Curvature Effects in Turbulent Premixed Flames of H2/Air: a DNS Study with Reduced Chemistry

Data from a three-dimensional Direct Numerical Simulation of a turbulent premixed Bunsen flame at a low global Lewis number are analyzed to address the effects of the curvature on the local flame front. For this purpose, the chemical kinetics is modeled according to a reduced scheme, involving 5 reactions and 7 species, to mimic a H-2/Air flame at equivalence ratio I center dot=0.5. An increase of the local temperature and reaction rate is found for fronts elongated into the fresh gases (concave), while local quenching is observed for fronts elongated in the opposite direction (convex), i.e. towards the burnt mixture. Data show that the occurrence in the reaction region of these super-reactive (concave fronts) and quenched zones (convex fronts) is predominant compared to a behavior compatible with the corresponding unstretched laminar flame. In particular, well inside the reaction region, the probability density function of the OH radical concentration shows a bi-modal shape with peaks corresponding to negative (concave) and positive (convex) curvatures, while a locally flat front is less frequently detected. The two states are associated with a higher and lower chemical activity with respect the laminar case. Additional statistics conditioned to the local hydrogen concentration provide further information on this dual-state dynamics and on the differences with respect to the corresponding laminar unstretched flame when moving from the fresh to the burnt gas regions. Finally we discuss the effects of the turbulence on the thermo-diffusive instability showing that the turbulent fluctuations, increasing the flame front corrugations, are essentially responsible of the local flame quenching.