A four-step reduced chemical-kinetic mechanism for syngas combustion is proposed for use under conditions of interest for gas-turbine operation. The mechanism builds upon our recently published three-step mechanism for H 2 -air combustion (Boivin et al., Proc. Comb. Inst. 33, 2010), which was cate that the reduced description can be applied with reasonable accuracy in numerical studies of gas-turbine syngas combustion.

Subgrid scale (SGS) variance of a scalar field in large eddy simulations is only properly defined in relation to a probability density function. This solves a reported problem in the variance definition [Cook and Riley, Phys. Fluids 6, 2868 (1994); Cook, Riley, and Kosály, Combust. Flame 109, 332 (1997)] and allows to write a simple evolution equation for the scalar variance. This equation shows that a recently proposed model for scalar dissipation in terms of the large-scale gradients [Pierce and Moin, Phys. Fluids 10, 3041 (1998)] implies dissipation and production canceling out, preventing variance decay and complete mixing at SGS level. An alternative simple model for dissipation in terms of a SGS mixing characteristic time is proposed and tested here.

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A three-dimensional direct numerical simulation of a propagating turbulent premixed flame is performed using one-step Arrhenius model chemistry. The interaction of the flame thermochemical processes with the local geometries of the scalar field and flow topologies is studied. Four regions (“fresh reactants,” “preheating,” “burning,” and “hot products”), characterized by their reaction rate and mass fraction values, are examined. Thermochemical processes in the “preheating” and “burning” regions smooth out highly contorted iso-scalar surfaces, present in the “fresh reactants,” and annihilate large curvatures. Positive volumetric dilatation rates, −P = ∇ · u, display maxima for elliptic concave and minima for convex scalar micro-structures. Constant average tangential strain rates, aT, with large fluctuations, occur throughout the flow domain, whereas normal strain rates, aN, follow the trends of volumetric dilatation rates. Focal topologies, present in the “fresh reactants,” tend to disappear in favor of nodal structures as moving towards the “hot products.” The vorticity vector is predominantly tangential to the iso-scalar surfaces. The Unstable Node/Saddle/Saddle and Stable Focus/Stretching topologies, present in the “fresh reactants,” correlate with large values of aN and aT providing hints on the flow topologies fostering scalar mixing.