Stratified jet flames in a heated (1390 K) air cross-flow with autoignition

Measurements are reported of the heat release profiles, the flame lengths, flame structure and other properties of a reacting jet-in-cross-flow (JICF) for two fuels. The air was heated to a static temperature of 1390 K, which is above the autoignition temperature, and the air velocity was 468 m/s, which is much larger than values that were considered previously. Aerodynamic strain rates are so large that the flame was expected to fall into either the “distributed reaction”, “thickened flamelet”, or “shredded flamelet” regimes. Fluorescence images of CH, OH and formaldehyde identified the flame structure. The jet-in-cross-flow is a unit physics problem that occurs in turbojets and scramjets. While scaling relations are known for the non-reacting case, more information about the reacting case is needed, especially when autoignition and strain rates become important. Three regions were identified. In the liftoff region autoignition reactions occur which create a strong formaldehyde PLIF signal. However, flames and heat release do not occur in the liftoff region since CH and CH∗ signals were negligible. The second region is the lifted flame base, which has the character of a premixed flame, as evidenced by a very rapid rise in the heat release rate as indicated by the CH∗ and OH∗ signals. The third region contains a turbulent non-premixed flame and the CH images indicate the presence of thickened and shredded flamelets. The 2–3 mm thickness of each CH layer is more than 10 times the laminar flamelet thickness. In the third region the heat release rate decays slowly downstream, which is typical of a non-premixed flame. Because both upstream autoignition and downstream thickened flamelets were observed, we classify this combustion to be an “autoignition-assisted flame”. Flame lengths increase linearly with fuel mass flow rate, indicating that mixing is controlled by the air velocity rather than the fuel velocity.