Comparaison de deux types d'écoulements (tangentiel plan et tourbillonnaire) dans des modules d'ultrafiltration. Influence de la contrainte pariétale

This work consists of evaluating the performances of a new membrance separation process: an annular, swirling, decaying flow induced by a tangential inlet. For this purpose, we have compared the permeation flux obtained in swirling flow with that of a crossflow. We ultrafiltered a model suspension of bentonite in order to compare the performances of the two modules for different operating conditions. Parameters such as flow rate, Reynolds number and wall velocity gradient were studied, and this last one appears to be a significant hydrodynamic parameter to compare the performances of the two configurations. Indeed, this parameter takes into account the flow field nature and, more precisely, the shear stress at the membrane surface. Improvement of the permeation flux induced by the swirling, decaying flow in comparison with that observed in cross‐flow can reach more than 30% under specific conditions. The study of the energy consumption of these two cells allows us to confirm the interest of the swirling, decaying flow in order to design new types of membrane separation modules.     

Transfert de matière et chute de pression lors d'un écoulement tourbillonnaire annulaire non-entretenu induit par une entrée tangentielle

This work is devoted to an experimental study of overall mass transfer and pressure in decaying swirlng annular flow induced by means of a tangential inlet. Overall mass transfer coefficients, obtained using an electrochemcal method, are found to be greater on the outer cylinder that on the inner core of the annulus, and remains largely greater than that obtained in fully developed axial flow on both cylinders. This enhancement in mass transfer rate is coupled with an increase in pressure drop, mainly due to the tangential inlet of the fluid. Nevertheless, an energetic correlation of overall mass transfer on both cylinders of the annular cell shows that decaying swirling flow induced by a single tangential inlet allows an important increase in mass transfer coefficients for a given energy consumption, compared with a fully developed annular axial flow.     

Distribution des temps de séjour en écoulement tourbillonnaire annulaire induit par une entrée tangentielle du liquide

In this work, a thorough model for the flow in annular cell provided with various feed systems, based on an experimental determination of the residence time distribution, was presented for values of the Reynolds number in the range 100 to 5000. Using the plug flow model with axial dispersion, followed by a series of perfectly stirred plug-reactors, it was shown that the non-maintained swirling flow induced by a tangential inlet exhibits a very pronounced piston behavior in conjunction with a stirred zone due to the flow inlet/outlet system in the cell. When an axial inlet or an inlet perpendicular to the cell was used, the percentage of the overall reactor volume having a plug flow with respect to the one with a tangential inlet was decreased.     

Overall mass transfer in the swirling flow induced by a tangential inlet between coaxial cones

An electrochemical method is used to measure mass transfer coefficients between an electrolytic solution and the inner core of a system formed by stationary coaxial cones of the same apex angle. A swirling decaying flow is induced by means of a tangential inlet at the system base. The average mass transfer coefficients are measured at three axial positions from the tangential inlet for both laminar and turbulent flow regimes. Pressure drops between the inlet and the outlet of the experimental device are also investigated. Flow visualization revealed the existence of axially fixed toroidal vortices. The overall mass transfer coefficients along the conical gap are found to be greater than those measured in annular swirling decaying flow for the same values of the annular gap thickness, of the tangential inlet diameter and of the Reynolds number based on the mean axial velocity at the bottom of the conical gap. The enhancement in mass transfer, up to 50% compared with that measured in a cylindrical arrangement, is not counter-balanced by an increase in pressure drop, which remains of the same order of magnitude as that measured in a classical annular configuration.     

旋风分离器内气相旋转流不稳定性的实验研究

旋风分离器内气相旋转流具有较强的不稳定性,其表现形式是旋转流的旋转中心围绕几何中心的偏心摆动,导致流场的瞬时速度随时间发生脉动变化。这种旋流的不稳定特性难以用时均流场参数进行描述,需要用动态流场参数描述,为此采用热线风速仪测量了?300 mm旋风分离器内瞬时切向速度随时间的变化。实验结果表明瞬时切向速度是由气体湍流形成的高频脉动和旋转流偏心摆动形成的低频脉动两部分叠加构成,据此探讨了旋转流摆动形成的机制。瞬时切向速度的低频脉动来源于刚性涡的偏心摆动,脉动幅值与偏心距成正比。通过瞬时切向速度频域建立了旋转流的摆动频率与入口速度、筒体直径和排气管直径的计算模型。     

旋风分离器内气相旋转流不稳定性的实验研究

旋风分离器内气相旋转流具有较强的不稳定性,其表现形式是旋转流的旋转中心围绕几何中心的偏心摆动,导致流场的瞬时速度随时间发生脉动变化。这种旋流的不稳定特性难以用时均流场参数进行描述,需要用动态流场参数描述,为此采用热线风速仪测量了?300 mm旋风分离器内瞬时切向速度随时间的变化。实验结果表明瞬时切向速度是由气体湍流形成的高频脉动和旋转流偏心摆动形成的低频脉动两部分叠加构成,据此探讨了旋转流摆动形成的机制。瞬时切向速度的低频脉动来源于刚性涡的偏心摆动,脉动幅值与偏心距成正比。通过瞬时切向速度频域建立了旋转流的摆动频率与入口速度、筒体直径和排气管直径的计算模型。     

Modélisation numérique du transfert de matière dans un écoulement annulaire faiblement tourbillonnaire non-entretenu

Mass transfer and hydrodynamics in a laminar swirling flow induced by a tangential inlet in a cylindrical annulus was investigated. The simplified version of the momentum and convection-diffusion equations using the boundary layer theory, was resolved by an implicit finite difference method. Although the proposed method cannot be applied to vortex flows and recirculating phenomena, the present work identified a relevant problem about the validity of the boundary layer theory for the mass transfer determination at high Schmidt numbers.     

A new liquefaction method for natural gas by utilizing cold energy and separating power of swirl nozzle

A swirl nozzle with a central body was newly designed to make full use of the cold energy and separating power, and the coupling of swirling flow and condensation was realized based on a condensation model, a droplet surface tension model and a Reynolds stress model turbulence model. The flow and condensation characteristics of methane gas under supersonic swirling flow conditions were studied. The results show that the flow and condensation parameter distribution in the swirl nozzle are similar under varying swirling intensities, but the swirling performance improves with the increase in swirling intensity, and a tangential velocity is beneficial before the gas enters the nozzle. As the inlet temperature decreases or the inlet pressure increases, the liquefaction efficiency increases, and the gas condensation process can be promoted. With the advancement of the initial nucleation position and the increase in the droplet radius, the separation efficiency of the swirl nozzle increases.     

Transfert photonique dans un photoréacteur à écoulement tourbillonnaire

Photonic transfer has been analyzed for two hydrodynamic conditions in an annular photoreactor: a swirling decay- ing flow generated by a tangential inlet, and an axial flow generated by a radial inlet. The photonic transfer coefficient was determined by carrying out the homogeneous photoreduction of potassium ferrioxalate. A comparison of the two hydrodynamic conditions is made in aqueous solution and in a suspension of black carbon in order to reduce the light penetration depth. The last condition emphasizes the interest in using tridimensional flow to design a photoreactor.     

圆管内轴向旋转流切向速度湍流强度

采用热线风速仪(hot wire anemometry,HWA)测量了ø300 mm×2000 mm圆管内轴向旋转流瞬时切向速度随时间的变化,重点分析了切向速度湍流强度的分布特点和旋转流摆动对切向速度湍流强度的影响.测量结果表明,瞬时切向速度由高频的湍流脉动速度和低频的波动速度叠加构成.由于受旋转流旋转中心偏离圆管几何中心造成的旋转流摆动的影响,在圆管中心区域瞬时切向速度随时间的波动速度变化较大,边壁区域瞬时切向速度随时间的波动速度变化较小.通过对瞬时切向速度数据进行概率密度分析可知,切向湍流强度不仅受气流脉动的影响,还受旋转流中心摆动的影响,是由自身气流脉动产生的湍流强度和旋转流摆动产生的湍流强度两部分叠加构成.旋转流摆动导致了中心区域的湍流强度远大于边壁面区域的湍流强度.     

旋风分离器内旋转流的不稳定性

<正>引言旋风分离器内的流场是一个复杂的旋转流流场,以往的流场研究主要集中在稳态流场的时均特性上,重点关注流场的时均速度分布和湍流特性方面,从实验测量和模拟两方面进行,而对瞬态速度的分析不够,对流场的不稳定性尚缺乏分     

Flow Characteristics in a Vortex Chamber

In this article we examine confined swirling flows using the integral equations of continuity and energy, along with the minimum pressure criterion. The pressure drop and the core size have been studied in the swirling confined vortex chamber. Both the n = 2 vortex model, with reverse and non‐reverse flow, and the free vortex model have been used at the vortex chamber exit plane. The influence of vortex chamber geometry, such as contraction ratio, inlet angle, area ratio, aspect ratio, and Reynolds number, on the flow field has been analyzed and compared with the present experimental data. The pressure drop across the vortex chamber differs from that in pipe flow, due to the mechanism of swirl flow that depends mainly on the intensity of tangential velocity. If the chamber length is increased, the vortex decays producing a weaker tangential velocity (less centrifugal force) that leads to less pressure drop. Based on the present theory, a new approach to determine the tangential velocity and radial pressure profiles inside the vortex chamber is developed and compared with the available experimental data. It shown that the n = 2 vortex model with reverse flow gives better results for strongly swirling flow.     

超短接触旋流反应器混合腔内气固混合特性的数值模拟

采用欧拉双流体模型对超短接触旋流反应器内的气固两相流场进行了数值模拟,主要研究了混合腔内固相的体积分数分布情况。计算结果表明：混合腔内的气流在切向进气的作用下得到了一定的混合加速效果,切向的高速射流有效地缩短了气固停留时间,保证短接触反应效果。通过对两种不同混合腔结构反应器的对比计算发现,在相同入口速度条件下结构2（切向进气管位于混合腔顶部）较结构1（切向进气管位于混合腔下部）气固停留时间短,由于切向气流的迅速作用,增加了混合腔内的湍动强度,使催化剂颗粒迅速有效扩散、增强气固接触效果而更有利于催化裂化反应的进行,更易实现短接触操作要求。计算结果与实验测量结果的比较表明模型能有效地描述超短接触旋流反应器内气固两相流动形态。     

Study of the swirling flow field induced by guide vanes using electrical resistance tomography and numerical simulations

In this work, the swirling flow field induced by guide vanes was studied using electrical resistance tomography (ERT) and numerical simulations. The results show that the two-phase water and oil mixture moves in the same axial direction for this type of flow field, which is very unlike the flow behavior of a traditional hydrocyclone with a tangential inlet. In the pipe behind the guide vanes, the smallest axial velocity and tangential velocity are located at the center of the pipe. From the pipe center to the pipe wall, both pressure and velocity increase gradually. Downstream of guide vanes, the maximal oil volume fraction is observed at the center of the pipe. From the center of the pipe to the inner wall, the oil volume fraction gradually decreases. Moreover, ERT can precisely show the oil distribution in the pipe section. These studies prove the possibility of efficient oil and water mixture separation by guide vanes, and the results may be very important for guiding the optimal design of vane-type pipe separators.     

Stereoscopic PIV studies on the swirling flow structure in a gas cyclone

Understanding the swirling flow in a gas cyclone is of great importance in improving the cyclone design. Once the three-dimensional strong swirling flow is fully understood, cyclone performance such as pressure drop and separation efficiency can be improved by optimizing the cyclone design. The swirling flow was investigated by the stereoscopic particle image velocimetry (Stereo-PIV) in this work. The instantaneous whole-field tangential, axial, and radial velocities were measured simultaneously in the cylindrical and conical separation zone, and in the dust hopper area of the cyclone with gas inlet velocity of .The time-averaged flow pattern in the cylindrical and conical sections of the cyclone showed: a typical Rankine vortex with inner quasi-forced vortex and outer quasi-free vortex which is generated by tangential gas velocity; inner upward flow and outer downward flow of axial gas velocity; and centripetal flow in the region close to the wall due to the presence of radial gas velocity. In the dust hopper, a secondary longitudinal circular flow is formed in the annulus area between the conical body and the cylindrical wall. Experimental results indicate that the separated particles may be re-entrained into the cyclone from the bin to degrade the separation efficiency of the cyclone.     

Liquid mixing intensification by adding swirling flow in the transverse jet mixer

In this study, effect of swirling addition on the liquid mixing behavior of multiorifice-impinging transverse jet mixer has been investigated by planar laser-induced fluorescence as well as large eddy simulation (LES). In the case of swirling addition into the jet flow, there exists an optimized swirling jet angle or optimized jet-to-cross velocity ratio for the fixed mixer configuration. A larger swirling jet angle will make the flow dominated by the swirling, resulting in a slower mixing process. Interaction of swirling crossflow with no-swirling-injected streams, or with swirling-injected streams in the opposite direction is beneficial for the mixing. LES predictions show that many small vortices are produced homogenously due to intensified impingement in the case of opposite swirling directions, leading to a relative fast mixing process in few milliseconds. Whereas the mixing is restrained when the swirling directions of two flows are the same.     

旋风分离器有无灰斗对气相流场动态特性的影响

旋风分离器底端的排尘口依据气固分离工艺的要求分别采用有灰斗或无灰斗结构。但灰斗是否存在对旋风分离器内部流场影响的研究尚显不足。为此，采用热线风速仪对排尘口有灰斗和无灰斗的旋风分离器气相流场的切向速度进行了测量。结果表明旋风分离器内旋转流具有较强的不稳定性，表现为瞬时切向的速度低频高幅值波动变化。灰斗的存在进一步导致了排尘口附近瞬时切向速度的强烈波动。通过对瞬时切向速度的频谱分析表明，有灰斗结构的旋风分离器瞬时切向速度有2个主频，分别是存在于整个空间的全空间主频和出现在锥体下端排尘口附近区域的局部主频。无灰斗结构的旋风分离器仅有1个全空间主频。全空间主频是气体旋流中心围绕旋风分离器几何中心摆动造成的，而局部主频是灰斗气体回流造成的。灰斗气体回流主频与全空间旋转流摆动的主频叠加形成了锥体下端排尘口附近区域瞬时切向速度的2个主频。     

Turbulent mixing in the confined swirling flow of a multi‐inlet vortex reactor

Turbulent mixing in the confined swirling flow of a multi‐inlet vortex reactor (MIVR) was investigated using planar laser induced fluorescence (PLIF). The investigated Reynolds numbers based on the bulk inlet velocity ranged from 3290 to 8225, and the Schmidt number of the passive scalar was 1250. Measurements were taken in the MIVR at three different heights (¼, ½, and ¾ planes). The mixing characteristics and performance of the MIVR were investigated using instantaneous PLIF fields and pointwise statistics such as mixture fraction mean, variance, and one‐point concentration probability density function. It was found that the scalar is stretched along velocity streamlines, forming a spiral mixing pattern in the free‐vortex region. In the forced‐vortex region, mixing intensifies as the turbulent fluctuations increase significantly there. The mixing mechanisms in the MIVR were revealed by identifying specific segregation zones. At Re = 8225 the mixing in the free‐vortex region was dominated by both large‐scale structures and turbulent diffusion, while in the forced‐vortex region mixing is dominated by turbulent diffusion. © 2016 American Institute of Chemical Engineers AIChE J, 63: 2409–2419, 2017     

旋风分离器内气相旋转流动态特性分析与表征

旋风分离器内的气相旋转流具有很强的动态特性,表现为流场瞬时参数随时间波动变化。为了对旋风分离器内气相旋转流动态特性进行表征,本文基于热线/热膜风速仪（HWFA）和动态压力传感器测量的旋风分离器内瞬时切向速度和瞬时压力,从时域和频域两个方面进行了分析。结果表明,旋风分离器内瞬时切向速度信号时域上的波形分布与旋转流的摆动存在联系,时域上的标准偏差可以直观地表征旋风分离器内旋转流的波动强度;频域的主频和功率谱密度（PSD）可以表征旋转流动态参数波动的准周期行为、传递行为和强度衰减特征,也是旋转流摆动行为及其影响范围的反映。基于瞬时切向速度和瞬时压力的时域和频域分析能较好地反映旋转流流场的波动特点,均可用于表征旋风分离器内气相旋转流的动态特性。     

Classification performance of model coal mill classifiers with swirling and non-swirling inlets

The classification performance of model coal mill classifiers with different bottom incoming flow inlets was experimentally and numerically studied. The flow field adjacent to two neighboring impeller blades was measured using the particle image velocimetry technique. The results showed that the flow field adjacent to two neighboring blades with the swirling inlet was significantly different from that with the non-swirling inlet.With the swirling inlet, there was a vortex located between two neighboring blades, while with the nonswirling inlet, the vortex was attached to the blade tip. The vorticity of the vortex with the non-swirling inlet was much lower than that with the swirling inlet. The classifier with the non-swirling inlet demonstrated a larger cut size than that with the swirling inlet when the impeller was stationary(～0 r·min~(-1)). As the impeller rotational speed increased, the cut size of the cases with non-swirling and swirling inlets both decreased, and the one with the non-swirling inlet decreased more dramatically. The values of the cut size of the two classifiers were close to each other at a high impeller rotational speed(≥120 r·min~(-1)). The overall separation efficiency of the classifier with the non-swirling inlet was lower than that with the swirling inlet, and monotonically increased as the impeller rotational speed increased. With the swirling inlet, the overall separation efficiency first increased with the impeller rotational speed and then decreased when the rotational speed was above 120 r·min~(-1), and the variation trend of the separation efficiency was more moderate. As the initial particle concentration increased, the cut sizes of both swirling and non-swirling inlet cases decreased first and then barely changed. At a low initial particle concentration(b 0.04 kg·m~(-3)), the classifier with the swirling inlet had a larger cut size than that with the non-swirling inlet.