2016 Volume 2 Pages 16-00374
The criterion for transition from a quasi-steady state to a non-steady state of laminar stagnating lean premixed flame in the presence of oscillation in fuel concentration is discussed experimentally. Two types of mixtures with different Lewis numbers are examined: lean methane/air and lean propane/air mixtures. Sinusoidal oscillation in fuel concentration is effected by an oscillator with two cylinder-piston units that supply leaner and richer mixtures alternately. The flame response under fuel concentration oscillation is measured as a function of frequency. The frequency of fuel concentration oscillation varies within 2-20 Hz, and the burner exit velocity varies within 0.6-1.1 m/s. For both mixtures, oscillator characteristics cause the amplitude of oscillation of the flame position to increase with frequency. The increase in amplitude becomes less sharp as frequency increases further. In the present study, two additional Strouhal numbers, one based on heat transfer and another based on mass transfer, are newly introduced in addition to the ordinary Strouhal number which is based on momentum transfer. When any of these Strouhal numbers exceeds unity, the increase in the amplitude starts to fall off. This indicates that the three Strouhal numbers play equally important role on phenomena with momentum, heat and mass transfer, such as combustion.
