Ignition by moving hot spheres in H_2-O_2-N_2 environments

A combined experimental and numerical study was carried out to investigate thermal ignition by millimeter size (d=6 mm) moving hot spheres in H_2-O_2-N_2 environments over a range of equivalence ratios. The mixtures investigated were diluted with N_2 to keep their laminar flame speed constant and comparable to the sphere fall velocity (2.4 m/s) at time of contact with the reactive mixture. The ignition thresholds (and confidence intervals) were found by applying a logistic regression to the data and were observed to increase from lean (Φ=0.39; T_(sphere) = 963 K) to rich (Φ=1.35; T_(sphere) = 1007 K) conditions. Experimental temperature fields of the gas surrounding the hot sphere during an ignition event were, for the first time, extracted using interferometry and compared against simulated fields. Numerical predictions of the ignition thresholds were within 2% of the experimental values and captured the experimentally observed increasing trend between lean and rich conditions. The effect of stoichiometry and dilution on the observed variation in ignition threshold was explained using 0-D constant pressure delay time computations.