Effects of pressure on propagation characteristics of methane-air edge flames within two-dimensional mixing layers: A numerical study

The effects of pressure on propagation of laminar edge flames were numerically investigated using an OpenFOAM-6 platform developed with multicomponent transport properties. With various equivalence ratio gradients (V phi) as inlet conditions, propagation of non-stationary edge flames in two-dimensional mixing layers at three pressures (0.5 bar, 1.0 bar and 2.0 bar) were studied. The heat release rate increases with increasing pressure and decreases with increasing V phi due to flame curvature and stretch. The linearity between flame curvature K and V phi is revealed. The K and slope of this linearity are both larger under high pressure. Flame stretch is dominated by flow strain and flame curvature rather than unsteady flame motion. With increasing V phi and pressure, stretch rate kappa shows increasing feature due to larger curvature and stronger flow strain. The obvious negative linear dependence of local flame speed Sd on kappa is revealed under three pressures. The range of Karlovitz number under three pressures are 0.2- 1.4, indicating that edge flames are weakly stretched and the linear correlation between Sd and kappa could be explained by weakly stretched flame theory. Compared with positive dependence of mass diffusion term Sdd on K, the dominating negative dependence of reaction term Srd on K leads to negative correlation of Sd with K. Global flame speed UF shows non-monotonic feature with increasing V phi under three pressures, which is mainly due to non-monotonic upstream velocity reduction Udrop. Flame stretch is important for the shift from critical gradient V phi dc (for maximum Udrop) to smaller V phi Fc (for maximum UF) and stronger decreasing feature of UF thanUdrop under large V phi.