Numerical Modeling of NO Formation in Laminar Bunsen Flames—A Flamelet Approach

Abstract

Based on the flamelet concept, a numerical model has been developed for fast predictions of NO X and CO emissions from laminar flames. The model is applied to studying NO formation in the secondary nonpremixed flame zone of fuel-rich methane Bunsen flames. By solving the steady-state flamelet equations with the detailed GRI2.1 methane–air mechanism, a flamelet library is generated containing thermochemical information for a range of scalar dissipation rates at the ambient pressure condition. Modeling of NO formation is made by solving its conservation equation with chemical source term evaluated based on flamelet library using the extended Zeldovich mechanism and “NO reburning reactions.” The optically-thin radiation heat transfer model is used to explore the potential effect of heat loss on thermal NO formation. The numerical scheme solves the two-dimensional Navier–Stokes equations as well as three additional equations: the mixture fraction, the NO mass fraction, and the enthalpy deficit due to radiative heat loss. With an established flamelet library, typical computing times are about 5 hours per calculation on a DEC-3000 300LX workstation. The predicted mixing field, radial temperature profiles, and NO distributions compare favorably with recent experimental data obtained by Nguyen et al. [2]. The dependence of NO X emission on equivalence ratio is studied numerically and the predictions are found to agree reasonably well with the measurements by Muss [3]. The computed results show a decreasing trend of NO X emission with the equivalence ratio but an increasing trend in the CO emission index. By examining this trade-off between NO X and CO, an optimal equivalence ratio of 1.4 is found to yield the lowest combined emission.