Development of two-beam femtosecond/picosecond one-dimensional rotational coherent anti-Stokes Raman spectroscopy: Time-resolved probing of flame wall interactions

Abstract

Hybrid femtosecond/picosecond rotational coherent anti-Stokes Raman spectroscopy (CARS) is developed utilizing a two-beam phase-matching approach for one-dimensional (1D) measurements demonstrated in an impinging jet burner to probe time-resolved head on quenching (HOQ) of a methane/air premixed flame at Φ=1.0 and Reynolds number=5000. Single-laser-shot 1D temperature profiles are obtained over a distance of at least 4mm by fitting the pure-rotational N2 CARS spectra to a spectral library calculated from a time-domain CARS code. An imaging resolution of ∼61μm is obtained in the 1D-CARS measurements. The acquisition of single-shot 1D CARS measurements, as opposed to traditional point-wise CARS techniques, enables new spatially correlated conditional statistics to be determined, such as the position, magnitude, and fluctuations of the instantaneous temperature gradient. The temperature gradient increases as the flame approaches the metal surface, and decreases during quenching. The standard deviation of the temperature gradient follows the same trend as the temperature gradient, increasing as the flame front approaches the surface, and decreasing after quenching.