Investigation of OH chemiluminescence and heat release in laminar methane–oxygen co-flow diffusion flames

OH* chemiluminescence is one of the major spontaneous emission in flames, and often applied in combustion diagnostics to indicate flame structure, strain rate, equivalence ratio, heat release rate, etc. In this work, OH* chemiluminescence in the laminar methane–oxygen co-flow diffusion flames was investigated. A high resolution ultra-violet imaging system was used to capture the OH* chemiluminescence images. Numerical simulations of the experimental cases were performed based on OH* chemiluminescence reaction mechanism. The numerical results show good agreement with the experimental measurements. It's found that there are two OH* distribution zones in laminar methane–oxygen co-flow diffusion flames. Analysis on the production pathway of OH* chemiluminescence shows that the reaction H + O + M = OH* + M (R1) is the major formation reaction of OH* chemiluminescence in laminar methane–oxygen diffusion flames. The increase of diluent addition in oxidizer will lead to the dominant OH* production pathway changing from the reaction R1 to the reaction CH + O2 = OH* + CO (R2). The OH* distribution characteristics under different global oxygen-fuel equivalence ratios indicate that OH* chemiluminescence can be employed as an appropriate indicator to characterize the combustion condition. Moreover, the correlation between integrated heat release rate and integrated OH* concentration is derived for the oxygen-deficient flames. The integrated heat release rate can be predicted in terms of integrated OH* concentration, methane flow rate and global oxygen-fuel equivalence ratio.