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Velocity profiles measured across wrinkled flames--to assess flamelet assumptions

Velocity profiles were measured across a freely propagating premixed flame, as the flame undergoes a wrinkling process, in order to assess certain flamelet assumptions. Several models have been developed that use flamelet theory, as proposed by N. Peters (1986), which hypothesizes that a segment of a turbulent flame will retain some of the properties of an analogous laminar counterflow flame. Counterflow flames have idealized properties, including (a) zero vorticity everywhere, (b) zero strain rate variations in the reactants, (c) larger velocity gradients in the products than in the reactants, and (d) tangential gradients that are negligible compared to normal gradients. Axisymmetric wrinkling of a premixed flame was achieved by interacting a flame with a vortex to achieve conditions typical of a segment of a turbulent flame: the wrinkle size is 4 times the flame thickness, and rotational velocities were 3-10 times the laminar flame speed. It was found that during the wrinkling process the flamelet assumptions were valid providing that conditions were far from extinction (Case A) or were approaching extinction (Case B). After the onset of local extinction (Case C) it is shown that the flamelet assumptions break down. Prior to extinction, the velocity profiles and strain rate profiles did maintain the general features of counterflow flames, and tangential gradients were negligible compared to normal gradients. Surprisingly, even the presence of intense large-scale vorticity did not invalidate the flamelet assumptions.