Structure analysis of righ fuel-air flames in the forward stagnation region of a porous cylinder

Abstract

An experimental study was made of the structure of rich fuel-air flames established in the forward stagnation region of a porous cylinder, from the surface of which premixed methane and air were ejected into a uniform air stream (the secondary air). Aerodynamic, temperature, and stable species concentration profiles were measured for flames at atmospheric pressure. These distributions were analyzed to yield the reaction-rate profiles of stable species throughout the flame zone. The stabilized flame is really a double flame. For the equivalence ratioof the ejected mixture >2.7, the stagnation point lies between the inner flame and the cylinder surface, and net reaction-rate profiles of various species are similar to those of a pure diffusion flame. Therefore, for >2.7, the flame can be treated essentially as a diffusion flame. For mixtures inside the rich limit of flammability (<1.64), two reaction zones are completely separated, and the inner flame plays the role of fuel (hydrogen and carbon monoxide) supply to the outer diffusion flame. For <2.7, the stagnation point lies on the air side of the inner flame. Even for mixtures outside the rich limit of flammability (1.64<<2.7), the inner flame maintains the characteristics of a self-sustaining flame and has a propagation velocity, although the inner flame is affected thermally by the outer flame. A flame can propagate in the fuel-air mixture outside the rich limit of flammability if the adiabatic state is retained or if heat is added from the burned side of the flame. Besides the rich limit of flammability usually defined, there is another limit of flame propagation. In the mixture beyond this limit a flame will never propagate. This limit can be regarded as fundamental in the sense that it depends only on the internal properties of the mixture. The equivalence ratio of this limit is 2.7 for methane-air mixture.