Strategies for Quantitative Planar Laser-Induced Fluorescence of NH Radicals in Flames

Laser-induced fluorescence imaging of the NH radical using excitation in the (0-0) and (1-0) vibrational bands of the A(3)Pi-X-3 Sigma(-) electronic transition is characterized in premixed NH3-air flames. Filtered detection for excitation of the (1-0) band is found to be beneficial for measurement conditions with a challenging background and/or interferences. Concentration evaluation for excitation in the (0-0) and (1-0) bands is feasible by means of fully or partially saturated fluorescence, respectively. Detection limits of a few tens of ppm for averaged data and 100 ppm on a single-shot basis was achieved for 3-cm imaging size using both excitation alternatives. Thinning and broadening of the NH layer in a turbulent flame indicate turbulence/flame chemistry coupling, which occurred at lower turbulence levels compared with hydrocarbon combustion. Fluorescence imaging of the NH flame-front marker in such flames thus provide an experimental configuration for detailed studies of turbulence/flame chemistry interactions.