SAFT Regimes and Laminar Burning Velocities: A Comparative Study of NH3 + N2 + O2 and CH4 + N2 + O2 Flames

Abstract

Super adiabatic flame temperature (SAFT) is a distinctive phenomenon in the adiabatic flame where the local maximum temperature exceeds the adiabatic flame temperature. The flame temperatures exhibiting the extent of SAFT are difficult to measure with low uncertainties in experiments, while the laminar burning velocity also represents global flame features, thus could possibly be related to the SAFT. The present study investigated the SAFT regimes, laminar burning velocities (SL), and their relationships for the CH4 + O2 + N2 and NH3 + O2 + N2 flames over large equivalence (ϕ) and oxygen ratio (xO2) ranges. The laminar burning velocities were experimentally measured using the heat flux method at ϕ = 1.4–1.8 and xO2 = 0.22–0.44, where some conditions have never been reported before in the literature. Comparisons were made with simulated SL results using five CH4 mechanisms and five NH3 mechanisms, and none of them well reproduce all of the experimental data. From the simulation results, three CH4 SAFT regimes (I, II, and III) and two NH3 SAFT regimes (I and II) have been identified, among which regime III for CH4 and regime II for NH3 were found for the first time. The kinetic origins of these regimes were discussed, and different flame features regarding the flame temperature and dominant species were clarified. The relationship between the SAFT extent and the laminar burning velocity is revealed by equation derivation based on the classical flame theories, proving that a mechanism reproducing well the SL and its temperature dependence can at the same time yield accurate predictions of the SAFT. The present study also provided the most sensitive reactions in the SAFT predictions accompanied by the rate constant uncertainties, which can be helpful for further mechanism development since none of the mechanisms reproduces well the present SL experimental data, let alone the SAFT extent.