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

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 Le_eff < 1.0 and Le_eff > 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 Le_eff < 1.0 is correlated to short wavelength and the flame front evolution of Le_eff > 1.0 is strongly determined on flame size.

KEYWORDS:

• Self-acceleration
• self-similarity
• thermodiffusive instability
• hydrodynamic instability
• flame front evolution

Acknowledgments

The authors appreciate the financial supported by National Natural Science Foundation of China (No. 51674059 and No. 51874066), Key Laboratory of Building Fire Protection Engineering and Technology of MPS (No. KFKT2016ZD01), Liaoning Provincial Natural Science Foundation of China (20170540160) and the Fundamental Research Funds for the Central Universities (DUT16RC(4)04). Furthermore, the authors thank Prof. K. Kuwana (Yamagata University) and Shi-Jun Li (Alibaba Group) for providing assistance in calculating Sivashinsky equation.

Additional information

Funding

This work was supported by the National Natural Science Foundation of China [51674059,51874066]; Liaoning Provincial Natural Science Foundation of China [20170540160]; Key Laboratory of Building Fire Protection Engineering and Technology of MPS [KFKT2016ZD01]; Fundamental Research Funds for the Central Universities [DUT16RC(4)04].