Time-Resolved Spray-Droplet Velocity and Size Measurements via Single Camera Laser Sheet Imaging and Planar DPIV

Abstract

This paper describes a first effort to investigate the feasibility of droplet size and shape characterization by direct laser sheet imaging using time resolved Digital Particle Image Velocimetry. A 60-degree conical, high-pressure spray generated a poly-dispersed droplet distribution. Measurements were preformed for seven planes parallel to the spray axis, and separated by 4mm. A CMOS camera recorded the DPIV images at sampling rate of 10 KHz. Advanced image processing techniques were employed to identify the droplets and individually resolve their velocity using a hybrid cross-correlation particle-tracking algorithm. Subsequently, the size distribution of each droplet was quantified using geometric optics theory to convert the droplet image information to the true droplet size. Finally, the entire volume of the spray velocity and size distributions was reconstructed in a time-averaged sense. The droplet sizes from our direct imaging DPIV system were validated using a Phase Doppler Particle Analyzer (PDPA). The calculated sizes from the direct imaging methodology were found to agree with the measured PDPA results for droplets images larger than the diffraction limited diameter. Resolution limitations introduced inaccuracy for smaller droplets. In addition, the shedding frequency of the spray ligament was observed to be on the order of 1KHz, demonstrating the feasibility of using a high speed, direct imaging system in the characterization of unsteady, liquid sheet breakup properties. This preliminary effort illustrates the potential of performing global time resolved velocity and size measurements using a simple DPIV configuration based on CMOS imaging technology.

Planar Mie Scattering Method for Measurement of Spray Characteristics

“…Things are even more complicated when a wider range of particles is present within an image; differences in calculation methods where the droplet is imaged onto only a few pixels are much different than methods used when the droplets cover larger areas of an image. In addition to the considerable computational effort required to analyze particles within the images, another big limitation of direct imaging techniques is the challenge of balancing desire for a large area of interrogation against the need for relatively high magnification of individual droplets (at some limit of particle density the images will begin to overlap and individual particles can no longer be identified or evaluated) in order to accurately determine their size, as even the best methods must have at least 2-10 pixel "diameters" for best accuracy [5] 6]. Of course, as the price of high-resolution and high-speed cameras and laser equipment decreases and computational power increases, direct imaging is becoming significantly more accessible and usable in practical applications.…”

“…% For example, the intensity at zcorr blk 7, ycorr blk 9is % i blk (7,9) % SUPERPIXEL data: sup cor y and sup cor z contain exact coordinates for % the intensities in i sup, which is of size output super pixels yz. % For example, the intensity of the superpixel located at % sup cor z(3) and sup cor y(5) is i sup (3,5). % IMAGES: img blk is simply the sub−block image itself.…”

“…5: Errors in calibration seen when changing f-stop settings. Calibration was done with f-stop set to 3.2, and then applied to data collected at f-stop of 8.0.…”

Stream Discharge Measurement Using a Large-Scale Particle Image Velocimetry (LSPIV) Prototype

“…The PIV algorithms are some of the most robust available and well-tested in laboratory settings Vlachos, 2002a, 2002b;Brady et al, 2002). The DPIV algorithm is based on a hybrid scheme that integrates a dynamically adaptive cross-correlation method with a particle tracking velocimetry algorithm.…”