ID 360

Effect of the boundary layer thickness on the hydrodynamic instabilities of two-fluid coaxial atomization at varying Reynolds numbers and swirl ratios

 

Abstract:

We investigate the break-up and spray development of a liquid jet by a high-speed turbulent coaxial gas jet under a wide range of gas Reynolds numbers and swirl ratios. Despite its extended use in engineering and natural processes to generate a high quality spray, the instabilities that control the liquid droplet size and their spatio-temporal distribution in the spray are not fully understood. This quantitative understanding is necessary for a first-principles approach to spray control. We present measurements of the liquid interface instability (wavelength and break-up frequency) and droplet size and velocity distributions in the near- and mid-field of a canonical coaxial gas-liquid atomizer. The liquid jet is kept laminar, while the turbulent gas jet momentum ratio varies from 5 to 125. The gas boundary layer velocity at the exit of the nozzle is measured using a combination of hot-wire anemometry and 3D/Stereo PIV, in the absence of liquid, resolving the boundary layer thickness and the azimuthal to axial momentum ratio that causes the three-dimensionality of the flow in the swirl cases. The development of the hydrodynamic instabilities on the liquid-gas interface is quantified using high speed visualizations at the exit of the nozzle and related to the frequency and growth rates predicted by stability analysis of this boundary layer flow. The resulting spray structure is characterized via Particle Phase Doppler Anemometry and compared to stability analysis statistical predictions, as well as to LES simulations of the mid-field by our ONR-MURI collaborators at University of Florida.