Soot Modeling in Gas Turbine Combustors

A method is presented for predicting soot in gas turbine combustors. A soot formation/oxidation model due to Fairweather et al [1992] has been employed. This model has been implemented in the CONCERT code which is a fully elliptic three-dimensional (3-D) body-fitted computational fluid dynamics (CFD) code based on pressure correction techniques. The combustion model used here is based on an assumed probability density function (PDF) parameterized by the mean and variance of the mixture fraction and a β-PDF shape. In the soot modeling, two additional transport equations corresponding to the soot mass fraction and the soot number density are solved. As an initial validation, calculations were performed in a simple propane jet diffusion flame for which experimental soot concentration measurements along the centerline and along the radius at various axial downstream stations were available from the literature. Soot predictions were compared with measured data which showed reasonable agreement. Next, soot predictions were made in a 3-D model of a CF6-80LEC engine single annular combustor over a range of operating pressures and temperatures. Although the fuel in the combustor is Jet-A, the soot computations assumed propane to be the surrogate fuel. To account for this fuel change, the soot production term was increased by a factor of 10X. In addition, the oxidation term was increased by a factor of 4X to account for uncertainties in the assumed collision frequencies. The soot model was also tested against two other combustors, a CF6-80C and a CFM56-5B. Comparison of the predicted scot concentrations with measured smoke numbers showed fairly good correlation within the range of the soot model parameters studied. More work has to be performed to address several modeling issues including sensitivity to oxidation rate coefficients and scalar diffusion.