Effect of oscillatory stretch on the flame speed of wall-stagnating premixed flame

Abstract

The effect of unsteady stretch on the wall-stagnating lean methane, premixed flame was investigated experimentally. The influence of the oscillatory stretch on the flame speed was examined when the stagnation wall was oscillated sinusoidally in the direction of the main flow at a constant amplitude. Using LDV, the velocities of unburned gas and of flame in laboratory coordinate were measured to obtain flame speed (propagating velocity relative to unburned gas). The oscillatory characteristic of the flow field is indicated by the Strouhal number, representing the ratio of the angular frequency of the wall times the mean wall location to the flow velocity at the nozzle exit. There exist two factors dominating the oscillatory stretch, that is, the change of the distance between the wall and the nozzle, which is a quasi-steady factor, and the change of the velocity of the wall, which is an unsteady factor. When the flow is oscillated at low Strouhal number (≤1), the quasi-steady factor, which is independent of the Strouhal number, is dominant. And the oscillation of the flame speed is not observed. In this case, quasi-steady factor is not large enough to oscillate the flame speed since the ratio of the amplitude of the wall oscillation to the mean distance between the wall and the nozzle is small. On the other hand, when the flow is oscillated at high Strouhal number (≤1), the unsteady factor, which is proportional to the Strouhal number, becomes dominant. In this case, the oscillation of the flame speed is observed and the amplitude of the oscillation of flame speed increases with increasing Strouhal number.