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

Superadiabatic flame temperature (SAFT) is a distinctive phenomenonin the adiabatic flame where the local maximum temperature exceedsthe adiabatic flame temperature. The flame temperatures exhibitingthe extent of SAFT are difficult to measure with low uncertaintiesin experiments, while the laminar burning velocity also representsglobal flame features, thus could possibly be related to the SAFT.The present study investigated the SAFT regimes, laminar burning velocities(S L), and their relationships for theCH4 + O2 + N2 and NH3 +O2 + N2 flames over large equivalence (phi) and oxygen ratio (x O2 ) ranges. The laminar burning velocities were experimentallymeasured using the heat flux method at phi =1.4-1.8 and x O2 = 0.22-0.44,where some conditions have never been reported before in the literature.Comparisons were made with simulated S L results using five CH4 mechanisms and five NH3 mechanisms, and none of them well reproduce all of the experimentaldata. From the simulation results, three CH4 SAFT regimes(I, II, and III) and two NH3 SAFT regimes (I and II) havebeen identified, among which regime III for CH4 and regimeII for NH3 were found for the first time. The kinetic originsof these regimes were discussed, and different flame features regardingthe flame temperature and dominant species were clarified. The relationshipbetween the SAFT extent and the laminar burning velocity is revealedby equation derivation based on the classical flame theories, provingthat a mechanism reproducing well the S L and its temperature dependence can at the same time yield accuratepredictions of the SAFT. The present study also provided the mostsensitive reactions in the SAFT predictions accompanied by the rateconstant uncertainties, which can be helpful for further mechanismdevelopment since none of the mechanisms reproduces well the present S L experimental data, let alone the SAFT extent.

Automated summary

This study investigates the phenomenon of SAFT in the CH4 + O2 + N2 and NH3 + O2 + N2 flames and examines the relationship between SAFT and laminar burning velocities. Experimental measurements and simulations reveal different SAFT regimes for CH4 and NH3 flames, and demonstrate the connection between SAFT extent and laminar burning velocity.