Quantitative Temperature & Formaldehyde Concentration Imaging for High-Pressure Turbulent Fuel Jet Ignition

Herzog, J. M. Quantitative Temperature & Formaldehyde Concentration Imaging for High-Pressure Turbulent Fuel Jet Ignition. 2020.

Understanding the process of turbulent fuel jet ignition in engines is critical to improve engine designs. Unfortunately, there is much we do not understand about the ignition process, and current diagnostic methods are insufficient to improve our understanding of the coupling between temperature, velocity, and chemistry in high-pressure turbulent jet ignition. Here, a diagnostic approach is designed that can simultaneously measure temperature, velocity, and formaldehyde concentration in a turbulent fuel jet during low-temperature ignition in an optically-accessible engine. Particle image velocimetry (PIV), aerosol phosphor thermometry (APT), and formaldehyde planar laser-induced fluorescence (PLIF) are used in combination. A detailed characterization of formaldehyde photophysics is performed using spectral simulations and experimental data. Several thermographic phosphors are characterized in detail (including physical and luminescence properties), models are developed for phosphor signal intensity and APT performance, and a method is outlined and demonstrated to combine simultaneous APT techniques to increase the temperature measurement range. The APT methods are applied to atmospheric pressure heated jets to validate the performance estimates, and identify issues in their application. A thorough analysis of design considerations for particle-based techniques is also provided, and performance estimates are made for the combined diagnostic. It was found that the Ce:LuAG phosphor with 355-nm excitation can be most easily integrated with PIV and formaldehyde PLIF measurements, and has good performance characteristics over the 700-1000 K temperature range of interest, with ~20 K estimated precision on average. Multiple scattering was found to impose a significant limitation on particle seeding density. Several phosphor materials and techniques were also found to be viable for temperature imaging well above 1000 K (Ce:GdPO4, Eu:BAM, and Ce:CSSO). Performance predictions for formaldehyde suggest that detection limits are on the order of 100 ppm throughout much of the temperature and pressure range expected during ignition, and a ratiometric background correction approach was discussed to avoid interference from phosphor luminescence, or other broadband background sources. PIV is readily integrated into the APT measurement. The proposed approach is capable of simultaneous temperature, velocity, and formaldehyde concentration imaging of low-temperature ignition processes, and provides a significant step towards improving our understanding of high-pressure turbulent jet ignition.