Atomization and Droplet Dynamics Analysis of Air-assisted Spray under Different Gas-liquid Ratios

The principal objective of the present work is to experimentally investigate the droplet behavior of air-assisted spray. Simultaneous measurements of droplet sizes and velocities were made by phase-Doppler particle analyzer with particular emphasis on droplet size distribution, gas-droplet interactions and droplet collision. The experimental results show that air-assisted spray is composed of bulk of tiny droplets entrained by high-speed air flow. The calculated Stokes numbers of selected tracer droplets with diameter less than 5 μm show that these droplets follow the air flow faithfully and thus can be used to estimate the local air flow velocity. The Reynolds number of local gas phase was found to be larger than the order of 10⁴. The analysis of the disintegration of discrete droplets indicates that the sampled droplets basically will not undergo shear breakup, but there is a possibility of turbulent breakup (Lasheras et al. J. Fluid Mech., vol. 357, 1998, pp. 351–379). A simplified calculation method based on classical concepts of binary droplet collision were adopted to describe the collision of unequal-size droplets and collision outcome within spray under different gas to liquid ratios (GLR). The coalescence possibility of all the collision outcomes is relatively considerable and found to be influenced by GLR of air-assisted spray.