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

The effects of air preheat on flame structure are studied in counterflow me thane-air diffusion flames, considering air temperatures in the range 300 t o 560 K. Species concentrations for H-2, O-2, N-2, CH4, CO, CO2, C2H2, and C2H4 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: measure ments. The results of the numerical calculations and the measurements show excellent agreement for Q(2), N-2, CH4, and good agreement for CO2, H-2, an d CO. However, they show poor agreement for C2H2 and C2H4. Independent of t he air temperature in the range 300 to 560 K, measured and predicted concen trations of CH4, CO2, O-2, and N-2 collapse reasonably well when plotted ag ainst the local equivalence ratio. The peak CO and H-2 concentrations incre ase with increasing: air preheat. The peak CO concentrations increase becau se of enhanced dissociation of CO2. The peak H-2 concentrations increase be cause of an increase in I-I atom concentrations causing enhanced rates of t he reaction CH4 + H --> CH3 + H-2. Both the measured and the predicted NO p rofiles showed approximately a 70% increase in the peak mole fractions with the increase in air temperature. The predictions of NO mole fractions in t he fuel-lean legion and near the peak are within 10% of the measurements. H owever, 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 m ole fractions with air preheat occurs primarily through the enhanced reacti on rate of the prompt initiation reaction N-2 + CH --> HCN + N. The NO prod uction by the thermal mechanism increases significantly with air preheat, b ut still remains a very small portion of the total. The effects of air preh eat on many species and reaction rates manifest through the increased H ato m concentrations. (C) 2000 by The Combustion Institute.