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1. Flame-drift velocimetry and flame morphology measurements with dual-perspective imaging in a shock tube

   Susa, Adam J; Hanson, Ronald K (May 2021, 12th U.S. National Combustion Meeting)

   null (Ed.)

   The application of simultaneous, dual-perspective, high-speed imaging to expanding flame experiments in a shock tube provides new opportunities to characterize the post-reflected-shock flow field. The shock-tube flame speed method has recently been demonstrated as an experimental approach to enable flame speed measurements at high unburned-gas temperatures inaccessible to previously established methodologies. The fidelity of these experiments are predicated on two underlying assumptions: quiescence of the unburned gas and symmetry of the expanding flames. While both are ubiquitous in the related literature, neither of these assumptions had been previously explicitly evaluated in relation to shock-tube flame experiments. This work reports the first measurements in which side-wall emission imaging, in addition to simultaneous end-wall imaging, is applied to expanding flame experiments in a shock tube. The fact that the burned gas within an expanding flame is nominally stagnant relative to the local flow field is leveraged to perform single-point, 3D velocimetry measurements of the core gas based upon the motion of the flame centroid, or “flame drift”. These measurements reveal that minimal motion is present in the radial directions, while the velocity of the core gas in the axial direction is larger in magnitude and displays strong temperature dependence. The 3D morphology of flames is also characterized for the first time. Side wall imaging reveals that, while the expected flame symmetry is observed under some conditions, it breaks down under others, particularly at increasing temperatures. These results shed new light on previously reported flame structure observed in shock-tube flame experiments, which can now be explained as the axial integration of emission from an axially distorted flame. These observations serve as a demonstration of a novel diagnostic application, provide new insight as to how future shock-tube flame experiments might be refined, and motivate the continued use of side-wall imaging to ensure the fidelity of future shock-tube flame speed measurements. 

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2. Effect of Diluents on Flame Stability in Constant Flame Speed Mixtures for Piloted, Swirl-Stabilized Flames

   Rodriguez_Camacho, Javier; O'Connor, Jacqueline (March 2024, Combustion Institute)

   This study considers the effect of exhaust gas recirculation diluents on the static and dynamic stability of a swirl-stabilized flame in a model gas turbine combustor. The test matrix is designed such that the unstretched laminar flame speed of the mixture is maintained constant as the diluent level and composition is varied. Results show that the constant flame speed test matrices exhibit similar flame stability and shape. This can be attributed to the similar stretched flame properties that these conditions exhibit when the unstretched laminar flame speed is matched. 

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3. Cool-flame dodecane-droplet extinction diameters

   https://doi.org/10.1016/j.combustflame.2019.10.036  

   Williams, Forman A.; Nayagam, Vedha (February 2020, Combustion and Flame)
4. Double flame dynamics in hotspot ignition

   https://doi.org/10.1016/j.combustflame.2024.113660  

   Akita, Keisuke; Zhao, Peng; Morii, Youhi; Maruta, Kaoru (November 2024, Combustion and Flame)
5. Laminar Flame Speeds in Degenerate Oxygen–Neon Mixtures

   https://doi.org/10.3847/1538-4357/ab6f03  

   Schwab, Josiah; Farmer, R.; Timmes, F. X. (February 2020, The Astrophysical Journal)

   Abstract The collapse of degenerate oxygen–neon cores (i.e., electron-capture supernovae or accretion-induced collapse) proceeds through a phase in which a deflagration wave (“flame”) forms at or near the center and propagates through the star. In models, the assumed speed of this flame influences whether this process leads to an explosion or to the formation of a neutron star. We calculate the laminar flame speeds in degenerate oxygen–neon mixtures with compositions motivated by detailed stellar evolution models. These mixtures include trace amounts of carbon and have a lower electron fraction than those considered in previous work. We find that trace carbon has little effect on the flame speeds, but that material with electron fraction has laminar flame speeds that are times faster than those at . We provide tabulated flame speeds and a corresponding fitting function so that the impact of this difference can be assessed via full star hydrodynamical simulations of the collapse process. 

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