Effects of different kinds of fuel and fuel equivalence ratio on flame structure of triple flame

In order to clarify the effects of different kinds of fuel and fuel equivalence ratio on flame structure, a numerical simulation of triple flame developed in a co-flowing methane-air or hydrogen-air mixture and air stream was made taking into account the elementary chemical reaction mechanism. The following conclusions were reached: (1)The relation between the apparent burning velocity of the triple flame and the fuel equivalence ratio shows a similar tendency to that of the one-dimensional premixed flame of the corresponding fuel. However, the fuel equivalence ratio at which the apparent burning velocity is the largest is a little larger than that of the one-dimensional premixed flame. The apparent burning velocities are two and three times higher than that of the one-dimensional premixed flame for the methane-air or hydrogen-air mixture. ( 2 )The flame thrusts out forward in the downstream of the boundary between mixture and air stream, and a part of the flow is bent and forks out in this protruding flame so that a triple flame is originated ; this triple flame is composed of fuel rich and lean premixed flame branches and a diffusion flame branch. The change in shape of the convex part, caused by the effect of the one-dimensional premixed flame, is further promoted by the effect of hydrodynamic instability originated in the expansion brought about by heat release. A considerably strong diffusion flame branch exists almost in the center of the two premixed flame branches for the methane-air mixture, while a considerably weak diffusion flame branch approaches the fuel lean premixed flame branch for the hydrogen-air mixture. ( 3 )Near the fuel equivalence ratio at which the burning velocity of the one-dimensional premixed flame is the largest, the effect of the one-dimensional premixed flame becomes large and the fuel rich premixed flame advances and becomes vertical to the flow direction. As a result, the effect of hydrodynamic instability is weakened. Thus, both of these effects demonstrate that the fuel equivalence ratio at which the apparent burning velocity is the largest is a little larger than that of the one-dimensional premixed flame.