The nonsteady behaviour of counterflow diffusion hydrogen-air flames is studied in this article, using a finite difference implicit method and a complex kinetics model. The flame responses to step and sinusoidal strain rale variations are obtained for flames submitted to moderate strain rates and also to strain rates corresponding to extinction conditions. Frequency response curves are obtained for both cases and for different values of the equivalence ratio. The behaviour of flame characteristics like the maximum temperature, the heat release rate and typical species mass fraction profiles is examined. It is found that H2O and OH radicals change approximately like the maximum temperature while hydrogen atom H mass fraction exhibits larger excursion. The dynamics of the flame are governed by the imposed mean strain rate on one hand and by the critical extinction strain rate on the other. Far from the extinction limit the cut-off frequency is set by the mean strain rate and the flame behaviour is essentially linear. Close to the extinction limit the flame behaves non-linearly. It may be extinguished at low frequencies when the instantaneous strain rate exceeds the critical extinction value by a sufficient amount and for a sufficiently long period of time.
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