Modulation of the liquid flow rate and the gas swirl ratio as an actuation strategy for control of a two-fluid coaxial atomizer spray*

We study the effect of harmonic modulation of the liquid flow rate and the the gas swirl ratio on the droplet size and number density distributions. A canonical coaxial two-fluid atomizer with laminar liquid and turbulent gas streams has been designed to investigate feedback control on spray. High-speed stereo visualization of the primary instability that drives the atomization will be presented, along with droplet size and velocity interferometry measurements in the region where the break-up spray is finished and the spray structure is developing. The liquid mass loading is kept smaller than 1, while the gas momentum ratio is varied from 5 to 125, and the swirl ratio ranges from 0-1 (equal stream-wise and cross-stream flow rates). Harmonic low frequency/ high amplitude (50% of maximum) forcing of the control valves and the nozzle back-pressure modulate the gas and liquid injection rates. The effect of the frequency and amplitude of the modulation on the spray primary instabilities is characterized to enable feedback control of the droplet break-up. Modulating the liquid injection produces larger drops than steady injection, while modulating swirl significantly homogenizes the droplet size and number density distributions.