High-speed flamefront imaging in premixed turbulent flames using planar laser-induced fluorescence of the CH C-X band

We describe efforts to develop kHz-rate or high-speed planar laser-induced fluorescence (PLIF) of the CH radical for application to premixed flames. The basic approach used here involves excitation and detection of the CH radial via the C²σ⁺-X²Π (v'=0, v″=0) band, which has transitions in the wavelength range λ≈310-320nm. Transitions in this band are generally stronger than those in the A-X and B-X bands of CH and the radiative lifetimes are shorter too. Thus, the C-X band should have advantages with regard to CH detectability in atmospheric flames, and we show that good CH-PLIF signal-to-noise and signal-to-background ratios can be attained at a 10-kHz interrogation rate and that the spatial resolution (of the CH layer) is reasonably good as well. Of course, strong OH lines, from the A²σ⁺-X²Π (0,0) and (1,1) bands, lie nearby the CH C-X lines. While this can create some interference in the detection of CH, we demonstrate that OH lines can be avoided, if desired, or excited, if desired. Indeed, easy access to either CH or OH is a substantial benefit of the method outlined herein. Furthermore, we show that simultaneous imaging of CH and OH-using a single laser system and camera-is possible too. We demonstrate the utility of this approach for resolving the flamefront dynamics with 10-kHz measurements in a turbulent, premixed methane-air Bunsen flame: we see intrusion of flame and products into the reactant-zone, which appears to accelerate the consumption of the reactant core, and fingers of flame and reactants that extend into the product-zone and then pinch off and burn out.