Generation Of Entropy Waves By Fully Premixed Flames In A Non-Adiabatic Combustor With Hydrogen Enrichment

Abstract

Thermoacoustic combustion instability is a significant concern in gas turbines with hydrogen-enriched fuels. Unsteady combustion generates acoustic waves and fluctuations in burnt gas temperature, known as entropy waves. The latter are convected by the mean flow through the combustor and can cause indirect combustion noise when accelerated. This work demonstrates that entropy waves occur in a fully premixed burner due to unsteady heat transfer at the combustion chamber wall. This mechanism of entropy generation is often neglected in the literature. This work indicates an additional mechanism in CH4-H2-air flames, which may produce entropy waves even in the fully premixed case - differential diffusion of hydrogen results in local fluctuations in equivalence and carbon-to-hydrogen ratios. The adiabatic flame temperature is defined based on these quantities to examine the effect of differential diffusion on the generation of entropy fluctuations. System identification (SI) is applied to time series data obtained from a broadband-forced large eddy simulation (LES) to investigate the generation of entropy waves. The entropy transfer function (ETF) and flame transfer function (FTF) identified are compared to experimental data obtained with TDLAS-WMS for measuring temperature fluctuations and the multi-microphone method. After validating the computational setup, the entropy frequency response is identified at several positions within the combustion chamber, and the effects of generation and convective dispersion of entropy waves are qualitatively investigated. We show that a fully premixed turbulent system may exhibit significant entropy waves caused by wall heat losses and differential diffusion of hydrogen.