Advancing Spray Analysis: Combining PDA and High-Speed Velocimetry

Understanding local droplet size and velocity distributions is crucial for enhancing spraybased technologies. Phase Doppler Anemometry (PDA) measures droplet sizes and velocities simultaneously, yet its application is typically limited to the far-field, dilute region of a spray, where droplets are smaller and more spherical compared to those in the breakup region. To complement PDA, a new method involving high-speed shadowgraphy has been developed. This method integrates droplet and ligament detection with a multiimage stitching algorithm to analyse the breakup region, determining local droplet size and velocity distributions in a steady-state, continuous, flat fan water spray. By collating non-overlapping images, a composite spray image covering the entire spray region was created. To remove artefacts along the borders, the data along the ’stitches’ was reconstructed using four grids, each shifted vertically and/or horizontally by half the height and/or width of the field of view. Altogether, 1,200 composite images were generated for each grid at a frame capture rate of 84 kHz. Droplet sizes from 10 µm and greater were identified in the images, with PDA extending this range down to 1-2 µm. Particle Image Velocimetry was then applied to the temporally resolved composite images to measure droplet velocities. The droplet characteristics at flow rates of 4, 4.5, and 5 kg/h were analysed and compared using both the new method and PDA. The findings showed good qualitative agreement between the two methods, demonstrating consistent trends, although quantitative discrepancies were noted. Additionally, velocity measurements showed up to a 20% difference between the two methods. These differences may stem from the exclusion of non-spherical droplets or droplets approaching the probe volume at an angle in PDA. In imaging, lower resolution and droplets outside the depth of field, which move slowly, may lead to bias towards lower velocities during crosscorrelation.