Dynamics and structure of unsteady, strained, laminar premixed flames
- Univ. of Southern California, Los Angeles, CA (United States). Dept. of Mechanical Engineering
The influence of unsteadiness on the structure and dynamics of counterflowing, strained, premixed methane/air flames was investigated by conducting a detailed numerical study. The unsteady conservation equations of mass, momentum, energy, and species were solved along the stagnation streamline of the counterflow, in a opposed-jet, finite-domain configuration, by using detailed description of chemistry and transport. The unsteadiness was introduced by imposing sinusoidal variations of the reactant velocity at the exit of the nozzles. Results demonstrate that the flame response is quasi-steady at low frequencies, while at high frequencies it fails to respond to the imposed velocity oscillations. The flame frequency response was physically explained by analyzing the instantaneous flame structure and realizing that the velocity oscillations in the hydrodynamic zone cause reactant concentration oscillations in the diffusion zone. The diffusion processes attenuate these oscillations in a manner analogous to the Stokes` second problem, and the angular frequency at which substantial attenuation commences was found to be of the order of [2(S{degree}{sub u}){sup 2}/a], where S{degree}{sub u} is the laminar flame speed and a an average heat diffusivity of the mixture. The validity of the analogy was confirmed by examining flames at a wide range of unsteadiness. The effect of unsteadiness on extinction was also assessed, and the phenomena of partial extinction, reignition, and extinction delay were identified. These phenomena were found to be controlled by the relative magnitudes of the time scales of the imposed oscillations and the time required for permanent extinction.
- OSTI ID:
- 93289
- Report Number(s):
- CONF-940711-; TRN: IM9537%%333
- Resource Relation:
- Conference: 25. international symposium on combustion, Irvine, CA (United States), 31 Jul - 5 Aug 1994; Other Information: PBD: 1994; Related Information: Is Part Of Twenty-fifth symposium (international) on combustion; PB: 1838 p.
- Country of Publication:
- United States
- Language:
- English
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