Characteristics of shear-driven micro flow between stationary and rotating disks 静止-回転円板間のせん断駆動マイクロ流れの特性
Characteristics of shear-driven micro flow between stationary and rotating disks
Recently, quantitative flow visualization has become an important tool to investigate three-dimensional complex flow structures in microfluidic. The development of laser, computer and digital image processing techniques made it possible to extract velocity field information from visualized flow images of tracer particles. Particle image velocimetry (PIV) / particle tracking velocimetry (PTV) method has become one of the most useful flow diagnostic technologies in the modern history of fluid mechanics. The particle based velocimetry techniques measure the whole velocity field information in a plane by dividing that is placements Δx and Δy of tracer particles with the time interval Δt during which the particles were displaced. Since the flow velocity is inferred from the particle displacement, it is also important to select proper tracer particles that follow the flow motion accurately without changing the flow properties. These methods have been accepted as a reliable and powerful velocity field measurement technique. The PIV method in a strict sense provides the representative velocity vector averaged over each interrogation window. As the PTV method can identify individual particles and track them from image to image, the PTV method has higher spatial resolution than the PIV method for low-particle-density flows such as micro-scale flows in microfluidics. We develop two/three dimensional micro-PIV/PTV that can be applied to micro-scale passage. In chapter 1, the detailed understanding of the flow inside the micro-scale passage is very important for their optimum design and active/passive control of flow due to rapid development in MEMS technology. In order to observe the flow phenomena in microfluidics, a suitable experimental technique that can resolve the temporal and spatial resolutions of the given micro-scale flow is definitely needed so that two/three dimensional PIV techniques are introduced. In chapter 2, since the first micro-PIV experiment was carried out by Santiago et al (1998), micro-PIV measurement technique has been developed rapidly. This chapter is to provide the theoretical and technical methods to understand a micro-PIV system and technical technique. In chapter 3, the numerical simulation is carried out by using the commercial fluid analysis software STAR-CD. It solves the three-dimensional, incompressible, laminar Navier-Stokes equations by finite volume method. The SIMPLE method is chosen for the pressure-velocity coupling and the algebraic multi-grid solver is used for the velocity and pressure corrections. These results for flow field are shown by the purpose of comparing with two/three-velocity components of micro-PIV and stereoscopic micro-PTV. In chapter 4, digital image filtering is introduced in order to improve measure accuracy of near-wall flow from the present micro-PIV technique. Frequency sampling method is used to design a simple, digital, high-pass filter. In chapter 5, the micro rotating flow between a pair of rotating and stationary disks, whose separation is 500μm, was studied experimentally and numerically with an objective to clarify the characteristics of the basic flow found in rotation-shearing chemical reactors. The micro-PIV technique was used to measure two-component velocities in the liquid layer. The commercial CFD software was used to provide data to compare and validate the micro-PIV results. As for the overall velocity profiles in the liquid layer, the micro-PIV and the CFD results are in fair agreement; both are showing (1) the linear increase of tangential and radial velocities with radial position, and (2) the presence of a secondary flow that consists of an outward flow near the rotating disk and an inward flow near the stationary disk. This secondary flow is strengthened with the rotational speed and is responsible for the deviation of tangential velocity component from its linear profile in the direction of the thickness. Measurement of near-wall from the present micro-PIV technique is appreciably improved by the use of a simple, digital, high-pass filtering technique that is applied to the acquired particle images. It is shown that the cut-off frequency of 0.1~0.15 pixel-1 (or cut-off wavelength of 6.7~10.0 pixels) works well with this technique. This cutoff wavelength is two to three times larger than the typical diameters of the in-focus particle images that are acquired in this study. It is demonstrated that the micro-PIV measurement with the high-pass filtering technique can provide detailed information about the flow field in the thin liquid layer between the rotating and stationary disks. In chapter 6, a stereoscopic micro-PTV technique was shown using a single camera with a stereo optical attachment. The attachment developed here was shown to have a higher capability of fine optical adjustment than the previous one proposed by the authors. This stereoscopic micro-PTV technique was validated through a threedimensional measurement of a rotating flow in a thin liquid layer between a rotating disk and a stationary disk. This rotating liquid layer is 10mm in diameter and the gap between the disks is 500μm. The rotational speed examined is 500rpm. It is shown that the effective depth-of-field of the present technique is 300μm for the imaging optics using a 5× objective (NA=0.14). The measured velocities compare reasonably well with the previous results obtained with a conventional 2D-2C PIV technique. In chapter 7, in these rotating systems, the problem of friction torque by shear stress, power loss, and of heat transfer is strongly related to inside flowing induced by the rotating disk such as the circulation and secondary flows. These induced flows depend on the geometries of the stationary and rotating disks, for example, gap between stationary and rotating disk as well as radius and angle of rotation disk and so on. The approximate analytical equation was inferred as the function of z/H, r/R, and ω.
横浜国立大学, 平成23年12月31日, 博士(工学), 甲第1422号