Local extinction characteristics of turbulent premixed H <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si15.svg" display="inline" id="d1e1077"> <mml:msub> <mml:mrow/> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> </mml:math

The performance of high-altitude relight which is characterized by low-pressure conditions has been a critical indicator during the development of high-efficiency and low-emission aero-engines. To understand pressure effects on forced ignition, the evolution of turbulent premixed H 2 /air flame kernels under both sub-atmospheric (0.2 bar) and atmospheric (1 bar) pressure conditions are investigated comparatively in this work using direct numerical simulations, with special attention placed on the local extinction of flame kernels. It is found that the local extinction is significant during the kernel evolution and that there are two extinction modes. The first mode is induced by reduced reactivity due to the combined effects of positive curvature and differential diffusion, while the second mode is induced by excessive flame stretch that occurs at low-curvature regions. In the second mode, two subtypes of extinction are observed, either reactant-to-reactant (R-R) or reactant-to-product (R-P), the R-R type with higher probability than the R-P type. Finally, compared to the atmospheric case, the extinction of the kernel at 0.2 bar is less prone to extinction by Mode 1 (differential diffusion with positive curvatures) but tends to be extinguished by Mode 2 of R-R type. Novelty and significance statement The novelty of this research is summarized into 3 points: (1) turbulent premixed flames at such low pressure (0.2 bar) are, to our knowledge, investigated in detail for the first time; (2) local extinction mechanisms are categorized into thermochemical effect and aerodynamic effect; both are analyzed systematically; (3) the R-R type extinction and R-P type extinction are, to our knowledge for the first time, clearly distinguished and separated in the same turbulent flame. This is significant because sub-atmospheric combustion (especially ignition and extinction) is important for aircraft engines, while such studies are scarce, especially for turbulent flames.