Flame front identification typically relies on chemiluminescence or planar laser-induced fluorescence (PLIF) diagnostics. Yet, existing approaches are often limited by line-of-sight integration, low signal-to-noise ratio, or limited applicability to hydrogen flames. This study introduces a novel OH PLIF-based method that isolates the flame front by subtracting fluorescence signals obtained from two excitation lines with distinct temperature dependencies. The method is validated using LIFSIM coupled with 1D freely propagating flame simulations and is experimentally demonstrated in a laminar premixed H2[jls-end-space/]-air Bunsen flame. The resulting flame front marker exhibits a well-defined peak located, on average, 0.56 flame thicknesses from the heat release rate (HRR) peak and an average width of 1.57 times the HRR width. These values are comparable to those obtained from OH∗ chemiluminescence and the OH PLIF gradient, while offering substantially higher SNR and providing spatial resolution. Beyond introducing a new flame-front marker, suitable for hydrogen flames, this work provides a numerical and experimental assessment of OH-based HRR markers, evaluating their performance for hydrogen flame diagnostics.

Hydrogen combustion is gaining attention for its potential to enable low-emission energy conversion in gas turbines. Since hydrogen is carbon-free, it produces no carbon-based pollutants and primarily forms water vapor. However, due to its higher adiabatic flame temperature with respect to hydrocarbons, hydrogen combustion is more prone to increased nitrogen oxides (NOx[jls-end-space/]) formation. Accurately predicting NOx formation remains a major challenge, particularly when scaling from laboratory experiments to industrial applications. While scaling laws are widely used in fluid dynamics, their application to NOx emissions is challenging due to the complex nature of NOx formation. This study develops a semi-empirical, physics-based correlation to estimate NOx emissions in swirl-stabilized, partially premixed lean hydrogen–air burners. The proposed correlation expresses the Emission Index of NOx (EINOx[jls-end-space/]) as a function of key operating and design parameters, including fuel mass flow rate, pressure, adiabatic flame temperature, equivalence ratio, residence time and swirl number. It builds upon Westenberg’s NOx formation rate equation, incorporating composition-dependent effects via equivalence ratio. The slow formation nature of NOx is accounted for via the combustor mean residence time. Additionally, the influence of swirl on the mixing process is modeled through a swirl-modified effective equivalence ratio, acknowledging that while partial premixing is a design choice, swirl intensity can either enhance or disrupt the degree of premixing before combustion. The model’s parameters were calibrated using experimental data from published literature. By providing a predictive tool for NOx scaling across different operating conditions, this model supports the development and design of experiments and devices for hydrogen–air combustion.

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.

Resonance igniters are a promising alternative to conventional ignition devices for rocket engines using non-hypergolic propellants. This paper presents the development and analysis of a resonance igniter using gaseous oxygen and methane, supported by experimental measurements and numerical modelling. The effect of nozzle gap distance on acoustic resonance heating is investigated using oxygen and nitrogen as driving gases. Microphone data are used to determine the operating mode of the igniter; thermocouple data acquired on the outside of the resonator tip are used to evaluate heating performance across various nozzle pressure ratios and nozzle gap distances. A numerical model based on the open-source CFD software SU2 is developed and validated against resonance heating experimental data. This non-reacting flow model accurately captures the transition from the high-frequency Jet Screech Mode to the lower-frequency Jet Regurgitant Mode. Furthermore, it identifies the operational parameters leading to the highest rates of resonance heating observed in the experiments. Ignition attempts in non-premixed conditions, using gaseous oxygen and methane, show that the separate injection of methane in cross-flow into the combustion chamber causes severe disruption of resonance heating, preventing ignition.

HYLENA will investigate, develop and optimize an innovative, highly efficient integrated hydrogen powered, electrical aircraft propulsion concept for short and medium range. It will achieve significant climate impact reduction by being completely carbon neutral with radical increase of overall efficiency. The full synergistic use of: a) an electrical motor (as the main driver for propulsion), b) a contoured hydrogen fueled SOFC stacks (geometrically optimized for nacelle integration), c) a gas turbine (to thermodynamically integrate the SOFC), will act as an enabler for hydrogen aviation and will allow for efficient and compact engine concepts. This disruptive propulsion system will be called HYLENA concept. HYLENA aims to evaluate and demonstrate the feasibility of a “game changing” engine type which integrates Solid Oxide Fuel Cells (SOFC) into a turbomachine, in order to utilize the heat generated by the fuel cells on top of its electrical energy. The combination of e-motor, turbomachine and contoured SOFCs fueled with H2 will deliver high overall efficiency and performance versus state-of-the-art turbofan engines. Indeed, HYLENA Figures of Merit consist of minimizing CO2 emission; negligible NOX and an unmatched overall efficiency versus state-of-the-art turbofans which corresponds to an outstanding performance increase. It will also enable to extend the flight range for the same fuel tank size. The HYLENA project will deliver: 1. On SOFC cell level: Experimental investigations on SOFC cell technologies and identification of the most promising one(s) for aeronautical applications; 2. On SOFC stack level: Studies and tests to determine the most compact/light/manufacturable way of stack integration; 3. On thermodynamic level: Cycles simulations of the proposed novel HYLENA concept architecture and down selection of the most performing one; 4. On engine design level: Exploration, through resilient calculation and simulation, of the best engine design, sizing and overall components integration; 5. On overall engine efficiency level: Demonstration that HYLENA concept can reach very high efficiency levels with limited weight and complexity; 6. On demonstration level: A decision dossier for a potential ground test demonstrator to prove that the HYLENA concept works in practice during a second phase in the continuity of this project.

Laser-Induced Grating Spectroscopy (LIGS) was applied in a high-pressure combustion facility. Instantaneous (sub-μs), spatially resolved (within 5 mm) measurements of temperature and molar fractions of water were obtained using thermal and electrostrictive LIGS signals. Temperatures up to 1800 K and water molar fractions between 0.01 and 0.12 were measured. A new analytic approach was developed to extract temperature from the frequencies of the measured signal within the flame brush region, where mixtures contain both burnt and unburnt gases. Mean product temperatures are shown to be 8% lower than the adiabatic temperatures for the nominal equivalence ratio, and 14% higher than measurements made with a thermocouple, uncorrected for radiation losses. This work represents the first application of LIGS to a high-pressure, turbulent swirling flame, opening up the potential for future uses in other real world applications. Challenges associated with the deployment of the technique are described, as are potential measures to overcome these difficulties.

A high-frequency (1.5 kHz) spontaneous Raman spectroscopy measurement technique is developed and applied to measure external fluctuations generated in the local concentration of an isothermal binary gas mixture of methane and air. Raman excitation is provided by a high-frequency laser at 527 nm in dual-pulsed mode. The Stokes Raman signal is collected using an EMCCD camera coupled to a high-frequency intensifier as a shutter. The emitted signal is collected over the 596–627 nm wavelength range, which allows for the simultaneous tracking of methane and nitrogen Stokes Q-branch mode signals. Calibration curves are initially obtained for each species (CH ₄ and N ₂) based on steady-state concentrations, and further corrected during use to detect local unsteady mixture fluctuations at gas pulsation frequencies up to 250 Hz. The main novelty is the demonstration of Raman spectroscopy for the simultaneous multispecies measurement of unsteady concentrations of gas-phase methane and air mixtures using a laser beam with a high-repetition rate, low energy per pulse, combined with a high-frequency intensifier and a single camera.

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.