Femtosecond Holography of Sprays
The study of liquid jet breakup near the nozzle exit under the complex conditions, where the liquid/gas density ratio is relatively low, where rapid shearing and droplet formation is intense, and where the optical scattering obscures the condensed phase structure, has been an important sub-field of spray and spray combustion research for decades. With combustion chamber pressures on the rise, and engine performance improvement demands unrelenting (reduced emissions, fuel tolerance, increased power density, and efficiency), gaining diagnostic access to this critical jet breakup domain is more important than ever. This research project contributes to the understanding of the near nozzle region of realistic fuel sprays by developing and quantitatively demonstrating ultra-short pulse off-axis holography (USPODH) as a coherence filtering tool for 3D evaluation, obscuration avoidance, and transient imaging. USPODH is an interference-based single-shot technique that uses coherence filtering to allow single-shot imaging through media with optical density (OD) up to 12, conditions where other techniques resort to hardware time-gating solutions, multiple shot averaging or a synchrotron light source. The results show how USPODH can be successfully used in such environments. In particular, Monte Carlo photon transport simulations and experimental results show how coherence filtering can outperform picosecond time-gating for imaging in optically dense environments. Droplets as small as 25 µm, generated by a water spray hidden by a field of polydisperse droplets with OD up to 12 were successfully resolved. Performance in engine-relevant environments was demonstrated by imaging a USAF resolution chart at pressures up 20 bar and temperatures up to 450 K. The effects on pulse broadening and image quality from pressure and temperature gradients and the presence of thick fused quartz windows were quantified. A gas-liquid interface in the near-nozzle region of a single-orifice dodecane spray injected at 1550 bar was imaged in a vessel at pressures up to 20 bar. Surface perturbations and ligaments detaching from the spray liquid core were observed with an object-side resolution of approximately 14 µm at a working distance of 35cm, and off-line numerical focusing allowed to focus on features of interest in depth planes tens of centimeters apart from each other. USPODH reconstructions were also compared to lensed shadowgraph images and inline holograms reconstructions acquired in the same conditions, and the results show that USPODH is effectively rejecting the scattering light noise signature from the shroud of droplets surrounding the spray core in the nozzle proximity.