Effects of oxygen concentration on the thermal and chemical structures of laminar coflow CO/H2 diffusion flames burning in O2/H2O atmosphere

Abstract

The thermal and chemical structures of laminar coflow syngas diffusion flames burning in O2/H2O atmosphere were experimentally and numerically studied at 1 atm and initial temperature of 400 K. The O2/H2O molar ratio was varied from 20/80 to 100/0 to investigate the effects of oxygen concentration. An intensified CCD was used to capture the OH*-chemiluminescence based on which the measured flame heights were obtained. Numerical modelling was conducted using a detailed validated chemical mechanism including OH* formation, the discrete-ordinates method coupled with the statistical narrow-band correlated-K(SNBCK) model for the radiative properties of combustion products, and the conjugate heat transfer model to account for the heat transfer between the burner wall and the fuel and oxidizer streams. Results show that the measured flame heights agree well with the simulated values. The flame height and the maximum flame temperature decrease and increase respectively with increasing the O2 concentration. With increasing the O2 concentration, the maximum OH mole fraction first increases but finally decreases and peaks at the oxidizer composition of 90%O2-10%H2O. Increasing the O2 concentration has little influence on the flame attachment but significantly enhances the heat release rate of H + O2(+M) = HO2(+M) and this reaction is found to be the primary cause of the strong influence of the oxygen concentration on the burner tip temperature. As the oxygen concentration increases, syngas pyrolysis inside the fuel tube is promoted, mainly through the H consumption reactions due to the enhanced back diffusion of H from the flame front into the burner.