Phase Doppler anemometry optics

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. 

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]. 

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]...

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). 

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. 

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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