DNS study of flame speed enhancement in turbulent premixed hydrogen flames

Hydrogen combustion is a desirable alternative to fossil fuel combustion due to the reduced emissions of pollutants such as greenhouse gases and soot. Lean hydrogen combustion has a propensity to develop thermodiffusive instabilities due to differential diffusion. In the presence of turbulence, these instabilities significantly enhance the flame speed, leading to safety concerns such as flame flashback. In this study, a suite of DNS are conducted across a range of Karlovitz numbers and integral length scales to study in detail this flame speed enhancement. The relationship between global effects (e.g. flame speed and area) and local effects (local flame structure) is studied through a generalized expression for the burning efficiency. The effect of detailed chemistry is discussed, in particular the effect of Soret diffusion. The integral length scale has a minor impact on the flame structure, whereas the turbulence intensity has a significant impact. As the Karlovitz number is increased, the flame structure asymptotes towards the unity Lewis flamelet solutions. Across the range of tested Karlovitz numbers, the flame speed and area are shown to increase before decreasing, while the burning efficiency continues to increase.