Effects of the activation energy on intrinsic instability of adiabatic and non‐adiabatic premixed flames

Abstract

The activation energy on the intrinsic instability of adiabatic and non-adiabatic premixed flames is studied by twodimensional unsteady calculations of reactive flows based on the compressible Navier-Stokes equation. A sufficiently small disturbance is superimposed on planar flame to obtain the relation between the growth rate and wave number, i.e. the dispersion relation. When Le=1.0, the activation energy had no effect on the dispersion relation, flame shape and cell depth because the Le=1.0 flame was affected only by hydrodynamic instability. The unstable behavior of cellular flames such as the growth rate and the unstable range decreased, the cell size and cell depth became larger as the heat-loss parameter increased due to intrinsic instability. For Le<1.0, the diffusivethermal instability played an important role and the lateral movement of cellular flame front was observed by increase of the activation energy. Moreover, the unstable behavior such a lateral movement of flame front was stronger when flame temperature decreased due to heat-loss. However we also obtained the normalized burning velocity of cellular flames increased as the activation energy increased and became stronger when heat-loss increased due to hydrodynamic instability. In contrast, at Le>1.0, the growth rate gradually decreased and the unstable range narrowed as the activation energy increased owing to influence of stabilization. When heat-loss increased, the growth rate became lower, the cell size and cell depth increased by the activation energy. Moreover, we obtained that the normalized burning velocity of cellular flames decreased as the activation energy increased for
non-adiabatic flames. The obtained results show that the activation energy has a great influence on the intrinsic instability of premixed flames.