A study of the effects of air preheat on the structure of methane/air counterflow diffusion flames

Abstract

The effects of air preheat on flame structure are studied in counterflow methane–air diffusion flames, considering air temperatures in the range 300 to 560 K. Species concentrations for H 2, O 2, N 2, CH 4, CO, CO 2, C 2H 2, and C 2H 4 were measured using sampling and gas chromatography. Concentrations of NO were measured using sampling and chemiluminescence analysis. Results of numerical calculations using GRI-Mech 2.11 were compared with the measurements. The results of the numerical calculations and the measurements show excellent agreement for O 2, N 2, CH 4, and good agreement for CO 2, H 2, and CO. However, they show poor agreement for C 2H 2 and C 2H 4. Independent of the air temperature in the range 300 to 560 K, measured and predicted concentrations of CH 4, CO 2, O 2, and N 2 collapse reasonably well when plotted against the local equivalence ratio. The peak CO and H 2 concentrations increase with increasing air preheat. The peak CO concentrations increase because of enhanced dissociation of CO 2. The peak H 2 concentrations increase because of an increase in H atom concentrations causing enhanced rates of the reaction CH 4 + H → CH 3 + H 2. Both the measured and the predicted NO profiles showed approximately a 70% increase in the peak mole fractions with the increase in air temperature. The predictions of NO mole fractions in the fuel-lean region and near the peak are within 10% of the measurements. However, in the fuel-rich region, the predicted NO mole fractions are lower by up to 70% than the measured NO mole fractions. The increase in peak NO mole fractions with air preheat occurs primarily through the enhanced reaction rate of the prompt initiation reaction N 2 + CH → HCN + N. The NO production by the thermal mechanism increases significantly with air preheat, but still remains a very small portion of the total. The effects of air preheat on many species and reaction rates manifest through the increased H atom concentrations.