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Interactions of turbulence, molecular and energy transport coupled with chemistry play a crucial role in the evolution of flame surface geometry, propagation, annihilation and local extinction/re-ignition characteristics of intensely turbulent premixed flames. This study seeks to understand how these interactions affect flame surface annihilation of lean hydrogen-air premixed turbulent flames. Direct numerical simulations (DNS) are conducted with detailed reaction mechanism and transport properties for hydrogen-air flames, at different parametric conditions. Flame particle tracking (FPT) technique is used to follow specific flame surface segments. An analytical expression for the local displacement flame speed (S d ) of a temperature isosurface is considered and the contributions of transport, chemistry and kinematics on the displacement flame speed at different turbulence-flame interaction conditions are identified. In general, the displacement flame speed for the flame particles is found to increase with time for all conditions considered. This is because, eventually all flame surfaces and their resident flame particles approach annihilation by reactant island formation at the end of stretching and folding processes induced by turbulence. Principal curvature evolution statistics obtained using FPT suggest that these islands are ellipsoidal on average, enclosing fresh reactants. Further examinations show that the increase in S d is caused by the increased negative curvature of the flame surface and eventual homogenization of temperature gradients, as these reactant islands shrink due to flame propagation and turbulent mixing. Finally, the evolution of the normalized, averaged, displacement flame speed vs. stretch Karlovitz number was found to collapse on a narrow band, suggesting that a unified description of flame speed dependence on stretch rate may be possible in the Lagrangian description.

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Paper published as part of the special topic on In Memory of Edward E. (Ted) O'Brien ARTICLES YOU MAY BE INTERESTED IN Turbulence topology evolution in weakly turbulent premixed flames

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