Three-dimensional clustering characteristics of large-stokes number sprays interacting with turbulent swirling co-flows

Three-dimensional (3D) clustering characteristics of large-stokes number sprays interacting with turbulent swirling co-flows are investigated experimentally. The Astigmatic Interferometric Particle Imaging (AIPI) technique is utilized for simultaneous measurement of the spray droplets position in 3D space and their corresponding diameter. The Stokes number estimated based on the Kolmogorov time scale varies from 34 to 142. The results show that the degree of droplet clustering plateaus at about 0.4 and at large Stokes numbers. It is obtained that the mean length scale of the clusters normalized by the Kolmogorov length scale follows a power-law relation with the Stokes number, and the mean void length scale normalized by the integral length scale plateaus at about 1.5. It is shown that the ratio of the number density of the droplets residing within the clusters to the global number density increases with increasing the Stokes number and is about 8 for the largest stokes number examined in this study. The joint characteristics of cluster's normalized volume and the mean diameter of droplets residing within the clusters show that small-volume clusters accommodate droplets with a relatively broad range of diameter. However, large clusters carry droplets with the most probable diameter. The developed AIPI technique in the present study and the corresponding spray characteristics are of importance for engineering applications that aim to understand the 3D clustering characteristics of large-stokes number droplets sprayed into turbulent swirling co-flows.