High-data-rate one-dimensional thermometry using femtosecond CARS
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abstract
We report here-for the first time-single-laser-shot one-dimensional thermometry in flames using femtosecond coherent anti-Stokes Raman scattering (fs-CARS) line imaging. Fs-CARS enables high-bandwidth (1-10 kHz), nearly collision-free measurement of temperature and species concentration in reacting flows. Two high-power, 800-nm beams are used as the pump and probe beams, while a tunable 983-nm beam is used as the Stokes beam for CARS signal generation from the N2 Q-branch transitions at ~ 2330 cm-1. The probe beam is frequency-chirped for single-laser- shot imaging of the time-dependent frequency-spread dephasing of the Raman coherence. All three laser beams are formed into sheets and crossed in a line through the probe volume. The resulting line CARS signal at ~ 675 nm is spatially and spectrally resolved and recorded as a 2D image. Temperature measurements are demonstrated in flat-field flames up to temperatures exceeding 2000 K and the potential for high-speed measurements through sharp thermal gradients is explored. Such high-bandwidth, one-dimensional temperature measurements can provide valuable data to numerical modelers for validating complex turbulent-combustion model as well as increase the understanding of the spatio-temporal instabilities in practical combustion devices such as modern gas-turbine combustors and augmentors.
