Experimental Study on OH*, CH*, and CO₂* Chemiluminescence Diagnosis of CH₄/O₂ Diffusion Flame with CO₂-Diluted Fuel

OH* and CH* chemiluminescence in hydrocarbon flames are often applied to characterize flame structure, equivalence ratio, strain rate, heat release rate, etc. In this study, chemiluminescence images of OH*, CH*, and CO2* in the CH4/ O2 diffusion flame were obtained using a CCD camera imaging system. The effect of CO2 dilution on the flame structure, strain rate, and other flame characteristics of CH4/O2 diffusion flame was discussed. The results show that CO2 dilution greatly affects flame morphology and chemiluminescence intensity. There are quantitative functions between the chemiluminescence peak intensity of OH* and CH* and the CO2 dilution level. The CO2* average intensity in the flame zone is better suited to characterize the dilution level than the CO2* peak intensity. Moreover, the strain rate of CO2-diluted laminar flame is defined. It is found that there is a linear relationship between the thickness of the OH* reaction zone and the square root of the strain rate.