Structure and extinction of near-limit flames in a stagnation flow

Abstract

Experimental studies of the structure and extinction of near-limit premixed flames in astagnation flow were made using counterflow twin flames established in the forward stagnation region of a porous cylinder. The stagnation surface between the two flames can be expected to be adiabatic and noncatalytic wall surface. Near-limit rich- and lean-methane/air and propane/air flames were used in the experiment. The structure of the twin flames, the flame temperature, the distance between the two flame zones, and the concentrations of reactants on the stagnation surface were measured, and the extinction mechanism is discussed. Two distinct modes of flame extinction exist in the stagnation flow field: one is flameextinction which occurs close to the stagnation surface due to incomplete combustion and the other is flame extinction which occurs at a finite distance from the stagnation surface due to flame stretch. The Lewis number of the deficient reactant in the premixed combustible gases (fuel in the lean mixture and oxygen in the rich mixture) is responsible for the existence of these two distinct modes of flame extinction. These results strongly support the theoretical predictions by Sivashinsky and by Sato and Tsuji. The extinction of a flame in which the diffusion coefficient of the excess reactant is muchlarger than that of the deficient reactant is also affected by dilution of the reaction zone by the excess reactant with the larger diffusion coefficient.