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Structure and Response of Spherical Diffusion Flames (s-Flame)

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Results from the Structure and Response of Spherical Diffusion Flames (s-Flame) Experiment on the International Space Station (ISS)

The s-Flame Experiment science team (Princeton U., Rutgers U., and Glenn Research Center) completed the second and final round of tests in July. Those tests, conducted as part of the Advanced Combustion via Microgravity Experiments Project, featured helium dilution of the fuel and chamber atmosphere as a substitute for nitrogen. Preliminary examination of the results shows that the structure and dynamics of the spherical flames were greatly affected by helium dilution. Helium’s large thermal conductivity increased burner heating and appeared to increase the characteristic flame size (perhaps due to the lower density). The helium furthermore affected the quasi-steady flame behavior and extinction dynamics, where detailed analysis, including computational modeling, of the flame instability and extinction limits is underway. The long-duration microgravity environment afforded by the ISS eliminates buoyancy-induced convection enabling the science team to study flame ignition, growth and extinction in a simplified geometry not achievable in terrestrial laboratories.

The purpose of the Spherical Flame (s-Flame) experiment is to advance our ability to predict the structure and dynamics, including extinction, of both soot-free and sooty flames. The spherical flame, which is only possible in microgravity, will be created through use of a porous spherical burner from which a fuel/inert gas mixture will issue into the CIR chamber. Flames will be ignited at non-steady conditions and allowed to transition naturally toward extinction. Tests will be conducted with various inert diluents, in both the fuel and chamber atmosphere. The fuel gases include hydrogen and methane for soot-free flames, and ethylene for sooty flames. One experiment objective is to identify the extinction limits for both radiative and convective extinction (i.e., at high and low system Damkohler numbers, respectively). Another objective is to determine the existence, onset, and nature of pulsating instabilities that have been theoretically predicted to occur in such flames with fuel/diluent mixtures that are above a critical Lewis number.

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