锌锅中低Pr流体混合对流的数值模拟

以带钢连续热镀锌生产工艺为背景,对抽象出的低Pr数流体混合对流流动和换热模型进行了数值模拟,给出了在不同Re、Ra及Ri时的流场和温度场.数值结果表明,当Re、Ra都不等于0时,在所考虑的参数范围内,流动和换热受自然对流和强制对流两种机理控制.Re不变,增大Ri,自然对流作用加强,并且当Ri增加到一定值时,流动和换热发生振荡.所给出的速度相图显示,对应不同的Re、Ra及Ri,流动和换热会出现稳态解、周期性振荡解和混沌.     

Microfluidic valves for flow control at low Reynolds numbers

Fluidics is a technology of generating and controlling fluid flows — preferably without the action of mechanical moving components. Microfluidics perform these tasks in small, typically micronsized structures. Essential part of almost all microfluidic systems are flow control valves. The basic problem is the low Reynolds numberRe: inertial effects used in large-scale fluidics are too small relative to viscous dissipation. New approaches, such as pressure or electrokinetic driving are required. In the subdynamic, viscosity dominated flow regime, Re ceases to be of importance and for pressure-driven valves a new characterisation number was to be introduced. An example of a diverter valve, developed by the author, is described and the meaning of the new dimensionless parameter is demonstrated.     

Velocimetry microsensors driven by linearly polarized optical tweezers

We report on a new type of single-point velocimetry microsensor that can be positioned in microfluidic devices by use of optical tweezers. The flag-shaped microsensor is readily made by a low-cost two-photon polymerization technique. At rest the linearly polarized optical tweezer traps the microsensor at the focal point, and the flag-plate gets aligned in the polarization direction. Under a fluid flow, the plate rotates to an equilibrium angle that is used to measure the fluid velocity with a micrometer-size spatial resolution. Experimental results are in good agreement with theoretical calculations of optical and hydrodynamic torques on such a flag-shaped microsensor.     

高超临界雷诺数区间内二维圆柱绕流的实测研究

实测强风工况下高度167 m的徐州彭城电厂冷却塔的表面风荷载,并归纳历史上其他研究人员给出的实测结果,以丰富高超临界雷诺数(Re)区间二维圆柱绕流的试验成果.在低湍流度均匀流场和高湍流度大气边界层流场中分别开展4种风速8类粗糙度条件下的冷却塔刚性模型测压风洞试验,通过对比低雷诺数(Re=2.1×10~5—4.19×10~5)条件下的风洞试验结果和高雷诺数(Re=5.4×10~7—1×10~8)条件下的现场实测结果研究各种静动态绕流特征随雷诺数的变化规律,重点考察雷诺数无关现象的产生条件.研究结果表明,对于物表相对粗糙度在0.01以上的圆柱绕流,雷诺数不相关现象存在于很宽的雷诺数范围(2×10~5Re1×10~8)内;增大来流湍流度亦能引起的雷诺数无关现象,但此时该现象可能仅存在于一个较窄的低雷诺数范围内.     

Shear flow pumping in open micro- and nanofluidic systems

Based on a novel interplay of wetting adhesion and concurrent flow concepts, we propose to drive open microfluidic systems by shear in a covering fluid layer, e.g., oil covering water-filled chemical channels. The advantages are simpler forcing and prevention of evaporation of volatile components. We calculate the expected throughput for straight channels and show that devices can be built with off-the-shelf technology. Molecular dynamics simulations suggest that this concept is scalable down to the nanoscale.     

Dynamic pattern formation in a vesicle-generating microfluidic device

Spatiotemporal pattern formation occurs in a variety of nonequilibrium physical and chemical systems. Here we show that a microfluidic device designed to produce reverse micelles can generate complex, ordered patterns as it is continuously operated far from thermodynamic equilibrium. Flow in a microfluidic system is usually simple-viscous effects dominate and the low Reynolds number leads to laminar flow. Self-assembly of the vesicles into patterns depends on channel geometry and relative fluid pressures, enabling the production of motifs ranging from monodisperse droplets to helices and ribbons.     

A continuum model for the flow of thin liquid films over intermittently chemically patterned surfaces

It is known from both experiments and molecular dynamics simulations that chemically patterning a solid surface has an effect on the flow of an adjacent liquid. This fact is in stark contrast with predictions of classical fluid mechanics where the no-slip boundary condition is insensitive to the chemistry of the solid substrate. It has been shown that the influence on the flow caused by a steep change in the wettability of the solid substrate can be described in the framework of continuum mechanics using the interface formation theory. The present work extends this study to the case of intermittent patterning. Results show that variations in wettability of the substrate can significantly affect the flow, especially of thin films, which may have applications to the design of microfluidic devices.     

碳富勒烯在水中的运动阻力特性

采用分子动力学(MD)方法,模拟了Cn(n=60,180,240,540)以及C60@C240富勒烯在水中的速度衰减过程,得到了它们的阻力一速度与阻力,雷诺(Re)数关系曲线;进而将MD模拟的阻力结果和低Re数宏观流体力学中球的Stokes阻力解进行了对比,研究表明,低Re数下,富勒烯在水中的阻力要比传统流体力学中球的Stoke解高两个数量级;富勒烯在水中的阻力可通过Stokes解乘以(110.38-1.48/Re)近似估算.     

Optical manipulation of microscale fluid flow

A novel optical method is used both to probe and to control dynamics in experiments on the spreading of microscale liquid films over solid substrates. The flow is manipulated by thermally induced surface-tension gradients that are regulated by controlling the absorption of light in the substrate. This approach permits, for the first time, the measurement of the dispersion relation for the well-known contact line instability; the measurements are compared with theoretical predictions from the slip model for spreading films. The experiments also demonstrate the use of feedback control to suppress instability. These results show that optical control can provide dynamically reconfigurable manipulations of fluid flow, thereby suggesting a general approach for constructing reprogrammable microfluidic devices.     

非牛顿流体在周期性渐扩渐缩通道内层流流动与换热的实验研究

本文用实验方法研究了浓度为１５００ｗｐｐｍ的聚丙烯酰胺（ＰＡＭ）溶液在周期性渐扩渐缩通道内层流流动与换热问题。研究表明,大约经过８个周期后流动进入充分发展阶段,而换热则要经过大约２０个周期后才能进入充分发展阶段。在充分发展阶段,阻力系数和换热Ｎｕｓｓｅｌｔ数随广义 Ｒｅｙｎｏｌｄｓ数的变化关系式分别为： ｆ＝ １３５ × Ｒｅ＊－０．６３２和 Ｎｕ＝ １０．５ × Ｒｅ＊０．５９８。     

微流控制备单分散微米级SiO2微球

采用软模板法制备出了聚二甲基硅氧烷微流控装置。利用该装置讨论了正硅酸乙酯和氨水的用量分别对反应体系凝胶化时间的影响,确定了制备SiO2微球的优化反应体系,即二甲基乙酰胺、正硅酸乙酯和氨水的体积比为8∶4∶1,实验所需的反应温度为60 ℃。实验发现:在微流体通道中,分散相的流速越大,粒径越大;连续相流速越大,粒径越小。因此,通过控制微流控装置中分散相和连续相的流速制备了粒径40~220 m的单分散SiO2微球,并对其形貌进行表征。光学显微镜和粒径分析均表明所制备的SiO2微球球形度高,单分散性好。     

Probing single cells using flow in microfluidic devices

Enabling fluids to be manipulated on the micron-scale, microfluidic technologies have facilitated major advances in how we study cells. In this review, we highlight key developments in how flow in microfluidic devices is exploited to investigate the behavior of individual cells, from trapping and positioning single cells to probing cell deformability. Exploiting the properties of fluids and flow patterns in microchannels makes it possible to study large populations of single cells at micron-length scales with increased throughput and efficiency.     

Hydrodynamic interaction of blood cells with micro-rough surfaces in a shear flow

In this paper, we address hydrodynamic interactions of individual red blood cells (erythrocytes) with micro-rough surfaces in a shear flow of a viscous fluid using computer simulations. We study the influence of the microrelief on the dynamics of cell movement. It has been shown that periodic microrelief with typical length scale comparable with the size of an erythrocyte may cause an increase of the repulsive hydrodynamic force directed outwards on the micro-rough wall. The results can be used in the design of microfluidic devices that are intended to operate with whole blood samples.     

Microfluidic turn-down valve

Flow visualisation and numerical flowfield computations were used in the development of a microfluidic valve for control of reaction product in microreactors. The flow rate is modulated by headon collision with an opposed flow of control fluid. This principle makes possible unique capability of turn-down (rather than diverting) control at very low operating Reynolds numbers (- design valueRe=75). The planar layout with shallow constant-depth cavities is suitable for microfabrication, especially by one-sided etching.     

纳米尺度圆柱绕流现象的分子动力学模拟研究

本文采用分子动力学模拟方法,利用Lennard-Jones(L-J)势能模型,模拟了Re为28时Ar流体流过Pt纳米圆柱的绕流现象.采用小步长时均值作为涡的瞬时值的方法,在纳米尺度、纳秒量级下得到了涡的周期性产生、组合、发展和脱落现象;在大步长时均条件下,得到了稳定的对称涡,表现出了绕流现象在不同时间范围内的不同特征.绕流过程还体现了流体的密度变化,圆柱上游密度大于下游密度、对称的两侧离轴线越远密度越大.结果表明,分子动力学模拟方法可以有效地细节刻画圆柱绕流现象.     

Performing chemical reactions in virtual capillary of surface tension-confined microfluidic devices

In this paper we report a new method of fabrication of surface tensionconfined microfluidic devices on glass. We have also successfully carried out some well-known chemical reactions in these fluidic channels to demonstrate the usefulness of these wall-less microchannels. The confined flow path of liquid was achieved on the basis of extreme differences in hydrophobic and hydrophilic characters of the surface. The flow paths were fabricated by making parallel lines using permanent marker pen ink or other polymer on glass surfaces. Two mirror image patterned glass plates were then sandwiched one on top of the other, separated by a thin gap-created using a spacer. The aqueous liquid moves between the surfaces by capillary forces, confined to the hydrophilic areas without wetting the hydrophobic lines, achieving liquid confinement without physical side-walls. We have shown that the microfluidic devices designed in such a way can be very useful due to their simplicity and low fabrication cost. More importantly, we have also demonstrated that the minimum requirement of such a working device is a hydrophilic line surrounded by hydrophobic environment, two walls of which are constituted of air and the rest is made of a hydrophobic surface.     

“Fluid plug” microfluidic valve for low Reynolds number fluid flow selector units

A novel no-moving-part valve was developed mainly for applications in microchemistry, where Reynolds numbers tend to be extremely low. The valve of very small size and working with viscous biological fluids was designed for operating at Re values between 10 and 100. In this range, the inertial jet flow effects used in standard fluidics could not be relied upon and a qualitatively new operating principle was introduced: the control fluid forms a “plug” at the entrance of the collector, precluding an entry of the main fluid flow. In particular, an array of new valves was used in a selector unit to allow a single reagent flow into a reactor from several available sources. The flow visualization validated the assumptions on which the design is based and compared well with numerical flowfield computations.     

Multipoint holographic optical velocimetry in microfluidic systems

We show how holographic optical trapping can be used for the multipoint measurement of fluid flow in microscopic geometries. An array of microprobes can be simultaneously trapped and used to map out the fluid flow in a microfluidic device. The optical traps are alternately turned on and off such that the probe particles are displaced by the flow of the surrounding fluid and then retrapped. The particles' displacements are monitored by digital video microscopy and directly converted into velocity field values. This technique enables the measurement of a two-dimensional flow field at points arbitrarily distributed in a three-dimensional volume. The validity of the technique is demonstrated for the case of the flow around a spinning sphere and the flow at the outlet of a microchannel.     

Mode-switching: A new technique for electronically varying the agglomeration position in an acoustic particle manipulator

Acoustic radiation forces offer a means of manipulating particles within a fluid. Much interest in recent years has focussed on the use of radiation forces in microfluidic (or “lab on a chip”) devices. Such devices are well matched to the use of ultrasonic standing waves in which the resonant dimensions of the chamber are smaller than the ultrasonic wavelength in use. However, such devices have typically been limited to moving particles to one or two predetermined planes, whose positions are determined by acoustic pressure nodes/anti-nodes set up in the ultrasonic standing wave. In most cases devices have been designed to move particles to either the centre or (more recently) the side of a flow channel using ultrasonic frequencies that produce a half or quarter wavelength over the channel, respectively.It is demonstrated here that by rapidly switching back and forth between half and quarter wavelength frequencies - mode-switching - a new agglomeration position is established that permits beads to be brought to any arbitrary point between the half and quarter-wave nodes. This new agglomeration position is effectively a position of stable equilibrium. This has many potential applications, particularly in cell sorting and manipulation. It should also enable precise control of agglomeration position to be maintained regardless of manufacturing tolerances, temperature variations, fluid medium characteristics and particle concentration.