Particle size imaging velocimetry

Wide Field Particle Sizing and Velocimetry is a new technique developed by Everest and Atreya [11] and Putorti et al. [12] to photographically determine the size and spatial distributions of particles in a large flow field. Here, laser induced fluorescence from a tracer dye in the droplets is imaged by a photographic camera at high resolution and analyzed to determine its size... 

PIV has also been applied to gas-liquid systems (Reese and Fan, 1994 Lin et al, 1996) to study the flow structure in bubble columns. A specific complication here is caused by the presence of the gas bubbles. On the basis of a prior knowledge of the size distribution of the tracer particles and the gas bubbles it is possible to discriminate bubbles from particles and thus phase-specific postprocessing of the images can be undertaken whereby both the flow pattern of the bubbles and the liquid in principle can be obtained. Particle image velocimetry has also been applied (Chen and Fan, 1992) to study the flow structure in 3D gas-liquid-solid fluidized beds. 

Herpfer and Jeng [146] used streak particle imaging velocimetry for planar measurement of droplet velocities and sizes. They encountered problems due to out-of-focus effects of the PIV imaging procedure. 

In most electroosmotic flows in microchannels, the flow rates are very small (e.g., 0.1 pL/min.) and the size of the microchannels is very small (e.g., 10 100 jm), it is extremely difficult to measure directly the flow rate or velocity of the electroosmotic flow in microchannels. To study liquid flow in microchannels, various microflow visualization methods have evolved. Micro particle image velocimetry (microPIV) is a method that was adapted from well-developed PIV techniques for flows in macro-sized systems [18-22]. In the microPIV technique, the fluid motion is inferred from the motion of sub-micron tracer particles. To eliminate the effect of Brownian motion, temporal or spatial averaging must be employed. Particle affinities for other particles, channel walls, and free surfaces must also be considered. In electrokinetic flows, the electrophoretic motion of the tracer particles (relative to the bulk flow) is an additional consideration that must be taken. These are the disadvantages of the microPIV technique. 

Various flow visualisation techniques have been utilised to obtain experimental results from local gas hold-ups and bubble size distributions (BSD) in a gas-liquid mixed tank. Particle Image Velocimetry (PIV), Phase Doppler Anemometry (PDA), Capillary suction probe (CSP), High-speed video imaging (HSVI) and Electrical Resistance Tomography (ERT) techniques have been applied. The applicability of various techniques is dependent on the location of the measurement, the physical properties of the gas-liquid flow, the gas hold-up and the size of the tank. 

Laser Doppler Velocimetry (LDV) (Joshi et al. 2001) and PDA (Schafer et al. 2000) are optical techniques that have been used to determine BSDs, gas hold-up and flow patterns. Detectors observe the Doppler shift and phase difference when bubbles pass through the volume of the intersection of two laser beams. Doppler effect is related to the velocities of bubbles and the phase difference is related to the sizes of bubbles. Particle Image Velocimetry (PIV)... 

A number of experimental measurement techniques are discussed, with a focus on noninvasive optical techniques such as particle image velocimetry and digital image analysis, as well as a number of academic numerical modeling tools such as discrete particle model and two-fluid model. Not only hydrodynamic aspects, such as the emergence of defluidized zones and solids circulation profile inversion, but also the effect on the bubble size distributions are discussed for wall-mounted membranes and horizontally immersed membranes. 

By confining the fluidized bed in one direction and using a translucent waU, visual access is restored so that the bed behavior can be studied fuUy and non-intrusively using optical techniques, such as particle image velocimetry (PIV) or digital image analysis (DIA), which are discussed in detail below. With these techniques, it is possible to obtain information on the instantaneous flow fields, but it remains difficult to translate the 2D results quantitatively to 3D. As a learning tool that allows to see and verify different aspects of the bed behavior (e.g., bubble size distribution, instantaneous particle fluxes) however, such techniques are unrivaled. The main focus of this chapter therefore lies on these optical techniques. 

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