Mapping of local droplet size and velocity distributions in flat-fan sprays using high-speed videography

Abstract

A methodology based on high-speed photography has been developed to measure local droplet size and velocity distributions across the entire flow domain of a continuous, quasi-steady flat-fan water spray. The motivation is to complement phase Doppler anemometry (PDA) point measurements, which are limited to the far-field, dilute region of sprays. The nozzle was systematically shifted to capture a series of non-overlapping images of the spray, which were subsequently ’stitched’ to form composite images. To reconstruct distributions along the stiches, four grids were used, each offset vertically and/or horizontally. Image analysis was employed to quantitatively evaluate the primary breakup zone and reconstruct droplet size and velocity distributions at all locations. Results showed that droplet sizes and velocities decrease downstream, with larger droplets and higher velocities observed at the periphery, attributed to rim tearing. Droplet sizes ranged from 10 to 250 µm, with velocities between 6 and 24 m/s. The local droplet size distributions were found to follow a lognormal distribution, consistent with previous observations. The local velocity distributions within the main spray region exhibited a Gaussian profile, whereas those along the spray periphery displayed a negatively skewed distribution, varying from monomodal to bimodal. At lower flow rates, the breakup zone contracted and shifted further from the nozzle. Rim tearing became dominant, leading to increased polydispersity and velocity fluctuations along the periphery.</p