Experimental investigation of laser-ignited hydrocarbon sprays and droplets

Abstract

Spray ignition involves complex multiphase interactions wherein atomized fuel droplets undergo evaporation, secondary breakup, and mixing with the oxidizer. These processes result in highly heterogeneous fuel-air mixtures, characterized by significant local equivalence ratio variations, complicating the establishment of consistent and reliable ignition conditions. Additionally, substantial energy losses occur in spray ignition, as a considerable portion of the input energy is consumed by droplet heating and vaporization rather than directly contributing to plasma formation and flame kernel initiation. The presence of turbulence further exacerbates ignition challenges by introducing local velocity gradients and strain that distort or extinguish nascent flame kernels. This dissertation aims to bridge the existing knowledge gaps in spray ignition through systematic experimental investigations under realistic yet controlled laboratory conditions. The research is structured into two complementary components: (1) laser-droplet interactions and droplet breakup dynamics, and (2) laser ignition of fuel sprays. In the first component, experiments focus on elucidating how laser pulse energy and droplet size influence droplet fragmentation dynamics, plasma formation, and species evolution in the breakdown region. Utilizing diagnostics such as shadowgraphy and Laser-Induced Breakdown Spectroscopy (LIBS), droplet breakup regimes, species composition, electron densities, and temperatures are characterized. A novel energy-based metric is developed to effectively distinguish and classify different droplet fragmentation regimes. The experiments demonstrated that low laser energy densities (<~70 mJ/mm3), designated as regime 1, resulted in a single plasma breakdown event accompanied by broadband emission and C2 Swan bands, suggesting weak plasma formation. Conversely, high energy densities (>~70 mJ/mm3), designated as regime 2, resulted in multiple plasma breakdowns that resulted in emission of Hα, O, and N, implying a full laser breakdown in the gaseous reactive mixture. The second component includes laser ignition experiments that 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. The results presented in this dissertation not only advance the fundamental understanding of spray ignition phenomena but also contribute toward the development of more reliable and efficient ignition systems. The broader significance of this research lies in its direct applicability to practical combustion systems, particularly aero-turbines where dependable ignition and stable flame propagation are essential for safe and efficient operation. Moreover, the insights gained from this work can guide future experimental studies and enhance computational modeling efforts in the field of multiphase ignition.