Special LDA-Systems for Two-Phase Flow Studies

Lens with mask Colo, fife, (green) 

The information provided by the intensity of the diffractively scattered light is an equivalent diameter De obtained from the projected area of the particle Sp  

The relation between equivalent diameter and scattering intensity can be determined from Fraunhofer diffraction theory. The intensity at a point on the receiving plane diffracted by a circular aperture is determined from (Hecht and Zajac 1982)  

The authors concluded from their results that the size measurement was not very sensitive with respect to particle orientation in the measurement volume, due to the concentric receiving aperture. Also for transparent particles, where refracted light will be also collected, reasonably accurate measurements could be obtained. One should however keep in mind that intensity measurements are very sensitive to variations in laser power, photodetector sensitivity, and contamination of windows. Moreover, variations of the particle concentration within a cross-section of a flow field will result in variations of light absorption for the incident and scattered light, depending on the measurement location (see for example Kliafas et al. (1990) for a detailed analysis of the turbidity). Therefore, the application of this method is again limited to very dilute two-phase flows. 

As a particle passes through the measurement volume, the magnified image sweeps across the detector in the direction of particle motion. Hence, the output signal of those elements of the linear photo-diode array exposed to the shadow 

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