Recent research [Porcarelli et al., Int. J Hydrogen Energy 49, 2024] shows NOx can be suppressed in lean premixed hydrogen flames at high levels of strain without loss of efficiency. However, the thermoacoustic response of such flames in high strain regimes remains unexplored. The present work addresses this by investigating the thermoacoustic characteristics of very lean premixed hydrogen flames under increasing levels of applied strain rates. High-fidelity simulations are conducted in a counterflow, reactants-to-products configuration, where strain rate is controlled by inlet velocities. Several simulations are conducted under a range of different strain rates and forcing frequencies. The simulations are run using a modified version of the reactingFOAM solver within OpenFOAM-9 that accounts for the different diffusivities in hydrogen flames and uses the low-Mach formulation of the Navier–Stokes equations. The forcing was imposed by applying sinusoidal oscillations on top of the uniform velocity of the reactants at the inlet boundary with a specific frequency and an amplitude of 10% of the mean value. Results indicate that above a threshold, increasing strain raises the gain and shifts its peak to higher frequencies. This behaviour stems from the flame speed response to strain at negative Markstein lengths and can be leveraged for the development of ultralow-emission combustion devices where applied strain can be used as an additional mechanism to shift an unstable flame response to different frequencies. Moreover, the highest applied strain rate investigated triggers an inversion of the Markstein length, leading to a 180 phase shift in the flame dynamic response. This aspect and further implications are discussed in the paper.

This study investigates the effect of increasing strain rate on thermodiffusively unstable, lean premixed hydrogen flames in a 2D counterflow configuration through detailed-chemistry numerical simulations for the first time. The analysis of transient flame dynamics without imposed perturbations reveals that a steady-state flame front is achieved only when the strain rate exceeds a certain threshold. Below this threshold, the curved flame tips exhibit an unstable sequential onset and suppression pattern. When subjected to a range of perturbation wavelengths, the flame front exhibits an exponentially increasing wavelength over time, driven by the flame-tangential velocity component, with the applied strain rate acting as the amplification factor. It is shown that any perturbation is damped at sufficiently high applied strain rate conditions after a transient phase. At these high strain regimes, the growth rate transient follows a characteristic onset that depends uniquely on the initial perturbation wavelength and exhibits a linear dependence on the applied strain rate.

The influence of Soret effect on the prediction of flame characteristics and NO_x emissions in lean premixed hydrogen flames is studied in a reactant-to-product counterflow configuration under high strain conditions. By means of one-dimensional detailed chemistry simulations, the impact of Soret effect on the response of the flame to strain is first analyzed. The results show that leaner mixtures exhibit a stronger sensitivity to strain, and modeling thermal diffusion further intensifies this behaviour by affecting the prediction of temperature, peak of radicals, and consumption speed. Moreover, the Markstein length prediction is found to be affected by the thermal diffusion, with the main effect being to shift the point of sign inversion to a richer equivalence ratio as compared to the case where Soret effect is not considered. Isolating the hydrogen preferential diffusion and Lewis number effect, it is found that the response to strain is mainly driven by the Lewis number effect. Nevertheless, preferential diffusion behaviour is still observed to play a significant role in the leaning of the mixture ahead of the flame when Soret effect is taken into account. In terms of NO_x emissions, including thermal diffusion in the modeling causes an increase in both the peaks of NO mass fraction and its formation rate, especially under ultra-lean conditions where NO formation is primarily through the NNH pathway. The profiles of NO production rate with strain are also influenced, with prediction discrepancies ranging from 10 % in moderately lean conditions to 30 % in ultra-lean conditions. These effects are observed to be mainly associated to the preferential diffusion (as opposed to non-unity Lewis number effect) and its coupling with strain. Effect of pressure is also investigated, showing that the thermal diffusion can significantly alter the rate of production of NO even at high pressure conditions.

This paper investigates water injection effects in a simplified Ansaldo GT36 reheat system under realistic conditions of 20 atm using large eddy simulation (LES) coupled with thickened flame modeling and adaptive mesh refinement. The water injection conditions are optimized by performing a parametric study based on global sensitivity analysis (GSA) with a surrogate model based on Gaussian process (GP) to reduce computational cost. In particular, the influence of four design parameters, namely, Sauter mean diameter (SMD), water mass flow, and the angles of the spray's hollow cone, is tested to achieve an optimized solution. In the "dry"case, the LES simulations show several flashback events attributed to compressive pressure waves resulting from auto-ignition in the core flow near the crossover temperature. The use of water injection is found to be effective in suppressing the flashback occurrence. In particular, the global sensitivity analysis shows that the external angle of the spray cone and the mass flow of water are the most important design parameters for flashback prevention. NOx emissions are reduced by about 17% with water injection. Once an optimized condition with water injection is found, a recently proposed method to downscale the combustor to lower pressures is applied and tested. Additional LESs are performed for this purpose at the dry, unstable condition and the "wet,"stable condition. Results show that similar dynamics are predicted at 1 atm, validating the method's robustness. This provides avenues for experimentally testing combustion dynamics at simplified conditions which are still representative of high-pressure practical configurations.

Electromagnetic fields influence flame behavior by altering the transport of paramagnetic species such as oxygen and OH radicals in hydrogen flames, affecting reaction pathways and combustion dynamics. This study presents a numerical investigation of the effects of magnetic fields on a premixed swirl-stabilized hydrogen flame using a modified combustion solver in OpenFOAM. Additional body force and diffusion terms were incorporated into the governing equations to model interactions with paramagnetic species, and the solver was validated against experimental data and simulations from the literature. The study focuses on analyzing flame structure, species redistribution, and mixture fraction variations under magnetic conditioning. Large Eddy Simulations (LES) with the Eulerian Stochastic Fields (ESF) method were employed to capture turbulence-chemistry interactions. The results indicate that the presence of a magnetic field induces an upstream-directed force on oxygen, leading to localized changes in mixture fraction and combustion characteristics. A reduction in temperature, heat release rate, and OH concentration was observed, with peak reductions of approximately 2%, 5%, and 6%, respectively. These effects are attributed to the redistribution of oxygen, which makes the flame locally leaner. This study extends the understanding of hydrogen combustion under electromagnetic influence and demonstrates the potential of magnetic fields for controlling the flame behavior. The findings provide new insights into magnetic field-assisted combustion strategies, offering a framework for further research in advanced propulsion and energy applications.

We present a data-driven approach to Reynolds-averaged Navier-Stokes (RANS) turbulence closure modelling in magnetohydrodynamic (MHD) flows. In these flows the magnetic field interacting with the conductive fluid induces unconventional turbulence states such as quasi two-dimensional (2D) turbulence, and turbulence suppression, which are poorly represented by standard Boussinesq models. Our data-driven approach uses time-averaged Large Eddy Simulation (LES) data of annular pipe flows, at different Hartmann numbers, to derive corrections for the - SST model. Correction fields are obtained by injecting time averaged LES fields into the MHD RANS equations, and examining the remaining residuals. The correction to the Reynolds-stress anisotropy is approximated with a modified Tensor Basis Neural Network (TBNN). We extend the generalised eddy hypothesis with a traceless antisymmetric tensor representation of the Lorentz force to obtain MHD flow features, thus keeping Galilean and frame invariance while including MHD effects in the turbulence model. The resulting data-driven models are shown to reduce errors in the mean flow, and to generalise to annular flow cases with different Hartmann numbers from those of the training cases.

Direct numerical simulations (DNS) are conducted for reactants-to-products counterflow configurations at turbulent conditions to understand how strain affects the structure and NOx emissions of lean premixed hydrogen flames. Two nominal equivalence ratio conditions, 0.5 and 0.7, are investigated. Under unstretched conditions, the Markstein length is negative for the former and slightly positive for the latter, indicating distinct responses of heat release rate and flame consumption speed to strain in each case. For each equivalence ratio condition, three levels of applied strain rate are considered, resulting in a total of six DNS. Results indicate that overall NOx emissions decrease with increasing strain at turbulent conditions, consistent with recent results for laminar conditions presented in Porcarelli et al. (2024). However, the relative decrease of NOx with strain is faster under turbulent conditions because turbulent mixing limits the occurrence of super-adiabatic temperatures. Moreover, the decrease of NOx is strongly correlated only to the mean applied tangential strain rate, while local fluctuations of strain due to vortices exhibit more stochastic behaviour. The detailed analysis presented in this article indicates that the applied strain can be used to substantially decrease NOx emissions in premixed hydrogen flames under practical conditions. Novelty and Significance statement: This work examines for the first time in detail the coupled effects of strain and turbulence in hydrogen flames, for various conditions spanning different signs of the Markstein length and increasing applied strain levels. In particular, it clarifies the different roles of applied strain, turbulence-driven strain, and curvature on both flame structure and NOx generation. Results further show for the first time that both in-flame and post-flame NOx can be suppressed at high strain levels under turbulent conditions. This result is of paramount importance as it implies that NOx can be suppressed at combustor-relevant conditions by straining the flame.

The transition to hydrogen as a primary fuel in aviation requires innovative strategies to mitigate challenges associated with flashback in combustion systems. Due to its low volumetric energy density, hydrogen is preferably stored at cryogenic temperatures (T ≤ 100 K). Since stoichiometric hydrogen/air laminar premixed flames have been observed to sustain combustion even at temperatures as low as 100 K, low-temperature injection can be used as a strategy to control flashback and stabilize the flame. This study investigates the performance of low-temperature rich premixed hydrogen/air combustion within a model Rich-Quench-Lean (RQL) combustor configuration equipped with a Trapped Vortex Cavity (TVC). Large Eddy Simulations (LES) with Eulerian Stochastic Field (ESF) approach are conducted for this purpose. The LES are first validated against a recent experimental campaign involving high-speed chemiluminescence diagnostics. Subsequently, parametric studies explore the operating limits and flashback resistance of rich hydrogen flames under varying low-temperature conditions. Results indicate a strong sensitivity of the flame position within the TVC to the injection temperature. The strong reduction in laminar flame speed observed in experiments, however, is counteracted by the increase in turbulent/laminar flame speed ratio, adding to the complexity of the problem. Insights on how to control the flame dynamics in the cavity are provided within this study with the purpose of optimizing RQL-TVC designs for robust, low-emission, and flashback-resistant operation in hydrogen-based propulsion systems.

The interplay between strain and preferential diffusion in lean premixed hydrogen flamelets is investigated numerically. Lean conditions are established at an equivalence ratio of 0.5. Detailed chemistry, one-dimensional simulations are performed on a reactants-to-products counterflow configuration, both including and artificially excluding preferential diffusion effects. A comprehensive analysis of the flame physical properties is performed, showing that preferential diffusion tends to weaken the flame as compared to the case where it is artificially suppressed, as it triggers a local leaning of the mixture ahead of the flame front. Counterintuitively, strain is observed to counteract or limit this preferential diffusion effect, with the peaks of radicals and reaction rate, flame thickness, and consumption speed, progressively approaching and in some cases overtaking the corresponding solution obtained with equal diffusivities as strain increases. This is shown to be a consequence of the fluid elements being increasingly preferentially transported in the flame tangential direction rather than diffusing in the flame normal direction. Hence, the flame weakening effect due to different diffusive fluxes of fuel and oxidizer across the flame front is progressively compensated by their differential transport on the flame tangential direction triggered by increasing applied strain rate, which instead enables an overall enrichment of the burning mixture. This analysis provides a different view as compared to previous studies attributing to strain an enhancing influence on the effects of preferential diffusion. In this work the opposite interpretation is proposed instead, where strain acts as a limiting factor to the weakening effect of preferential diffusion on lean hydrogen flames.

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.

Mixed convection of an electrically conductive fluid in a square duct with imposed transverse magnetic field is studied using Large Eddy Simulation (LES) paradigms. The duct walls are electrically conductive, with the wall conductivity parameter c_w ranging from 0 to 0.5. The Reynolds number is Re=5602 and the Prandtl number is Pr=0.0238. The focus of the study is on flows at Hartmann numbers Ha⩽125, Richardson numbers Ri⩽10 and two different thermal boundary conditions are considered: four wall uniform heat fluxes and one-sided heating (fixed wall temperature). The results show that the transition from laminar to turbulent flow depends not only on the ratio Ri/Ha, but also on c_w and on the local thermal boundary conditions. In the turbulent regime with one-sided heating, the turbulent heat fluxes play an important role in the total heat transfer, in contrast with the typical behaviours of liquid metals. Moreover, the turbulent and thermal structures are highly dependent on the thermal boundary conditions, which completely alter the flow structure. It is also found that at c_w≥0.01 the turbulent heat fluxes decrease.

This paper presents an investigation of the effects of water injection within a simplified version of the Ansaldo GT36 reheat system. The investigation is carried out under realistic operating conditions of 20 atm and using large eddy simulation (LES) coupled with the thickened flame model (TFM) and an adaptive mesh refinement. The water injection conditions are optimized by performing a parametric study based on global sensitivity analysis and a surrogate model based on Gaussian process is employed as a way to reduce computational cost. In particular, the influence on the system performance of four design parameters, namely Sauter mean diameter, water mass flow and the angles of the spray's hollow cone, is tested to achieve an optimized solution. In the 'dry' case, the LES simulations show several flashback events, which are a defining aspect of the considered conditions, and attributed to compressive pressure waves resulting from autoignition in the core flow near the crossover temperature. The use of water injection is found to be effective in suppressing the flashback occurrence. In particular, the global sensitivity analysis shows that the external angle of the spray cone and the mass flow of water are the most important design parameters for flashback prevention. Moreover, NOx was shown to be reduced by about 17% by the use of the water injection at the tested conditions. Once an optimised condition with water injection is found, a recently proposed method to downscale the combustor to lower pressures is applied and tested. Additional LES are performed for this purpose at the 'dry', unstable condition and the 'wet', stable condition. Results show that similar dynamics, respectively unstable and stable, is predicted at 1 atm, suggesting the robustness of the method. This provides avenues for experimentally testing combustion dynamics at simplified conditions which are still representative of high-pressure practical configurations.

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.

In this work, we propose a data-driven framework to identify precursors of extreme events in turbulent reacting flows. Specifically, we tackle the problem of flashback prediction in a lean hydrogen reheat combustor. Our framework is composed of two parts. The first consists in the use of a co-kurtosis based approach to identify the components of the thermochemical and flow state which are the most relevant for the onset of flashback. This allows for an efficient low-dimensional representation. From this reduced representation, a modularity-based clustering algorithm is then employed to segregate between clusters which contain normal and extreme (flashbacking) states, and the cluster located in-between these states, which are the precursor states of extreme events. We show that this method is able to identify the most important features at the onset of flashback in the considered reheat combustor and then provide precursor states based on those. The prediction time obtained with the identified precursors is relatively large when compared to the duration over which the combustor is stable. Additional analyses on the specific choice of features for the precursor identification and the sampling locations are made. The robustness of the method when using shorter time series to identify the precursor is also investigated. Results show that the method is generally robust with respect to such changes. A first step towards practical measurements is also attempted with wall pressure measurements, which shows only a moderate reduction in prediction time. This work proposes for the first time a data-driven technique to automatically identify precursors of flashback in hydrogen combustion opening the path for such applications on other extreme events in reacting flows.

In this study, the effects of water injection are analysed in a simplified geometry of a reheat combustor, which is based on a state of the art sequential reheat combustor from Ansaldo Energia. The inlet temperature of this burner and the chemical properties of hydrogen in terms of flame speed and autoignition imply that combustion occurs in an autoignition-assisted propagation regime. Large eddy simulations with an Artificial Thickened Flame modelling have been employed to analyse the dynamic behaviour of the hydrogen flame in lean premixed combustion mode. Results show that this approach can accurately predict the position of the flame front and the autoignition process as compared to a pre-existing DNS dataset. The flame front behaviour is further investigated, and observed to quickly move upstream and downstream, driven by the effect of pressure waves on the autoignition process. The study conducted here shows how the injection of water droplets can prevent the autoignition upstream and thus effectively help in achieving flame stability. Some light is also shed on the influence of the spray design parameters on the stability of the H₂ flame and the NOx emissions.

NO_x formation in premixed, highly-strained, pure hydrogen-air flamelets is investigated at lean conditions. Detailed-chemistry, one-dimensional and two-dimensional simulations are performed on a reactants-to-products configuration with varying applied strain rate. The results highlight for the first time for lean pure-hydrogen flamelets that NO_x emissions are suppressed as strain rate is increased. The most substantial decrease is observed across the thermal NO_x formation pathway. Here the suppression of NO_x is triggered by a redistribution across the flamelet of the radicals involved in the pathway reactions. A comparison with methane flamelets is further performed to understand to what extent the observed trends with strain can be specifically linked to hydrogen burning features. Similar NO_x suppression trends with strain are also observed for increased pressure and different equivalence ratios, although with different rates of decrease. A correlation is proposed based on the observed results and further theoretical considerations, to predict NO_x emissions according to applied strain rate and equivalence ratio.

Turbulent structures in a concentric annular pipe within a uniform transverse magnetic field are examined for a liquid metal flow. Large-eddy simulations are performed to study the effect of magnetic field on turbulence suppression and heat transfer within this geometry. At the characteristic Prandtl number of liquid metals, the smallest scales based on temperature fluctuations are much larger than those of the velocity, which allows to resolve all the temperature scales with sufficient accuracy. The calculations are run at Reynolds number 8900 for three different Hartmann numbers, H a = 40, 60, 120. The comparison with available direct numerical simulation data shows encouraging agreement. The main findings of this work show a circumferential dependency of the flow characteristics on the local orientation of the magnetic field, with increased anisotropy observed at all Hartmann numbers studied. Anisotropic effects of the magnetic field are predominant for Ha = 60 and Ha = 120 causing turbulence to deviate from its conventional state. At these Hartmann numbers, a partial redistribution of the turbulent kinetic energy from the axial and radial components to the azimuthal component is observed. This effect, observed here for the first time, appears to be related to the appearance of coexisting quasi two-dimensional (2D) and three-dimensional (3D) turbulence states. Moreover, large skin friction increments are also observed at Ha = 60 and Ha = 120, while coherent structures stretching and streak suppression are found for all three Hartmann numbers.

A lean premixed ethylene-air flame in a backstep configuration is simulated on multiple grids using both direct numerical simulations (DNS) with reduced order kinetic mechanism and large eddy simulations (LES) with flamelet-based thermochemistry. The configuration includes preheated reactants and a recirculation zone that provides radicals and high temperature gases to stabilize the flame. Heat losses are present due to the proximity of cooled walls. The reacting flow obtained from DNS at different resolutions is first analyzed to investigate the property of heat transfer within the recirculation region. LES based on adiabatic flamelets with a correction of the heat capacity is then tested, and its ability to account for heat losses is compared to results obtained using a three-dimensional non-adiabatic flamelet approach. Mean fields and subgrid properties are compared to those obtained from DNS to assess the capability of the LES models. The results show that the non-adiabatic flamelet approach can predict recirculation region and temperature fields with good accuracy. The model with heat capacity correction is able to effectively correct the heat capacity behavior as observed by a priori comparisons. However, in the a posteriori context, it is observed to overestimate the temperature field, although the correct size of the recirculation region is predicted. The combined a priori and a posteriori analyses on the same configuration and at different mesh resolutions allow for a precise separation of modeling effects due to heat transfer at the wall and combustion closure, thus providing indications on the LES performance in the context of flamelets.

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.

Large eddy simulation is used to investigate the flashback mechanism caused by the combustion-induced vortex breakdown (CIVB) in a high-pressure lean-burn annular combustor with lean direct injection of kerosene. A single sector of the geometry, including a central pilot flame surrounded by a main flame, is simulated at take-off conditions. A previously-developed flamelet-based approach is used to model turbulence-combustion interactions due to its relatively low cost, allowing to simulate a sufficiently long time window. In stable operations, the flame stabilises in an M-shape configuration and a periodic movement of the pilot jet, with the corresponding formation of a small recirculation bubble, is observed. Flashback is then observed, with the flame accelerating upstream towards the injector as already described in other studies. This LES, however, reveals a precursor partial blow-out of the main flame induced by a cluster of vortices appearing in the outer recirculation region. The combined effect of vortices and sudden quenching alters the mixing level close to the injector, causing first the main, then the pilot flame, to accelerate upstream and initiate the CIVB cycle before the quenched region can re-ignite. Main and pilot flames partly extinguish as they cross their respective fuel injection point, and re-ignition follows due to the remnants of the reaction in the pilot stream. The process is investigated in detail, discussing the causes of CIVB-driven flashback in realistic lean-burn systems.