Extinction of near-stoichiometric flames diluted with nitrogen in a stagnation flow

Abstract

Experimental studies of the behavior and extinction of the near-stoichiometric flames diluted with nitrogen in a stagnation flow were made using counterflow twin flames established in the forward stagnation region of a porous cylinder. Methane-air and propane-air flames were used in the experiment. The extinction limit of the twin flames, the flame separation distance at extinction, the concentrations of some stable species on the stagnation surface of the flames very near the extinction limit and the limit flame temperature at extinction were measured, and the behavior, extinction mechanism and the flammability limits of the near-stoichiometric flames diluted with nitrogen are discussed. The position of the maximum allowable dilution by nitrogen for twin flames burning in a stagnation flow depends on preferential diffusion of the more diffusive reactant (either fuel or oxygen). For methane flames, this position is found on the slightly fuel-lean side (the equivalence ratio of the mixture =0.95), but for propane flames, this position on the considerably fuel-rich side (=1.25). With the increase of , the limit flame temperature and the flame separation distance at extinction increase for methane flames, but decrease for propane flames. For the stoichiometric methane flame and propane flame, flame extinction occurs at a finite distance from the stagnation surface. These experimental results are discussed on the basis of preferential diffusion and the Lewis number effect. Moreover, some discussion on the flammability-limit problems of the near-stoichiometric fuel-air-nitrogen mixtures is made.