Scaling transition of turbulent flame speed for thermodiffusively unstable flames

This work presents an experimental set of Bunsen flames characterized by a moderate Reynolds number and a variable turbulence intensity. Ten lean hydrogen-enriched methane–air mixtures at three turbulence levels are investigated, ranging from methane–air to hydrogen–air mixtures. Such mixtures are selected to have an almost constant laminar flame speed while inducing the onset of thermal-diffusive (TD) instability by gradually increasing the hydrogen content of the blend. The flames' global consumption speed, stretch factor, and flame surface area are investigated and discussed as functions of the effective Lewis number of the mixture. As the interplay between TD instability and turbulence enhances the overall flame propagation, below a transitional Lewis number, flames are observed to be particularly sensitive to external turbulent forcing. This synergistic interaction is discussed in terms of Karlovitz and Lewis numbers. A parameterization of the turbulent flame speed is thus proposed, based on a functional form depending, concurrently, on both Karlovitz and Lewis numbers. The proposed form is shown to fit the experimental results at different turbulence levels and to capture the flame speed enhancement across the transitional Lewis number.