Automatic digital holographic analysis of near-field aerated liquid jets in crossflow

Abstract

Scope and Method of Study:

This report has discussed the advantages that holography has over other spray diagnostic approaches; namely, holography is a volumetric measurement as opposed to a point measurement and drops are not limited to a particular shape. This project has advanced holographic spray analysis by automating the image processing algorithms. The program was then applied to sprays in a realistic testing environment. Aerated liquid jets in a high subsonic crossflow were analyzed, including the rarely investigated near injector region (from the injector to the far field, x/d = 0..100). Usable data was presented in a variety of figures. These results demonstrate that practical 3D analysis of sprays has been pushed well into the near injector region, specifically up to x/d ~ 15. Additional data was obtained even closer to the injector, but its confidence drops significantly.

Findings and Conclusions:

The hologram image processing algorithm has been validated and its uncertainty quantified. The drop detection false positive rate is 6% and false negative rate is 20%. These detection rates are independent of the drop size, for drop images with a diameter greater than six pixels.

The area near the injector floor in the figures are composed of a high number density, wide span of very small droplets separated from the main, high density core. This region immediately downstream of the injector, under the jet core, features a continuous distribution of drops. Hence, while a wake of relatively large drops forms downstream of the injection site, the near-field wake is not completely empty.

The downstream location is important to the Sauter Mean Diameter in the near field; larger drops break up during secondary atomization. However, even with the near field data, the influence of x is much less than GLR or M for the given data set. SMD is inversely proportional to all three factors.