Detailed measurements of transient two-stage ignition and combustion processes in high-pressure spray flames using simultaneous high-speed formaldehyde PLIF and schlieren imaging

The low- and high-temperature ignition and combustion processes in a high-pressure spray flame of n-dodecane were investigated using simultaneous 50-kHz formaldehyde (HCHO) planar laser-induced fluorescence (PLIF) and 100-kHz schlieren imaging. PLIF measurements were facilitated through the use of a pulse-burst-mode Nd:YAG laser, and the high-speed HCHO PLIF signal was imaged using a non-intensified CMOS camera with dynamic background emission correction. The experiments were conducted in the Sandia constant-volume preburn vessel equipped with a new Spray A injector. The effects of ambient conditions on the ignition delay times of the two-stage ignition events, HCHO structures, and lift-off length values were examined. Consistent with past studies of traditional Spray A flames, the formation of HCHO was first observed in the jet peripheries where the equivalence ratio (Φ) is expected to be leaner and hotter and then grows in size and in intensity downstream into the jet core where Φ is expected to be richer and colder. The measurements showed that the formation and propagation of HCHO from the leaner to richer region leads to high-temperature ignition events, supporting the identification of a phenomenon called “cool-flame wave propagation” during the transient ignition process. Subsequent high-temperature ignition was found to consume the previously formed HCHO in the jet head, while the formation of HCHO persisted in the fuel-rich zone near the flame base over the entire combustion period.