High dynamic range thermometry at 5kHz in hydrogen-air diffusion flame using chirped-probe-pulse femtosecond coherent anti-stokes Raman scattering

Chirped probe pulse femtosecond coherent anti-Stokes Raman scattering (CPP fs-CARS) thermometry was performed at 5kHz in a hydrogen jet diffusion flame with an air co-flow. Measurements were performed at different heights and radial locations within the jet diffusion flame, up to 16 nozzle exit diameters downstream (x/d=16). The near-nozzle measurements were characterized by large, organized, buoyancy-driven instabilities that become more chaotic at the downstream locations x/d4. The diffusion flame results highlight temperature fluctuations characteristic of the buoyancy-driven Kelvin-Helmholtz-type instability and provide new insights into the transient structure of these flames. At some measurement locations, the time-varying temperatures ranged from 300K to nearly 2400K. The CPP fs-CARS signal intensity is a factor of approximately 1000 times lower at 2400K compared with 300K. A dual-channel detection system was used to increase the dynamic range of the CARS measurements. The determination of temperature from the single shot spectra is discussed in detail. Laser and detection system parameters were determined from CPP fs-CARS spectra obtained from a near-adiabatic laminar calibration flame apparatus. The temperature precision of the system was determined from these calibration measurements and was found to be better than 2.0% at 2200K. The influence of an instrument response function on spectral fitting parameters is systematically assessed. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.