Hot Jet Re-ignition of Hydrocarbons and Hydrogen in Air: Effect of Jet Chemical Activity

A jet of hot reactive gas can reliably ignite premixed gases in combustion engines. The presence of chemically active species in the hot jet may influence the time delay and ignitability limits for ignition of a combustible mixture. Rapid ignition and combustion may also involve complex interactions of pressure waves with flames in the transient jet, especially in a wave-rotor constant-volume combustor for gas turbine performance enhancement. In the present work, detailed numerical simulations are carried out to understand the relative role of chemical kinetics and turbulent mixing in a transient hot jet on ignition in a constant-volume combustor. A transient hot jet is modeled in two ways: as a relatively inert hot jet and as a radical-rich chemically active hot jet for slightly rich ethylene-air mixture. Combustion is modeled in the main constantvolume chamber for methane, hydrogen and blended methane-hydrogen mixtures. A hybrid eddy-break-up combustion model is applied, with detailed or reduced reaction mechanisms and the two-equation k-ω model. For stoichiometric methane mixture in the main chamber, the predicted ignition of the blended fuel mixture was surprisingly not any faster than that of pure methane. The ignition delay time of the more chemically active jet is slightly shorter than a jet of similar composition with only stable species. These observations generally point to a complex interaction of physical and chemical process in hot-jet ignition in a confined volume with shock dynamics and jet mixing processes playing a role that may be as important as chemical kinetics.