Droplet behavior in overexpanded supersonic two-phase jets

The interaction of liquid droplets generated by spray atomization with high-speed co-flowing gas is of interest in various applications including fuel injection, spray cooling, and surface coating. The underlying processes involve interactions between the droplets themselves, and with the carrier gas. Additionally, they are influenced by shock and expansion waves present in high-speed flows, accompanied by resulting velocity and pressure fluctuations. Non-intrusive diagnostics including Shadowgraphy and Phase Doppler Particle Anemometry, along with flow simulations, are used to study droplet behavior, characterized by size and velocity changes, as a finely atomized spray is propelled by a carrier gas through a supersonic, overexpanded jet structure. Measurements of droplet size and velocity are correlated to the shock train structure of the overexpanded jet and further to gas velocity as obtained from simulation results. While the shock structure is observed to be unaffected by spray injection, periodic increases and decreases are observed in average droplet size. Analysis using gas flow field obtained from simulations, estimated critical Weber numbers for droplet breakup and timescales for droplet dynamics, compared to residence time in the flow, suggests that droplet size changes can be attributed to alternating coalescence and breakup. The two-dimensional flow structure involving converging and diverging sections as well as a spread in droplet sizes likely promote coalescence and breakup processes leading to periodic variation in droplet SMD. The streamwise variations of gas velocity along the centerline caused by the shock train are not noticed in the droplet velocity, which can be explained by the high Stokes number causing droplets at the centerline to behave as ballistic particles moving through the shock structure. Experimental measurements, simulation results, and their subsequent analyses show that droplet collision and coalescence regimes established in previous work for ambient conditions show deviations when subject to high-speed gas flow conditions.