Experimental and computational study of CH, CH*, and OH* in an axisymmetric laminar diffusion flame

In this study, we extend the results of previous combined numerical and experimental investigations of an axisymmetric laminar diffusion flame in which difference Raman spectroscopy, laser-induced fluorescence (LIF), and a multidimensional flame model were used to generate profiles of the temperature and major and minor species. A procedure is outlined by which the number densities of ground-state CH (X²II), excited-state CH (A²δ, denoted CH^*), and excited-state OH (A²Σ, denoted OH^*) are measured and modeled. CH^* and OH^* number densities are deconvoluted from line-of-sight flame-emission measurements. Ground-state CH is measured using linear LIF. The computations are done with GRI Mech 2.11 as wel as an alternate hydrocarbon mechanism. In both cases, additional reactions for the production and consumption of CH^* and OH^* are added from recent kinetic studies. Collisional quenching and spontaneous emission are responsible for the de-excitation of the excited-state radicals.

As with our previous investigations, GRI Mech 2.11 continues to produce very good agreement with the overall flame length observed in the experiments, while significantly under predicting the flame liftoff height. The alternate kinetic scheme is much more accurate in predicting lift-off height but overpredicts the overall flame length. Ground-state CH profiles predicted with GRI Mech 2.11 are in excellent agreement with the corresponding measurements, regarding both spatial distribution and absolute concentration (measured at 4 ppm) of the CH radical. Calculations of the excited-state species show reasonable agreement with the measurements as far as spatial distribution and overall characteristics are concerned. For OH^*, the measured peak mole fraction, 1.3×10⁻⁸, compared well with computed peaks, while the measured peak level for CH^*, 2×10⁻⁹, was severely underpredicted by both kinetic schemes, indicating that the formation and destruction kinetics associated with excited-state species in flames require further research.