Laser-induced fluorescence measurements and modeling of nitric oxide in high-pressure counterflow diffusion flames

Quantitative laser-induced fluorescence (LIF) measurements of NO concentrat ion ([NO]) have been obtained along the centerline of prompt NO dominated, methane-air counterflow diffusion flames at two to five atm. Global strain rates of 20, 30 and 40 s(-1) were investigated at each pressure, with the a ddition of a 15 s(-1) case at three and four arm. Linear LIF measurements o f [NO] are corrected for variations in the electronic quenching rate coeffi cient by using major species profiles generated by an opposed-flow flame co de and quenching cross-sections for NO available from the literature. Corre cted linear LIF measurements of [NO] are compared with numerical prediction s from the opposed-flow flame code by utilizing the GRI (version 2.11) mech anism for the NO kinetics. The effect of radiative heat loss on code predic tions is accounted for by using an optically thin radiation model. A modest decrease in predicted temperature owing to radiative heat loss causes a si gnificant decrease in predicted [NO]. indicating the temperature sensitivit y of the prompt-NO kinetics. Comparisons between [NO] measurements and pred ictions show that the GRI mechanism underpredicts prompt-NO by a factor of two to three at all pressures. The underprediction peaks at 2 to 3 atm, and decreases with pressure from 3 to 5 atm. Although the GRI mechanism does n ot display this trend, predictions with a modified rate coefficient for the prompt-NO initiation reaction give qualitative agreement with the experime ntally observed variation. However, modifying the prompt-NO initiation reac tion is not sufficient to account for the differences between measurements and predictions, thus indicating a need for refinement of the CH chemistry.