High repetition-rate thermometry in a piloted spray burner using femtosecond chirped-probe-pulse coherent anti-stokes raman scattering

This paper presents results from the first temperature measurements in spray flames using chirped-probe-pulse femtosecond coherent anti-Stokes Raman scattering (CPP fs-CARS) thermometry. The CPP fs-CARS system consists of an amplified femtosecond laser system, an optical parametric amplifier for frequency shifting the Stokes beam, a glass rod for chirping the probe beam, various routing optics, a spectrometer, and an EMCCD camera. Temperature calibration of the CPP fs-CARS system involves performing measurements at known conditions in a Hencken burner flame. CARS spectra are then recorded in the experimental burner, and temperature is determined at each individual laser shot resulting in a 5-kHz history of temperature measurements at each test location in the spray burner. The Sydney Needle Spray Burner (SYNSBURN™) consists of a retractable liquid fuel delivery needle in an airblast atomizer surrounded by an annular pilot flame. This novel spray burner has independent control of the airblast velocity, recess of the fuel delivery needle in the airblast tube, fuel flow rate, and fuel type. The burner was mounted on a translation stage and scanned in the radial and axial dimensions with respect to the jet velocity to move the CARS laser probe volume location for each point measurement. Two spray flames with identical airblast velocity, fuel loading, and needle recess length, but with different fuel type (acetone vs. ethanol) are presented in this paper. Average temperature profiles, distributions, and spectra are presented and discussed. The acetone flame exhibited a higher average temperature profile in the radial dimension compared to the ethanol flame at 20 diameters downstream of the jet exit. This temperature increase, which indicates the acetone spray flame stabilized more quickly, is likely due to the increased volatility and subsequent droplet evaporation of acetone compared to ethanol.