Femtosecond chirped-probe-pulse coherent anti-stokes Raman scattering thermometry of nitrogen in a piloted spray burner

Chirped-Probe-Pulse, femtosecond Coherent anti-Stokes Raman Scattering (CARS) spectroscopy of nitrogen has been shown effective in making high-speed (5 kHz) single-shot temperature measurements in both laminar and turbulent combustion environments. The chirped-probe pulse technique offers one of the shortest integration times of any laser diagnostic technique – appx. 3 ps, and excellent spatial resolution – appx. 1 mm along beam propagation and 75 μm in the transverse dimension. This paper outlines the current work, applying this technique to the two-phase combustion environment in a liquid-fuel spray burner. The research collaboration presented here involves fs CPP CARS measurements in a dilute spray flame of ethanol. This paper presents results from two flame cases: one premixed methane-air flame and one ethanol dilute spray flame. Time-averaged temperature profiles as well as temperature time-histories are presented for both flames. Sources of measurement error are discussed relating to the interference of the methane signal with the nitrogen signal and relating to liquid droplets passing through the laser probe volume. The 5 kHz data acquisition rate was rapid enough to capture smoothly-varying temperature profiles for all but the most turbulent regions. The buildup and decay of temperature oscillatory structures was observed at the flame-coflow boundary in both flames and is discussed. Continuing research is aimed at the expansion of this data set into more complex spray flames of intermediate density.