Lean or ultra-lean stretched planar methane/air flames |
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Authors: | Zhongxian Cheng Joseph A Wehrmeyer Robert W Pitz |
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Institution: | aMechanical Engineering Department, Vanderbilt University, Box 1592 Station B, Nashville, TN 37235, USA |
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Abstract: | Lean premixed combustion has potential advantages of reducing pollutants and improving fuel economy. In some lean engine concepts, the fuel is directly injected into the combustion chamber resulting in a distribution of lean fuel/air mixtures. In this case, very lean mixtures can burn when supported by hot products from more strongly burning flames. This study examines the downstream interaction of opposed jets of a lean-limit CH4/air mixture vs. a lean H2/air flame. The CH4 mixtures are near or below the lean flammability limit. The flame composition is measured by laser-induced Raman scattering and is compared to numerical simulations with detailed chemistry and molecular transport including the Soret effect. Several sub-limit lean CH4/air flames supported by the products from the lean H2/air flame are studied, and a small amount of CO2 product (around 1% mole fraction) is formed in a “negative flame speed” flame where the weak CH4/air mixture diffuses across the stagnation plane into the hot products from the H2/air flame. Raman scattering measurements of temperature and species concentration are compared to detailed simulations using GRI-3.0, C1, and C2 chemical kinetic mechanisms, with good agreement obtained in the lean-limit or sub-limit flames. Stronger self-propagating CH4/air mixtures result in a much higher concentration of product (around 6% CO2 mole fraction), and the simulation results are sensitive to the specific chemical mechanism. These model-data comparisons for stronger CH4/air flames improve when using either the C2 or the Williams mechanisms. |
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Keywords: | Hot products Ultra-lean Sub-limit Counterflow flames Raman scattering |
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