Liquid spraying and break-up of droplets in a transonic air flow

Sprays of water, ethanol, and acetone of micrometre-scale droplets are investigated in a transonic flow in a flat converging–diverging nozzle with a shock wave structure downstream. High-magnification two-frame shadowgraphy with a short interframe delay is used to investigate the distributions of droplet sizes, velocities, and deformations. The investigation is focused on droplets with diameters less than 1 0 0 μ ⁢ m and Weber numbers less than 100. Visualisation of the droplet shape evolution demonstrated that droplet break-up follows the bag and bag-and-stamen modes, which is consistent with other observations of this process for this range of Weber numbers. It is shown that droplet flattening before the shock wave in the direction of their movement increases when the Weber number increases if this number is less than the critical Weber number ( 𝑊 ⁢ 𝑒 c r = 1 2 ). Droplet elongation is observed after the shock wave. In the accelerating pre-shock region, droplet velocity is only weakly dependent on droplet size for sufficiently large droplets.