Large eddy simulations of hydrogen/methane jets in crossflows under engine-relevant conditions
As the global energy sector shifts toward low-carbon alternatives, hydrogen has emerged as a promising fuel due to its high reactivity and zero-carbon emission potential. However, realising its full potential requires careful integration into existing combustion systems, particularly through fuel-flexible strategies such as hydrogen-methane blends. Among various fuel injection strategies, the jet in crossflow (JICF) presents a promising configuration for hydrogen in gas turbine applications, offering enhanced near-field mixing and favourable flame stabilisation characteristics. While the fundamental fluid dynamics of non-reacting JICF have been extensively studied, a detailed understanding of hydrogen combustion in these flows, especially under practical high-pressure conditions, remains limited. Moreover, the transitional development of hydrogen-methane blends introduces additional complexity in flame behaviour, fuel-air mixing, and pollutant formation, particularly when combustion occurs over a broad range of mixture fractions in partially premixed modes. This thesis addresses these concerns by employing high-fidelity Large Eddy Simulations (LES) with a presumed joint probability density function (PDF) model to investigate partially premixed combustion of hydrogen and hydrogen-methane blends in JICF configurations. The study focuses on three key aspects: (1) the influence of methane addition (∼ 0 to 25% by volume) on the combustion characteristics of a nitrogen-diluted hydrogen jet at atmospheric pressure, (2) the effect of elevated pressure (∼ 4 to 15 bar) on flame stabilisation and mixing behaviour in helium-diluted hydrogen jets, and (3) the combined and relative influence of pressure and hydrogen content on fuel consumption rates and chemical and flow timescales of a methane jet. At atmospheric pressure, the addition of methane broadens the flame and reduces the local heat release rate due to changes in chemical kinetics and mixing, with the peak value decreasing by approximately a factor of two as the methane content increases from 0 to 25% (by volume). The flame stabilises in two branches, leeward and windward, and combustion along both branches remains predominantly premixed, spanning a wide mixture fraction range that evolves along the jet. Near the jet exit, burning occurs under lean and near-stoichiometric conditions, shifting toward richer mixtures between approximately 4 and 20 jet diameters downstream before returning to lean combustion further along the centreline. While methane slightly increases the non-premixed contribution, it remains below 12%, confirming that the premixed mode dominates across all blending conditions. At elevated pressure, flame anchoring shifts closer to the jet exit, with both windward and leeward branches stabilising at the jet exit. This alters the entrainment mechanisms and suppresses near-field fuel-air mixing. While non-reacting flow fields show increased absolute entrainment due to a ∼15-fold rise in mass flow rate as pressure increases from 1 to 15 bar, the overall mixing pattern remains largely unchanged. Under reacting conditions, however, elevated pressure increases local dilatation, strain rates, and kinematic viscosity, thereby slowing the decay of the mixture fraction and its variance. Despite these changes, combustion remains dominated by the premixed mode, with the non-premixed contribution further suppressed as pressure increases. These findings emphasise that flame stabilisation, not pressure alone, plays a decisive role in combustion and mixing characteristics in the JICF configuration. Increasing both hydrogen content and pressure leads to a notable reduction in the chemical timescale - on the order of 30-40% with 20% hydrogen enrichment and around 10-15% with a 5 bar pressure increase - particularly within the flame stabilisation region, due to enhanced reactivity. While pressure increases local turbulence intensity, especially at subgrid scales, hydrogen enrichment tends to reduce turbulence intensity. Damköhler number analysis shows that subgrid scale mixing remains faster than chemical reactions across all conditions, where mixing at the resolved scales is comparatively slower in flame stabilisation zones. Across all cases, combustion resides in the thin reaction zone, with only a marginal shift towards the corrugated flamelet regime when the hydrogen fraction is increased by 20% and the pressure by 5 bar. These findings indicate that, although both hydrogen enrichment and pressure influence reactivity and mixing dynamics, the overall combustion regime remains largely unaffected, particularly for the range of conditions investigated. The findings highlight the influences of fuel composition and pressure on the local combustion characteristics in partially premixed JICF systems. This work aims to contribute to the fundamental understanding necessary for developing low-emission combustors that operate on hydrogen-based fuel blends under practical conditions.
Turbulent reacting flows, Jet in crossflow, Hydrogen-methane combustion
Networked Digital Library of Theses and Dissertations