Steady and unsteady laminar premixed methane/air and hydrogen/air plane-jet flames with different equivalence ratios ranging from fuel-lean to fuel-rich are investigated under atmospheric conditions using DNS with detailed molecular transport and chemistry. The objective is to gain a deeper understanding of the influence of unsteady and nonuniform stretching on flame propagation.
A nonuniform velocity profile used at the inlet leads to a stretched flame. For steady-state flames, consumption speeds, flame stretch, curvature, strain and Markstein numbers are evaluated. By increasing the mass flow rate at the inlet, the flames become longer and different Markstein numbers are obtained.
The inflow is then harmonically excited with different frequencies and the flames oscillate in the unsteady flow. For these unsteady flames, flame relaxation times are evaluated from the phase shift between the movement of the flame and the fluid flow velocity at the flame surface. The amplitude of the flame front movement is attenuated with increasing frequency and chemical time scale. Also, the phase shift between the movement of the flame and th ... mehre local flow field becomes larger with increasing frequency or chemical time scale. Due to the flame relaxation time, different Markstein numbers are obtained from different phase angles within one oscillation period. Time averaged and frequency dependent Markstein numbers are computed which become smaller with decreasing frequency. This behavior can be reproduced by a power function in dependence on the Damköhler number.