Curvature and velocity strain dependencies of burning speed in a turbulent premixed jet flame

In this work the dependency of the turbulent burning speed on flame stretch in a premixed jet flame is analyzed. Considering a reference system attached to the front, the flame stretch is split into three contributions based on flame front curvature, normal fluid velocity and divergence of tangential velocity. The turbulent burning velocity is derived from the measure of the divergence of the mean unconditioned velocity field, that is taken as an estimate of the mean reaction rate in the context of flamelet hypothesis. The results are in a reasonable agreement with the literature data on turbulent combustion rates. Though the present methodology is more complex than the usual one based on reactant consumption rate, it provides the local burning speed and not the overall one. Combining these measurements with the local flame stretch, we show that, for a given flame, it exists a wide region along the flame height where the increase of the local flame speed in respect to the laminar unstretched one (stretching factor) is constant. Since the Reynolds number controls the small-scale behavior of turbulence, these findings denote a direct connection between the local, turbulence-induced, flame front deformation and the increase of the local flame propagation speed. The aim of this work is to establish correlations between the three different terms of flame stretch and the turbulent combustion speed that can lead to the definition of suitable closure models for turbulent combustion numerical simulations.