Models for high-intensity turbulent combustion

Since direct numerical simulation of the Navier-Stokes plus combustion chemistry equations will not be practical in the foreseeable future, models are required for the parameter range of practical interest, i.e. high Reynolds Numbers and a wide range of Damkohler Numbers. Models based on the notion of a flamelet are not appropriate when the turbulence intensity is much greater than the laminar flame speed, but a stochastic model based on the joint PDF of velocity and composition is promising. If the velocity field and inhomogeneities in physical space are ignored in the joint PDF equation, the “Partially Stirred Reactor” or PaSR model is obtained. The PaSR model has recently been studied in detail. Full chemical schemes are computationally tractable. Because the composition PDF has a large number of dimensions (e.g. N_s > 20 for methane), finite-element/volume techniques are not viable, but particle-tracking Monte-Carlo algorithms work well. An enabling feature of the PaSR is that, with the IEM scalar mixing sub-model, it is well suited to parallel computers. The PaSR can describe the effect of turbulence (coupled to a full kinetic scheme) on combustion, including the behavior of emissions such as NO_x and CO, of minor species such as free radicals, and the ignition-extinction bifurcation.