Investigation of the OH∗ chemiluminescence characteristics in CH4/O2 lifted flames

Abstract

Premixed and non-premixed flames of CH4/O2 are widely used in industrial fields, including the non-catalytic partial oxidation (NC-POX) of natural gas to produce hydrogen-rich syngas. As a major spontaneous emission present in flames, OH∗ chemiluminescence is often applied in combustion diagnostics to indicate flame structure, reaction mechanism and equivalence ratio. In this study, the flame imaging technique was applied in order to investigate the flame structure, lift-off height, peak OH∗ intensity and core reaction zone for a CH4/O2 lifted flame impinging a plate. The results demonstrated that both the increase in the global equivalence ratio (λ) as well as the reduction in the plate-to-nozzle distance (L/De) can contribute the generation of the lifted flame. In regard to the fixed L/De, the lifted flame exhibited a narrower flame front and lower chemical reaction intensity in the case of a small λ. As L/De decreased, the position of the flame core reaction zone shifted to the flame base. Meanwhile, the core reaction zone of the flame and peak OH∗ intensity was found to increase progressively. In addition, with the increase in λ, the flame lift-off height (H) initially rose gradually, after which it then declined sharply and thereafter increased. As indicated by the flame lift-off characteristic, as λ rose, a circulation zone developed at the flame base, which was located between the flame inner shear layer and the jet of oxidizer. Accordingly, the circulation zone can promote the generation of a lifted flame due to the enhanced mixing of the reactants.