Nitric oxide formation in turbulent jet diffusion flames

Abstract

Combustion and nitric oxide formation in the turbulent mixing region between a semi-infinite stream of oxidant and a fuel jet issuing from a finite orifice are analyzed for both planar and axisymmetric flows. The flowfield is considered to consist of three major components: fuel, oxidant, and combustion products. Nitric oxide is treated as a trace species. The fast fuel-oxidant combustion reaction is assumed to be diffusion-limited; whereas the relatively slow nitric oxide formation is taken as rate-limited. Turbulent transports of momentum, enthalpy, and species are described by Prandtl's mixing length theory. The chemical kinetics of nitric oxide formation is assumed to follow the Zeldovich mechanism. Local similarity relations are obtained for the flow properties and a nonsimilar continuity equation is obtained for nitric oxide concentration; these are solved by standard numerical methods. Molecular mixing of the hot combustion product eddies with the cool oxidant or fuel eddies is shown to have a great influence on the amount of nitric oxide formed, although its effect on the time-averaged fluid properties is small. Both limits of no local molecular mixing and complete mixing are examined. Results for the flame contour and for the profiles of velocity, temperature, and concentrations of fuel, oxidant, products, and nitric oxide are illustrated. The nitric oxide concentration profile is shown to have a maximum in the fuel-rich region throughout the flame.