Numerical and experimental study of swirl premixed CH4/H2/O2/CO2 flames for controlled-emissions gas turbines

Abstract

Stoichiometric hydrogen-enriched oxy-methane (CH4/H2/O2/CO2) flames are investigated numerically by large eddy simulations (LES) and experimentally in a premixed dry-low-emission (DLE) swirl combustor to study the combustion behavior as well as stability characteristics. The study focuses on the effects of hydrogen fraction (HF), oxygen fraction (OF), and inlet bulk velocity (Uin) of the combustible mixture on the stability and combustion characteristics of the flames. Some distinctive features of the flames are reported in this study for possible application in controlled-emissions gas turbines. The results show that the reaction rates are increased with increasing percentages of hydrogen and oxygen in the reactant mixture, and the flame structures are observed to be more stable and compact. It is noted that the reaction rates are more sensitive to OF than they are to HF. The reaction rates are reduced with increasing inlet flow velocity, owing to the associated shorter residence time. The outer recirculation zone plays a significant role in flame stabilization at smaller values of HF and OF; however, its role diminishes at higher values. The CO concentration at the combustor exit increases with increasing HF and OF, due to higher CO2 decomposition at elevated temperatures. The effluent CO concentration, on the other hand, decreases with increasing inlet velocity. Higher HF and OF are characterized by higher Damköhler number (Da) and reduced flame thickness. The effect of velocity on flame thickness, however, was found to be negligible, indicating that the flame microstructure is governed by the overall stoichiometry. The Da showed declining behavior with increasing the inlet velocity because of smaller residence time. The reaction rates thus dominate the flame behavior, while turbulence affects only the macro-structure of the flame.