Stability of diffusion flame formed in a laminar flat plate boundary layer (effect of fuel dilution)

A stability limit of the diffusion flame with fuel injection from a porous wall in a laminar flat plate boundary layer is measured as functions of fuel (CH₄) concentration of CH₄/N₂ injectant mixture (x) and its injection velocity (v). The free stream velocity (U8) is set as 0.6 m/s. The thermal condition at the wall is controlled by setting temperature at the upstream end of the porous wall as a reference temperature. When v>20 mm/s, the flame becomes unstable with the separation of leading flame edge with decreasing x. The value of x at the stability limit is constant without regard to v as long as the wall temperatrure is kept constant. As the wall temperature is decreased the value of x increases. The separation is supposed to take place as a result of the limit of the reaction rate. When υ<20 mm/s, the flame becomes unstable with the oscillation. The value of x at the stability limit increases drastically with decreasing v. The oscillation takes place mainly due to the repeat of the extinction due to heat loss to the wall and the flame propagation in the combustible layer.