Origin of the bending of turbulent burning velocity at high intensity
In plain words
Stirring a premixed flame harder makes it burn faster, but at strong stirring the gain levels off or even reverses. Is this caused by stretching of the flame surface, local blow-out of the flame, unequal diffusion of fuel and heat, or by how burning speed is defined in each experiment?
Precise statement
For premixed flames the turbulent burning velocity $S_T/S_L$ grows nearly linearly with $u'/S_L$ at low intensity and then bends to slower growth at $u'/S_L$ from a few to several tens depending on geometry and mixture; whether the onset is set by the Karlovitz number Ka (flame time over smallest-eddy time) is part of the question. Identify the mechanism (saturation of flame-surface production by stretch, local extinction, Lewis-number Le and differential-diffusion effects, geometry-dependent definitions of $S_T$, finite residence time) and give a predictive $S_T(u'/S_L, L/\delta_L, \mathrm{Le}, \mathrm{Ka})$. An answer is a closed theory confirmed by direct numerical simulation and by Bunsen, spherical and stagnation-flame experiments.
What would settle it
Direct numerical simulations with detailed chemistry across Ka and Le, analysed with a common $S_T$ definition and matched to experiments in at least two flame geometries.