High-speed 10kHz measurements of CN-PLIF in cofired NH3 hydrocarbon flames

This paper presents novel high-speed (10 kHz) Planar Induced Laser Fluorescence (PLIF) measurements of the cyano radical (CN) in turbulent ammonia cofired hydrocarbon flames (HC) of: Methane (CH 4 ), Ethylene (C 2 H 4 ) and Dimethyl Ether (DME). The 3rd harmonic of a multi-mode Nd:YAG laser (355 nm) is employed to excite the CN radical. The validity of this approach was ascertained using a 10 Hz dye laser which has a narrow linewidth (∼0.2 cm -1 ) enabling a fine line scan around the center of the Nd:YAG 3rd harmonic (∼ 28,183 cm -1 ). The convolution of the 3rd harmonic linewidth (∼3 cm -1 ) onto the dye laser, included multiple vibrational levels, producing signals that are at least three times higher than the dye laser, centered at the equivalent laser position of ῦ ∼ 28,183 cm 1 . The addition of high quantities of NH 3 in these flames (5 – 60 % by volume) produces significant CN and can therefore be used as a marker of the reaction layer, and alternative to CH, which reduces in HC NH 3 flames. Ethylene and DME were used to investigate the difficulties of using 355 nm, where these fuels have strong interferences from PAH and CH 2 O respectively. A two-camera approach was used to identify and remove these interferences from the CN camera and therefore still provide interference-free CN fluorescence signals. For the CH 4 flames, no interference occurs on the CN emission, such that joint OH and CN PLIF was utilised in the Sydney inhomogeneous burner. A thin CN layer was observed to form on the inside of the OH layer, marking the reaction zone, where the CN skeletal length was used to highlight local extinction. For all CH 4 NH 3 blended flames (with and without inhomogeneity) local extinction occurred at x / D = ∼20, the location that coincides with high local strain rates and a reduction in pilot influence.