The ongoing transition towards carbon-neutral aviation has triggered extensive research and development efforts into alternative fuels, with hydrogen emerging as a promising candidate due to the absence of any direct carbon emissions. However, 100% hydrogen combustion poses several operational challenges, even in ground-based gas turbine applications, including flashback risk, thermoacoustic instabilities, and bulky storage requirements for long-haul aircraft. In response, multi-fuel combustion strategies, simultaneously burning liquid kerosene and gaseous hydrogen, are gaining traction in both industrial initiatives and European R&D projects. This approach aims at mitigating the drawbacks associated with pure hydrogen combustion while enhancing the performance of kerosene combustion through improved flame stability at reduced equivalence ratios, thereby reducing carbon dioxide, NOx, and soot production in comparison to pure kerosene combustion. This study, carried out in the context of the HOPE Horizon Europe project, presents the development and preliminary application of a novel extension of the Thickened Flame Model (TFM), tailored for simulations of multi-fuel kerosene-hydrogen combustion within a scale-resolving framework. First, the standard TFM setup available in ANSYS Fluent is validated against two 100% hydrogen turbulent swirled flames - the HYLON and HOPE experimental burners - demonstrating good predictive capabilities in pure hydrogen regimes. Subsequently, the model is modified to operate in a multi-fuel context and preliminarily assessed against experimental data from the HOPE test rig, characterized by simultaneous kerosene-hydrogen injection and combustion. The results highlight the model’s ability to capture some of the key characteristics of the studied multi-fuel system, thereby providing a step towards the CFD-based design and optimization of next-generation, low-emission aeronautical multi-fuel combustors.

Low emissions and fuel flexibility are two important criteria required for gas turbine combustors to facilitate the energy transition to low-carbon fuels for propulsion and power applications. A jet-stabilized combustor, having both these characteristics, was operated with CH ₄–H ₂ fuel mixtures with H ₂ varying from 0 to 100 % and with varying equivalence ratios (ϕ). Comprehensive measurements were carried out of the velocity field using Particle Image Velocimetry (PIV), temperature and gas composition by traversing probes in the chamber, and flame topology using chemiluminescence imaging. The flow field in this combustor consists of a jet that undergoes recirculation, generating Central and Peripheral Recirculation Zones (CRZ and PRZ). The recirculation ratio in the PRZ is found to be twice that of the CRZ. Increasing H ₂ % for the same ϕ leads to higher NO _x. Ultra-low ϕ flames could be stabilized only at H ₂≥50 %, which in turn leads to low NO _x due to low adiabatic flame temperatures. The combination of temperature, gas composition (CO/NO), and chemiluminescence images is used to identify the extent and location of the reaction zone. Distributed reaction zones, stabilizing at around 30 % of the length of the chamber, are achieved at lean conditions, whereas an increase in H ₂ % makes the reaction zone more compact and shifts upstream towards the burner head. Flame kernels are extracted from the instantaneous chemiluminescence images, and probability distribution functions for their aspect ratio and axial location are constructed. It is seen that reducing ϕ leads to low aspect ratio kernels that tend to occur further downstream, whereas increasing H ₂ % leads to higher aspect ratio kernels, stabilizing upstream. These flame kernel statistics are also used to identify ignition modes (autoignition/flame propagation) for varying fuel H ₂ % and inlet ϕ based on a hypothesis of flame stabilization mechanisms.

Introducing H ₂ as fuel in gas turbines is a promising step towards decarbonizing the energy sector. However, the future availability of H ₂ in large quantities remains uncertain. Consequently, designing fuel flexible (CH ₄/H ₂) combustion chambers for various fuel blends is necessary. The distinct combustion characteristics of H ₂, such as high flame speeds and high adiabatic flame temperatures, pose challenges when designing systems that can operate in a stable manner and with low emissions across a wide range of fuel mixtures. This paper investigates the fuel-flexibility of an atmospheric laboratory scale, partially premixed swirl stabilized combustor. By deploying a non-rotating axial air jet (AAI) in the center-line of the swirling flow, the flashback risk for high H ₂ content fuels is minimized. This study provides detailed insights into AAI's interaction with CH ₄/H ₂ fuel blends, analyzing the resulting flow field from Particle Image Velocimetry, emissions from exhaust gas analyser measurements, and flame structures from OH* chemiluminescence and OH Planar Laser Induced Fluorescence. The results show that AAI enables flame stabilization across the full range from 100% CH ₄ to 100% H ₂ in the same injector geometry. However, a high portion of the total airflow must be injected axially to stabilize H ₂ flames. Increasing the level of AAI increases NO emissions and alters flame stabilization mechanisms. This is likely due to a decrease in mixing quality, resulting in the fuel staying close to the periphery of the mixing tube. Switching the fuel from 100% CH ₄ to 100% H ₂ leads to an increase in NO emission, despite lower adiabatic flame temperatures for the perfectly premixed case. This indicates that the mixing process and flame location within the combustion chamber are essential in controlling NO emissions. Moreover, the flow field transforms significantly from a swirl-stabilized flow field featuring an inner recirculation zone to one resembling the one of a jet flame.

The power sector accounts for ∼40% of global energy-related CO₂ emissions. Its decarbonization by switching to low-carbon renewables is essential for a sustainable future. Existing electrical grids, however, have limited capacity to absorb the variability introduced by these new energy sources and rely largely on natural-gas-based power generation. For deep decarbonization, alternative solutions to increase grid flexibility are needed. Among these, energy storage is expected to have a key role. This paper proposes a unique energy storage and re-conversion system by coupling the hydrogen combustion in supercritical CO₂ (HYCOS) cycle, a zero-emission combustion cycle, with long-term/seasonal energy storage based on green H₂ production. This power cycle is expected to be highly scalable and compact and can deliver power at net electrical efficiency between 55% and 60% at distributed generation levels. Thus, it can be highly competitive with existing solutions such as fuel cells, reciprocating engines, and gas turbines.

Lean-premixed swirl-stabilized combustion is a successful strategy to reduce pollutant emissions. However, these combustion systems are especially prone to thermoacoustic instabilities. The precessing vortex core (PVC) plays a significant role in suppressing or exciting those instabilities. Therefore, it is necessary to predict the PVC dynamics in different operating conditions. The introduction of alternative aviation fuels like hydrogen in fuel-flexible gas turbines might require changes in the combustor geometry. However, the influence of particular geometric parameters on the PVC dynamics in less conventional combustion chamber configurations is not yet clear. To contribute to the knowledge of PVC dynamics in different combustor geometries, this paper presents an experimental study of the PVC dynamics in isothermal conditions in a counter-rotating dual swirler configuration in different confinement ratios. Additionally, a non-rotating axial air jet can be injected on the center line of the primary swirler as a provision for increased flashback resistance in the reacting case with H ₂. PVC frequencies and amplitudes are obtained by spectral proper orthogonal decomposition (SPOD) of time-resolved PIV measurements, and by time-resolved pressure measurements. The study shows that the frequency of the PVC scales with StPVC = 0.78, based on the diameter and the bulk velocity of the mixing tube. The PVC frequency is only determined by the conditions in the primary swirler and is fully independent of the amount of airflow going through the secondary counter-rotating swirler. Introducing an axial air jet on the center line decreases the PVC frequency significantly, which can be related to the change in effective swirl number. It is also shown that the smallest combustion chamber diameter results in the highest spectral energy for the PVC mode for all investigated points, hence the PVC motion is the strongest. Meanwhile, the biggest combustion chamber diameter shows the weakest pressure fluctuations. The results obtained in this study provide evidence that the periodic oscillations arising in the swirling flow field can be predicted and follow a Strouhal scaling independent of the geometry, even for more unconventional configurations.