Development and application of CN PLIF for single-shot imaging in turbulent flames

Abstract

A laser-induced fluorescence concept for improved interference-free imaging detection of the cyano radical (CN), an intermediate species in nitrogen combustion chemistry, has been investigated. Both a standard Nd:YAG-pumped dye laser and a solid-state alexandrite laser were employed for excitation of CN B–X (1–0) lines near 359nm in combination with detection of the (1–1) band near 387nm, providing a direct comparison of interferences and achievable detection sensitivity. Measurements in ammonia-doped premixed methane–air flames showed that the alexandrite laser, with longer pulse duration (∼30ns) and broader line width (2.5cm−1), can provide around 4 times stronger CN signal than that achievable with conventional Nd:YAG pumped dye laser system. Enhanced CN detection using alexandrite laser excitation is demonstrated that signal-to-noise ratio ∼5 is achieved for single-shot fluorescence imaging at conditions with CN concentration levels experimentally estimated to be a few hundred ppbs. This improved detection sensitivity, however, might still not be sufficient for pure methane/air flames due to low CN concentration. Observations of local extinction, represented by CN layer dis-continuation, in turbulent flames suggest that the improved CN detection will be useful for deepened understanding of interactions between fuel-nitrogen chemistry and local turbulent transport in combustion.