Multi-modal counterflow flames under autoignitive conditions

In practical systems, combustion occurs in multi-model regimes rather than in asymptotic limits of nonpremixed flames, premixed flames, or autoignition as commonly assumed in turbulent combustion models. In canonical configurations, multi-modal combustion is critical in the stabilization of lifted jet flames. At low temperatures, stabilization occurs kinetmatically through a "triple" flame with regions of both premixed and nonpremixed combustion. At high temperatures, autoignition is activated, and the role of autoignition versus premixed flame propagation for flame stabilization depends on the residence times and flow speed. While detailed simulations of laminar lifted jet flames are computationally tractable, for turbulent lifted jet flames, the lift-off heights require very large computational domains, and simulations of such flames with DNS becomes extremely expensive. Therefore, in work, the counterflow configuration is investigate as a more compact alternative. A series of detailed simulations of DME/air counter flames at elevated pressure are performed spanning a range of boundary conditions in both streams. The counterflow configuration is shown to exhibit multi-reaction zone structures, that is double and triple "flame" structures, analogous to lifted jet flames but only if the fuel and oxidizer streams are changes from pure components to partially premixed components. In other words, fuel/air boundary conditions gives only one reaction zone, but, under certain conditions, rich/lean boundary conditions can give up to three reaction zones.