Spectroscopic diagnostic techniques were used to analyze the fundamentals of sub- to supercritical jet disintegration and mixing. The research could lead to a better understanding of the precise dynamics of fuel breakup and dispersion, impacting the way rocket engines, gas turbines and diesel engines are built. In the same facility and under the same fluid conditions, researchers from the University of Florida applied shadowgraph visualization and planar laser-induced fluorescence (PLIF) to an analysis of fuel injection of a single orifice and compared the results. Unlike the shadowgraph data, PLIF provided quantitative measurement of central jet plane density and density gradients. While imaging studies of jets have been performed by many different institutions, limited quantitative density data has been reported. “The planar laser-induced fluorescence technique and the process of correcting for absorption effects is a tool that is unique to the Combustion and Propulsion Laboratory,” said researcher Shaun DeSouza. “This method provides quantitative data for comparison with the qualitative data produced by the shadowgraph technique.” Researchers ran 48 tests of jets injected from a single orifice into a chamber with one of a range of sub- to supercritical temperature and pressure combinations. A fluid called fluoroketone was used in the testing because of its low critical temperature and pressure and its distinct spectral features, which are well-suited to shadowgraph and PLIF detection. Shadowgraph images with planar laser-induced fluorescence (PLIF) density gradient maps of a subcritical injection into an environment of subcritical conditions compared with the two imaging results for injection with all conditions supercritical. Courtesy of DeSouza and Segal and American Institute of Physics, Washington DC. Research results demonstrated the accuracy of PLIF for imaging single planes of the flow field through the center of the jet, leading to noticeable differences in the measured spreading angle compared to shadowgraphy. Unlike shadowgraphy, which integratively imaged the entire jet, PLIF provided more detailed density information, illuminating features that shadowgraphy could not detect. According to the researchers, the imaging techniques could offer complementary advantages, with PLIF providing quantitative density results and shadowgraphy providing very detailed flow visualization. While the shadowgraph data agreed with previous visualization studies, the PLIF results that provided quantitative measurement of central jet plane density and density gradients offered new and differing results. The research also revealed trends, such as an increase in normalized drop diameter and a decrease in droplet population as chamber temperatures increased, that could be key to understanding and improving applications like jet propulsion. “The next step for this line of research is to expand the thermodynamic conditions explored and to improve imaging hardware to gain a better understanding under a larger variety of conditions,” said DeSouza. The research was published in Physics of Fluids, a publication of the American Institute of Physics (doi: 10.1063/1.4979486).