PDF calculations of turbulent nonpremixed flames with local extinction

A joint velocity-composition-turbulence frequency probability density function (PDF) model is used to calculate a series of piloted-jet flames of methane. The ingredients of the present model include the simplified Langevin model for velocity, a stochastic model of turbulence frequency, the Euclidean minimum spanning tree (EMST) mixing model, and the 16-species augmented reduced mechanism for methane. The solutions are obtained using a particle/mesh algorithm, and the chemistry mechanism is implemented via the in situ adaptive tabulation (ISAT) algorithm. These flames exhibit an increasing amount of local extinction with increasing jet velocity, and are good cases to test the capabilities of turbulent combustion models to account for local extinction in turbulent nonpremixed flames. The calculation results are compared extensively with the experimental data, and demonstrate the ability of the PDF model to represent, quantitatively, the processes of local extinction and reignition that occur in these flames. (C) 2000 by The Combustion Institute.