Linear stability and long-time evolution of planar lean premixed H2/air flames*

In the absence of buoyancy and acoustic interactions, two mechanisms can render premixed propagating flames unstable: hydrodynamic instabilities, stemming from the density jump across the flame, and thermal-diffusive instabilities in subunity Lewis number mixtures. In the present study, the linear stability of lean H$_2$/air flames as well as their long-term evolution is studied using both single-step and detailed chemistry in 2D rectangular domains of height h=3 to 80 laminar flame thicknesses with periodic boundary conditions imposed along the horizontal boundaries. At the inflow boundary, the lean (equivalence ratio $\phi=0.6$) mixture enters with a velocity equal to the laminar flame speed, temperature T=298 K and pressure p=5 atm. At these conditions, the Lewis number of the fresh mixture is sufficiently lower than unity (Le=0.4) and the expansion ratio ($\sigma=6.2$) is large enough so that both mechanisms play a destabilizing role.