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Interferometric techniques, which use information encoded in the Doppler burst resulting from the interaction of a particle with a two-beam interference pattern have become practical diagnostic tools for the analysis of complex, two-phase flows. In the original application of these techniques to sprays, the signal visibility, a measure of the modulation of the scattered light intensity, was analyzed to determine the size of the particle. Light collection was made on-axis, using the diffracted portion of the scattered light. The particle shape was a secondary consideration, and its index of refraction was not required in the analysis. On-axis collection, however, restricted application of the technique to flows of impractically low particle number density. Off-axis collection of the refracted component of the scattered light represented a significant refinement to the original method. However, with this approach, the particle to be sized had to be spherical and its index of refraction had to be known a priori. With this refinement, application to practical number density flows became possible, although the collection efficiency remained adversely affected by high particle loading. The final major development came when the basis of the interferometric measurement changed from visibility to phase shift. The phase shift approach has the advantages of the off-axis visibility technique and also has a broader sizing dynamic range and is less sensitive to the droplet number density. The technique has so markedly improved droplet measurements that extensions of the technique to measure liquid mass flux and local droplet number density are being evaluated. This paper traces the technical evolution of the interferometric particle sizing. It also examines the capabilities of the current approach in making two-phase flow measurements in complex flows.
particle sizing, interferometry, phase Doppler, laser Doppler velocimeter, droplets, sprays, atomization
Research engineer, Aero Propulsion and Power Laboratory, Air Force Wright Research and Development Center, Wright-Patterson AFB, OH