Stretch extinction characteristics of CH4/CO2 versus O2/H2O/CO2 and O2/H2O counterflow non-premixed flames at different oxidizer temperatures

- The effects of H2O concentration in oxidizer on methane oxygen-enriched flame are investigated at different oxidizer temperatures.
- The flame temperature and the extinction strain rate increase with increasing H2O concentration of oxidizer.
- The relative increasing of the extinction strain rate is depended on the H2O concentration and is independent of the oxidizer temperature.
- The effect of H2O in the oxidizer on oxygen-enriched flame is clarified.

The concentrations of CO2 and H2O in the combustion environment are important for the combustion characteristics of oxygen-enriched flames using hot oxidizers with large amount of CO2 and/or H2O. To understand the variations of flame temperature and stretch extinction limit of oxygen-enriched flames with H2O and CO2 concentrations, the CH4/CO2 versus O2/H2O/CO2 and O2/H2O counterflow non-premixed flames with the oxygen mole fraction of 0.35 were investigated numerically and compared with those of the CH4/CO2 versus O2/CO2 (XO2 = 0.35) flames at the oxidizer temperatures of 500, 700 and 1000 K. Two O2/H2O/CO2 mixtures with different H2O and CO2 concentrations were used as the oxidizers in the study. The results show that the flame temperature and the extinction strain rate are increased when the CO2 in the oxidizer has been replaced by H2O. The analysis based on the theoretical solution and computational result suggests that the total amount of OH radicals of the flame can be used as a criterion for the comparison of stretch extinction limit. The computational results show that the production rate of OH radicals via the reaction O + H2O = 2OH is comparable to that of H + O2 = O + OH when H2O is added in the oxidizer. And the amount of OH radicals generated by O + H2O = 2OH increases with increasing H2O molar fraction of the oxidizer, resulting in a significant increase of the extinction strain rate.