Large eddy simulations of weakly turbulent diffusion flames in an oxygen-reduced co-flow using a new subgrid combustion model

Abstract

The recently proposed sub-grid combustion model (SCM) for weakly turbulent buoyant diffusion flames is applied to simulate 10 and 15 kW flames produced by a circular porous burner in an oxidizer co-flow with normal and reduced oxygen concentrations. Turbulence is modelled by the large eddy simulations technique. Soot production and radiative emission is predicted based on local resolved species concentrations and temperature, eliminating the need to prescribe soot yield and global radiative fraction. Finite-rate chemistry is incorporated via a single-step global reaction of fuel oxidation with temperature-dependent effective kinetic parameters, which are derived to fit the autoignition delay times predicted by detailed chemical mechanisms. When the oxidizer co-flow is ambient air, good agreement between the predicted and the measured spatial distributions of mean temperatures and soot volume fractions is demonstrated. In flames with reduced oxygen concentrations in the co-flow, the experimental combustion efficiency and radiative fraction are replicated in the simulations, and the critical oxygen concentration causing complete flame extinguishment is predicted well by the SCM. The predictions are shown to be weakly dependent on the values of model constants. Adequate representation of the flame shape requires the large turbulent fluctuations to be sufficiently resolved by the computational grid.