Multi-modal counterflow flame structure under autoignitive conditions

In practical systems, combustion does not occur in asymptotic limits of nonpremixed flames, premixed flames, or autoignition but rather multiple modes that interact. In canonical configurations, multi-modal combustion is critical in the stabilization of lifted jet flames. At low temperature conditions, such flames are stabilized by a 'triple' flame consisting of regions of premixed and nonpremixed combustion. At high temperature conditions, autoignition is activated, and the role of autoignition versus premixed flame propagation in flame stabilization depends on the local residence time and the local flow speed. While detailed simulations of laminar lifted jet flames are computationally tractable, extension to DNS of turbulent lifted jet flames at reasonable Reynolds numbers is computationally intractable due to the large domain size required. Therefore, the counterflow configuration is investigated as a more compact alternative. In this work, a series of detailed simulations of DME/air laminar counterflow flames at elevated pressure are performed with variations in the stream compositions, temperatures, and velocities to provide flames spanning different combinations of combustion modes, specifically a nonpremixed flame, a 'triple' flame, and a series of flames with all three modes interacting. Similarities and differences between the counterflow flames and the lifted jet flames are explored.