Stretching and quenching of flamelets in premixed turbulent combustion

The stretch rate of flamelets in premixed turbulent combustion is computed using (1) detailed numerical simulations of vortex-flame interactions and (2) a model for intermittent turbulence taking into account all possible turbulence scales acting on the flame front. Simulations of interactions between isolated vortices and a laminar flame front are used to obtain a relation between the characteristics of a given vortex and the actual flame stretch generated by this structure. Quenching conditions and quenching times are also given by these simulations. A net rate of stretch is then defined in the case of a complete turbulent flow field as the difference between the total rate of flame stretch and the quenching rate due to scales that have a high enough energy and a long enough lifetime to quench locally the flame front. The net rate of stretch is computed for a variety of parameters of interest in practical applications. It is a function of the large-scale turbulence parameters and the laminar flame speed and flame thickness and may be used as an input in most flamelet models for premixed turbulent combustion. Different criteria for total flame quenching in premixed turbulent combustion are derived and compared (1) to the classical Klimov-Williams theory, (2) to a criterion proposed by Poinsot et al. [8, 9], who studied quenching according to the presence near the flame front of a single eddy able to locally quench combustion, and (3) to the experimental results of Abdel-Gayed and Bradley [6, 7].