RANS Simulations of Premixed Turbulent Flames

Book chapter, 2018

While Reynolds-Averaged Navier-Stokes (RANS) simulations are widely used in applied research into premixed turbulent burning in spark ignition piston engines and gas-turbine combustors, fundamental challenges associated with modeling various unclosed terms in the RANS transport equations that describe premixed flames have not yet been solved. These challenges stem from two kinds of phenomena. First, thermal expansion due to heat release in combustion reactions affects turbulent flow and turbulent transport. Such effectsmanifest themselves in the so-called counter gradient turbulent transport, flame-generated turbulence, hydrodynamic instability of premixed combustion, etc. Second, turbulent eddies wrinkle and stretch reaction zones, thus, increasing their surface area and changing their local structure. Both the former effects, i.e. the influence of combustion on turbulence, and the latter effects, i.e. the influence of turbulence on combustion, are localized to small scales unresolved in RANS simulations and, therefore, requiremodeling. In the present chapter, the former effects, their physical mechanisms and manifestations, and approaches to modeling them are briefly overviewed, while discussion of the latter effects is more detailed. More specifically, the state-of-the-art of RANS modeling of the influence of turbulence on premixed combustion is considered, including widely used approaches such as models that deal with a transport equation for the mean Flame Surface Density or the mean Scalar Dissipation Rate. Subsequently, the focus of discussion is placed on phenomenological foundations, closed equations, qualitative features, quantitative validation, and applications of the so-called Turbulent Flame Closure (TFC) model and its extension known as Flame Speed Closure (FSC) model.