CH(A-X) and OH(A-X) Optical Emission in an Axisymmetric Laminar Diffusion Flame J. LUQUE, J. B. JEFFRIES,* G. P. SMITH, AND D. R. CROSLEY Molecular Physics Laboratory, SRI International, 333 Ravenswood Ave., Menlo Park, CA 94025 K. T. WALSH, M. B. LONG, AND M. D. SMOOKE Department of Mechanical Engineering, Yale University, New Haven, CT 06520 Steady-state concentrations of electronically excited CH(A 2 ) and OH(A 2 ¥ + ) are extracted from previous quantitative measurements of optical emission from an axisymmetric laminar diffusion flame [K. T. Walsh, M. B. Long, M. A. Tanoff, and M. D. Smooke, Twenty-Seventh Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1998, pp. 615– 623]. The flame is modeled with a two-dimensional transport and detailed chemistry explicitly augmented with the reactive, radiative, and energy transfer collisional processes to produce and remove electronically excited CH(A) and OH(A). Computations predict concentra- tions of CH(A) and OH(A) which agree with the measurement within a factor of 6 or better, a significant improvement compared to the earlier report. © 2000 by The Combustion Institute INTRODUCTION Even though the practical diagnostic uses of the optical emission from flames have been long apparent, very little work quantitatively investi- gating the intensity of the optical emission from excited molecules in flames has been published since Gaydon [1] in 1974. Joklik et al. investi- gated CH(X) and CH(A) concentrations in a low-pressure acetylene flame [2], and Deviendt et al. [3] measured CH(A) production rates. Recently, Walsh et al. [4] quantitatively mea- sured the optical emission intensity from the CH(A-X) transition near 431 nm and the OH(A-X) transition near 308 nm; they found very poor agreement between these measure- ments and model calculations for a methane/air diffusion flame. In the work reported here, we correct two errors in the published analysis of Ref. 4. First, the removal of the excited molecules by colli- sional quenching was double counted by both adding collisional removal into the model and then additionally correcting the observed emis- sion for quenching. This double-counting re- sulted in an overprediction of measured CH(A) and OH(A) by factors of 176 and 327 respec- tively. Second, Ref. 4 used a branching fraction of unity for CH + O 2 7 OH(A) + CO; this ignores the dominant product channel OH(X) + CO. Using more appropriate branch- ing ratio reduces predicted OH(A) by a factor of 540. In this paper, we redetermine the steady- state concentrations of CH(A) and OH(A) from the emission intensities measured in Ref. 4, and we compare these concentrations with values predicted from the same model including two- dimensional transport and modified chemistry for CH(A) and OH(A). THE EXPERIMENT The lifted axisymmetric laminar methane/air diffusion flame studied here has been exten- sively characterized both experimentally and computationally [4 – 8]. Walsh et al. [4] mea- sured the optical emission from CH(A) and OH(A) on a cooled CCD camera with a f/4.5 UV camera lens using narrow bandpass filters (10 nm FWHM at 431 nm and 307 nm respec- tively). Emission intensity measurements are line-of-sight-integrated and the two-dimen- sional, in-plane intensity distribution is recov- ered with an Abel deconvolution [9, 10]. The number density of electronically excited molecules in the flame is a balance between production by chemiluminescent reactions and removal by electronic energy transfer collisions, *Corresponding author. E-mail: Jeffries@Navier.Stanford.edu Present Address: Department of Mechanical Engineering, Thermosciences Div Bldg 520, Stanford University, Stan- ford CA 94305-3032 COMBUSTION AND FLAME 122:172–175 (2000) 0010-2180/00/$–see front matter © 2000 by The Combustion Institute PII S0010-2180(00)00112-7 Published by Elsevier Science Inc.