Experimental studies of the effects of global equivalence ratio and CO2 dilution level on the OH* and CH* chemiluminescence in CH4/O2 diffusion flames

Focusing on the chemiluminescent properties of the flame is an effective way to study the flame structure and reaction mechanism. In this paper, based on the CCD camera imaging system, the chemiluminescent distributions of OH* and CH* in CH4/O2 diffusion flames with different global equivalence ratios and CO2 dilution levels were obtained. The changes in the reaction zones of OH* layer and CH* layer were further compared. The peak intensity ratios of OH* and CH* were used to quantitatively characterize the global equivalence ratio and the CO2 dilution level. The results show that variations in the global equivalence ratio and the CO2 dilution level do not obviously alter the shape and thickness of the CH* layer. The distribution and intensity changes of the OH* layer are much more significant than those of the CH* layer when the experimental conditions are varied. For the flame with a low dilution level, the CH* layer is located on the inner rim of the OH* layer, and the radial positions of two peaks are not much different. The increase of the CO2 dilution level leads to the displacement of the OH* peak, which becomes closer to the flame axis. In addition, due to the narrowing of the OH* layer, the CH* layer eventually becomes at the outer rim of the OH* layer. The peak intensity ratio of OH* and CH* decreases linearly with the rise of the global equivalence ratio, and changes as a power function with the increasing CO2 dilution level.