Numerical study of ignition and combustion of hydrogen-enriched methane in a sequential combustor

TL;DR

This study uses Large Eddy Simulation to analyze how hydrogen addition to methane affects ignition and combustion in a sequential combustor, revealing that H2 promotes auto-ignition upstream due to enhanced radical chemistry.

Contribution

It provides new insights into the impact of H2 blending on ignition behavior and highlights the importance of out-of-equilibrium chemistry in sequential combustor operation.

Findings

H2 addition causes upstream auto-ignition kernels.

Radicals like OH promote rapid fuel decomposition.

Out-of-equilibrium conditions significantly influence ignition behavior.

Abstract

Ignition and combustion behavior in the second stage of a sequential combustor are investigated numerically at atmospheric pressure for pure CH4 fueling and for a CH4/H2 fuel blend in 24:1 mass ratio using Large Eddy Simulation (LES). Pure CH4 fueling results in a turbulent propagating flame anchored by the hot gas recirculation zone developed near the inlet of the sequential combustion chamber. Conversely, CH4/H2 fueling results in a drastic change of the combustion process, with multiple auto-ignition kernels produced upstream of the main flame brush. Chemical Explosive Mode Analysis indicates that, when H2 is added, flame stabilization in the combustion chamber is strongly supported by auto-ignition chemistry. The analysis of fuel decomposition pathways highlights that radicals advected from the first stage flame, in particular OH, induce a rapid fuel decomposition and cause the reactivity enhancement that leads to auto-ignition upstream of the sequential flame. This behavior is promoted by the relatively large mass fraction of OH radicals found in the flow reaching the second stage, which is approximately one order of magnitude greater than it would be at chemical equilibrium. The importance of the out-of-equilibrium vitiated air on the ignition behavior is proven via an additional LES that features weak auto-ignition kernel formation when equilibrium is artificially imposed. It is concluded, therefore, that parameters affecting the relaxation towards chemical equilibrium of the vitiated flow can have an important influence on the operability of sequential combustors fueled with varying fractions of H2 blending.