Size measurements of droplets and bubbles by advanced interferometric laser imaging technique

This paper describes an advanced size and spatial distribution measurement technique for small spherical droplets and bubbles in two-phase flows. In the field of spray analysis, various kinds of measurement techniques have been proposed. Interferometric laser imaging for droplet sizing (ILIDS) is the method that can provide an instantaneous spatial distribution of droplet size. The number of fringes on the projected images by the ILIDS technique is correlated to their individual particle sizes. The measurement accuracy depends upon the image processing technique for estimating the size of the circular image and the fringe spacing. The relation between the fringe number and the particle diameter was derived by noting the angular phase difference of the external reflection and the direct refraction. Conventional ILIDS techniques, which observed the circular image with fringes on a film, have difficulties in discriminating the fringe images due to the overlap between them. Moreover, enlargement of the defocused image for measuring the larger particle caused the complicated overlapping. The objective of the present study is to describe the advanced ILIDS image acquisition optics and the accelerated data processing by employing a high resolution CCD camera and automated computing system. In the technique presented, the circular fringes are optically compressed into linear images that are horizontally defocused and vertically focused. The present partial compression technique, which is the integration of the vertical information of the image, reduced the noise involved and consequently the signal to noise ratio of the interference signals become much better than that of the conventional technique. The linear interferograms were easily detected by scanning the horizontal pixels of the recorded image. The signal processing procedure to determine the particle diameter needs only one-dimensional information for calculating the fringe spacing by discrete Fourier transforms. The present ILIDS technique was examined for the planar measurement of size and location of spray droplets, and the size transition of CO₂ bubbles by absorption in water.