Nitric oxide formation in turbulent diffusion flames

Abstract

Combustion and NO formation are investigated in the turbulent mixing region between parallel fuel and oxidant streams. Chemical reactions are divided into two classes: (i) the fast, diffusion-limited combustion reaction, and (ii) the relatively slow, rate-limited NO formation. For the fast reaction, the turbulent mixing zone contains fuel, oxidant, and reaction products. The formation of NO is calculated separately as a trace species, since it has negligible effect on the flowfield. Transport of momentum, enthalpy, and chemical species is calculated, using a mixing-length theory. Because NO generation is highly temperature sensitive, the history of combustion product gases, subsequent to their formation, is decisive in determining the total NO production. Upper and lower bounds on NO production are obtained by considering that: (i) the combustion products remain undiluted and intact in the form of eddies as the turbulent field transports them throughout the mixing layer, and (ii) the combustion products are locally mixed with cool oxidizer or fuel. These yield upper and lower limits, respectively. The time-averaged velocity, temperature, and concentrations of fuel, oxidant, products, and NO distributions, are illustrated. Molecular mixing of turbulent eddies is shown to have a great influence on the amount of NO formed, although its effect on the time-averaged fluid properties is negligible. For a sample problem, the NO concentration obtained by assuming complete local molecular mixing is nearly an order-of-magnitude lower than the value predicted for no mixing.