Two-zone subgrid combustion model for large eddy simulations of buoyant diffusion flames

Abstract

This work presents the development of a novel two-zone subgrid combustion model (SCM) recently designed to incorporate the finite-rate kinetics in the large eddy simulations of buoyant turbulent diffusion flames in which turbulence might not be fully developed. The reaction rate is evaluated as the mass-weighted average of contributions by the higher-temperature flamelet zone and the lower-temperature surrounding. The volume fraction of the flamelet zone and the time scale responsible for the inter-zone transfer are determined by the expected thickness of the laminar diffusion flamelet existing in the grid cell, and by wrinkling its surface by the unresolved turbulent eddies. This approach is applicable to an arbitrary level of turbulence (quantified by the subgrid Reynolds number) thereby overcoming the limitation of the conventional eddy dissipation concept formulated for the high-Re flames. Advancing the previous study [1], the flamelet status at every time step is evaluated by solving transient equations of heat and mass transfer between the subgrid zones. Effective kinetic parameters for the simplified global reaction of fuel oxidation are derived from experimental data for autoignition delay in methane-air and n-heptane-air mixtures and are shown to capture transition from the low- to high-temperature measurement datasets including the NTC (negative temperature coefficient) interval. With these kinetic parameters, the SCM is validated against the measurements in the unconfined methane-air flame produced by the UMD line burner and then applied to predict the development and extinction of the heptane-air flame in an under-ventilated enclosure. In the first case, flame extinction is predicted at the oxygen concentrations consistent with the experimental values. In the second scenario, global low-frequency oscillations of the heat release rate are predicted prior to flame extinction.