Laser Doppler phase shift, particle sizing

The sizing methods involve both classical and modem instrumentations, based on a broad spectrum of physical principles. The typical measuring systems may be classified according to their operation mechanisms, which include mechanical (sieving), optical and electronic (microscopy, laser Doppler phase shift, Fraunhofer diffraction, transmission electron miscroscopy [TEM], and scanning electron microscopy [SEM]), dynamic (sedimentation), and physical and chemical (gas adsorption) principles. The methods to be introduced later are briefly summarized in Table 1.2. A more complete list of particle sizing methods is given by Svarovsky (1990). 

A mean particle size can be measured using laser Doppler phase shift. Two laser beams cross in a measurement volume and scattered light is measured in addition to the usual velocity measurement obtained from the frequency shift, the phase shift can provide the mean particle size. 

When a spherical particle enters the crossing volume of two laser beams, a Doppler effect occurs not only in frequency shift but also in phase shift of the scattered light. The frequency shift yields the velocity of the sphere, whereas the phase shift gives the particle size. The phase Doppler principle has been employed to measure the size and size distributions of spheres in addition to the particle velocity. The phase Doppler principle was first reported by Durst and Zare (1975) and became a viable measurement tool one decade later [Bachalo and Houser, 1984]. 

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. 

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)... 

Phase Doppler anemometry or phase Doppler analysis was proposed two decades ago, based on the then well-established flow measurement technology-laser Doppler velocimetry [110]. PDA is widely used in sizing spherical and homogeneous particles, such as liquid sprays, aerosols, air bubbles in liquid, or other spherical particles. When single particles move through the intersection of at least two focused laser beams, they scatter light from each beam with different individual Doppler frequency shifts. Multiple detectors are placed at... 

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