Unsteady Strained Premixed Laminar Flames

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Abstract
The transient response of laminar flames to strain is studied numerically using a simplifying transformation not previously applied to these problems. Two time scales were found to characterize the flame response during straining. For slow strain, a slow time scale associated with the preheat zone dominates. For high strain, a fast time scale related to the reaction zone is important. The cross-over between these time scales is a function of the activation energy and occurs when the reaction zone thickness is a significant fraction of the total flame thickness. The overall time constant of the flame varies with the Damköhler number to the 0.7 power. During relaxation from the strained state, the flame responds in a more self-similar manner and a single time constant suffices to describe its behaviour. The implications of these results for flamelet models based on the local strain rate are that flame response time and history effects should be included.

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