THE EFFECT OF BACK DIFFUSION OF INTERMEDIATE HYDROGEN ON METHANE-AIR AND PROPANE-AIR FLAMES DILUTED WITH NITROGEN IN A STAGNATION FLOW

Experimental studies on the behavior, structure, and extinction of laminar, premixed methane-air and propane-air flames diluted with nitrogen in a stagnation flow are made using counterflow, twin flames established in the forward stagnation region of a porous cylinder. The extinction limits of the twin flames, the flame separation distance at extinction, and the concentrations of some stable species on the stagnation surface for flames very near the extinction limit are measured over the whole composition range of fuel-air-nitrogen flames. The experiments are made under the condition of a constant stagnation velocity gradient. The concentration distributions of stable species across the twin flames are also measured for flames near extinction. An anomalous relationship between the flame separation distance-DELTA-c at extinction and the equivalence ratio-phi of the mixture is found for rich methane flames, but not for propane flames. Comparisons between the concentrations of some stable species measured on the stagnation surface and those expected from equilibrium considerations reveal that the underlying cause of the occurrence of this anomalous relationship between DELTA-c and phi is attributed to preferential diffusion of the intermediate hydrogen across the divergent flow lines toward the unburned gas mixture (i.e., back diffusion). On the other hand, it is found that, for rich propane flames, preferential diffusion of oxygen, back diffusion of the intermediate hydrogen, and the Lewis number effect of the deficient reactant (i.e., oxygen) remarkably shift the local gas composition in the flame zone toward a lowe fuel-oxygen ratio.