  The present work reports on the numerical investigation of NOx in three
turbulent piloted diffusion flames of different levels of extinction. The study
involves two-dimensional axisymmetric modeling of combustion in these flames
with fairly detailed chemistry, i.e. GRI 3.0 mechanism. The main focus of the
study is to analyze the effects of the two different combustion model
approaches, such as infinitely fast chemistry based unsteady flamelet and
finite rate chemistry based EDC, in predicting the NOx formation in three
piloted methane jet flames (Sandia D, E, and F). The EDC approach is able to
predict the passive scalar quantities but shows over-prediction in the reactive
scalar quantities and NO prediction, while the unsteady flamelet modeling is
found to be essential in predicting the accurate formation of slow kinetic
species like NOx. The inability of flamelet and EDC approach in capturing
localized flame extinction is observed, which lead to an over-prediction of NOx
at larger downstream locations. Further, the dominance of NOx formation
pathways is investigated in all three flames.
