Detailed numerical simulations of intrinsically unstable two-dimensional planar lean premixed hydrogen/air flames

Lean premixed hydrogen/air flames are characterized by subunity Lewis numbers and the strong influence of thermal-diffusive instability in addition to the omnipresent hydrodynamic instability caused by thermal expansion. Such intrinsically unstable flames acquire cellular structures that modify significantly their propagation dynamics. In this paper, numerical simulations with detailed chemistry and transport are used to study premixed hydrogen/air flames at an equivalence ratio of phi = 0.6 and p=5 atm in 2D rectangular domains with height ranging between 3 and 80 flame thicknesses and periodic boundary conditions along the lateral boundaries. The initial linear growth of perturbations superimposed on the planar front as well as the long-time nonlinear evolution are studied as a function of the periodicity height of the domain which affects the relative importance of hydrodynamic and thermodiffusive effects on the evolution of the flame front. Flame propagation in narrow domains is strongly affected by thermodiffusive effects, while in wide domains a cascade of transitions of the flame shape occurs. Further increase of the periodicity height, enhances the hydrodynamic effects, and the flame shape is dominated by the single-cusp structure predicted by the hydrodynamic theory. (C) 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.