| dc.description.abstract | Among conventional fossil fuel–based power generation technologies, gas turbines are among the cleanest and most efficient options. Replacing coal-fired power plants with natural gas–fired open-cycle gas turbines reduces specific carbon emissions by approximately 25–50%. Further enrichment of natural gas with hydrogen offers a viable pathway for deeper decarbonization; however, hydrogen’s high laminar flame speed, molecular diffusivity, and strong propensity for flashback and autoignition introduce significant flame-stabilization challenges in premixed combustion systems. In this context, cavity-based flameholders such as the trapped vortex combustor (TVC) are particularly promising. The present work evaluates the feasibility of operating a TVC with hydrogen-blended natural gas using an optically accessible, high-pressure combustor test facility, focusing on key challenges including the control of NOₓ formation associated with elevated adiabatic flame temperatures, altered combustion dynamics arising from thermoacoustic coupling and vortex-shedding interactions, achievement of an acceptable turbine-inlet pattern factor, and systematic characterization of pressure effects up to approximately 5 bar. The study begins with a detailed investigation of non-reacting flowfields in the TVC using acetone planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV), examining vortex dynamics for both forewall and aft-wall injection strategies. Forewall injection produced a dual-vortex structure within the cavity that was highly sensitive to the momentum flux ratio (J), where lower J enhanced residence time and higher J promoted greater penetration into the mainstream flow. Hydrogen enrichment was found to significantly increase the extent of flammable zones, particularly at higher J, due to hydrogen’s extended flammability limits, with direct implications for ignition strategies in cavity-stabilized combustors operating on hydrogen-rich fuels. Subsequent reacting-flow experiments demonstrated that hydrogen-enriched methane substantially alters flame dynamics, with hydrogen addition up to 50% by volume extending the stability limits of the TVC and significantly reducing the lean blowout equivalence ratio, thereby enabling stable combustion in high–strain-rate regions relative to pure methane. The hydrogen-enriched flames were more compact and attached, consistent with higher extinction strain rates, while emissions measurements showed reduced CO and unburned hydrocarbons due to more complete combustion, accompanied by increased NOₓ emissions driven by higher flame temperatures and an overall improvement in combustion efficiency. Hydrogen addition also induced periodic vortex shedding from the cavity and thermoacoustic oscillations that manifested as stable limit cycles with pressure amplitudes of approximately 2% of the mean chamber pressure, although combustion remained stable under all tested conditions. Recognizing that gas turbines operate at elevated compressor discharge pressures, the effects of pressure increase from atmospheric conditions to 5 bar were investigated, revealing enhanced combustion efficiency and extended lean stability limits, with the cavity equivalence ratio at blowout decreasing from 0.9 to 0.85 for pure methane and from 0.55 to 0.50 for a 50% H₂–CH₄ blend, albeit with increased NOₓ formation and noticeable changes in flame structure, including reaction-zone thinning and the onset of high-amplitude, yet stable, limit-cycle oscillations associated with vortex shedding. Finally, operation with 100% hydrogen at atmospheric conditions showed initial stabilization as a diffusion flame anchored by the cavity vortex, while subsequent premixed hydrogen–air injection into the cavity produced more compact flames, lower sound pressure levels, and an improved pattern factor; the lean blowout process was observed to occur in three distinct stages, with complete blowout governed by a critical Damköhler number. Overall, the study demonstrates the viability of hydrogen-enriched methane and pure hydrogen operation in a trapped vortex combustor, outlining effective stabilization strategies while highlighting the performance enhancements achieved through hydrogen addition. | en_US |
