Effects of oxygen concentration on the thermal and chemical structures of laminar coflow CO/H_2 diffusion flames burning in O_2/H_2O atmosphere

摘要

The thermal and chemical structures of laminar coflow syngas diffusion flames burning in O-2/H2O atmosphere were experimentally and numerically studied at 1 atm and initial temperature of 400 K. The O-2/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 discreteordinates 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 O-2 concentration. With increasing the O-2 concentration, the maximum OH mole fraction first increases but finally decreases and peaks at the oxidizer composition of 90%O-2-10%H2O. Increasing the O-2 concentration has little influence on the flame attachment but significantly enhances the heat release rate of H + O-2 ( + 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.