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Description
Achieving higher fuel efficiency and reducing NO$_x$ emissions from internal combustion engines and gas turbine requires operation at lower equivalence ratios, when ignition and flameholding may become unstable. A similar problem occurs during ignition of supersonic flows, when ignition delay time may become comparable with the flow residence time and fuel-air mixing time in the combustor. Laser Induced Breakdown (LIB) is being studied as a promising non-intrusive approach to initiate combustion and enhance fuel-air mixing at these conditions. The objective of the present work is to characterize the flow induced by laser breakdown at near atmospheric pressure, and to measure the temperature distribution in the breakdown-induced plume.
The time evolution of the temperature profile of a laser breakdown plume was studied by Planar Laser Induced Fluorescence (PLIF) at 226$\,$nm. The 2$^{nd}$-harmonic output of an Nd:YAG laser (47$\,$mJ/pulse) produced optical breakdown in a slow flow at near atmospheric pressure. The delay time between the LIB and PLIF laser pulses was varied to monitor the time evolution of the breakdown-induced plume.
For characterization of the induced flow, the laser-induced depletion of NO in a 1000$\,$ppm NO-N$_2$ mixture at 600$\,$Torr was imaged for ensembles of laser shots at varying LIB-PLIF time delays up to 500$\,$μs. Statistical analysis of the plume boundary for the individual laser shots reveals stochastic behavior at time delays longer than 100$\,$μs, with little or no shot-to-shot reproducibility.
Time resolved, two-dimensional temperature profiles in dry air at 600$\,$Torr were inferred from the intensity ratio of PLIF images of two rotational lines obtained by exciting NO generated in the laser breakdown plasma. Images of the two rotational transitions of the NO($X^2\Pi,v^\prime =0\rightarrow A^2\Sigma^+,v^{\prime\prime}=0$) $^QR_{12}+Q_{2}$ band, $J^{\prime\prime}=6.5$ and $J^{\prime\prime}=12.5$, are averaged over 600-1200 laser shots. Peak temperatures over 1200$\,$K are detected in the plume at time delays up to 500$\,$μs.
