Effect of radical quenching on CH4/air flames in a micro flow reactor with a controlled temperature profile

Abstract

The effect of radical quenching on CH4/air flames was investigated using a micro flow reactor with a controlled temperature profile experimentally and numerically. To evaluate the radical quenching, two methods were employed in this study; changing the inner diameter of the reactor at atmospheric pressure, d; and changing pressure in the reactor, P. The stabilized flame locations were measured and gas sampling analysis was conducted. The results were compared with two-dimensional computation including detailed gas-phase kinetics with/without a radical quenching surface reaction mechanism. At atmospheric pressure, there was no significant difference in the flame locations between the inert and quench wall cases at d=2.0 and 1.5mm. The measured flame location agreed with the computational flame locations. Thus the effect of radical quenching on the flame locations were negligible. The measured CO mole fraction agreed with the computed CO mole fraction for the inert wall, whereas significant amount of CO was predicted for the quench wall. At reduced pressures (0.5, 0.1 and 0.05atm), the flame locations in computations for the inert wall fairly agreed with those in experiments. Flames blew out in computations for the quench wall with the sticking coefficient, S, of unity. The present surface reaction mechanism overestimated the effect of the radical quenching on flames. The equivalence ratio dependence of the CO mole fraction of burned gas was computed for the quench wall with S=0.0005 at all pressures. When ϕ>1, the effect of the radical quenching on the CO mole fraction was not significant at P=0.5atm but some effects were observed at P=0.1 and 0.05atm. When ϕ<1, the enhancement of CO oxidation was numerically observed for the quench wall with S=0.0005. Reaction pathways of OH production by the interaction between gas-phase reactions and radical quenching surface reactions were identified for interpreting this CO oxidation enhancement.