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.

Large eddy simulation (LES) paradigms are used in the present work to predict premixed and partially premixed turbulent flames with flamelets based thermochemistry and presumed filtered density function approach for turbulence-chemistry interaction modelling. The combustion model requires a closure for the scalar dissipation rate of a progress variable, in which a modelling constant must be chosen. The present work focuses on the computation of the model constant through dynamic procedures based on the scale-similarity assumption, which requires the application of test-filters. In particular, two test-filtering approaches for LES, based respectively on an algebraic formulation and a newly proposed differential equation, are tested for flame configurations at different levels of turbulence, and using block-structured and unstructured meshes. The analysis shows that the differential filter, unlike the algebraic one, is handled well in situations of weak turbulence at comparable computational costs. At higher turbulence conditions the outcome looks less dependent on the test-filter and mesh topology used, although quantitative differences in the behaviour of the dynamically-computed model constant are still observable and discussed. Further analyses to understand the behaviour of the two filters are presented in the paper.

Large eddy simulations (LES) with flamelet and presumed filtered density function closure are used to simulate turbulent premixed and partially premixed hydrogen flames. Different approaches to model differential diffusion are investigated and compared. In particular, two existing models are extended to the LES framework to correct the resolved diffusive flux of the controlling variables due to differential diffusion. A lean premixed turbulent hydrogen flame in a slotted burner configuration is simulated first to compare the capability of the considered models in capturing local mixture fraction redistribution, super-adiabatic temperatures and thermo-diffusive instabilities. Results show that both models describe the formation of cellular burning structures. Next, a partially premixed lifted hydrogen flame in vitiated hot coflow is simulated to gain insight on the relevance of differential diffusion modelling at a higher turbulence level, a different combustion mode and in the presence of a complex stabilisation mechanism. Good predictions of the turbulent mixing and temperature fields are observed. Moreover, results show that the flame lift-off height has an appreciable sensitivity to the differential diffusion model. When differential diffusion is included only in the thermochemistry database, only mild effects on the predicted temperature fields, mixing and flame height are observed. On the contrary, a considerable shift of the flame base is observed when corrections are applied in the LES at the resolved level, depending on what controlling variables are considered. Further analyses reveal how the corrections of diffusive fluxes in the thermochemistry and at the LES level affect differently the flame burning mode, whose details are given throughout the paper.

Large eddy simulation (LES) paradigms are employed to analyse the internal flow field of a lean premixed swirl-stabilized combustor with axial air injection at both non-reacting and reacting conditions, for a methane and a methane-hydrogen fuel mixture. The thickened flame combustion model (TFM) with detailed chemical kinetic mechanism is employed to simulate the flow. An adaptive mesh strategy is used to maximise the mesh resolution in the flame and boundary layer regions. The numerical results for the methane flame are firstly validated against experimental velocity measurements obtained via particle image velocimetry (PIV). Subsequently the LES is employed to simulate hydrogen-enriched methane flames by keeping the same output power in the combustor, in order to obtain insights on the flow behaviour when hydrogen is added, in terms of flame stability and emissions. A POD analysis reveals the presence of a precessing vortex core (PVC) in both reacting and non-reacting conditions, and how this PVC is affected by the reactants mixture is discussed in the paper. Moreover, the flame is observed to propagate upstream in the jet core despite the use of axial air injection, although flashback is not observed. In terms of emissions, significant reduction in CO and NO_x is observed when adding the hydrogen to the reactants mixture despite the higher flame speed, the reason for are discussed in the paper.