Direct numerical simulation of heat release and NOx formation in turbulentnonpremixed flames

Attempts to use complex chemistry and transport in direct numerical simulat ions (DNS) of premixed combustion (even for kinetically simple systems, suc h as H-2/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 maint aining computational efficiency. The methodology includes the use of a limi ted number of species and reactions with parameters which are derived to ma tch a number of flame properties. It is illustrated through a four-step rea ction mechanism appropriate for a stoichiometric methane/air flame, and whi ch compares favorably with predictions of the detailed GRI 2.11 mechanism. The proposed scheme includes one reaction for the methane oxidation, one fo r the thermal, one for the Fenimore, and one for the nonpremixed reburn che mical NOx routes. The kinetic parameters for the hydrocarbon oxidation were determined by matching the GRI 2.11 predictions for laminar burning veloci ty and adiabatic flame temperature, main reactants concentrations, and exti nction strain rates for both premixed (steady) and nonpremixed (steady and unsteady) strained laminar flames. The chemical parameters for the three st eps corresponding to NOx chemistry were determined by matching the NOx prof iles obtained for strained diffusion flames with GRI 2.11. Finally, this fo ur-step mechanism was used in DNS of two- and three-dimensional turbulent n onpremixed 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 e xistence of compressed zones where products accumulate and increase the NOx production. (C) 1999 by The Combustion Institute.