High-Speed Hydroxyl and Methylidyne Chemiluminescence Imaging Diagnostics in Spherically Expanding Flames

Fundamental understanding of chemical kinetics pathways involved in ignition and flame propagation in hydrocarbon flames is essential for designing efficient and clean combustion devices for aerospace applications. In this study, spherically propagating methane-air and methane-ethane-air flames inside a constant-volume vessel were characterized using a species-specific, high-speed chemiluminescence diagnostic to reveal the position of the spatially and temporally resolved flame front and the primary combustion zone. The emission recorded by a high-speed camera with an image intensifier was used to investigate the effects of equivalence ratio on the chemiluminescence from electronically excited hydroxyl (OH*) and methylidyne (CH*) radicals. High-speed movies of OH*, CH*, and total broadband signals were recorded. The line-of-sight integrated images were Abel-inverted to obtain the spatially resolved two-dimensional flame structure and the temporal evolution of radical zone thickness. Features such as direct laminar flame-speed determination, flame wrinkling at elevated pressures, and the reaction zones of flames propagating under well-characterized initial turbulent conditions were all well-captured by the high-speed chemiluminescence imaging. In addition to visualizing the flame fronts and subsequent determination of the flame speed, this technique provides the simultaneous information of reactive chemical species that is not available using commonly used schlieren-based imaging methods.

Automated summary

Understanding the chemical kinetics involved in hydrocarbon flame ignition and propagation is crucial for developing efficient and clean combustion devices for aerospace applications. This study utilized high-speed chemiluminescence diagnostics to characterize the spatial and temporal flame front and primary combustion zone in methane-air and methane-ethane-air flames, providing insights into the effects of equivalence ratio on electronically excited radicals like OH* and CH*. High-speed imaging techniques allowed for direct flame-speed determination and observation of flame characteristics under various conditions.