Structures and stability of lifted combustion zones in preheated oxidizer

The structures and stability of lifted combustion zones have been simulated with detailed chemistry and transport properties in an axisymmetric laminar fuel (CH ₄) jet and outer co-flow of the (O ₂ + N ₂) oxidizer whose initial temperature is 300 K, 700 K and 1 200 K. A set of numerical simulations was executed by increasing the N ₂ dilution ratio, Z (mole fraction of N ₂ in the oxidizer). The results showed that at 300 K, the lifted combustion zone had a triple flame structure where the rich premixed wing is smaller than the lean one and the trailing diffusion flame immediately inclined to the fuel side from the triple point as well as the leading edge of the triple flame was shifted away from the jet axis as Z increased. As the initial temperature increased, the combustion zones were lifted at larger Z values than the one at 300 K. Especially, for 1 200 K, it was found that the lifted combustion zones, when expressed in terms of the heat release rate, have become so weak that a flameless triple combustion zone was formed due to the high dilution ratio and high preheat temperature. The numerical simulations on the response of the lifted triple combustion zone to the initial fuel velocity were also carried out, and the results showed that the lifted combustion zone using a high preheated temperature was very stable in the near field.