The effect of hydrogen enrichment on the forced response of CH4/H2/Air laminar flames

TL;DR

This study examines how hydrogen enrichment affects the unsteady response and stability of methane/hydrogen/air flames, revealing that higher hydrogen levels increase the risk of thermoacoustic instability by shifting the flame response to higher frequencies.

Contribution

It introduces a detailed analysis of hydrogen enrichment effects on flame response using the Level Set Approach and Flame Describing Functions, highlighting the impact on thermoacoustic stability.

Findings

Hydrogen enrichment shifts FDF gain drop-off to higher frequencies.

Increased hydrogen reduces effective flame time delay.

Changes in FDF are mainly driven by flame burning speed.

Abstract

Hydrogen-enrichment of conventional natural gas mixtures is an actively-explored strategy for reducing pollutant emissions from combustion. This study investigates the effect of hydrogen enrichment on the unsteady flame response to perturbations, with a view to understanding the implications for thermoacoustic stability. The Level Set Approach for kinematically tracking the flame front was applied to a laminar conical premixed methane / hydrogen / air flame subjected to 2D incompressible velocity perturbations. For hydrogen enrichment levels ranging from 0% to 80% by volume, the resulting unsteady heat release rate of the flame was used to generate the Flame Describing Functions (FDFs). This was performed across a range of perturbation frequencies and levels at ambient pressure. The mean heat release rate of the flame was fixed at $\overline{\dot{Q}} = 2.69\ \textrm{kW}$ and the equivalence ratio was set to $\varphi=1.08$ for all hydrogen enrichment levels. Hydrogen-enrichment was found to shift the FDF gain drop-off to higher frequencies, which will increase propensity to thermoacoustic instability. It also reduced the effective flame time delay. Sensitivity analyses at $\varphi = 0.8$ revealed that the changes in FDF were driven predominantly by the flame burning speed, and were insensitive to changes in Markstein length.