Interactions between liquid sprays and shock waves in underexpanded flows

Abstract

The interactions between droplets and shock waves have many applications, but few studies have investigated how the distributions of droplet diameters and droplet velocities in a spray are modified after passing through a shock. This study examines the droplet statistics upstream and downstream of shock waves in an underexpanded jet by performing phase Doppler interferometry in combination with Schlieren imaging. A mixture of water and propylene glycol is employed as the liquid. It is observed that, when the spray passes through an oblique shock, the droplet diameters decrease and then increase but the droplet velocities remain steady. When the droplets pass through a weak normal shock, on the other hand, the most probable droplet diameter decreases, some very large droplets appear, and the velocity distribution splits into three distinct regions. However, a strong normal shock causes the mean droplet diameters and the mean droplet velocities to decrease consistently. Reasons for these different behaviors are given, and secondary breakup regimes are estimated in terms of the Weber number. Additionally, droplet probability distribution fits are compared to the measured values for each of the different cases. The droplet statistics presented here can be used to improve computational modeling of spray-shock interactions.