Hydrogen Enrichment Effects on Thermoacoustics of Turbulent Partially Premixed Flames

Abstract

Power generation and aircraft gas turbine combustors are frequently operated under fuel lean conditions to mitigate harmful emissions, rendering them susceptible to undesirable thermoacoustic instabilities. Additionally, meeting decarbonisation targets require incorporation of hydrogen into the existing combustion infrastructure. This doctoral thesis addresses the challenges of hydrogen combustion and thermoacoustic instability in Gas Turbine Model Combustors (GTMC). A bimodal thermoacoustic instability where two different modes are excited are investigated using Large Eddy Simulations (LES) for a partially premixed swirl stabilised methane-air flame in a GTMC. Simulations are also performed for cases with methane-hydrogen blends revealing a similar bimodal instability, where the frequency of thermo acoustic oscillations differs significantly from the cavity acoustic mode frequency due to the presence of intrinsic thermoacoustic modes. These modes are analysed using a 1D low-order model which offers insights on the appropriate area-ratio required at different junctions of the geometry to mitigate these modes. This thesis further explores pure hydrogen combustion in a Lean Direct Injection (LDI) combustor. Experimental results demonstrating thermoacoustic instabilities in this combustor are characterised using dynamical systems theory, revealing the presence of period-1 LCO, period-2 LCO, intermittent, quasi-periodic, and chaotic states, as either bulk velocity or equivalence ratio is varied. The unstable acoustic modes and their spatial behaviour are investigated using a reduced-order model. This thesis also addresses the challenge of modelling direct combustion noise, essential for mitigating its harmful effects on frequent flyers and residents living close to airports. Direct combustion noise is closely related to the Heat Release Rate (HRR) spectra, which this work investigates through LES analysis of three different configurations operating with various CH₄-H₂-air mixtures at atmospheric pressure. The global HRR spectra reveal an atypical spectral decay of f⁻⁵ at high frequencies across all cases. Local HRR spectra are qualitatively similar to the global spectra but are influenced by local mixing and coherent hydrodynamic structures. This analysis facilitates modelling local HRR spectra from non-reacting velocity spectra.