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PLIF imaging is used to identify the internal structure of a partially-premixed lifted turbulent jet flame within a cross-flow of air that is heated to a temperature of 1364 K, which is above the autoignition temperature. Large aerodynamic strain rates were imposed since the velocity of the cross-flow air is 480 m/s. A jet flame-in-a-cross-flow is a simple unit physics problem for which scaling relation are known for the non-reacting case. However, the present work addresses the roles of two factors: autoignition chemistry and large aerodynamic strain rates. This unit physics problem is important in propulsion devices, including ramjets and in internal combustion engines. Formaldehyde PLIF is used identify the distributed auto-ignition regions, while CH and OH PLIF identifies the (downstream) primary reaction regions. The primary reactions are shown by the CH images to have the structure of broken and thickened flamelets under these high-strain rate conditions. That is, the CH occurs within a 2-3 mm thick wrinkled interface that is not continuous but contains many holes due to local extinction. The formaldehyde PLIF images show that there is a broad 10 mm thick region of formaldehyde upstream of the primary thickened flamelets. This broad formaldehyde region indicates that distributed (non-flamelet) reactions occur early in the chemical reaction process due to auto-ignition. Therefore we classify this combustion to be an “auto-ignition-assisted flame”. The measured profiles of heat release rate are presented, which are useful in assessing modeling efforts.

Stratified Jet Flames in a Heated (1364K) Cross-Flow with Auto-ignition