In this study, the macroscopic properties of kerosene-H₂ blended flames are investigated in a multi-phase, multi-fuel combustor, focusing on the effects of increasing H₂ blending fractions. The non-reacting flow field of the swirl-stabilized combustor is characterized using PIV, and macro-structures in the flow and spray-swirl interactions are analyzed. Kerosene atomizers are tested to estimate variations in spray quality across different fuel blends. The changes in the optical properties of the flames are recorded using broadband chemiluminescence imaging while the changes in the acoustic emissions are recorded using a microphone. Results show that H₂ addition significantly alters the flame topology, transitioning from a lobed flame for pure kerosene to a single contiguous swirling flame for blended or pure H₂ cases. The flame luminosity decreases, with the emission color shifting from bright yellow (pure kerosene case) to dull yellow (multi-fuel cases) to a red-blue hue (pure H₂ case). These changes are attributed to variations in fuel distribution, heat release patterns, combustion mode, flame speed, and soot formation tendencies. The acoustic analysis reveals that a strong tonal behavior is observed under pure fuel conditions (prominent peaks at higher harmonics of 150 Hz) while broadband characteristics are exhibited under blended fuel conditions. The overall acoustic emissions in multi-fuel cases are reduced by ~80% compared to pure H₂ and ~55% compared to pure kerosene. This study highlights the effects of high levels of H₂ blending on flame dynamics and acoustic behavior in a multi-phase, multi-fuel combustor, offering valuable insights for the development of fuel-agnostic combustion systems.

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 mixing of fuel and air is a key factor in determining NOx emissions during combustion. Lean-premixed burning strategies allow to control the flame temperature and therefore NOx emissions. However, for highly reactive fuels like hydrogen, the high flame speed makes full premixing dangerous due to the increased risk of flashback. In these cases, current combustor geometries are often operated in partially premixed modes with the fuel injected as close as possible to the combustion chamber. This highlights the need for effective mixing strategies to achieve a high degree of mixing over a short distance. This is even more critical in fuel-flexible combustion systems (e.g., combustors capable of burning both CH4 and H2), as the mixing process is heavily influenced by the varying properties of the fuel mixture. In such cases, a comprehensive understanding of the mixing process is required to minimize NOx emissions under all fuel blends conditions. This paper investigates the mixing of fuel jets into a swirling air cross-flow of a partially-premixed, swirl stabilized combustor using a combined experimental and numerical approach. The injector features an axial swirler and a mixing tube where the air and the fuel jets mix before entering the combustion chamber. The experiments are performed in cold flow conditions. A variable mixture of helium–air is used to represent different blends of CH4-H2 fuel, and the mixing process is visualized by seeding the fuel stream with DEHS droplets. Large-Eddy Simulations (LES) confirm the suitability of helium as a surrogate for H2 by demonstrating similar macro-mixing behavior for the two gases. This study examines the impact of varying fuel composition and momentum flux ratio (Jswirl) between the fuel jet and the swirling cross-flow on mixing performance. The results indicate that fuel with lower density achieve better mixing with the air at the mixing tube outlet. A numerical analysis of the radial transport terms reveals that higher H2 content in the fuel makes it less subject to outward convection which causes stratification close to the mixing tube outlet. Furthermore, the contribution of the molecular diffusion term increases with higher levels of H2, resulting in improved mixing. When increasing Jswirl (up to Jswirl = 10) increases the penetration of the fuel jet into the swirling flow. Above a critical value of Jswirl, the mixture homogeneity at the mixing tube outlet becomes insensitive to Jswirl for the investigated geometry. Overall, the fuel composition was found to have a greater influence on the level of mixing close to the mixing tube outlet than variations in Jswirl.

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.

Due to climate change concerns, hydrogen is being considered for future aviation, but its commercial availability is limited, storage is bulky and its combustion with 100% concentration still poses numerous technical challenges. This leads to a certain interest in multi-fuel systems using both hydrogen and kerosene to facilitate the transition without completely redesigning the existing engines. Within the HOPE project, the present study focuses on an innovative multi-fuel combustion concept for aircraft propulsion, considering a laboratory-scale combustor hosted at TU Delft. Such a device, originally fueled with hydrogen and methane, is schematically composed of an axial swirler, four ducts for gaseous fuel injection, a mixing tube and a cylindrical combustion chamber. To avoid flashback, also an axial air injection duct is present that bypasses the swirler and directly reaches the air-fuel mixture in the mixing tube.
In this work, reactive CFD simulations are used to explore different spray injection configurations and assess the impact of kerosene on the flow field, the flame shape and the NO emissions of the modified system. In particular, three different injection positions are studied, featuring injection points on the backplane of the combustion chamber, inside the fuel/air mixing tube or on the axis of the burner. It is found that the most suitable position for kerosene injection is on the axis of the burner, so that the spray is surrounded by the swirling flow and undergoes a rapid mixing with the oxidising stream, limiting the maximum temperature reached by the mixture. Moreover, in this case, the addition of hydrogen leads to reduced NO emissions since it decreases the size of the hot spots generated by the combustion of kerosene.

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.