Synergistic interactions of thermodiffusive instabilities and turbulence in lean hydrogen flames

Abstract

Interactions of thermodiffusive instabilities and turbulence have been investigated by large-scale Direct Numerical Simulations (DNS) in this work. Two DNS of turbulent premixed lean hydrogen/air flames have been performed in a slot burner configuration at the same jet Reynolds number of Re=11,000 and Karlovitz number of Ka≈15 using a detailed chemical mechanism. Realistic transport models are employed in one case, which features the characteristic patterns of thermodiffusively unstable flames, such as strong variations of the heat release and super-adiabatic temperatures. In the other case, the diffusivities of all species are set equal to the thermal diffusivity (unity Lewis numbers assumption) and thermodiffusive instabilities are therefore suppressed.The local burning of the turbulent flame without thermodiffusive instabilities is similar to an unstretched laminar flame and the turbulent flame speed increases only due to the increase of flame surface area in agreement with previous studies for flames at similar conditions. In contrast, the thermodiffusively unstable flame features a strong enhancement of the turbulent flame speed, which is not only caused by flame wrinkling, but is greatly increased due to significant variations of the local reaction rates. These are caused by variations of the local equivalence ratio due to the differential diffusion of hydrogen. A comparison with a thermodiffusively unstable laminar flame at the same conditions reveals that the variations of the local equivalence ratio and local reaction rates are significantly enhanced in the turbulent flame due to higher fluctuations of curvature and an enhanced average strain rate induced by turbulence. Thus, turbulence and thermodiffusive instabilities show synergistic effects, which are reflected in a significantly higher fuel consumption rate per flame surface area. The flame surface area generation, which is governed by the tangential strain and the flame propagation in curved flame segments, is also different in the two cases. Most noteworthy, the tangential strain rate is shown to be determined by the smallest turbulent structures in both turbulent flames and to be unaffected by the thermodiffusive instability mechanism. However, thermodiffusive instabilities lead to a production of flame surface area in convexly curved flame segments, featuring the formation of tongue-like structures that penetrate into the unburned gas, which do not exist in the turbulent flame with unity Lewis numbers. This is linked to an enhancement of the flame displacement speed with curvature in the thermodiffusively unstable flame, while in the absence of instabilities, a reduction of the flame displacement speed with increasing curvature is observed, leading to a destruction of flame surface area.These findings suggest that thermodiffusive instabilities are sustained in turbulent flows and even show synergistic interactions with turbulence, which needs to be accounted for in turbulent combustion models.