Large-Eddy Simulation of Bluff-Body Flame Using the Equilibrium Combustion Model

Abstract

Flame stabilization is an important issue in practical applications of combustion systems. The bluff-body configuration is a common technique that stabilizes the flame by producing a strong recirculation zone downstream of the bluff body. Gas turbines provide a typical application of the bluff-body configuration. In this paper, a computational study of a bluff-body flame has been undertaken using an unstructured large-eddy simulation. The filtered Navier-Stokes equations are closed using a standard Smagorinsky model, and an equilibrium combustion model is employed for the chemistry calculation. All filtered scalar quantities are evaluated using a presumed probability density function based on a beta distribution function. It was found that accurate prediction of velocity profiles are strongly dependent upon the inflow conditions. It was also found that the equilibrium model for chemistry calculation does not capture well the dynamics of the reaction in the recirculation zone, where the diffusion timescales are very small. Based on this observation, a criterion was established for the implementation of the equilibrium model in terms of diffusion and chemical timescales. The diffusion timescales are inversely proportional to the scalar dissipation rate. It was found that the ratio of the diffusion timescale to the chemical timescale must be at least 2 × 10 2 to obtain accurate results with the equilibrium model. Nomenclature BC1 = Inlet boundary condition based on fixed axial velocity BC2 = Inlet boundary condition based on instantaneous velocity components from precursor simulation C s = Smagorinsky constant C d = dynamic coefficient L ϵ = dissipation length scale PZ = probability density function TZ = instantaneous temperature T = filtered temperature ν SGS = eddy or subgrid scale viscosity Z 0 02 = mixture fraction variance δ ij = Kronecker delta ϵ v = viscous dissipation σ T = turbulent Prandtl number τ c = chemical timescale τ d = diffusion timescale χ = scalar dissipation rate