All combustion processes lead to formation of the unwanted nitrogen oxides (NOx). These species are among the most important air pollutants. During the last decades, many efforts have been performed to reduce the formation and emission of these species. Unfortunately, all the available measures to alleviate the formation and emission of NOx inhibit the combustion efficiency. Therefore, accurate estimation of nitrogen oxide in combustion phenomena is an important factor in designing the combustion chambers. The purpose of this study is to address this issue. It starts with a comprehensive survey on different physical mechanisms of formation and emission of NOx . Then, the effect of different parameters on the formation and emission of nitrogen oxides is assessed. Different techniques for the reduction of nitrogen oxide formation/emission that comprises of pollution prevention methods( Less Excess Air (LEA), Burners Out Of Service (BOOS), Over Fire Air (OFA), Low NOx Burner (LNB), Flue Gas Recirculation (FGR), air staging, fuel staging, use of oxygen instead of air for combustion, injection of water or stream and reduction of preheated air) and add-on technologies (Selective Non-Catalytic reduction (SNCR), Selective Catalytic reduction (SCR), adsorption and absorption) is introduced. Next, a complete study on the mathematical models of the thermal NO and prompt NO mechanisms is done . The effect of the turbulence on the NOx formation/emission mechanisms is examined and different ways for including turbulence-chemistry interaction using assumed Probability Density Function (PDF) containing Equilibrium Model (EQM), Steady Laminar Flamelet Model (SLFM) and Unsteady Laminar Flamelet Model (ULFM) is explained. Then the combustion problem for three Non-premixed diffusion flame is solved. Finally, a post processing code is developed to solve the traort equation of NO. This code is written in C++ and is able to handle unstructured mesh. The code uses fluent combustion results to calculate NO mass fraction. The turbulence-chemistry interaction is taken into account via steady laminar flamelet model and several sub models for calculating NO sources is presented. The validity of NO models and correctness of the program have been assessed by simulating aforementioned diffusion flames and comparing the results with available experimental data. The results are given both by considering turbulence-chemistry interaction and without it. It is concluded that the turbulence chemistry interaction has a profound effect on the accuracy of predictions and the use of SLFM method in combustion simulations leads to good degree of accuracy in the prediction of NO formation. Key Words NO, SLFM, Unstructured mesh, turbulence-chemistry interaction.
