Optical and spectroscopic diagnostics of laser-induced air breakdown and kerosene spray ignition

Abstract

This study focuses on the transition from a laser-induced breakdown plasma to a flame kernel in two-phase flows. The test rig was a vertical flow channel with a full-cone spray nozzle installed inside. The fuel was Jet A-1 aviation kerosene. Breakdowns were generated by the focused laser pulses from a frequency-doubled and Q-switched Nd:YAG laser. The investigation of laser-induced breakdowns in ambient air provided valuable supplementary data to understand the interaction of the breakdown plasma and the fuel spray. To determine the breakdown energy, the amount of absorbed laser pulse energy was measured, and the blast wave energy consumption was estimated. Blast waves were visualized with high-speed schlieren imaging. Their energies were estimated by the application of Jones’ blast wave expansion model. High-speed imaging of air and spray breakdowns visualized their transient morphologies. Expansion velocities of air breakdowns were determined and revealed a supersonic expansion during the first few microseconds. Air breakdowns decayed and disappeared within 30μs. Spray breakdowns were observed over a period of 90μs, which covered their transition into flame kernels. Optical emission spectroscopy was applied to ambient air breakdowns, spray ignitions and spray breakdowns in nitrogen. The temporal decrease of nitrogen ion and atom lines was investigated, and mean lifetimes were determined. CN∗, C2∗ and CH∗ radicals were observed in spray ignitions, but no CH∗ was confirmed in spray breakdowns in nitrogen, while CN∗ and C2∗ occurred with a similar intensity as in spray ignitions. Simulated spectra were fitted to the CN∗ B2Σ+–X2Σ+ band between 384.2 and 388.4nm to determine temperatures at the breakdown region during the transition from breakdown plasma into spray flame kernels.