Self-Acceleration and Self-Similarity of Hydrogen–Methane–Air Flame at Elevated Pressure

Abstract

ABSTRACT By varying equivalence ratio and methane addition at elevated pressure, this paper is aimed at revealing acceleration characteristics of hydrogen–methane–air flame under thermodiffusive instability and hydrodynamic instability. On the basis of capturing flame characteristics, the flame destabilization mechanism is analyzed using effective Lewis number, density ratio and flame thickness. Then the acceleration characteristics of spherically expanding flame is evaluated using critical flame radius, critical Peclet number and acceleration exponent. Finally, the flame front evolutions of Leeff < 1.0 and Leeff > 1.0 are compared experimentally and theoretically. The results demonstrate that as the methane addition increases, the hydrogen–methane–air flame of Φ = 0.8 and Φ = 1.0 tends to be stable due to decreasing destabilization effect of both thermodiffusive instability and hydrodynamic instability. For Φ = 1.4, the stabilization effect of thermodiffusive instability and destabilization effect of hydrodynamic instability reduce monotonously with increasing methane addition, which gives rise to the transition from cellular flame to smooth flame. Both critical flame radius and critical Peclet number continue to increase with increasing methane addition. For Φ = 0.8 and Φ = 1.0, the self-similarity are emerged at x CH4 ≤ 0.5. For Φ = 1.4, the self-similarity could be only observed at x CH4 = 0.1. Especially as the methane addition increases, maximum acceleration exponent in self-similarity stage has a decreasing tendency. The flame front evolution of Leeff < 1.0 is correlated to short wavelength and the flame front evolution of Leeff > 1.0 is strongly determined on flame size.