RANS and LES Modelling of Premixed Turbulent Combustion

Premixed combustion is becoming more common in practical combustion systems in response to increasing regulatory pressure to reduce unwanted emissions. The inherent ability of premixed flames to propagate into the unburned mixture leads to a more active response to turbulence by comparison with non-premixed flames. It is clear that the sheet-like reaction zone within the premixed flame is highly resistant to disruption by turbulent eddies. Hence flamelet structure is prevalent in premixed combustion over a broad range of turbulent velocity fluctuation magnitudes and turbulent eddy length scales. Modelling of turbulent transport in premixed flames is rendered more difficult by the occurence of countergradient transport in the presence of strong heat release. Modelling of the mean turbulent reaction rate has involved a variety of approaches involving either algebraic expressions or additional transport equations. A brief review of current modelling practice is presented, covering some simple models together with the flame surface density approach, the G equation and the more recent scalar dissipation rate model. The emphasis is on models that are applicable in the context of both Reynolds-averaged Navier Stokes (RANS) and Large Eddy Simulation (LES). © Springer Science+Business Media B.V. 2011