Influence of Laser Energy and Ignition Location on Spray Flame Evolution

Laser ignition experiments were performed in a heptane spray using an Nd:YAG laser to investigate its ignitability. In the first set, Laser-Induced Breakdown Spectroscopy (LIBS) and imaging were employed to quantify Hα/O ratios, kernel size, and kernel number at various locations within the spray. While these parameters generally followed the spray profile, they did not reliably predict ignition. In the second set, OH chemiluminescence, LIBS, and high-speed imaging was utilized to understand the effect of laser energy and ignition location on ignitability. Two distinct modes of ignition failure – short and long – were identified based on kernel extinction time. For the spray conditions studied, ignition was achieved at laser energy of 250 mJ, while long-mode failure occurred at 80 mJ, and short-mode failure at 30 mJ. Optical intensities of OH and CH showed that higher laser energies generated more radical species and sustained the flame long enough to establish stable ignition. Additionally, kernel trajectories extracted from high-speed images showed that ignition is more probable for cases where the spark is generated in or moves into recirculation zone. These findings enhance our understanding of spray ignitability and can inform the development and validation of models for laser or plasma-assisted combustion in sprays.