Assessment of a flame surface density-based subgrid turbulent combustion model for nonpremixed flames of wood pyrolysis gas

Abstract

A flame surface density (FSD) model for closing the unresolved reaction source terms is developed and implemented in a large eddy simulation (LES) of turbulent nonpremixed flame of wood pyrolysis gas and air. In this model, the filtered reaction rate ω¯α of species α is estimated as the product of the consumption rate per unit surface area mα and the filtered FSD Σ¯. This approach is attractive since it decouples the complex chemical problem (mα) from the description of the turbulence combustion interaction (Σ¯). A simplified computational methodology is derived for filtered FSD Σ¯, which is approximated as the product of the conditional filtered gradient of mixture fraction and the filtered probability density function. Two models for flamelet consumption rate mα are proposed to consider the effect of filtered scalar dissipation rate. The performance of these models is assessed by direct numerical simulation (DNS) database where a laminar diffusion flame interacts with a decaying homogeneous and isotropic turbulent flow field. The chemistry is modeled by a four-step reduced mechanism that describes the oxidization process of gaseous fuel released from high temperature pyrolysis of wood occurring in a wildland fire. Two-dimensional (2D) and 3D LES computations based on the FSD models are conducted for the same conditions as the DNS. The comparative assessments confirm the applicability of the proposed FSD model to describe the filtered reaction rate and the time evolution of temperature and species concentration in the turbulent nonpremixed flame.