Droplet dynamics and spray mechanism of air-assisted intermittent atomization under different injection timings

The droplet dynamics and spray mechanism of air-assisted intermittent atomization were experimentally investigated, with a particular focus on the effect of injection timing (duration of fuel injection ranged from 1 to 10 ms, and the interval between fuel injection and air injection varied from −0.5 to 2 ms) on jet atomization. Droplet velocities, sizes, and arrival times at a selected location were obtained by applying a phase Doppler particle analyzer (PDPA) with the combination of high-speed photography for spray visualization. The results indicate that droplet dynamics and velocity distribution are substantially influenced by fuel injection duration. Specifically, two peaks in the droplet velocity distribution are identified when the fuel injection duration exceeded 6 ms. Reconstruction of the gas-phase velocity using tracer particles reveals that the gas-phase velocity and stability are similarly affected by the fuel injection duration, and the gas flow tends to be stable for fuel injection durations longer than 6 ms. Droplets in the two velocity peaks exhibit different dynamic characteristics. The phenomenon of two peaks in droplet velocity is attributed to fuel accumulation inside the air-assisted atomizer, with the analysis given in terms of spray concentration and fuel state in the atomizer. Fuel accumulation and twin-peak droplet velocity caused by extended fuel–air injection intervals are predicted and validated.