Planar two-photon fluorescence imaging of dense spray to estimate spray characteristics: application in pressure-swirl atomizers

The dense spray produced at the primary stage of atomization in a pressure-swirl atomizer is characterized in this work. The optically dense regime from continuous liquid stream to first step breakup into liquid structures is acquired using a two-photon planar laser-induced fluorescence (2p-PLIF) technique. A notable advantage of 2p-PLIF over conventional PLIF is the attenuation of multiple scattering by simultaneous absorption of two photons in an ultra-short pulse duration. This approach is able to capture the complex interface morphology of spray structures. A curvature-based analysis of near-field is carried out to predict far-field spray characteristics. This methodology was recently introduced by \cite{Palanti2022} to investigate numerical simulation of atomizing liquid flows. The present work extends its application to experimental images. The atomization process is described through the curvature distribution in different regimes. The spray characteristics are predicted from the early stage of atomization and are reasonably comparable with those of direct measurement by Phase Doppler Anemometry (PDA) in the later stage of atomization. The present analysis shows how it is possible to obtain information about the dispersed phase of the spray in advance based on the dense spray curvature distribution.