This study analyzes the structure of highly curved, lean, premixed flames in the tubular geometry using both advanced laser diagnostics and detailed computer simulations. Fuels studied are hydrogen, methane, and propane. A tubular, premixed flame, where premixed reactants flow radially inward to form a stretched cylindrical flame, is an ideal fundamental configuration for the numerical study of curvature and stretch. The tubular configuration can be mathematically simplified to a two-point boundary-value problem where solutions incorporating complex chemistry and detailed molecular transport can be determined in a short time on a personal computer. The study uses a unique optically accessible tubular burner that incorporates optical ports for application of advanced laser diagnostics (e.g., Raman scattering and fluorescence). Recent Raman measurements of gas composition and temperature in lean hydrogen-air tubular flames show excellent agreement with calculations at low stretch values. However at high stretch, large theory-data discrepancies point to the need for improvement in chemical and/or molecular-transport modeling in lean tubular flames near extinction. This work studies lean premixed tubular flames with advanced laser diagnostics and numerical models incorporating detailed transport and complex chemistry. Near-extinction tubular flames are studied to determine the effects of molecular transport and chemical mechanism on the model-data comparisons. The studies focus on high-curvature flames where the flame radius to flame thickness is near unity. To vary Lewis number effects, various fuels are studied. The arc-length continuation method is used to properly predict the extinction condition. A full two-dimensional model of the tubular flame is developed to check the self-similarity assumption near extinction. Finally, the premixed tubular flame interaction with surfaces is studied with implications for design of micro-combustors.
Broader Impact
This study unites the capabilities of an advanced laser diagnostics laboratory at Vanderbilt University and a state-of-the-art combustion simulation group at Yale University. Graduate and undergraduate students including underrepresented minorities are involved in the research. Lean fuel/air premixing produces clean and efficient combustion in gasoline-fueled automobiles and methane-fueled gas turbines for electric power generation. Lean premixed flames in practical devices are stretched and curved by turbulence but there are few comprehensive experimental-numerical studies of the effects of both curvature and stretch. Most previous studies have focused on stretched planar premixed flames. Curvature in premixed flames is important as the combination of differential diffusion and curvature can greatly affect the flame temperature, gas composition, and pollutant formation. Flame extinction can be retarded or promoted by curvature and curved flames can exist below the standard lean flammability limit; sublimit. flames have the promise of producing very low pollutant emission at high levels of efficiency. Students will present their results at research conferences. Students will be exposed to the latest computer simulation and laser diagnostic facilities. Faculty will mentor junior and senior high-school students in engineering.
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