimation of the wake concentration by the RANS model is potentially due to inadequacy in the boundary layer approximation used in the model.     

基于浓度扩散的风力机尾流模型

将湍流扩散与浓度扩散进行类比,提出一种适用于复杂地形风电场微观选址的风力机尾流模型。求解Navier-Stokes方程和浓度扩散方程,得到风电场中的流场信息,建立尾流速度、湍动能与浓度之间多项式变换关系并通过实验数据拟合得到变换中的系数。研究表明,基于浓度扩散的风力机尾流模型通过边界条件将地形信息与浓度方程相耦合,所得尾流物理量能随着地形起伏而发生变化,且计算量适中,可用于预测复杂地形风电场的流场分布。     

Chemical Characterization of Flowing Polydisperse Aerosols by Raman Spectroscopy

We have applied Raman spectroscopy to the in-situ measurement of chemical composition of polydisperse flowing aerosols. Monodisperse and polydisperse aerosols in the size range 0.3 to 1.8 w m, composed of diethylsebacate (DES) and ammonium sulfate, were generated. The particles were irradiated with 514.5 nm laser light and Raman spectra were collected. The Raman intensities of DES at 2935 cm -1 and ammonium sulfate at 981 cm -1 , normalized by the nitrogen carrier gas Raman intensity at 2313 cm -1 , were approximately proportional to the aerosol mass loading over the particle size range studied. Calculations based on previous theoretical studies support this observation. The mass loading ranged from 0.17 to 12.8 g/m 3 for DES and 20 to 138 mg/m 3 for ammonium sulfate. The method was applied to mixing aerosol streams containing DES and ammonium sulfate in a turbulent jet. The Raman system, with a sensitive volume of 0.02 mm 3 , was used to measure radial and axial concentration profiles in the mixing region. The results compared well with turbulent mixing theory. The primary limitation for application of the method is the low signal to noise ratio.     

Wall Loss Rate of Polydispersed Aerosols

Wall loss rates of polydispersed aerosols in a stirred vessel were studied theoretically and experimentally. A formula for the poly- dispersity factor of the wall loss rate was derived using the moment method of log-normal size distribution and compared with numerical calculations. The representative theory of Crump and Seinfeld (1981) was used as the wall loss rate of monodispersed aerosols in which the Brownian diffusion, the turbulent eddy diffusion, and the gravitational settling are included as wall loss mechanisms. The results of the analysis show that the wall loss rate of a polydispersed aerosol is substantially higher than that based on a monodispersed size distribution model if the particle size distribution can be represented reasonably well by a log-normal function. The existing diagram showing the loss rate as a function only of the particle size was expanded to include the polydispersity effects. Experimental measurements of particle wall loss rate were performed by observing the time-dependent changes in particle number concentration for various stirring intensities in a cylindrical stirred chamber. It was shown that by correcting for the polydispersity effect, the dependence of the wall loss rate on particle size and stirring intensity agreed with the theory of Crump and Seinfeld (1981).     

Numerical simulation of mass transfer in a liquid–liquid membrane contactor for laminar flow conditions

Liquid–liquid phase membrane contactors are increasingly being used for mixing and reaction. The principle is the following: component A flows through the membrane device inlet to mix/react with component B which comes from the membrane pores. This study presents a numerical simulation using computational fluid dynamics (CFD) of momentum and mass transfer in a tubular membrane contactor for laminar flow conditions. The velocity and concentration profiles of components A–C are obtained by resolution of the Navier-Stokes and convection-diffusion equations. The numerical simulations show that mixing between A and B is obtained by diffusion along the streamlines separating both components. The mixing/reaction zone width is within the region of a few hundred of microns, and depends on the diffusion coefficients of A and B. Hollow fiber membrane devices are found to be of particular interest because their inner diameter is close to the mixing zone width.     

The turbulent behavior of novel free triple-impinging jets with large jet spacing by means of particle image velocimetry☆

A novel reactor that achieves rapid liquid–liquid mixing via free triple-impinging jets(FTIJs) is developed to improve mixing efficiency at unequal flow rates for liquid–liquid reactions. The flow characteristics of FTIJs were investigated using particle image velocimetry(PIV). The instantaneous and mean velocities data at different Reynolds numbers(Re) were analyzed to provide insights into the velocity distributions in FTIJs. The effect of jet spacing on the stagnation points, instantaneous velocity, mean velocity, profiles of the x- and ycomponents of mean velocity, and turbulent kinetic energy(TKE) distributions of FTIJs were investigated at Re = 4100 with a volumetric flow rate ratio of 0.5. The characteristics of the turbulent flows are similar for all jet spacings tested. Two stagnation points are observed, which are independent of jet spacing and are not located in the center of the flow field. However, velocity and TKE distributions are strongly dependent on the jet spacing.Decreasing jet spacing increases the expansion angle and the values of TKE, leading to strong turbulence, improving momentum transfer and mixing efficiency in FTIJs. The present study shows that optimization of the operating parameters is helpful for designing FTIJs.     

Texaco气化炉冷态流场和湍流混合的数值模拟

在直径为1000mm,高4000mm的双通道射流喷嘴的气化炉上,以Texaco气化炉冷模试验为基准对象,将空气经环隙和中心射入气化炉,以氢气为示踪剂,预测环隙和中心射流的混合程度。结合流体质量与动量守恒方程和k-ε湍流模型,用SIMPLER算法计算,对气化炉内的冷态流场和湍流混合进行了模拟,模拟了炉内速度分布、量纲一浓度分布、混合分数分布和轴向衰减的情况。结果显示:气化炉内浓度分布极不均匀;炉内存在富氧和贫氧区;环隙和中心通道射流动量比加大,混合分数沿轴向衰减加快,达到充分混合的时间缩短。模拟结果与冷模试验结果的比较表明计算值与试验值吻合良好。     

Effects of Electrohydrodynamic Flow and Turbulent Diffusion on Collection Efficiency of an Electrostatic Precipitator with Cavity Walls

The effects of electrohydrodynamic (EHD) flow and turbulent diffusion on the collection efficiency of particles in a model ESP composed of the plates with a cavity were studied through numerical computation. Electric field and ion space charge density in the ESP were calculated by the Poisson equation of electric potential and the current continuity equation of ion space charge. The EHD flow field was solved by the continuity and momentum equations of gas phase, including the electrical body force induced by the movement of ions under the electric field. RNG k - l model was utilized to analyze turbulent flow. Particle concentration distribution was calculated from the convective diffusion equation of particle phase. As the ion space charge increased, the collection efficiency of charged particles increased because the electric potential increased over the entire domain in the ESP. The collection efficiency decreased as the EHD flow became stronger when the electrical migration velocity of charged particles was high. However, the collection efficiency could increase for the stronger EHD flow when the electrical migration velocity of charged particles was relatively lower. Also, the collection efficiency decreased as the turbulent diffusion of particles increased when the electrical migration velocity of particles was high. However, the collection efficiency could increase with the turbulent diffusion when the electrical migration velocity of particles was relatively lower.     

仿柳叶形静态混合器的流动及混合特性

对仿柳叶形静态混合器内混合气流进行了速度场与浓度场的试验研究,结果表明该混合器内速度场与浓度场偏差均达到了非常理想的效果（优于国家标准偏差值）。同时采用CFD软件对该静态混合器内的流场进行了数值模拟,试验与模拟的数值结果以及两者的浓度云图分布都有着较好的一致性。随后的研究结果表明：在混合元件尾迹区域出现了纵向涡和发卡涡来促进混合;在经过混合元件区域时因为湍流动能耗散率增加形成的高湍流动能耗散率区能够使物质交换更加频繁;整个静态混合器的流动阻力也主要发生在该区域,随之出现的返混现象也在一定程度上加强了混合效果。     

Volume fraction gradient-induced flow patterns in a two-liquid phase mixing layer

This paper presents experimental and numerical results related to the dynamic behavior of a two-liquid phase mixing layer induced by a gradient of phase fraction at the flow inlet. A particle image velocimetry-based technique has been used together with a refractive index matching method in order to measure the volume phase fraction and the continuous phase velocity in the two-dimensional (2D) flow. The analysis of experimental results reveals the strong unsteady feature of the flow and the development of large-scale coherent structures. Experimental data have been compared to numerical simulations obtained using both a two-fluid model and a single-fluid mixture model (where only the density difference is accounted for). The similarities and discrepancies between numerical and experimental results provide an understanding of the relative importance of variable density effects compared to the two-phase interfacial exchanges in the momentum and the turbulent transport.     

PARTICLE CONCENTRATION AND SIZE MEASUREMENTS IN TWO-PHASE TURBULENT COAXIAL JETS

Particle concentration and particle size distributions have been measured for two-phase (solid/air) turbulent coaxial jets using the Laser Diffraction Method (LDM) and a tomography data transform technique. Effects of velocity ratio, particle loading ratio, and particle size on the dispersions of gas and particles were determined. Experimental results show that the gas disperses much more rapidly than the particles and particle dispersion decreases with increasing in particle size. Increasing velocity ratio significantly increases gas dispersion, while effects of other variables are less significant. The mean particle size at the jet edge is about 15-20% smaller than that at the jet centerline. The turbulent Schmidt number Sc_p for two-phase turbulent coaxial jets ranges from 1.4 to 1.5.     

喷嘴油气在提升管进料段的浓度径向分布及混合行为

在φ200 mm提升管冷态实验装置上,根据喷嘴油气在进料段4个轴向位置（H=0.375、0.675、1.075、1.375 m）及不同操作参数（U_r=2.25～4.30 m·s^-1,U_j=41.7～62.5 m·s^-1,M_j/M_r=0.29～4.21）的浓度径向分布形式,考察了喷嘴油气与预提升气体和颗粒在进料段内的混合过程。结果表明,喷嘴油气在进料段内存在6种浓度径向分布形式,反映了喷嘴油气与预提升气体和颗粒在进料段的不同混合行为,沿轴向由下至上分别为：未混合区（强M形分布）、混合区（弱M形分布、强三峰形分布、弱三峰形分布、单峰形分布）及完成混合区（环-核分布）。随着U_j的增加或U_r的减小,喷嘴油气与预提升气体和颗粒在进料段内的未混合区、混合区及完成混合区的轴向高度逐渐增加。采用喷嘴射流动量与预提升来流动量之比M_j/M_r考察了操作参数及装置结构尺寸等对喷嘴油气与预提升气体和颗粒在进料段内混合过程的综合影响。喷嘴油气与预提升气体和颗粒的未混合区、混合区及完成混合区的轴向位置在动量比M_j/M_r≤0.29时分别为：0～0.375 m、0.375～0.525 m、0.525～0.675 m;在动量比M_j/M_r=0.29～0.54时分别为：0～0.375 m、0.375～0.875 m、0.875～1.075 m;在动量比M_j/M_r=0.54～4.21时分别为：0～0.525 m、0.525～1.225 m、1.225～1.375 m。     

旋流数为1.0的强旋湍流两相流动的PDPA实验

采用相位多普勒颗粒测速仪 (PDPA)对旋流数为 1 0的轴向和切向进风的圆柱形旋风筒内强旋湍流气粒两相流动进行了测量研究 ,并与旋流数为 0 47、 1 5和 2 0 8的实验结果进行了对比分析 ,指出了旋流数变化对两相流场及两相湍流特性的影响 .     

Vertical distribution of aerosol particles near the air-sea interface in coastal zone

Measurements of vertical aerosol concentration profiles in the Mediterranean coastal zone are presented in the size band 0.1 to 20 μm, with focus on aerosol particles smaller than 10 μm. The results show that the profile shape depends mainly on turbulent processes induced by the wind rather thanthe whitecap production of freshly generated aerosols from the wind-wave interaction. Therefore, the data recorded during weak and moderate wind speeds show the prevalence of turbulent mixing processes induced by the wind for profiles in the size range 0.1–1 μm. For the size range 1–10 μm, in addition to the effect of wind velocity and direction (i.e. the coastal influence), we also noted the importance of the relative humidity gradient (very close to the interface). Good agreement was found between vertical concentration profiles recorded for winds of marine origin and the marine mixed layer model (Davidson and Shutz, 1983). However, we show that the model cannot be used for winds of continental origin. For wind speeds above >9 ms⁻¹, a maximum occurs in the concentration profiles (near 2–3 m height) for all particles larger than 1 μm, which confirms the predominant influence of an eddy which appears in the lee of the crest, referred to as the “wave rotor” model (De Leeuw, 1986).     

Hydrodynamics of turbulent slurry bubble column (Ⅳ)Modeling and simulation of bubble column with vertical pipe bundles

工业浆态床反应器一般都安装有密集的换热列管,列管束的存在使得速度和气含率的径向分布趋于陡峭,形成“烟囱效应”,目前对此还缺乏合适的流体力学模型进行定量描述。本文提出两点新思路以解决列管束的模拟问题：一是将密集列管束的影响作为体积源项而不是边界条件来考虑,在标准k-ε方程中增加与列管阻力相关的动量源、湍动源、耗散源本构关系;二是采用气泡群的径向升力和湍流扩散力平衡方程确定气含率径向分布。新模型中引入的阻力系数由流体力学文献给出,只有两个径向力和耗散参数需实验确定。模型计算的气含率和液速分布在宽泛的条件下与实验测量值符合良好,能够定量描述“烟囱效应”以及内构件参数对流动的影响。     

Experimental and numerical investigations of scale-up effects on the hydrodynamics of slurry bubble columns

Experiments and simulations were conducted for bubble columns with diameter of 0.2 m(180 mm i.d.), 0.5 m(476 mm i.d.) and 0.8 m(760 mm i.d.) at high superficial gas velocities(0.12–0.62 m·s-1) and high solid concentrations(0–30 vol%). Radial profiles of time-averaged gas holdup, axial liquid velocity, and turbulent kinetic energy were measured by using in-house developed conductivity probes and Pavlov tubes. Effects of column diameter, superficial gas velocity, and solid concentration were investigated in a wide range of operating conditions. Experimental results indicated that the average gas holdup remarkably increases with superficial gas velocity, and the radial profiles of investigated flow properties become steeper at high superficial gas velocities. The axial liquid velocities significantly increase with the growth of the column size, whereas the gas holdup was slightly affected. The presence of solid in bubble columns would inhibit the breakage of bubbles, which results in an increase in bubble rise velocity and a decrease in gas holdup, but time-averaged axial liquid velocities remain almost the same as that of the hollow column. Furthermore, a 2-D axisymmetric k–ε model was used to simulate heterogeneous bubbly flow using commercial code FLUENT 6.2. The lateral lift force and the turbulent diffusion force were introduced for the determination of gas holdup profiles and the effects of solid concentration were considered as the variation of average bubble diameter in the model. Results predicted by the CFD simulation showed good agreement with experimental data.     

方型多火嘴石油化工管式炉中辐射室三维流动及传热的数学模拟

提出方型石油化工管式炉中辐射室的三维流动和传热的数学模型.对某焦化炉计算的结果与实测数据进行了初步比较并作出讨论.     

Particle—gas dispersion effects in a round jet

The fully developed region for a two-phase jet exhibits similarity for velocity and concentration. A power law expression approximates the velocity distribution and the square root of this profile coincides with the concentration distribution. Measurements show that particle dispersion decreases with increase in particle concentration and the gas mixing rate is lower than the particle mixing rate. The turbulent Schmidt Number remains constant at a value equal to 0.47.     

Discrete element investigation of flow patterns and segregation in a spheronizer

Powder flow in a spheronizer is characterized by a toroidal motion and the flow patterns depend on the disc velocity and the fill level, but also on the particle properties. This work investigates numerically, with a discrete element method (DEM), the impact of these parameters on segregation patterns and the flow dynamics for bidisperse particle size distributions. Characterization of the segregation, by means of a mixing index and the relation with the shear stress in the toroidal domain, is presented. Characteristics, such as mixing curves, concentration profiles and azimuthal velocity correlations, are discussed. A logarithmic expression has been developed to account for the shear stress on the evolution of the azimuthal velocity inside the particulate bed. The combination of the mixing indexes and the concentration profiles is used to quantify the changes observed on the segregation when the fill level and the rotational rate of the spheronizer disk are varied.     

Axial thrust of axial flow impellers

This paper presents an analysis of the axial thrust of axial flow high-speed impellers under a turbulent regime of flow of an agitated liquid. The axial thrust is calculated from the measured total axial force affecting the cylindrical fully baffled mixing vessel and from the radial profile of the axial component of the ensemble-averaged mean velocity in the impeller discharge stream. The results of experimentally determined values of the dimensionless criteria (thrust number and momentum number) are successfully compared with the axial thrust of the pitched blade impellers calculated from the theoretically predicted simplified radial profiles of the axial component of the mean velocity in the impeller discharge stream.