Super-adiabatic flame temperatures in premixed methane-oxygen flames

Oxy-fuel combustion differs significantly from conventional air combustion. The absence of nitrogen leads to higher
flame temperatures and larger concentration of major species as well as intermediate species. In the present work,
freely propagating methane-oxygen flames were numerically calculated using a 1D model from lean to rich
conditions in order to investigate the appearance of super-adiabatic flame temperatures (SAFT). The calculations
were performed for equivalence ratios of 0.5 < Φ < 3.0 with an increment of 0.1, different inlet temperatures from
300 K to 700 K and a pressure range of 0.1 MPa to 1 MPa. Additionally, selected results were investigated with
different detailed chemical reaction mechanisms.
The results showed that the maximum flame temperature exceeds the equilibrium temperature for equivalence ratios
Φ > 0.9. Two different regimes were identified, where SAFT phenomenon appears. The first regime was found in
slightly rich conditions (1.0 < Φ < 2.1), whereas the second regime occurred in ultra-rich regime (Φ > 2.1).
A first maximum of temperature difference is observed at an equivalence ratio of Φ = 1.5. ... mehrApproximately 120 K to
180 K higher temperatures than the equilibrium ones at standard inlet conditions are locally observed, depending on
the applied reaction mechanism. The first maximum at Φ = 1.5 correlates with the maximum concentration of the Hradical, which plays a key role in the first SAFT regime. A minimum over-temperature of 50 K was identified at an
equivalence ratio of Φ = 2.1. By significantly increasing the equivalence ratio, the maximum flame temperature
exceeded the equilibrium up to almost 400 K at Φ = 3.0 in the second SAFT regime.
An increased preheating temperature enhanced the occurrence of SAFT in the first regime and degraded it in the
second regime. Elevated pressure leads to the opposite effects with decreased SAFT in the first and increased SAFT
in the second regime.