Laser Ignition and Laser-Induced Breakdown Spectroscopy of a Hydrocarbon Flame in an Annular Spray Burner

Laser-induced ignition of n-heptane and air mixtures in an annular co-flow spray burner is investigated in this paper. Laser-induced breakdown spectroscopy (LIBS) is conducted simultaneously with the laser ignition using the fundamental wavelength of an Nd:YAG laser to calculate the H/O ratio. The H/O ratio is used as a qualitative estimate of local fuel and air ratio as several past studies showed a linear relationship between H/O and local equivalence ratio. Imaging of the ignition kernels is also implemented to study the ignition process and to determine the accurate location of ignition in the spray. The ignition location is varied radially and axially with respect to the burner to understand the impact of local equivalence ratio along with varying spray and flow field characteristics (e.g., mean droplet diameter and mean droplet velocity) on ignitability. The H/O ratio increases closer to the centerline of the burner because of the dense fuel spray and a lack of oxygen diffusion. However, at higher axial locations the H/O ratio is broadened and more consistent across the radial cross section. The data acquired from this study can contribute to development of novel ignition techniques and validation of computational models for ignition and combustion in multiphase reacting flows.