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.
|Publication status||Published - 6 Jan 2020|