Numerical Prediction of the Soot and NO Formation in a Confined Laminar Diffusion Flame without and with Air Preheat

Abstract

The effect of air preheating on the formation of pollutants such as soot and NO in laminar diffusion flame is studied numerically. Only the thermal route is considered for the NO production. A numerical model has been developed to solve for the velocity, temperature, and species and soot concentration values in a confined, coflowing, laminar methane - air diffusion flame. The model uses simplified chemical kinetics following two-step reaction and a semi-empirical model for soot formation. It is shown that even with such reduced kinetic information, it is possible to predict the flame parameters and pollutant formation with significant accuracy, if suitable model parameters are adopted. It is observed from the model prediction that the soot is formed in the rich side of the flame front, but gets oxidized subsequently. The peak soot volume fraction is found to be more than five times within the flame with a 100 K increase in air inlet temperature. NO is formed around the flame just outside the flame surface where the temperature and oxygen concentrations are high. The axial flux of NO increases from the inlet plane until it reaches a constant value, indicating no further formation of thermal NO beyond a certain height because of the drop in temperature. The NO formation increases with increasing inlet air temperature.