Mixing and fast chemical reaction—VI: Extension of the reaction zone

Work reported earlier[1, 3] as well as here indicates that mixing at the molecular scale (micromixing) takes place by molecular diffusion in fluid elements, which are simultaneously undergoing laminar deformations. Whereas simple shear was considered before, the present analysis describes diffusion and reaction in regions deformed by extensional flow. After outlining various simple types of extension, results in the literature[4,7] are quoted, which were obtained from observations on the diffusion of heat spots in turbulent air. These indicated stretching motions in two directions with a corresponding shrinkage in the third. Direct application of these findings to diffusion in the direction of shrinkage leads to predictions of th product distribution from competitive, consecutive reactions. Without fitting any parameters, these predictions agree quite well with the distributions of two dyestuffs produced by two diazo coupling reactions. Insufficient fluid mechanical information is available to enablea more accurate comparison; matters requiring further study are listed.     

添加聚合物对混合层中拟序结构的影响

<正>混合层是一种在化工过程中经常见到的基本流场,对该流场拟序结构的研究具有理论和实际意义.自70年代以来,混合层中的拟序结构得到了深入和广泛的研究,然而对于加入聚合物而产生了粘弹性的混合层流场,国内外的研究工作还不多.实验方面,Hibberd发现聚合物的加入使流场中小尺度结构减少,拟序结构更加明显;Riediger发现拟序结构具有更长的生存期,动量厚度的增长受到抑制.理论方面,最新的进展是Azaiez和Homsy(简称AH)用Oldroyd-B模型求解了流场,但是在求解时,发现在Weissenberg数为5时数值解发散,并用有限拉伸率下聚合物法向应力会趋于无穷来解释这一现象.因而对流场没作仔细的研究.但事实上,数值解的发散并不表示此时应力已出现奇点,而是由于应力迅速增长的性质导致数值计算出现不稳定,正如计算激波时还没有真正到达间断,数值解就会由于振荡而发散.因此,可以采用适当的滤波措施来稳定计算而得到合理的近似解.     

微通道内单柱绕流特性的Micro-PIV实验研究

采用微观粒子成像系统（Micro-PIV）实验研究了6<Re<300范围内微通道内D=0.4mm圆柱的绕流特性，获得并分析了不同Re下不同高度流层的速度场、涡量场、湍流强度场及回流区漩涡结构。研究结果表明，微圆柱绕流出现漩涡的第一临界Re在10左右，随着Re的增大，尾流区涡长度和宽度增加，尾流区域增大，漩涡中心后移；由于黏性阻滞，越靠近微通道壁面，主流速度越低且分布越均匀；不同高度下回流区长度相同，远离壁面的平面尾流区漩涡中心沿流动方向后移；高涡量区与高湍流强度区分布在微圆柱两侧，说明该位置流体混合较为剧烈，随着Re的增大，涡量增加，高涡量区变窄、变长，湍流强度及高湍流强度区域增大，当Re>200，不同高度流层的湍流强度差别较小。     

搅拌反应釜中微观混和问题的研究——(Ⅰ)—频闪高速显微摄影法研究微观混和过程

本文介绍了作者首次开发成功的、用于观察亚微观及微观混和过程的微流元团变形历程的新型方法——高速频闪显微摄影法。作者用此方法首次拍摄到了无化学反应与伴有化学反应时的湍流场中亚微观及微观混和过程中微流元团的照片,对微观混和过程有了感性认识。并提出此过程由以下三个过程组成:(i)湍流涡旋的产生、死亡和再生,使得大涡团分裂成小涡团;(ii)小涡团内的无规则复杂的层片状伸长变形及(iii)分子扩散过程。所得到的这些结果为微观混和问题的进一步深入研究提供了重要的感性认识和物理基础。     

Simulation of supercritical water–hydrocarbon mixing in a cylindrical tee at intermediate Reynolds number: Formulation,numerical method and laminar mixing

The objective of this work is to study the flow dynamics and mixing of supercritical water and a model hydrocarbon (n-decane), under fully miscible conditions, in a small scale cylindrical tee mixer (pipe ID = 2.4 mm), at an intermediate inlet Reynolds number of 500 using 3-D CFD simulations. A Peng–Robinson EoS with standard van der Waals mixing rules is employed to model the near-critical thermodynamics with the mixture binary interaction parameter obtained from a Predictive Peng–Robinson EoS using group contribution theory (PPR78). The n-decane stream is introduced at the colder temperature of 700 K to ensure operation above the Upper Critical Solution Temperature (UCST, 632 K) of the water n-decane system while the water stream enters at a higher temperature of 800 K. Under these conditions, the flow in the tee mixer remains laminar and steady-state is reached. Mixing occurs predominantly due to the circulating action of a counter-rotating vortex pair (CVP) in the body of the hydrocarbon jet entering from the top. This CVP is formed due to the reorientation of the streamwise vorticity pre-existing within the hydrocarbon jet as it flows down the vertical pipe of the tee junction. The advective transport is further assisted by a secondary flow of water from the bottom stream, around the hydrocarbon jet, toward the space vacated near the top of the downstream pipe section by the downward motion of the HC jet. The CVP becomes progressively weaker due to vorticity diffusion as it is advected downstream and beyond 10–12 diameter lengths downstream of the mixing joint, transport is mainly controlled by molecular diffusion. It was found that the variations of density and transport properties with temperature do not have a significant impact on the flow and mixing dynamics for a ΔT = 100 K between the two streams. Local cooling of the fluid mixture was also observed in the mixing of water and n-decane streams entering at the same temperature (initially isothermal). This cooling effect is due to the diffusion of species along a gradient in their partial enthalpy in the mixture. Such gradients in species partial enthalpies are non-zero under near-critical conditions even for initially isothermal flows due to the non-ideality of the fluid mixture under these conditions. This local heating/cooling effect at near-critical conditions could give rise to unexpected formation of phases when operating close to critical points.     

Alternating mixing tabs in multifunctional heat exchanger-reactor

Streamwise vortices are used in many applications for mixing processes, fast chemical reactions and heat-transfer enhancement. In this work we studied experimentally and numerically the effects of vorticity field redistribution on the turbulence energy dissipation rate in a modified geometry of the high-efficiency vortex (HEV) mixer, in which the mixing tab arrays are periodically rotated by 45° to better distribute the vorticity field. Attention focuses on the evolution and distribution of turbulence energy dissipation, since this describes quantitatively the drop breakup and turbulent micromixing mechanism, which controls fast chemical reactions. It is found that redistribution of the vorticity field locally intensifies turbulent micromixing relative to the classical HEV mixer, producing a local mixing intensification of up to 120%. In addition, the alternating vortex generator arrays improve homogenization of the turbulence field in the mixer.     

内插梯形扰流片的矩形通道内涡流和传热特性

利用Realizable k-ε湍流模型对带缺口的梯形扰流片进行流动和传热特性的数值模拟,研究了梯形扰流片的缺口位置及流动方式对矩形通道内流场以及传热的影响,同时通过对涡量、流线、流速分布、压力变化、湍流强度等的分析,揭示了扰流片强化传热的机理。结果表明,逆流时Nusselt数比顺流时提高了21.7%,同时摩擦因子也提高了25%。顺流时内侧缺口绕流片提高了传热系数的同时也增加了摩擦阻力,而外侧缺口的绕流片降低了传热系数同时也降低了形状阻力。研究发现较低Reynolds数下(10000相似文献   

Heat transfer enhancement in fin-plate heat exchangers by wing type vortex generators

Heat transfer and fluid mechanical data were computed for laminar channel flows containing strong longitudinal vortex pairs. The strong vortices are generated by thin delta wings and delta winglet pairs of low aspect ratios and large angles of attack. These wings are attached to the channel walls. The data show that longitudinal vortices cause high local peaks in heat transfer and marked increases in the overall channel heat transfer. These increases occur over a wide region of channel wall, compared to the vortex generating wing area. The results are of special interest for compact heat exchangers. The heat transfer enhancement allows a considerable reduction in the heat transfer area which, in turn, reduces the manufacturing and operating costs.     

Experimental investigation of transition to laminar mixing of a homogeneous viscous liquid in a tilted-rotating tank

A tilted and partially filled rotating tank is investigated experimentally at O(1) Reynolds and small (?1) capillary numbers, to study the mixing of a viscous homogeneous fluid. Of particular interest is the transition from a previously studied low Reynolds number flow regime [Ward, T., Metchik, A., 2007. Viscous fluid mixing in a titled tank by periodic shear. Chemical Engineering Science 62, 6274-6284], that exhibited two large vortices, to the laminar flow regime which results in additional vortex generation. In the laminar Reynolds number limit O(1) the two primary vortices generated by the liquid rotation axis can interact with the bottom wall, generating two secondary counter-rotating vortices, via a cascade that is qualitatively similar to the well known Moffatt [1964. Viscous and resistive eddies near a sharp corner. Journal of Fluid Mechanics 18, 1-18] vortices in Stokes flow. While the secondary vortices aid in transporting material from the walls to the bulk, they also intensify in magnitude with increasing rotation rate leading to finite sized unmixed regions via the appearance of KAM-like surfaces [Alvarez-Hernández, M.M., Shinbrot, T., Zalc, J., Muzzio, F.J., 2002. Practical chaotic mixing. Chemical Engineering Science 57, 3749-3753]. This suggests that there may be an optimal tilt angle, for a given speed, with which to achieve the maximum mixed cross sectional area within a minimum amount of elapsed time. Experiments are performed using a 90% glycerol, 10% water mixture at two volume portions with angles ranging between 25^° and 65^° measured from the horizontal. Laser fluorescence is used to illuminate the vortices via experimental Poincaré mapping [Fountain, G.O., Khakhar, D.V., Ottino, J.M., 1998. Visualization of three dimensional chaos. Science 281, 683-686], and the resulting images are analyzed to determine the mixed cross sectional area versus elapsed time.     

Counter-rotating vortex shedding generated by acoustic excitations in confined mixing layers

Ultrafast mixing enhancement has been realized in confined mixing layers at an optimal forcing frequency, under which the flow exhibits the strongest mixing. The fast mixing is accompanied by the so-called counter-rotating vortices (CRVs). In this investigation, by using particle imaging velocimetry (PIV) and laser-induced fluorescence, the conditions of generating CRVs in confined mixing layers are theoretically and experimentally investigated. To generate CRVs, (a) the acoustic particle displacement must be larger than the summation of the curvature radius of the trailing edge and the thickness of the Stokes layer, and (b) the temporal derivative term of the vorticity equation should be larger than the convective term of the mean vorticity generated by the mean flow. This study enhances our understanding on vortex shedding dynamics and enables novel designs of fast mixers for continuous operations in process industry.     

幂律流体在Kenics型静态混合器中流动特性分析

为了探究Kenics型静态混合器内扭旋叶片剪切作用对幂律流体流动的影响,利用旋转流变仪测量了浓度为0.5wt%, 0.7wt%, 0.9wt%的羧甲基纤维素(CMC)水溶液的流变参数,采用数值模拟与实验研究了扭旋叶片作用下幂律流体流动阻力和剪切稀化特性。对流场研究表明,扭旋叶片诱导产生了内流涡旋、绕流涡旋和近壁面涡旋,有效强化了静态混合器内流体流动的剪切作用。受多个纵向涡旋分布的影响,扭旋叶片局域流场中周向45°位置速度最高,周向30°位置涡量与剪切应力最高而黏度最低。径向0.4倍半径位置速度最高,0.7倍半径位置黏度最高。静态混合器有效提高了流体的二次流流动速度和剪切应力,降低了幂律流体的黏度和流动阻力系数。     

Effects of embedded streamwise vorticity on turbulent mixing

This work concerns the characterization of turbulent flow underlying mixing in the presence of streamwise vorticity. An experimental test section made of a cylindrical tube equipped with seven rows of streamwise vortex generators was designed and constructed for this study. Each row is composed of four vortex generators fixed symmetrically on the tube wall. This new type of mixer, called a high-efficiency vortex (HEV) mixer, generates coherent structures in the form of longitudinal counter-rotating vortices. The resulting flow enhances radial mass transfer and thus facilitates particle dispersion and mixing. The energy cost of this mixer used as an emulsifier has been evaluated as up to a thousand times less than that of other static mixers for a given interface area generation (Lemenand et al. [1] and [2]).The aim of this work is to study experimentally and numerically the turbulence structure and mixing properties of the flow composed of streamwise vortices superimposed on a turbulent flow, in particular the more energetic structures present in the base flow. Experiments were carried out in the test section in a flow loop by measuring instantaneous velocities by laser Doppler anemometry. Numerical simulations of the velocity distribution and turbulence field inside the flow were conducted for various turbulence models using a computational fluid dynamic CFD package. Attention is focused on the evolution and distribution of turbulent kinetic energy dissipation as the underlying mechanism for turbulent mixing. Mean and turbulent quantities are compared with experimental results.Both laboratory experiments and numerical simulations show a vortex zone behind each tab that could explain the efficiency of the HEV mixer. This study provides a basis for understanding the physical mechanisms in the mixing and homogenizing of the flow and therefore the efficiency of the mixer.     

布置成涡结构微混合器内的流动与混合特性

基于成涡结构强化混合原理设计了一种改进型的平面被动式微混合器。采用商业软件CFD-ACE+对该结构微混合器的混合特性进行了三维数值模拟和结构优化,进一步揭示该微混合器结构对通道内流体流动与混合特性的影响。结合混合通道内流体的浓度和流型分布的数值和实验结果可知,该新型微混合器在布置成涡结构的弯曲通道内形成了扩展涡、分离涡和Dean涡,实现了涡系的叠加和强化,加大了流体间的扰动,从而增加了流体的接触面积强化混合;在综合考虑流体混合强度和压降分布等因素下,成涡结构正向布置且缝宽比W_d/W=1/4,厚宽比B/W=3/10,布置角度θ_a=120°的该微混合器在较广Re范围内的混合效果显著。     

Kenics型静态混合器充分发展段纵向涡演变分析

为分析Kenics型静态混合器内充分发展段沿轴向二次流纵向涡的形成诱因及演变过程,运用大涡模拟对混合器内流场进行研究,数值模拟结果与实验结果吻合较好. 结果表明,在第7个扭旋叶片所在区域内含3种旋涡,分别为叶片入口分割上一段流体后产生的合并旋涡、随扭旋叶片一起旋转的内流旋涡和绕流旋涡. 绕流旋涡是扭旋叶片高扭率产生的科氏力导致单侧流体压力不平衡,使边界层产生分离形成的诱导旋涡. 沿轴向将第7个扭旋叶片所在区域流场平均分成4段,第一和第二段的横截面上存在5个纵向涡,涡量和湍动强度平均值分别比第三和第四段高23.0%和8.93%. 在相邻叶片的分界面处,旋涡破碎和聚合产生能量损耗,使近壁面的涡量陡增,高出平均值73.0%.     

用流动可视化技术研究混合器内研究简报流场及混合效果

引　言不同流体的混合是化工、生物制品、食品工业等工艺过程中一道重要的工序 ,仅依靠流体在输送过程中的自然湍流扩散不能进行有效的混合 ,因此必须使用混合器 ,使液体在短时间内获得均匀的特性 ,达到满意的混合效果 ,并使损失的能量较小 .不同行业由于介质性能和混合要求不同 ,相应混合器的结构也不同 .通常国内外多采用经验类比法设计混合器 ,用取样分析法检验混合效果 .随着新技术的发展 ,深入研究流体混合机理对于高效节能混合器的设计具有重要意义 .尽管许多学者采用数值方法对混合器内部的流动和混合效果进行了研究[1,2 ],但由于数…     

用流动可视化技术研究混合器内研究简报流场及混合效果

引言 不同流体的混合是化工、生物制品、食品工业等工艺过程中一道重要的工序,仅依靠流体在输送过程中的自然湍流扩散不能进行有效的混合,因此必须使用混合器,使液体在短时间内获得均匀的特性,达到满意的混合效果,并使损失的能量较小.不同行业由于介质性能和混合要求不同,相应混合器的结构也不同.通常国内外多采用经验类比法设计混合器,用取样分析法检验混合效果.随着新技术的发展,深入研究流体混合机理对于高效节能混合器的设计具有重要意义.     

Mixing and fast chemical reaction—VII Deforming reaction zone model for the CSTR

Earlier work on batch reactors indicated that the final stage of homogenisation to the molecular scale (micromixing) occurs by molecular diffusion in deforming fluid elements. The product distribution of, for example, consecutive, competitive reactions is sensitive to concentration gradients at the molecular scale (segregation). Such reactions may be used as reactive tracers to throw light on the behaviour of small fluid elements. Moreover an understanding of micromixing allows the selectivity of fast, multiple reactions to be better controlled. This paper describes two ways of applying the unsteady-state equations for diffusion and reaction within a shrinking fluid element to determine the steady-state concentrations in a CSTR. The iterative model is more economical in computer time than the dynamic model and applies to fast reactions (i.e. when the time for diffusion and reaction is a small fraction of the mean residence time). It described well the measured effects on the product distribution of varying the volumetric feed rati the stoichiometric ratio and of changing the operating mode from semicontinuous to continuous. When the initial size of the deformable fluid element is identified with the Kolmogoroff microscale and a reasonable estimate of shear rate is introduced, a highly satisfactory prediction of experimental resu knowing the kinematic viscosity and the power consumption of the stirrer, becomes possible.     

Mass transfer between two turbulent liquid phases

The effect of forced turbulence on interfacial mass transfer between two liquid phases is investigated. A theoretical model is derived on the assumption that the mass transfer is controlled by unsteady diffusion into the vortices of the viscous subrange of the range of universal equilibrium. Mutual interaction of the turbulent fields in both phases is also accounted for. Experimental mass transfer rates for binary and ternary systems are presented; these were measured in a mixing cell of a new design. The model presented is shown to describe the process well, provided that the chosen liquid system is interfacially stable. The deviations due to interfacial instabilities are demonstrated for the case of the water—acetone—carbon tetrachloride system.     

Rushton涡轮搅拌槽内流场特性及颗粒运动行为数值模拟

基于计算流体动力学（CFD）方法,采用大涡模拟（LES）和拉格朗日颗粒追踪技术计算了Rushton涡轮搅拌槽内流场特性及三种St颗粒的运动行为。平均流场（切向速度、轴向速度和径向速度）、颗粒速度及浓度分布方面与实验值的吻合度较好,验证了数值模拟的可靠性。结果表明,搅拌流场及颗粒运动均呈现循环流特性,当转速N=313r/min不变时,St=0.24的小颗粒几乎实现了均匀分布;而St=37.3的大颗粒与流体的跟随性较差,底部沉积率较高,容器顶部会出现一定的颗粒空白区。叶轮附近产生一系列的湍流涡结构,并且由于剧烈的颗粒-壁面碰撞,该位置颗粒拟温度最高;小颗粒（St=0.24）的运移主要受叶片后方尾涡的控制,均匀分布在低涡量区;而大颗粒（St=37.3）由于具有较大的惯性,运动不再由涡主导,很快被叶轮甩向边壁,穿过了尾涡所形成的高涡量区,故而叶轮对附近大颗粒的搅拌效果较差。