Flame front dynamics during ignition of lean premixed H2/Air and CH4/Air flames

This study investigates differences in flame dynamics during the ignition of a premixed burner powered by lean CH4/air and H-2/air mixtures with time-resolved OH-PLIF and PIV measurements. Experiments are conducted for the same injection velocity and for mixtures featuring the same laminar burning velocity S-l(0). Despite different gas expansion ratios sigma = Tb/T-a. of burnt to unburnt gas, these mixtures exhibit similar pressure impulses during ignition but the flame leading edge trajectory differs substantially. For the CH4/air mixture, the flame is quenched near the injector outlet due to high strain rates in the outer shear layer of the jet flowing from the burner. The flame protrudes inside the shear layer further downstream, where the velocity and strain rates have sufficiently decayed, allowing the flame to propagate into the volume of unburned reactants in the core of the flow. For the H-2/air mixture, its higher reactivity and higher diffusivity enable the flame leading edge to propagate through the region of high strain rate produced by the outer shear layer. The distinction between methane and hydrogen flames is linked to the flame leading point dynamics through differences in absolute flame speed S-a and in displacement flame speed S-d. In the outer shear layer, the methane flame speed equals the flow velocity (S-a similar or equal to 0), while the H-2/air mixture still exhibits positive displacement (S-a > 0), even within the shear layer. It is found that methane flame leading point aligns with.... minima, while hydrogen flame leading point aligns with S-a maxima due to their different responses to stretch that alters their burning rate. Time-resolved data show that the CH4/air flame displacement speed scales with the thermal expansion ratio sigma of the gases with only a weak dependence on stretch, S-d similar or equal to sigma S-l(0), and is also more susceptible to quenching due to a lower resistance to stretch compared to the H-2/air flame. Conversely, the hydrogen flame resists stretch, penetrating the main jet due to substantial acceleration caused by preferential diffusion leading to a flame displacement speed almost twice the value deduced by only considering thermal expansion, S-d similar or equal to 1.9 sigma S-l(0).