Anemometry optical

Measurement of the velocity of a large particle. The investigation of the turbulence characteristics in the liquid phase of a bubbly flow has generated detailed studies on the use of thermal anemometry and optical anemometry in gas-liquid two-phase flows. These techniques have been proved to be accurate and reliable for the measurement of the instantaneous liquid velocity in bubble flow. However, the velocity of the gas bubbles—or, more precisely, the speed of displacement of the gas-liquid interfaces—is still an active research area. Three techniques that have been proposed to achieve such measurement were reviewed by Delhaye (1986), as discussed in the following paragraphs. 

Optical techniques like laser doppler anemometry (LDA) can be used to obtain knowledge about the local solids hydrodynamics in CFB units close to walls at low solids fluxes [14]. Such LDA measurements of FCC particles in a riser in circulating fluidized bed have been reported by [119, 120]. 

Rathbone et al. (1989) used a similar probe to determine particle velocity parallel to the surface. The difference is that the central fiber is the sensor of the Fiber Optic Doppler Anemometry (FODA), and it transmits light from a 5 mW He-Ne laser. Particles illuminated by the central fiber scatter light to the surrounding fibers as they passed the probe. When the particles passed over two fibers in succession, the transit time at cross correlation maximum can be obtained. In principle, by correlating the strongest signals from the fibers, it should be possible to determine the direction or different components of particle velocity. However, the measurement was carried out in a two-dimensional fluidized bed, and relatively few data were obtained. 

The dual beam configuration of LDA is most widely used today, where the Doppler difference frequency is directly measured and the receiving optics may be placed at an arbitrary position with respect to the transmitting beams. Laser-Doppler anemometry has been first applied to measurements of mean velocities and turbulence properties in single phase flows. In this case small particles, which follow the flow and the turbulent fluctuations, need to be present in the flow or must be added to it (i.e. seeding the flow with a tracer). The principles of LDA are, for example, described in detail by Durrani and Greated (1977), Durst et al. (1981), and Durst et al. (1987). 

The principle of phase-Doppler anemometry (PDA) relies on the Doppler difference method used for conventional laser-Doppler anemometry and was first introduced by Durst and Zare (1975). By using an extended receiving optical system with two or more photodetectors it is possible to measure simultaneously size and velocity of spherical particles. For obtaining the particle size the phase shift of the light scattered by refraction or reflection from the two intersecting laser beams is used. 

Known measuring techniques comprise local physical methods (hydrophones, thermoacoustic sensors, aluminum foil erosion, optical sound pressure and velocity sensors, radiation pressure scale, laser-Doppler anemometry), local chemical methods (Weissler reaction, chemiluminescence, electrochemical sensors), global chemical methods (model reactions), and sonoluminiscence (single-bubble sonoluminescence SBSL, multibubble sonoluminescence MBSL). 

Local instantaneous liquid velocity measurements in bioreactors that can quantify turbulence statistics are challenging using conventional laser-based techniques because optical access is critical for effective signal acquisition. Laser Doppler anemometry (LDA) and PIV have been used to determine local liquid velocities within multiphase flows. Reviews of LDA and PIV with applications to multiphase flows have appeared in the literature (Boyer et al., 2002 Chaouki et al., 1997 Cheremisinoff, 1986). 

Modified Laser-Doppler Anemometry is another alternative method to monitor liquid wicking in a 2D fabric plane based on the Doppler principle. When a laser beam is passed through a flowing liquid, light is scattered by the particles suspended in the liquid. The scattered light is subject to a frequency shift and contains information about the velocity of the particles, which can then be examined by electro-optical techniques. This measurement, therefore, requires the flow medium to be partly transparent and containing particles, that scatter light used this method, to obtain the local velocity of liquid flow in a nonwoven fabric. 

ABSTRACT. Characteristics and fluid dynamics of gas phase recirculation in a novel Riser Simulator Reactor have been investigated using constant temperature hot wire anemometry. In situ concentration and velocity measurements enabled to evaluate the mixing time and the inner recirculation ratio of the gas phase. In addition, fibre optic techniques allowed to characterize the degree of fluidization of the catalyst particles and the effect of gas phase density changes. By combining the anemometry and the fibre optic techniques, mixing patterns in the Riser Simulator have been evaluated. The importance of the study can be realized in the context of the potential use of the Riser Simulator for gas-solid reaction kinetics. 

Optical diagnostic techniques are desirable for fluid flow measurements due to their nonintrusive nature. Soon after the invention of the laser in the 1960s, the technique of laser Doppler anemometry (LDA) was developed. During the last three decades, the LDA technique has witnessed significant advancements. Three-component fiber optic-based LDA systems with frequency-domain signal... 

However difficulties arise in applying the phase-Doppler anemometry to process fluids with an optical absorption that lies between zero and very high. In addition these fluids are often inhomogeneous as in the case of metal flakes in paints or dissolved instant coffee in spray drying processes. 

Spray properties are mostly determined with optical measurement techniques. For the analysis of the droplet diameter Shadowgraphic methods, laser diffraction or Phase Doppler Anemometry (PDA) have been used elsewhere [1, 2, 11, 18]. Droplet velocities can be measured with Shadowgraphy, Particle Image Velocimetry (PIV), or PDA [1, 6, 19]. The determination of the spray temperature is possible with Global Rainbow Thermometry (GRT), Planar Laser Induced Fluorescence (PLIF), and Differential Infrared Thermography (DIT) [20-22]. 

During the last four-five decades, the laser-Doppler anemometry (LDA) has become a commonly used experimental technique to measure the instantaneous velocity of seeded single phase flows, and dispersed two-phase flows of very low concentration. A major reason is that LDA is a non-invasive optical technique and does not disturb the flow. Moreover, the LDA system has a high spatial resolution with a fast dynamic response and range. 

Fig. 13.4 The standard optical arrangement for the Phase Doppler Anemometry (PDA) [13, 57]. The incident beams correspond to the same optical arrangement as used in the LDA technique. The two detectors are positioned out of the plane of the incident beams at an angle (j>r (named the off-axis angle). The detectors are also placed symmetric out of the x — y plane by the angles V>(= ip, tl>2) (called the elevation angles). The intersection angle of the two beams is denoted by 0 (referred to as the beam crossing angle). The figure is drawn based on a similar sketch from Albrecht et al. [5]...

Our understanding of the hydrodynamics of multiphase flows has progressed substantially in the recent three decades, thanks to the development of advanced experimental techniques, particularly laser Doppler anemometry (LDA), particle image velocimetry (PIV), computer-automated radioactive particle tracking (CARPT), and optical bubble probes. In addition, computational fluid dynamics (CFD) simulations allow for inner views in two-phase process equipment. 

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