Use of PIV to measure turbulence modulation in a high throughput stirred vessel with the addition of high Stokes number particles for both up- and down-pumping configurations

A refractive index matching technique combined with particle image velocimetry (PIV) was used to measure turbulent properties of solid–liquid suspensions in a small high throughput scale cylindrical vessel of 45 mm diameter agitated with a 45° pitched blade turbine (PBT) for up-pumping (U) and down-pumping (D) configurations. This study analyses the effect of large 1.5 mm diameter particles (Stokes number>1), on liquid mean velocities, turbulent kinetic energy (TKE) and energy dissipation (ε) at particle concentrations of 0%, 1.5% and 5% by volume. Only small changes in the time-averaged liquid velocities were observed with increasing particle concentration. However, maximum TKE near the impeller decreased up to 40% with increasing particle concentration for both configurations. The Smagorinsky SGS method was used to estimate local energy dissipation rate near the impeller and the maximum value was found to decrease by 50% between 0% and 5% concentration for the (U) configuration. A lesser but still significant drop of 30% was observed for the (D) configuration. These data confirm that large Stokes number particles can suppress turbulence, in agreement with some previous experimental studies, but in contradiction with existing theories.     

Development of an Atmospheric Particle Dry Deposition Model

A model to predict the atmospheric dry deposition velocities of particles has been developed that is similar to a model developed for the prediction of particle deposition velocities in vertical pipes. The model correlates the particle deposition velocity (V_d ) with Stokes settling velocity (V _st), friction velocity (V*), dimensionless inertial deposition velocity (V ⁺ _di ), and dimensionless Brownian diffusion deposition velocity (V ⁺ _dd ). V ⁺ _di      

分级进风燃烧室内旋流反应流的湍流特性

This paper presents an experimental investigation of the turbulent reacting flow in a swirl combustor with staged air injection. The air injected into the combustor is composed of the primary swirling jet and the secondary non-swirling jet. A three dimension-laser particle dynamic analyzer （PDA） was employed to measure the instantaneous gas velocity. The probability density functions （PDF） for the instantaneous gas axial and tangential velocities at each measuring location, as well as the radial profiles of the root mean square of fluctuating gas axial and tangential velocities and the second-order moment for the fluctuating gas axial and tangential velocities are obtained. The measured results delineate the turbulence properties of the swirling reacting flow under the conditions of staged combustion.     

On the Settling Velocity in a Nonstationary Atmosphere

A general drag coefficient has been used in the equation of motion for solid spherical particles. The time constants, stopping times, and settling velocities in a still atmosphere are computed for a wide range of Reynolds numbers. The settling times are compared with the times calculated when a particle is falling in a fluctuating atmosphere. It is found that such particles will get significantly longer settling times owing to an enhancement in the drag coefficient caused by an increase of the relative velocity between the particle and the fluid. Surprisingly, this enhancement is present for a horizontal wind field due to a coupling between particle motion in different directions, but it is also present for a vertical field. The effect is most pronounced in the intermediate Reynolds number region, slightly above the Stokes range, where the increase in settling time can be more than 10% for certain fluctuation frequencies and amplitudes. This indicates that such particles must be carefully treated when they are falling in a nonstationary medium     

鼓泡塔内气液两相湍流实验研究

介绍了研究鼓泡塔气液两相流的实验装置、实验方法。液相用激光多普勒测速技术(LDV)测量,气相用粒子示踪测速技术(PIV)测量。实验表明,轴向液相速度的径向分布呈塔中心峰值、壁面附近倒流形式,且与气相表观速度大小有关,当液相表观速度一定时,随气相表观速度增大而愈加陡峭,返混也剧烈。当表观液速与表观气速之比小于19．6时,返混区总是存在,且返混区大小与高度有关：当表观液遣与表观气速之比大于19．6时,返混消失,含气率分布由塔中心峰值转向壁面峰值。径向液相速度既与气相表现速度有关又与位置高度有关,在塔底部呈现负值,这意味着向塔轴心方向流动。随着塔高增加。流动方向逐渐转变为向塔壁方向,且又有明显的峰值。     

Influence of the mass flow rate of secondary air on the gas/particle flow characteristics in the near-burner region of a double swirl flow burner

The influence of the mass flow rate of secondary air on the gas/particle flow characteristics of a double swirl flow burner, in the near-burner region, was measured by a three-component particle-dynamics anemometer, in conjunction with a gas/particle two-phase test facility. Velocities, particle volume flux profiles, and normalized particle number concentrations were obtained. The relationship between the gas/particle flows and the combustion characteristics of the burners was discussed. For different mass flow rates of secondary air, annular recirculation zones formed only in the region of r/d=0.3–0.6 at x/d=0.1–0.3. With an increasing mass flow rate of secondary air, the peaks of the root mean square (RMS) axial fluctuating velocities, radial mean velocities, RMS radial fluctuating velocities, and tangential velocities all increased, while the recirculation increased slightly. There was a low particle volume flux in the central zone of the burner. At x/d=0.1–0.7, the profiles of particle volume flux had two peaks in the secondary air flow zone near the wall. With an increasing mass flow rate of secondary air, the peak of particle volume flux in the secondary air flow zone decreased, but the peak of particle volume flux near the wall increased. In section x/d=0.1–0.5, the particle diameter in the central zone of the burner was always less than the particle diameter at other locations.     

稀疏气固两相湍流边界层拟序结构变动特性

应用PIV两相同时测量方法,对壁面Reynolds数为430的水平槽道稀疏气固两相湍流边界层拟序结构变动特性进行了研究。选取质量载荷为10^-4~10^-3的110 μm聚乙烯颗粒作为离散相。结果表明,低载荷颗粒仍能显著改变湍流拟序结构,进而影响宏观湍流属性。颗粒重力沉降形成的粗糙壁面增强了壁面附近湍流猝发行为,导致黏性底层中的气相法向脉动速度和雷诺剪切应力显著增大。颗粒与壁面的碰撞加强了低速流体上抛、削弱了高速流体下扫,同时增强了轨道交叉效应,从而抑制了湍流拟序结构发展,显著减小了黏性底层以上区域的法向脉动速度和雷诺剪切应力。此外,颗粒惯性还减小了黏性底层厚度、增大了流向速度梯度,导致气相流向脉动速度峰值增大,且其对应位置也更加靠近壁面。     

Reappearance of azimuthal waves in turbulent Taylor-Couette flow at large aspect ratio

Velocity field data were acquired for Taylor-Couette flow in the annular gap between an inner rotating cylinder and a stationary concentric outer cylinder using particle image velocimetry (PIV) in a meridional plane of the annulus. Data were acquired for several rotational Reynolds numbers with the ratio of the rotational and critical Reynolds numbers ranging from 6 to 200, corresponding to flow states ranging from laminar wavy Taylor vortex flow to turbulent Taylor vortex flow. Spatial correlations of velocity fluctuations were found to exhibit a sharp decrease as R, the ratio of Reynolds number to the critical Reynolds number, increases from 16, indicating the disappearance of azimuthal waves and the onset of turbulence, reaching a minimum at R=18. However, correlation lengths subsequently increase with increasing R, displaying a secondary peak from 20?R?38, suggesting the reappearance of azimuthal waves. The reemergence of azimuthal waves was confirmed through other methods including analysis of the axial velocity. At still higher Reynolds numbers, correlation lengths decay once again. The magnitude and Reynolds number associated with the secondary peak in the fluctuation velocity correlations were found to be dependent on the location of the basis point used in the calculations. Specifically, correlation lengths were longest near the outer cylinder in the inflow boundary and near the inner cylinder in the outflow boundary. This was shown to be due to the spatial dependence of Reynolds stresses in turbulent Taylor-Couette flow.     

Study on the Flow Field inside the Microfiltration Separator with Rotary Tubular Membrane

Abstract

By using a new type of laser surveying instrument named particle image velocimetry (PIV), the flow field inside a rotary tubular membrane separator with a rotating inner tubular microfiltration membrane and a transparent outer cylinder was measured experimentally, and from which some new observations were resulted. Pairs of stable Taylor vortices with similar dimensions and opposite flowing directions were directly visualized by the measured streamlines and vorticity of flow field in the annular gap of the membrane separator. No matter how the axial Reynolds number, radial Reynolds number and Taylor number changed, the dimensions of the Taylor vortices and the distances between the centers of adjacent Taylor vortices were almost the same, but the shapes of the Taylor vortices at lower Taylor numbers were more regular than those at higher Taylor numbers. The Taylor vortices disappeared because of the turbulence when the Taylor number was too high. The maximum axial velocity near the membrane surface was about 20 times larger than the mean velocity of axial flow inside the annular gap, and the maximum outward radial velocity near the membrane surface was even about 3000 times larger than the average velocity of the radial permeating flow through the tubular microfiltration membrane. The large velocities near the membrane surface, which were due to the Taylor vortices, could prevent solid fine particles from depositing onto the membrane surface and/or entering into the membrane pores and therefore result in reduced concentration polarization and reduced membrane fouling. The results in this study provided some valuable guidelines on the hydrodynamic way to reduce membrane fouling.     

Turbulent particle mass flux in a two-phase shear flow

This paper presents measurements of mean and rms of fluctuations of concentration, particle turbulent velocities, shear stress and covariance of the fluctuations of particle number density and particle velocities in a horizontal plane shear layer. Particle Image Velocimetry (PIV) was used to obtain simultaneously particle velocities and number densities to evaluate models for the prediction of particle dispersion in Reynolds-Averaged Navier-Stokes calculation approaches. The flow was horizontal with the low speed side on top and laden with nearly mono-dispersed 55 and 90 µm glass beads, which were injected at the upper, low speed side of the flow. The Stokes number of the particles was in the range of 0.41 to 4.3 and the drift parameter due to gravity was in the range 0.18 to 1.5. The experimental results quantified how particle ‘centrifuging’ by the large fluid vortices influenced the measured quantities. The turbulent particle mass flux was compared with models based on the gradient of mean particle concentration. Different dispersion coefficients were evaluated by introducing the measured quantities into the model equation and it was found that dispersion coefficients based on the fluid eddy diffusivity performed poorly leading to an order of magnitude errors. A dispersion coefficient in tensor form, based on the product of particle shear stress and particle integral time scale, led to good agreement with measured turbulent particle mass fluxes with errors between 0 and 50%.     

颗粒的湍流扩散

<正>1引言 在淤浆输送、流态化、分离、搅拌等化工过程中,经常涉及颗粒在湍流中的扩散,对这类问题,一般先确定颗粒的湍流扩散系数,然后用类似Fi。k定律的梯度扩散定律处理。因此,如何确定颗粒的湍流扩散系数是解决问题的关键。目前有关颗粒湍流扩散系数的计算公式主要适用于小颗粒,如常用的Hinze-Tchen公式     

Particle dispersion in a turbulent natural convection channel flow

Direct numerical simulations of particle dispersion in the turbulent natural convection flow between two vertical walls kept at constant but different temperatures are reported. It is assumed that the particles do not affect the flow (i.e. the dilute phase approximation is adopted). Particles with different levels of inertia, or Stokes numbers (0.843≤St≤17.45), are tracked according to the drag force imposed by the fluid. The gravity force is included for two cases, St=0.843 and St=17.45. The different levels of turbulence near the wall and near the center of the channel produce, as in isothermal turbulent channel or pipe flow, a larger concentration of particles near the wall. This effect becomes more important, and the deposition velocity of particles on the wall increases, as the particle inertia is increased. The simulations at St=8.38 and St=17.45 predict similar concentration profiles and deposition velocities according to the large inertia of these particles. The deposition velocities, obtained when the gravity force is ignored in the particle equations, follow the trend observed and measured for isothermal turbulent channel flows in the diffusion impaction regime. For the conditions considered, the gravity vector imposes a strong descending motion on the particles and this produces the increase of the particle concentration near the wall and a reduction of the deposition velocities in comparison with the results without the gravity force.     

A Stochastic Model for Particle Deposition and Bounceoff

A new model for particle deposition and bounceoff that combines current knowledge of turbulent bursts with the stochastic properties of turbulent fluctuations is presented. The model predictions for deposition velocities agree with experimental results in the literature for dimensionless particle relaxation time τ_p ⁺ > 2. For τ_p ⁺ > 10, most of the particles delivered to the edge of the viscous sublayer are able to deposit onto the surface due to their inertia; the deposition velocity approaches an asymptotic value because the process becomes limited by the rate of turbulent delivery to the viscous sublayer. Because of the penetration of turbulent fluctuations into the viscous sublayer, the minimum values of vertical velocities needed for particles to deposit onto the surface are smaller than those predicted by the free flight model. Most of the deposition occurs from those turbulent fluctuations at the upper tail of the distribution of the vertical component of air velocity.

In addition to the deposition velocity, the model is able to provide the distribution of particle velocities on reaching the surface which is used to compute the fraction of particle bounceoff. The model predictions for the fractions of rebound agree reasonably with the measured results from a wind tunnel experiment for τ_p ⁺ > 2. However, both the deposition velocity and the fraction of rebound are underestimated by the model for τ_p ⁺ < 2. Other mechanisms such as Brownian diffusion must be included in further revisions to this model in order to obtain satisfactory predictions for smaller values of τ_p ⁺.     

Behaviour and removal of fine particles in a liquid-solid spouted bed consisting of binary mixtures

In a liquid-solid spouted bed system containing coarse and fine particles, the entrainment and elutriation of the separated fines were investigated with reference to variations in the pressure drop through the bed. The effect of operating parameters on the velocity corresponding to the beginning of entrainment of fines (U_be) was examined and a dimensionless correlation was proposed. The effect of operating parameters on the elutriation rate coefficient of fines at liquid velocities larger than their terminal settling velocity (U_l) also was studied. An empirical equation was proposed for a column height above TDH, the height of freeboard at which the elutriation rate becomes constant. It was recognized that the elutriation phenomenon was closely related to the length of the stratified particle transport bed when the ascending velocity of the elutriable particles at the top of bed becomes constant.     

Influence and CFD analysis of cooling air velocity on the purification of aqueous nickel sulfate solutions by freezing

Finite energy resources and their rapidly waning imprint necessitate a sustainable wastewater treatment method. Nature could be exploited to freeze wastewater in locations which experience subzero temperatures during winter. The two most vital components that influence the efficiency of natural freezing are the ambient temperature and air velocity. The turbulent and unsteady air‐cooled natural freezing is simulated for ice crystallization from 0.1 wt % and 1 wt % NiSO₄ (aq) solutions. The efficiency of natural freezing is tested for different air velocities (2 ms^?1, 5 ms^?1) and levels of undercooling (ΔT = 0.5°C, 1°C) from the freezing temperature of the corresponding solution. The airflow in the winter simulator is modeled by computational fluid dynamics to investigate its behavior and to assess its effect on freezing. © 2017 American Institute of Chemical Engineers AIChE J, 63: 200–208, 2018     

Nitrogen dilution effect on flame stability in a lifted non-premixed turbulent hydrogen jet with coaxial air

The nitrogen dilution effect on flame stability was experimentally investigated in a lifted non-premixed turbulent hydrogen jet with coaxial air. Hydrogen gas was used as the fuel and coaxial air was injected to initiate flame liftoff. Hydrogen was injected into an axisymmetric inner nozzle (d_F = 3.65 mm) and coaxial air jetted from an axisymmetric outer nozzle (d_A = 14.1 mm). The fuel jet and coaxial air velocities were fixed at u_F = 200 m/s and u_A = 16 m/s, while the mole fraction of the nitrogen diluent gas varied from 0.0 to 0.2 with a 0.1 step. For the analysis of the flame structure and the flame stabilization mechanism, the simultaneous measurement of PIV/OH PLIF was performed. The stabilization point was in the region of the flame base with the most upstream region and was defined as the point where the turbulent flame propagation velocity was found to be balanced with the axial component of the local flow velocity. The turbulent flame propagation velocity increased as the nitrogen mixture fraction decreased. The nitrogen dilution makes the flame structure more premixed. That is, the stabilization mechanism shifts from edge flame propagation based mechanism toward premixed flame propagation based mechanism. We concluded that the turbulent flame propagation velocity was expressed as a function of the turbulent intensity and the axial strain rate, even though the mole fraction of the nitrogen diluent varied.     

Counter‐current gas‐liquid wavy film flow between the vertical plates analyzed using the Navier‐Stokes equations

The article is devoted to a theoretical analysis of counter‐current gas‐liquid wavy film flow between vertical plates. We consider two‐dimensional nonlinear waves on the interface over a wide variation of parameters. The main interest is to analyse the wave structure at the parameter values corresponding to the onset of flooding observed in experiments. We use the Navier‐Stokes equations in their full statement to describe the liquid phase hydrodynamics. For the gas phase equations, we use two models: (1) the Navier‐Stokes system and (2) the simplified Benjamin‐Miles approach where the liquid phase is a small disturbance for the laminar or turbulent gas flow. With the superficial gas velocity increasing and starting from some value of the velocity, the waves demonstrate a rapid decreasing of both the minimal film thickness and the phase wave velocity. We obtain a region of the gas velocity where we have two solutions at one set of the problem parameters and where the flooding takes place. Both the phase wave velocity and the minimal film thickness are positive numbers at such values of the velocity. We calculate the flooding point dependences on the liquid Reynolds number for two different liquids. The wave regime corresponding to the flooding point demonstrates negative u‐velocities in the neighbourhood of the interface near the film thickness maximum. At smaller values of the superficial gas velocity, the negative u‐velocities take place in the neighbourhood of the film thickness minimum. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Experiments on breakup of bubbles in a turbulent flow

The breakup of air bubbles in a turbulent water flow is studied experimentally. Water flows from a nozzle array, generating intense turbulence, and then flows downward through a cell. The velocity field is measured by PIV, and the local dissipation rate is estimated using a large‐eddy PIV technique. Bubbles (1.8 to 5 mm) are injected in the bottom of the cell and rise toward the region of intense turbulence, where they break. The time spent by bubbles in various zones without breaking and the number of breakups are evaluated, providing information about the breakup frequency. The number of daughter bubbles and their size distribution are determined. The number of daughters depends on a Weber number , where ? is the turbulent energy dissipation rate, D′ is the mother particle size, ρ and σ are the liquid density and surface tension. The daughter size distribution is a function of their number. © 2017 American Institute of Chemical Engineers AIChE J, 64: 740–757, 2018     

湍动流化床内固体颗粒扩散系数

将高速摄影及基于互相关原理的图像处理技术与颗粒扩散规律的研究进行结合,对湍动流化床中甲醇制烯烃催化剂SAPO-34颗粒的扩散系数进行了实验研究。实验表明,对于Geldart A类的SAPO-34颗粒,颗粒纵向扩散系数在10^-2～10^-1 m²·s^-1量级之间,横向扩散系数在10^-3～10^-2 m²·s^-1量级之间,两者均随流化风速的上升而增大。另外,在相同的流化风速下,粒径较小的颗粒具有更大的扩散系数。该结果对湍动床颗粒运动规律的研究有一定意义。     

CFD simulation and PIV measurement of the flow field generated by modified pitched blade turbine impellers

The hydrodynamics generated by modified pitched blade turbine (m-PBT) impellers with down-pumping mode were systematically investigated through particle image velocimetry (PIV) measurements and computational fluid dynamics simulations. The simulated mean axial velocity, mean radial velocity, and turbulent kinetic energy by the standard k–? turbulent model were validated against the measured PIV data. This shows that the standard k–? turbulent model predicts mean velocity well, but underestimates turbulent kinetic energy near the blade. The flow field and power consumption as well as pumping number for the m-PBT and the standard PBT impeller were predicted. The simulation results demonstrate that a few simple changes of the blade shape influence the velocity distribution, i.e., increasing the magnitude of mean velocity in the vicinity of impeller, and that the m-PBT impeller has a higher pumping efficiency than the standard one.