Structural Studies of Locally Strained Diffusion Flames : Analysis on Characteristics of Positively and Negatively Stretched Flames

Numerical analysis is performed on the H_2/N_2-air laminar counterflow diffusion flames affected by positive or negative stretch rate induced by locally sucking flow from fuel and air sides. Adjusting the suction velocity leads to the formation of three typical flame shapes, flat, or with concave or convex curvature. Numerical computations taking into account detailed chemical kinetics and multicomponent diffusion clarify the effects of negative stretch rate on the flame structure and characteristics. In addition, the effect of a preferential diffusion in relation to the flame curvature under the negative stretch regime is discussed. The results show that (1) Temperature increases with decreasing the stretch rate, and the tendency remains even in the regime of negative stretch rate. (2) H_2 concentration due to the preferential diffusion and excess enthalpy due to the non-unity Lewis number effect become relatively significant with decreasing stretch rate. This is one of the reasons for (1). (3) Temperature increase due to the negative stretch rate depends on the flame curvature. (4) The maximum flame temperature cannot be rationalized by the local stretch rate, and changes over a wide range depending on the preferential diffusion in relation to flame curvature.