Mathematical Modelling of Nitric Oxide Formation in Turbulent Diffusion Flames Doped with a Nitrogen Compound

Abstract

A numerical study has been effected to investigate nitric oxide (NO) formation in large-scale gas-oil spray flames doped with a nitrogen compound to simulate fuel-nitrogen. The mathematical model for aerodynamic/combustion incorporates a Reynolds-stress model for turbulence and an eddy-dissipalion model (Magnussen and Hjertager, 1978) for combustion. NO predictions are obtained using the Zeldovich mechanism for thermal-NO with two variants for the calculation of oxygen-atom concentration, and the kinetic rate expressions of De Soete (1975) for the formation of prompt- and fuel-NO. The effect of turbulence/chemistry interactions on NO formation rates is represented by a single variable beta probability density function. Predictions are compared with detailed in-flame and flue-gas data obtained in previous studies for different levels of doping with quinoline while maintaining the same flame conditions. These data allowed contributions of thermal- and fuel-NO to the total NO emissions to be estimated with a reasonable degree of accuracy. The predicted and measured gas temperature and major species concentrations are generally in good agreement though discrepancies exist in the shear layers near the burner. Validation of the NO post-processor against experimental data for different fuel-nitrogen doping levels has revealed good qualitative predictions and, in most cases, good quantitative agreements. The contributions of individual NO formation mechanisms to the total NO emissions are well simulated by the NO model.