Strain-curvature coupling in harmonically excited, turbulent premixed flames

Premixed flames in turbulent, shear flows are disturbed by a superposition of broadband, turbulent fluctuations, and coherent disturbances originating from acoustic disturbances and/or hydrodynamic instabilities. Prior studies have shown nonlinear interactions between such coherent and stochastic disturbances on the flame. This paper investigates the effect of these interactions on the correlation between tangential flame stretch and curvature, an important quantity in turbulent premixed flame physics. High speed particle image velocimetry is used to measure the flow field in an experimental facility where harmonic wrinkles are introduced into a premixed flame that is subjected to varying turbulence intensities, while Mie scattering images are used to extract flame edges. Results show a clear correlation between phase-averaged tangential flame stretch and flame curvature. The magnitude of the fluctuations in both of these quantities decreases with increase in turbulence intensity, even while their overall correlation remains strong. Results from a linearized analysis of the perturbation of the approach flow by a curved, laminar flame are shown to capture the key features of the experimental data. These results suggest that the phase-averaged tangential stretch-curvature correlation is due to gas expansion effects, causing divergence of the approach flow in regions of positive flame curvature, the same mechanism that is responsible for the Darrieus-Landau instability. This result shows that turbulent flames in shear flows with large scale coherent structures, are subjected to not only mean strain, but also coherent modulations in strain rate. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.