On the Bifurcations of Dean Flow between Porous Horizontal Cylinders with a Radial Flow and a Radial Temperature Gradient

The stability of a heat-conducting flow due to the pumping of a fluid around the annulus of horizontal porous cylinders is studied. The basic flow is under the action of radial flow and a radial temperature gradient. The objects of investigations are different regimes and bifurcations which may arise in this flow.

A Micromixer Using the Taylor-Dean Flow: Effect of Inflow Conditions on the Mixing

Chaotic mixing in a curved-square channel flow is studied experimentally and numerically. Two walls of the channel (inner and top walls) rotate around the center of curvature and a pressure gradient is imposed in the direction toward the exit of the channel. This flow is a kind of Taylor-Dean flows. There are two parameters dominating the flow, the Dean number De (∝ the pressure gradient or the Reynolds number) and the Taylor number Tr (∝ the angular velocity of the wall rotation). In the present paper, we analyze the physical mechanism of chaotic mixing in the Taylor-Dean flow by comparing experimental and numerical results. We produced a micromixer model of the curved channel several centimeters long with square cross section of a few millimeters side. The secondary flow was measured using laser induced fluorescence (LIF) method to examine secondary flow characteristics. We also performed three-dimensional numerical simulations for the exactly same configuration as the experimental system to study the mechanism of chaotic mixing. It is found that good mixing performance is achieved for the case of De ≤ 0.1Tr, and that mixing efficiency changes according to the difference in inflow conditions. The flow is studied both experimentally and numerically, and both results agree with each other very well.

A Micromixer Using the Taylor-Dean Flow: Effects of Aspect Ratio and Inflow Condition on the Mixing

Chaotic mixing in three different types of curved-rectangular channels flow has been studied experimentally and numerically. Two walls of the channel (inner and top walls) rotate around the center of curvature and a pressure gradient are imposed in the direction toward the exit of the channel. This flow is a kind of Taylor-Dean flow. There are two parameters dominating the flow, the Dean number De (∝ the pressure gradient or the Reynolds number) and the Taylor number Tr (∝ the angular velocity of the wall rotation). In this paper, we analyze the physical mechanism of chaotic mixing in the Taylor-Dean flow by comparing experimental results and numerical ones. We produced three micromixer models of the curved channel, several centimeters long, with rectangular cross-section of a few millimeters side. The secondary flow is measured using laser induced fluorescence (LIF) method to examine secondary flow characteristics. Also we performed three-dimensional numerical simulations with the open source CFD solver, OpenFOAM, for the same configuration as the experimental system to study the mechanism of chaotic mixing. It is found that good mixing performance is obtained in the case of De ≤ 0.1 Tr, and it becomes more remarkable when the aspect ratio tends to large. And it is found that the mixing efficiency changes according to the aspect ratio and inflow condition.

Enhancement of liquid-liquid micromixing performance in curved capillary microreactor by generation of Dean vortices

Micromixing efficiency is an important parameter for evaluating the multiphase mass transfer performance and reaction efficiency of microreactors.In this work,the novel curved capillary reactor with different shapes was designed to generate Dean flow,which was used to enhance the liquid-liquid micromixing performance.The Villermaux-Dushman probe reaction was employed to characterize the micromixing performance in different curved capillary microreactors.The effects of experiment parameters such as liquid flow rate,inner diameter,tube length,and curve diameter on micromixing performance were systematically investigated.Under the optimal conditions,the minimum value of the segmentation factor XS was 0.008.It was worth noting that at the low Reynolds number(Re<30),the change of curved shape on the capillary microreactor can significantly improve the micromixing performance with XS reduced by 37.5%.Further,the correlations of segment index XS with dimensionless factor such as Reynolds number or Dean number were developed,which can be used to predict the liquid-liquid micromixing performance in capillary microreactors.

Theoretical investigation on the impact of an oscillating time-dependent pressure gradient on Dean flow in a porous annulus

In this article,a theoretical analysis on flow in a curvilinear horizontal coaxial cylinder with permeable walls has been proposed.Specifically,the transient impact of an oscillating pressure gradient has been taken into account.The non-linear time-dependent partial differential equation accountable for the flow has been transformed using the classical Laplace transform technique.Exact solution of the momentum equation has been obtained in Laplace domain.Due to the intricacy of the Laplace domain solutions,a numerical inversing technique which is established upon the Riemann-sum approximation(RSA)has been utilized to transform the Laplace domain solutions to time domain.Findings reveal that the outcome of suction on the porous walls and boosting the frequency of oscillation renders skin frictions on both walls of the cylinder less effective.The instability of the Dean vortices in the annular gap can be suppressed by amplifying the frequency of oscillating pressure gradient while time is maintained.

膜分离过程中的Dean涡流及其作用分析

综述了螺旋式膜组件的Dean二次流基本原理、管程常用传质关联式及其在膜分离过程中的应用状况,并从Dean二次流对膜过程通量和压力降以及传质性能的影响等诸方面.阐述了螺旋式组件的优势及限制因素.分析结果表明,流体在螺旋流道中由向心力产生Dean流效应,使管内湍动状态增强、边界层效应减弱,从而缓解浓差极化,使管程传质和膜通量与直管组件相比有明显改善.此外,该结构还有助于消除壳程流体的沟流、死区等常见负面因素.

Impacts of Rotation on Unsteady Fluid Flow and Energy Distribution through a Bending Duct with Rectangular Cross Section

A depth understanding of fluid flow past a curved duct having rectangular cross-section with different aspect ratios(l)are essential for various engineering applications such as in chemical,mechanical,biomechanical and bio-medical engineering.So highly ambitious researchers have given significant attention to study new characteristics of fluid flow in a curved duct.The flow characterization in the rectangular duct has been studied over a wide range of numerical and selective experimental studies.However,proper knowledge with the effects of Coriolis force for different aspect ratios is important for better understanding of the transitional behaviour and the subsequent heat generation,which is required to improve further.The purpose of this study is to reveal insight into the transitional flow pattern and heat transfer in a curved rectangular domain.The Navier-Stokes equations are solved using the spectral method,while the Crank-Nicolson method is used to solve the energy equation.An in-house FORTRAN code is developed to get the numerical solution.For post-processing purposes,Tecplot-360 and Ghost-script tools are used.The present study exposes development of Dean vortices that affect heat generation as well as thermal enhancement in the flow with underlying the flow controlling parameters,the Dean number(Dn),the Grashof number(Gr)and the Taylor number(Tr).Time-dependent results followed by phase spaces show that transient flow undergoes in the scenario‘chaotic→multi-periodic→periodic→steady-state’generating 2-to 8-vortices for the periodic/multi-periodic flow at 2000≤Tr≤2205 for l=2,whereas similar sort of flow is observed in the range of 3100≤Tr≤3195 for l=3.More complicated 4-to 13-vortex solutions are obtained for the chaotic flow regime at l=2 in the range of 0≤Tr<2200 and at l=3 in the range of 0≤Tr<3100.The chaotic flow that occurs at the certain range of Tr proficiently intensifies the heat transfer than the unperturbed,periodic or multi-periodic flow.The overall investigation reveals that in the rotating duct,the temperature-influenced buoyancy compulsion and centrifugal-coriolis joint forces are dominant,influencing the characteristic of the fluid and thus optimizing the transfer of heat.The present investigation will contribute to enhancing the understanding of fluid flow and heat transfer of internal heating/cooling/gas turbines,electric generators,biological systems,and some separation processes.

Dean涡的微粒分离原理在污水2级处理中的应用

将弯道层流流动形成的Dean涡产生的微粒分离原理运用于污水的2级处理,通过研发水动力分离处理装置,采用投加适量絮凝剂后的某河水进行实验,研究在流道中不同体积流量和絮凝剂含量下装置的分离效果。结果表明：当进水粒径在94.55~161.4μm时,分离效果最好;当体积流量小于600 m L/min时,体积流量越大则分离效果越高,而体积流量大于600 m L/min后,分离效果基本不变;在体积流量为600 m L/min时对浊度的去除率在90%以上,对TP和SS去除率分别在85%和50%以上,经装置分离后的清水的浊度大致为1 NTU;在体积流量为600 m L/min时,絮凝剂Fe Cl3的适宜投加质量浓度为21 mg/L。

液压流道局部拓扑优化与增材制造

液压阀块是液压系统的重要组成部分,降低液压阀块的压力损失对实现液压系统节能化、提高功重比意义重大。选区激光熔化(Selective Laser Melting,SLM)技术是一种增材制造(Additive Manufacturing,AM)技术,基本突破了减材、等材等传统加工方式的设计约束,结合拓扑优化方法,可大大提高液压阀块及其流道的设计自由度。以降低流道局部压力损失为研究目标,对流道局部压力损失较大的拐弯处进行拓扑优化设计,并采用SLM技术成形,优化设计后流道的压力损失明显降低。进一步探究弯管压力损失的主要影响因素——迪恩涡,定量分析了拓扑优化流道降低压力损失的原理,结果表明,拓扑优化得到的变截面流域通过改变流域的弯曲程度,使迪恩涡对压力损失的影响降到最小,从而有效降低流域内的压力损失。对增材制造液压元件及其流道的设计具有重要指导意义。

Combined Effects of Centrifugal and Coriolis Instability of the Flow through a Rotating Curved Duct with Rectangular Cross Section

Combined effects of centrifugal and coriolis instability of the flow through a rotating curved duct with rectangular cross section have been studied numerically by using a spectral method, and covering a wide range of the Taylor number ?for a constant Dean number. The rotation of the duct about the center of curvature is imposed in the positive direction, and the effects of rotation (Coriolis force) on the flow characteristics are investigated. As a result, multiple branches of asymmetric steady solutions with two-, three-and multi-vortex solutions are obtained. To investigate the non-linear behavior of the unsteady solutions, time evolution calculations as well as power spectrum of the unsteady solutions are performed, and it is found that the unsteady flow undergoes through various flow instabilities in the scenario “chaotic?→ multi-periodic?→ periodic?→ steady-state”, if Tr is increased in the positive direction. The present results show the characteristics of both the secondary flow and axial flow distribution in the flow.

Effects of aspect ratio on unsteady solutions through curved duct flow

The effects of the aspect ratio on unsteady solutions through the curved duct flow are studied numerically by a spectral based computational procedure with a temperature gradient between the vertical sidewalls for the Grashof number 100 ≤ Gr ≤ 2 000. The outer wall of the duct is heated while the inner wall is cooled and the top and bottom walls are adiabatic. In this paper, unsteady solutions are calculated by the time history analysis of the Nusselt number for the Dean numbers Dn = 100 and Dn = 500 and the aspect ratios 1≤γ≤ 3. Water is taken as a working fluid （Pr =7.0）. It is found that at Dn = 100, there appears a steady-state solution for small or large Gr. For moderate Gr, however, the steady-state solution turns into the periodic solution if γ is increased. For Dn = 500, on the other hand, it is analyzed that the steady-state solution turns into the chaotic solution for small and large Gr for any γ lying in the range. For moderate Gr at Dn = 500, however, the steady-state flow turns into the chaotic flow through the periodic oscillating flow if the aspect ratio is increased.

螺旋管内局部二次流强度计算及演变规律

为了掌握螺旋管内二次流强度的变化规律,对螺旋管内局部二次流强度及演变规律进行了研究,提出了无量纲数H作为局部二次流强度指标。通过数值模拟得到H的体积平均值Hm,与描述整体二次流强度的迪恩数进行对比。结果显示,在不同的雷诺数、曲率、螺距下,H_(m)与迪恩数的相关系数约为1,且呈现较好的正比关系,说明H能够准确计量局部二次流强度。同时,探究了几何参数对螺旋管内的二次流演变规律的影响。依据二次流的强度,管内流动可分为发展段、过渡段和稳定段。二次流在发展段迅速增强;在过渡段出现最大值;在稳定段呈小幅度周期性波动。雷诺数增大会显著增大二次流强度,但是不影响二次流在3个阶段出现的位置。曲率的增大会略微增大二次流的强度,且延长了二次流达到稳定段的路程。螺距对二次流的强度和演变规律影响非常小。

导流片对弧度弯管输水性能影响的数值模拟

迪恩涡流及螺旋流作为管路中常见的湍涡结构,虽然具有相似的传热传质等功能,但迪恩涡流易引发管路系统振荡,对有压输水管路系统安全产生巨大威胁。通过前人的物理模型试验建立了管径D=50 mm,曲率半径为2.8 D的弯管管路系统,并根据试验数据验证了数学模型。基于模型管段设计了27种导流片组合用以形成螺旋流,模拟了管内多尺度涡流结构,重点分析了涡流在弯管段及出口直管段的流速、压强和湍动能分布。模拟结果表明:涡流的湍动能及湍流强度对管路系统存在不同程度的影响,弯管管路系统的输水能力在湍动能较大、湍流强度较低的条件下相对较高;螺旋流强度对弯管的输水效率及管路系统的安全性影响显著,当管路进口流速V=1.5 m/s时,弯管管路系统在导流片(个数N=8个、高度H=15 mm、偏转角度θ*=60°)作用下,管路输水效率相对提升10.62%,管路事故风险概率相对降低24.41%。研究成果可为长距离螺旋流输送理论及有压管路系统优化提供技术指导。

60°T形弯头内二次流特征

石油化工中普遍存在的T形弯头结构是诱发管路振动的重要因素,明晰弯头内流场特性,对于压力管道的长期稳定运行具有重要意义。设计并组装了模拟工业运行管路,观察段使用有机玻璃制作的60°T形弯头,采用三维粒子影像测速系统(particle image velocimetry,PIV)和压力传感器测量管路中流体的二次流特征。通过流线图和速度矢量图,对比分析不同流速下管道内涡流的分布和运移规律。结果表明,60°T形弯头中存在一对大小不等的周期性交替摆动的涡,在一个周期内出现、增加、减少和消失,具有卡曼涡街和迪恩涡旋的部分特征。这两个涡交替变化的频率随流速增加呈指数型增大,管内流体压力脉动的幅值也随流速增加而升高,但管道正面的涡流规律基本不变。采用重整化群(renormalization group,RNG)形式κ-ε湍流模型结合二次上游插值(QUICK)方法、压力-速度耦合方程(SIMPLE)模拟管道侧面二次流变化规律,模拟结果与实验数据吻合度较高。研究结果为明晰T形弯头内涡流诱发管道振动规律及预防措施优化提供了理论依据。

缠绕式中空纤维膜组件强化膜两侧传质过程

对缠绕式中空纤维膜组件强化管程及壳程的传质过程进行了实验研究,比较了缠绕式与平直式中空纤维膜组件的传质性能,并考察缠绕角对缠绕式膜组件传质系数的影响.结果表明:相同雷诺数Re时,缠绕式膜组件的传质系数,无论管程或壳程,均为平直式膜组件的2倍以上;缠绕角对缠绕式膜组件的传质性能有重要影响,缠绕角小于45°膜组件的传质效果优于缠绕角大于45°的膜组件.

基于狄恩流的微混合器的研究

所设计的微混合器是一种基于狄恩流的被动式混合器,该混合器的结构特点是由多个不同半径的弯曲管道双螺旋而成,中部通过"S"形管道将输入管道和输出管道相连。首先分析了狄恩流形成的机理及特点,再通过流体计算软件CFD-ACE+三维模拟并分析了不同流速对二次流强弱以及混合程度的影响,最后利用微细加工技术制作微混合器,罗丹明B溶液与去离子水用作待混合液体,利用Image J软件获取混合图像的灰度值来分析混合器的性能。

90°弯管流动预测评估及流动特性研究

采用两种不同的计算方案对弯管内的流动进行数值计算,并与试验值进行对比,分析6种不同湍流模型对弯管内流动预测的影响。结果表明,SKE模型配合壁面函数法和RNG以及RKE湍流模型的预测能力类似,都能大致预测出管内流动的分布规律,但对弯管内侧区域速度扭曲的计算结果与试验值存在明显差异,而SST k-ω湍流模型给出的计算结果与试验值最为符合。基于SST k-ω湍流模型的计算结果研究迪恩涡的发展规律,重点讨论迪恩涡的发展、消失过程,可为后续磨损预测的分析奠定理论基础。

采用二维气相色谱及毛细管色谱柱分析汽油中苯和甲苯的含量

采用微板流路控制技术(Deans Switch)和常见的商品毛细管柱HP-5(30m×0.32mm×0.25μm)和Rtx-TCEP(30m×0.25mm×0.4μm),建立了一种全新的分析车用汽油中苯和甲苯含量的二维气相色谱方法。用该方法分析汽油中0～5%的苯和0～20%的甲苯,两者校正曲线的线性关系良好,相关系数分别为0.9994和0.9999;标准样品5次重复测定的相对标准偏差均小于1.5%;车用汽油实际样品中苯和甲苯的加标回收率在96.8%～103.8%之间。车用汽油实际样品测定结果和SH/T0713-2002标准方法测定结果一致。该方法是车用汽油中苯和甲苯含量测定的一种简便快捷、准确可靠的分析方法。

迪恩涡强化传质的理论与优化设计试验研究

研究了编织型中空纤维膜迪恩涡强化传质理论、优化设计理论、各种几何参数对渗透性能的影响,试验研究了线型/编织型中空纤维膜渗透性能对比和不同螺距编织型中空纤维膜渗透性能对比。结果表明采用弯曲管道流体窄隙理论优化设计编织型中空纤维膜组件,试验结果与理论分析计算一致;迪恩涡强化传质编织型中空纤维膜渗透性能,不但对污染溶液作用显著,同时对降低不含溶质的纯水渗透能耗也有明显作用。

电磁力诱导二次流对微流体混合效果影响的研究

设计了一种微流道中液体混合的方法,基于电磁力的作用,诱导微流道中二次流动发生,从而产生Dean涡,使流道中流体出现往复运动及流体界面的弯曲延伸,使不同流体的接触面积大大地增加,从而提高混合效率.对3种结构的工况进行了比较.建立了微流道混合系统的理论模型并进行了数值模拟,对流体混合过程进行了分析并对混合效率进行了评价.进行了验证性的试验研究,对模拟结果与试验结果进行了比较,结果相吻合.