Qualitative measurements of pressure-atomized sprays through simultaneous collection of planar fluorescence, phosphorescence, and Mie scattering data

摘要

A laser diagnostic technique useful for qualitatively locating and describing regions of vapor and liquid structures of a pressure atomized fuel spray is examined. While Mie scattering is sensitive to the liquid phase within a spray, planar laser-induced fluorescence is sensitive to both the liquid and vapor phases. Hence, a comparison of images utilizing these two techniques could be used to qualitatively distinguish regions of vapor from regions dominated by droplets. Quantitative subtraction of the two signals is subject to significant error in polydisperse sprays, however, due to the fact that scattering is sensitive to droplet surface area (diameter squared) while fluorescence is sensitive to droplet volume (diameter cubed). Moreover, even qualitative comparison of the two signals may yield false identification of fuel vapor because of possible differences in signal behavior within dense regions of the spray. By simultaneously capturing phosphorescence in addition to fluorescence and Mie scattering, it is possible to gain further insight because phosphorescence is proportional to droplet volume, like fluorescence, but is sensitive only to droplets, like Mie scattering. Hence, phosphorescence can be used to determine whether differences between fluorescence and Mie scattering signals are due to the presence of fuel vapor or due simply to the different photophysics between the two techniques. The current work shows the utility of using phosphorescence for added information and advances the state of the art by (1) testing the use of fluorescence, phosphorescence, and Mie scattering (FPM) in a dense spray, (2) testing FPM in a multi-component fuel, (3) implementing FPM in a practical device, and (4) conducting tests with FPM under elevated temperatures. Signal collection techniques and data conditioning methods are presented and discussed for both laboratory and test cell applications. Results show that the measurement of fuel vapor from differences in fluorescence and Mie scattering data can be misleading due to variations in multiple scattering with these two techniques. By adding phosphorescence, it is possible to show that regions that appear to consist of fuel vapor from fluorescence are more likely attributable to diffuse scattering from a dense field of droplets within the spray. This is an important result that shows the significance of simultaneous collection of FPM signals in practical fuel sprays. Suggestions to improve and advance the technique are also presented.