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.
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Notes
But is vital for PIV because the image has to be captured in the ‘in focus’ condition.
Note that quantifying the degree of defocus in a given experiment is not straight forward since it is difficult to determine the distances I″ I′ and LI′ exactly.
Note that, this is an arbitrarily selected value and the corresponding distance in the experiment presented later is different.
This discrepancy, here related to the use of the geometric centre, arises even if we were able to use the ‘derived centre’ of the glare points/fringe pattern.
The location of the glare points and the corresponding fringe pattern in this case were misaligned in the vertical direction.
Another way of dealing with the problem of droplet centre discrepancy would be to have the ILIDS camera defocused during calibration: but then the detection of the “defocused crosses (calibration marks)” is more difficult. Also, this approach is based on an assumption that if the droplet centre and the centre of the “mark” coincide in the object plane initially, than after defocusing, the geometric centre of the “defocused mark” coincides with the geometric centre of the defocused droplet image as well. However this may or may not be true, because of the fact that the glare points are not symmetrically spaced around the droplet centre. Nevertheless the resulting error is likely to be small. Hence the approach of calibration before defocusing was preferred in this work which has the additional advantage of permitting the change of the degree of defocus, if required, during an experiment.
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Hardalupas, Y., Sahu, S., Taylor, A.M.K.P. et al. Simultaneous planar measurement of droplet velocity and size with gas phase velocities in a spray by combined ILIDS and PIV techniques. Exp Fluids 49, 417–434 (2010). https://doi.org/10.1007/s00348-009-0802-7
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DOI: https://doi.org/10.1007/s00348-009-0802-7