表面活性剂减阻流体湍流空间结构试验研究

本文应用PIV(Paticle Image Veloeimetry)和PDA(Phase Doppler Anemometry)在二维流道内对CTAC(Cetyltrimethyl Ammonium Chloride)减阻流体湍流流场进行试验研究,得到减阻流体湍流速度分布。研究表明：在完全减阻区内,减阻流体的减阻性能随雷诺数的增大而增大,在过渡减阻区内,减阻流体的摩擦系数则随着雷诺数的增大而逐渐回升,最终回到与溶剂相当的水平上;减阻流体的速度分布曲线在近壁面处与牛顿流体层状速度曲线趋近,但二者并不完全重合;在流道近壁面处,水湍流流动时所能观测到的强烈的旋涡波动在减阻流体中基本消失,与此同时,在此区域内减阻流体的速度轮廓线与流道几乎平行,且该平行轮廓线部分所占比例较牛顿流体湍流流动时相应部分要大很多,减阻流体的湍流强度受到了极大的抑制。     

添加筛网增强减阻流体传热性能的实验研究

本文对添加多种型式金属筛网后不同浓度及配比下减阻流体的传热及湍流特性进行了实验研究,对比分析了不同工况下减阻流体的压损率（PL）及传热增强率（HTE）,从而得到减阻流体传热及湍流特性的变化情况。研究结果表明：管道添加筛网后,在低雷诺数（Re）区,PL明显高于HTE;当雷诺数很低时,添加筛网不能改变减阻流体的传热性能,但雷诺数超过3.25×104时,添加筛网后其传热性能明显增强;相同雷诺数下浓度越高,筛网对胶束的破坏程度就越小,当雷诺数达到4.5×104时,其传热性能才开始增强;不同配比和浓度的减阻流体在相应临界雷诺数附近性能最佳。本文可为流体远距离输送、区域供热供冷等工程应用领域的节能降耗技术提供有力参考。     

CTAC减阻流体湍流高阶矩的研究

本文对不同工况下氯化十六烷基三甲基季铵盐(Cetyltrimethyl Ammonium Chloride,简称CTAC)减阻流体二维流道速度场进行了实验测量,分析了倾斜因子及平坦因子等湍流高阶矩的分布特征.实验结果表明:CTAC溶液流动主流区的倾斜因子远离零值,速度概率密度函数偏离高斯分布远,各向异性强,且平坦因子较大,存在拟序结构,主要是脉动频率低、湍流能量小的大涡结构;近壁面的过渡层区倾斜因子偏离零值不远,各向异性小;过渡区存在强烈的湍流混合.     

低浓度CTAC减阻流体流动性能试验研究

通过对CTAC溶液的减阻性能的测量,得到了CTAC溶液的减阻性能随伴随盐浓度变化的特性。研究结果表明,即使对同种减阻方式,减阻也存在极限和优选。同时,应用激光相位多普勒测速仪进行减阻流体的湍流特性试验研究,得到了50种工况下流体的速度脉动曲线。研究还发现,减阻流体的横向和轴向速度脉动及雷诺应力明显小于牛顿流体;减阻流体的轴向和横向速度脉动之间的关联被明显抑制。     

小槽道表面活性剂湍流减阻及流变特性的实验研究

深入理解表面活性剂减阻机理对于流体管道输送的减阻节能具有重要意义。为了理解表面活性剂湍流减阻特性和减阻机理,本文基于断面尺寸为10 mm×150 mm的二维有机玻璃槽道,对CTAC/Nasal表面活性剂湍流减阻开展了实验测试,并利用MCR302流变仪测试了减阻溶液的流变特性。研究表明,最高减阻率、临界减阻雷诺数、减阻雷诺数范围与减阻溶液的浓度密切相关,但它们之间并非线性关系;剪切作用对表面活性剂胶束剪切诱导结构(即网状胶束结构)的形成和破坏能够很好的诠释表面活性剂减阻机理。     

凝汽器水侧的数值研究及改造

引入多孔介质,采用k-ε湍流模型,利用二维数值计算方法对一双流程凝汽器的水侧流体流动进行了计算与分析。针对凝汽器水侧由于设计缺陷,存在漩涡及流动死区的情况,提出在凝汽器前后水室安装导流板,对凝汽器前后水室流体流动进行引导改善。改进后,凝汽器水侧流动漩涡以及流动死区完全消失,凝汽器水室的流动漩涡以及流动死区消失、流动阻力下降,水侧冷却水分布更加合理,凝汽器换热效果有所提高。     

用PIV技术研究汽油机的缸内气体流场

在一台四气门光学汽油机上,应用激光粒子速度法（PIV）研究了单、双进气门工作时缸内横截面速度矢量场的变化规律,对其空间速度场的平均速度和湍流强度进行了统计分析．结果表明,双气门进气时,缸内不存在明显的大尺度涡流结构,平均速度和湍流强度基本保持稳定;单气门进气时,涡流运动显著加强,进气上止点后150℃A时,速度矢量图上出现明显的大尺度涡流结构,进气上止点后260℃A时大尺度涡流结构边缘出现尺度相对较小的涡旋,平均速度和湍流强度大大加强．     

气泡粘度对水平槽道气泡减阻率影响的数值研究

采用多相流混合模型和雷诺应力湍流模型,对水平槽道气泡减阻进行了数值模拟;同时考虑了液相湍流诱导气泡聚合、气泡尾流诱导气泡聚合、湍流涡与气泡碰撞产生破碎作用对气泡减阻的影响。在保证雷诺数以及气相体积分数不变的情况下,调查了气相粘度变化以及气泡聚合、破碎物理因素对气泡减阻率和液相湍流的影响。研究表明:减阻率和液相湍流变化与气相粘度的大小具有直接关系;气泡减阻和液相湍流抑制是因气相粘度小于液相粘度而引起的,如气相的粘度大于液相的粘度时,液相湍流则被强化、气泡减阻现象消失;当气泡体积分数较低时,气泡聚合和破碎物理现象对液相湍流和减阻率的影响很小。     

甲烷扩散火焰空间拟序结构三维运动研究

提出了基于高速立体视觉的湍流火焰三维运动特性分析方法,该方法首先利用双视角立体镜,使单个CCD靶面同时获得两个不同角度的火焰图像,通过标定得到摄像机参数,最后基于双目视觉理论的三维重建方法获得火焰内部漩涡结构的三维分布及其扩散速度.通过实验重建了甲烷预混火焰中漩涡结构的三维分布及速度场,计算结果表明,由于燃烧速度的径向分布不均匀以及气流受热的膨胀作用,使得火焰面在靠近边界处向外弯曲.     

纳米通道内纳米流体流动特性的分子动力学模拟

利用分子动力学方法对铜-氩纳米流体和基础流体在不同剪切速度下的纳米尺度的Couette流进行模拟计算。结果表明:在纳米尺度通道内,纳米流体流动过程中颗粒存在旋转运动和平移运动,从而加强湍流效果,强化传热并影响整个流动区域内的流动速度分布,造成纳米流体速度呈非线性分布。壁面和纳米颗粒表面都会形成一层排布更为规则的液体原子吸附层,吸附层内液体分子在流体流动过程中一直伴随着壁面和纳米颗粒进行运动,且吸附层具有"类固"特性,可以增强纳米流体的传热能力。     

Experimental investigation of turbulent boundary layer flow with surfactant additives using PIV and PDA

Drag reduction of turbulent water flow with surfactant (CTAC) additives was experimentally investigated. By using PIV and PDA measurements, the spatial velocity distribution of surfactant solution flow was clarified in a two‐dimensional water channel. With an increasing Reynolds number, it was found that drag reduction of surfactant solution flow is enhanced within the region of drag reduction. However, in the region of post drag reduction, the drag‐reducing coefficient approaches one without surfactant when Reynolds number is increased. In the near‐wall region, velocity profiles of the drag‐reducing fluid are similar to, but not the same as, the laminar profiles of the Newtonian fluid. When compared to the case of water flow without surfactant, the velocity contour lines of the drag‐reducing fluid run approximately parallel to the wall. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(2): 99–107, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20047     

High performance printed circuit heat exchanger

Three-dimensional thermal-hydraulic simulations have revealed a new flow channel configuration for Printed Circuit Heat Exchanger (PCHE) recuperators of a carbon dioxide gas turbine cycle. Simulation calculations were done changing the fin shape and angle parametrically to obtain an optimal flow channel configuration considering pressure drop and heat transfer performance. The new configuration has discontinuous fins with an S-shape, similar to a sine curve, in contrast to the conventional continuous zigzag configuration. The new configuration has one-fifth of the pressure drop reference to the conventional zigzag configuration with equal thermal-hydraulic performance. The pressure drop reduction is ascribed a superior uniform flow velocity profile in the flow area and elimination of reverse flows and eddies that occur around bend corners of zigzag flow channels in conventional PCHE.     

Field synergy equation for turbulent heat transfer and its application

A field synergy equation with a set of specified constraints for turbulent heat transfer developed based on the extremum entransy dissipation principle can be used to increase the field synergy between the time-averaged velocity and time-averaged temperature gradient fields over the entire fluid flow domain to optimize the heat transfer in turbulent flow. The solution of the field synergy equation gives the optimal flow field having the best field synergy for a given decrement of the mean kinetic energy, which maximizes the heat transfer. As an example, the field synergy analysis for turbulent heat transfer between parallel plates is presented. The analysis shows that a velocity field with small eddies near the boundary effectively enhances the heat transfer in turbulent flow especially when the eddy height which are perpendicular to the primary flow direction, are about half of the turbulent flow transition layer thickness. With the guide of this optimal velocity field, appropriate internal fins can be attached to the parallel plates to produce a velocity field close to the optimal one, so as to increase the field synergy and optimize the turbulent heat transfer.     

Reynolds number dependence of turbulence statistics in the wake of wind turbines

A wind tunnel experiment has been performed to quantify the Reynolds number dependence of turbulence statistics in the wake of a model wind turbine. A wind turbine was placed in a boundary layer flow developed over a smooth surface under thermally neutral conditions. Experiments considered Reynolds numbers on the basis of the turbine rotor diameter and the velocity at hub height, ranging from Re = 1.66 × 10⁴ to 1.73 × 10⁵. Results suggest that main flow statistics (mean velocity, turbulence intensity, kinematic shear stress and velocity skewness) become independent of Reynolds number starting from Re ≈ 9.3 × 10⁴. In general, stronger Reynolds number dependence was observed in the near wake region where the flow is strongly affected by the aerodynamics of the wind turbine blades. In contrast, in the far wake region, where the boundary layer flow starts to modulate the dynamics of the wake, main statistics showed weak Reynolds dependence. These results will allow us to extrapolate wind tunnel and computational fluid dynamic simulations, which often are conducted at lower Reynolds numbers, to full‐scale conditions. In particular, these findings motivates us to improve existing parameterizations for wind turbine wakes (e.g. velocity deficit, wake expansion, turbulence intensity) under neutral conditions and the predictive capabilities of atmospheric large eddy simulation models. Copyright © 2011 John Wiley & Sons, Ltd.     

Effects of non-isothermal heating on drag reduction in surfactant aqueous solution flow

In this study we investigate the control of flow characteristics and heat transfer of a drag-reducing dilute cationic surfactant solution in a channel, in order to develop a highly efficient heat exchanger. As has been reported by many authors, addition of certain polymers or surfactants reduces heat transfer in drag-reduced water flow. Therefore, other measures must be taken in order to compensate the reduction in heat transfer. Specifically, this study investigates the effects of non-isothermal heating on drag-reduced flow: experiments were conducted in order to study passive control for effecting the drag-reduction state by employing temperature-dependent physical properties and heat transfer augmentation by complex flow. In addition, velocity and temperature profiles were measured under the coexistence of turbulent and drag-reducing flow in order to clarify the effect of drag reduction. It was confirmed that the drag reduction state was changed and diminished due to the temperature rise near the wall, especially the condition in the region of 50<y⁺<100 greatly influence on drag reduction of pipe flow.     

Molecular dynamics simulation of electroosmotic flow in rough nanochannels

A three-dimensional molecular dynamics model of electroosmotic flow in rough nanochannels is developed and numerically analyzed to investigate the role of surface roughness on microscale electroosmotic flow. The water and ion concentration distributions in the fluid, velocity profiles in rough nanochannels are examined and compared with the corresponding smooth nanochannel. In addition, the role of roughness height on electroosmotic velocity and zeta potential is presented. The results indicate that the electroosmotic behavior in nanochannels is sensitive to the surface roughness. The plug-like velocity in nanochannel is reduced by the presence of surface roughness, which owes to the variation in electrical double layer and additional viscous dissipation for flow past rough surface. There is a layering distribution of water molecules and Cl⁻ ions in the near wall region, and some ions and molecules are confined at the concave region due to fluid/solid interaction. In addition, increases in roughness height lead to a smaller electroosmotic velocity in bulk solution and also a smaller zeta potential.     

NUMERICAL SIMULATION OF LAMINAR FORCED CONVECTION IN AN AIR-COOLED HORIZONTAL PRINTED CIRCUIT BOARD ASSEMBLY

A numerical solution of the steady-state forced convection for air flowing through a horizontally oriented simulated printed circuit board (PCB) assembly under laminar flow condition has been developed. The considered assembly consists of a channel formed by two parallel plates. The upper plate is thermally insulated, whereas the bottom plate is attached with uniformly spaced identical electrically heated square ribs perpendicular to the mean air flow. The bottom plate is used to simulate the PCB, and the ribs with heat generation are used to simulate the electronic components. A second-order upwind scheme is adopted in the calculation and a very fine mesh density is arranged near the obstacle and the channel surface to achieve higher calculation accuracy. Four Nusselt numbers (Nu) are of particular interest in this analysis: local distribution along the rib's surfaces, mean value for individual surfaces of the rib, overall obstacle mean value, and overall PCB mean value between the central lines of two obstacles. The effect of the obstacle size and the separation between two obstacles is discussed systematically.     

Unsteady mixed convection flow of a micropolar fluid near the stagnation point on a vertical surface

The unsteady mixed convection boundary-layer flow of a micropolar fluid near the region of the stagnation point on a double-infinite vertical flat plate is studied. It is assumed that the unsteadiness is caused by the impulsive motion of the free stream velocity and by sudden increase or sudden decrease in the surface temperature from the uniform ambient temperature. The problem is reduced to a system of non-dimensional partial differential equations, which is solved numerically using the Keller-box method. This method may present well-behaved solutions for the transient (small time) solution and those of the steady-state flow (large time) solution. It was found that there is a smooth transition from the small-time solution (initial unsteady-state flow) to the large-time solution (final steady-state flow). Further, it is shown that for both assisting and opposing cases and a fixed value of the Prandtl number, the reduced steady-state skin friction and the steady-state heat transfer from the wall (or Nusselt number) decrease with the increase of the material parameter. On the other hand, it is shown that with the increase of the Prandtl number and a fixed value of the material parameter, the reduced steady-state skin friction decreases when the flow is assisting and it increases when the flow is opposing.     

Numerical study on laminar convection heat transfer in a rectangular channel with longitudinal vortex generator. Part A: Verification of field synergy principle

This study presents numerical computation results on laminar convection heat transfer in a rectangular channel with a pair of rectangular winglets longitudinal vortex generator punched out from the lower wall of the channel. The effect of the punched holes and the thickness of the rectangular winglet pair to the fluid flow and heat transfer are numerically studied. It is found that the case with punched holes has more heat transfer enhancement in the region near to the vortex generator and lower average flow frictional coefficient compared with the case without punched holes. The thickness of rectangular winglet can cause less heat transfer enhancement in the region near to the vortex generator and almost has no significant effect on the total pressure drop of the channel. The effects of Reynolds number (from 800 to 3000), the attack angle of vortex generator (15°, 30°, 45°, 60° and 90°) were examined. The numerical results were analyzed from the viewpoint of field synergy principle. It was found that the essence of heat transfer enhancement by longitudinal vortex can be explained very well by the field synergy principle, i.e., when the second flow generated by vortex generators results in the reduction of the intersection angle between the velocity and fluid temperature gradient, the heat transfer in the present channels will be enhanced. Longitudinal vortices (LVs) improve the synergy between velocity and temperature field not only in the region near LVG but also in the large downstream region of longitudinal vortex generator. So LVs enable to enhance the global heat transfer of channel. Transverse vortices (TVs) only improve the synergy in the region near VG. So TVs can only enhance the local heat transfer of channel.     

桥梁工程跨堤布置与防洪影响研究

为了解桥梁工程跨堤布置对两岸防洪堤及河道防洪的影响,基于概化的数值渠道,采用MIKE21二维水流数学模型,分别从桥墩近堤距离、桥墩轴线布置及桥墩墩型设计三方面设定不同的河道参数及墩堤布置情况,模拟分析了近堤水流流速和墩堤间局部流场变化情况,揭示了不同情形下近堤流速及流场变化的规律,研究了其对河道防洪的影响。结果表明,桥梁跨堤、桥墩近岸布置时,增大近堤距离、减小桥墩轴线与河道岸线夹角和采用单一矩形壁式墩型均可减小对河道岸坡稳定及防洪安全的不利影响。