Autoignition stabilization of a premixed jet in vitiated coflow

Next generation combustor technology for gas turbine engines may include lean premixed mixtures injected into high temperature vitiated products such that autoignition can become a potential stabilization mechanism. Fundamental understanding of when autoignition occurs in these complex flowfields is crucial to model future combustors. A simplified flowfield of a premixed jet in vitiated coflow was used in this work to understand how mixing between the two flows can stabilize a steady, laminar autoignition stabilized flame. Chemical simulations in conjunction with both formaldehyde and hydroxyl planar laser induced florescence measurements were performed to identify trends between different cases which influence the stabilization point. Simulations suggest that autoignition should occur much faster than the experimentally observed flow timescale to the stabilization point. Trends between simulation concentrations and experimental fluorescence signals show similarities, but at different timescales. Experiments show that the formaldehyde layer width, which can be thought of as marking the radical pool width, does not correlate to changes in the stability point. Peak formaldehyde signals at the stability point were less than 5% different across all cases examined, indicating that a critical concentration of formaldehyde and radicals formed prior to the flame is necessary to sustain the flame base. However, the flow time to reach the peak levels is inconsistent with the chemical simulation time scales. Therefore, it is proposed that the mixture fraction gradient significantly delays the ignition time.