Improvements of the interferometric technique for simultaneous measurement of droplet size and velocity vector field and its application to a transient spray

Abstract

The paper describes a planar measurement technique for droplet size and velocity vector for applications to transient flow fields. Interferometric laser imaging for droplet sizing (ILIDS) is a novel measurement technique that gives an instantaneous spatial size distribution of droplets. The inter-fringe spacing of the ILIDS interferogram is correlated with individual particle sizes and the measurement accuracy depends upon the image processing technique for estimating the fringe spacing. The relationship between the diameter and fringe spacing was found by considering the angular phase difference of the external reflection and the direct refraction. Conventional ILIDS, which observes a circular image with fringes on a film, has difficulties in evaluating the fringe spacing accurately due to overlapping of the circular image at high concentration. The objective of the present study is to develop a simultaneous measurement technique of size and velocity vector of individual spherical particles by using anamorphic image acquisition optics that have different focal lengths along mutually perpendicular radii and accelerated data processing on a fully automated computing system. In the present technique, the circular image with fringes is optically compressed into linear images that are horizontally defocused and vertically focused keeping the information of the location and the size of droplets. The present "squeezing" technique can reduce the fringe overlapping and avoid complexity for the evaluation of fringe spacing. In the case of 100 droplets in a 1 M-pixel image plane, the particle validation ratio of the present technique is more than 95% in contrast with the validation ratio of the conventional technique that is less than 20%. Moreover, the vertical integration of the image can eliminate noise and consequently the signal-to-noise ratio of the interference signals becomes much higher than that of the conventional technique.