Synergistic interplay of thermodiffusive instability and turbulence in premixed flames

In this work, we experimentally analyze the interplay of thermo-diffusive (TD) intrinsic flame instabilities and turbulence in premixed flame propagation. We utilize methane/hydrogen/air Bunsen flames at atmospheric pressure and variable hydrogen content, and variable turbulence intensity. Experiments are designed to maintain the laminar unstretched premixed flame speed constant by adjusting the equivalence ratio 𝜙 for each flame. As the hydrogen content is increased and 𝜙 is decreased, thermo-diffusive intrinsic flame instabilities are gradually promoted. We study the effect of thermo-diffusive instability on the global consumption speed by analyzing the contribution of flame surface area increase and flame mean reactivity measured via a stretch factor. We observe that the turbulence-instability interplay mainly occurs through an enhancement of flame reactivity and not flame area. In addition, a power spectral density (PSD) analysis of the flame curvature reveals that the spectra of unstable flames are consistently more energetic due to the wider range of linearly unstable scales interacting with the turbulent integral scale. A forced weakly nonlinear numerical model is also utilized to aid in the understanding of the experimental findings. The model exhibits a characteristic unforced PSD, representing the energy content of the typical spatiotemporal chaotic TD-unstable solution. When forced, the model exhibits PSD that emerge from the interplay of the turbulent spectrum and the characteristic TD-unstable spectrum, and, as a result are consistently more energetic than the TD-stable spectra.