Investigation of NOx in piloted stabilized methane-air diffusion flames using Finite-rate and Infinitely-fast chemistry based combustion models

TL;DR

This study compares finite-rate and infinitely-fast chemistry models in predicting NOx emissions in turbulent methane diffusion flames, highlighting the strengths and limitations of each approach in different flame conditions.

Contribution

It provides a detailed numerical comparison of two combustion modeling approaches for NOx prediction in turbulent flames, emphasizing the importance of model selection for accurate results.

Findings

Flamelet models better predict slow kinetic species like NOx.

EDC over-predicts reactive scalars and NO, especially downstream.

Both models struggle to capture localized flame extinction.

Abstract

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.