An experimental study of expanding spherical laminar lean H2/O2/N2/He flames

Experiments with confined spherical laminar flames expanding in lean (the equivalence ratio ϕ = 0.45) H 2 /O 2 /N 2 /He mixtures under room conditions are performed using high-speed Schlieren technique. Measured dependencies of (i) flame radius on time and (ii) flame speed on flame stretch rate are analyzed using four state-of-the-art methods. The following results are worth emphasizing. First, at large ratios of mole fractions X He and X N 2 of He and N 2 , respectively, e.g., X He / X N 2 = 1.81 , no visible cellularity was observed within the investigated flame radius range. Second, experimental data obtained at X He / X N 2 = 0.51 or 0.45 indicate that both (i) cells on flame surface and (ii) an increase in the flame speed due to flame instability appear at sufficiently large flame radii, more than 7 ms after the end of the spark discharge. This experimental finding is consistent with the standpoint that high stretch rates can suppress diffusional-thermal and Darrieus-Landau instabilities. Third, the performed measurements demonstrate that changing He:N 2 ratio in lean H 2 /O 2 /N 2 /He mixtures offers the opportunity to explore onset of cellular instability by separating it from ignition stage. Fourth, the experiments also show that although the truncation of the data obtained at large flame radii offers the opportunity to explore apparently stable flames in the case of a negative Markstein number M a b , such a method results in fitting-dependent and scattered values of laminar flame speeds and, especially, M a b .