Imaging atomizing sprays with high visibility using two-photon fluorescence laser sheet imaging

Two-photon excitation laser induced fluorescence (2p-LIF) is used here for imaging through an optically dense spray system. The main advantage of the approach is that a low level of unwanted fluorescence signal originating from multiple-light scattering is generated. This leads to high visibility and image contrast even through scattering media, thus providing faithful descriptions of the imaged fluid structures. While 2p-LIF imaging is a well-known point measurement approach in the field of life science microscopy [1], it has, to the best of the authors' knowledge, never been tested for observing atomizing sprays. We take advantage of this process here, at a macroscopic scale, by imaging a light sheet of ~1cm height. To generate enough 2p-LIF signal at such large scale and for single-shot detection, ultra-short laser pulses of high pulse energy are needed. This is obtained by using a laser system providing 25 fs pulses centered at 800 nm wavelength and having 2.5 mJ pulse energy. The technique is demonstrated by imaging a single spray plume from a 6 hole commercial Gasoline Direct Injection (GDI) system running at 200 bar injection pressure. The injected liquid is water mixed with Fluorescein dye. We show the important image contrast improvement of 2p-LIF light sheet imaging in comparison with the traditional shadowgraphy, laser sheet Mie scattering and back-fluorescence imaging. The proposed approach is very promising as a future imaging tool for detailed analysis of the dynamics of atomizing spray in the spray formation region.