Edinburgh Napier University

  To investigate the turbulent flame speed at high Karlovitz number (Ka) conditions, high fidelity direct numerical simulations (DNS) of lean hydrogen/air premixed flames propagating in a channel are performed with forced turbulence. The turbulent flame speed is analyzed with global and local perspectives. The global flame speed is evaluated from the fuel consumption rate while the local flame speed is computed from the displacement speed of the fuel species. It is found that for the global turbulent flame speed, the integral length scale plays a more important role rather than the turbulent intensity in that larger integral scales generate larger flame surface area which leads to the flame speed enhancement. The normalized flame speed is well correlated with the flame surface area, confirming that Damkhöler’s first hypothesis is still valid even at high Ka conditions up to Ka ≈ 700. Moreover, the local displacement speed with the statistic approach shows that the peak of the histogram of the displacement speed is found to nearly match the one computed from the one-dimensional laminar flame, implying that most of the turbulent flame elements burn like the laminar flame.