CH3 imaging via photo-fragmentation combined with CH planar laser-induced fluorescence employing C–X (0,0) band excitation and detection

Abstract

In this paper we demonstrate single-shot imaging of the methyl (CH3) radical using a single laser system. The diagnostic approach combines CH3 photo-fragmentation followed by methylidyne (CH) detection. Specifically, the 5th-harmonic beam of a Nd:YAG laser (λPF = 212.8 nm) facilitates CH3 photo-fragmentation, while the frequency-doubled output of the same laser pumps a dye laser, the output of which is frequency-doubled and tuned to excite CH R-branch transitions within the C–X (0,0) band (λLIF ≈ 311 nm). We detect the resulting fluorescence from the C–X band (primarily the Q and P branches), suppressing laser scattering using a custom long-wave-pass filter. Transitions in the C–X (0,0) band are exceptionally strong, which is an enabling feature for the proposed diagnostic. Indeed, the PLIF excitation pulse energy (at 311 nm) was limited to ∼2 mJ, which is sufficient for high-quality measurements of the CH produced via photo-fragmentation. We illustrate the utility the approach in premixed laminar and turbulent flames (over a range of stoichiometry) and in a methane lifted jet diffusion flame. A noteworthy feature of the proposed diagnostic is that one can detect nascent CH (by blocking the 5th harmonic beam) and hydroxyl (OH) in its A–X (0,0) band by tuning the dye laser to a nearby OH transition. We also propose a simple methodology for acquiring simultaneous OH and CH3 images employing a single camera via the use of structured illumination and demonstrate joint OH + CH3 imaging in the lifted jet diffusion flame.