Simultaneous planar measurement of droplet velocity and size with gas phase velocities in a spray by combined ILIDS and PIV techniques

Abstract

A new approach for simultaneous planar measurement of droplet velocity and size with gas phase velocities is reported, which combines the out-of-focus imaging technique 'Interferometric Laser Imaging Droplet Sizing' (ILIDS) for planar simultaneous droplet size and velocity measurements with the in-focus technique 'Particle Image Velocimetry' (PIV) for gas velocity measurements in the vicinity of individual droplets. Discrimination between the gas phase seeding and the droplets is achieved in the PIV images by removing the glare points of focused droplet images, using the droplet position obtained through ILIDS processing. Combination of the two optical arrangements can result in a discrepancy in the location of the centre of a droplet, when imaging through ILIDS and PIV techniques, of up to about 1 mm which may lead to erroneous identification of the glare points from droplets on the PIV images. The magnitude of the discrepancy is a function of position of the droplet's image on the CCD array and the degree of defocus, but almost independent of droplet size.Specifically, it varies approximately linearly across the image along the direction corresponding to the direction of propagation of the laser sheet for a given defocus setting in ILIDS. The experimental finding is supported by a theoretical analysis, which was based on geometrical optics for a simple optical configuration that replicates the essential features of the optical system. The discrepancy in the location was measured using a monodisperse droplet generator and this was subtracted from the droplet centres identified in the ILIDS images of a polydisperse spray without 'seeding' particles. This reduced the discrepancy between PIV and ILIDS droplet centres from about 1 mm to about 0.1 mm and hence increased the probability of finding the corresponding fringe patterns on the ILIDS image and glare points on the PIV image. In conclusion, it is shown that the proposed combined method can discriminate between droplets and 'seeding' particles and is capable of two-phase measurements in polydisperse sprays.

Numerical evaluation of droplet sizing based on the ratio of fluorescent and scattered light intensities (LIF/Mie technique)

“…The Interferometric Laser Imaging for Droplet Sizing [10][11][12][13] technique or ILIDS illuminates a plane of the measured spray by a laser sheet and the scattered light is imaged by an out of focus camera. The defocused image of each droplet exhibits a fringe pattern and the spacing between the fringes is proportional to the size of each droplet in the image.…”

Interaction of droplet dispersion and evaporation in a polydispersed spray

“…Droplet size, velocity and number density with vapor mass fraction around droplets (Sahu et al 2014b) Quantification of droplet centre discrepancy in both horizontal and vertical directions (Hardalupas et al 2010) …”

“…For this purpose, the position of the droplet centres by ILIDS was corrected for 'centre discrepancy' originating because of the defocusing performed in the ILIDS technique (Hardalupas et al 2010). We adopt a method similar to Hardalupas et al (2010) to quantify and minimize the positioning error. Finally, droplet size/velocity from ILIDS were assigned to the position of the same droplets on the PLIF image.…”

Measuring droplet size distributions from overlapping interferometric particle images

“…The wavelet approach of Hardalupas et al 3 has inspired the current paper. As the size and shape of the particle disks is known a priori, we will use a correlation technique to locate the particle disks in an image.…”

“…2 This technique has been refined in a recent paper by Hardalupas et al, 3 who used a rectangular mask and a cylindrical lens, resulting in narrow, rectangular interference patterns. Although these lines contain the pertinent information for droplet sizing, their identification in images is more difficult than that of circular disks.…”