Direct numerical simulation of heat release and NOx formation in turbulent nonpremixed flames

Abstract

Attempts to use complex chemistry and transport in direct numerical simulations (DNS) of premixed combustion (even for kinetically simple systems, such as H2/air and CH4/air) often result in excessive needs of memory and CPU time. This paper presents a methodology (integrated combustion chemistry [ICC]) capable of integrating complex chemistry effects into DNS while maintaining computational efficiency. The methodology includes the use of a limited number of species and reactions with parameters which are derived to match a number of flame properties. It is illustrated through a four-step reaction mechanism appropriate for a stoichiometric methane/air flame, and which compares favorably with predictions of the detailed gri 2.11 mechanism. The proposed scheme includes one reaction for the methane oxidation, one for the thermal, one for the Fenimore, and one for the nonpremixed reburn chemical NOx routes. The kinetic parameters for the hydrocarbon oxidation were determined by matching the gri 2.11 predictions for laminar burning velocity and adiabatic flame temperature, main reactants concentrations, and extinction strain rates for both premixed (steady) and nonpremixed (steady and unsteady) strained laminar flames. The chemical parameters for the three steps corresponding to NOx chemistry were determined by matching the NOx profiles obtained for strained diffusion flames with gri 2.11. Finally, this four-step mechanism was used in DNS of two- and three-dimensional turbulent nonpremixed combustion to assess the validity of flamelet approaches. While the flamelet approaches were found to perform well for heat release, their extension to NOx formation appears to be not as successful because of the existence of compressed zones where products accumulate and increase the NOx production.