Sustainable Aviation Fuels (SAF), encompassing fuels synthesized from biomass (biofuels) or zero-carbon electrical sources (electrofuels), offer a short-term solution for decarbonizing the aeronautical sector. Exhibiting main properties similar to conventional jet fuel, using SAF does not require completely redesigning current combustion chambers. However, given the diverse feedstock required to meet energy demands, SAF compositions may vary considerably. Recent studies indicate that these variations affect combustion chamber performance, influencing operability and emissions. Aeronautical engineers are also faced with the challenge of burning SAF under lean combustion regimes to minimize the production of pollutants. However, lean combustion can promote flame blowout and instability, requiring effective solutions for enhancing flame stabilization. The SAFARI project aims to gain fundamental insights into SAF combustion's multi-physic phenomena and develop numerical tools for designing future combustion chambers. In particular, the main objectives are: i) to understand the effects of pressure, heat transfer, and fuel composition variability on flame dynamics and pollutant emissions, such as NOx and soot particles, ii) to generate a detailed experimental database to validate turbulent combustion models, iii) develop and validate an advanced CFD modeling strategy, iv) test the relevance of plasma-assisted combustion for improving flame stabilization and preventing combustion instabilities in lean regimes. To achieve these objectives, the project is structured into three phases. Firstly, fundamental experimental diagnostics and advanced numerical tools will be developed to measure and model the chemical flame structure and its interactions with turbulence and heat transfer in a high-pressure and two-phase flow environment. Mixtures of chemical species will be identified to dissociate the influence of composition variability on flame chemical processes from differential evaporation processes. Secondly, these experimental and numerical tools will address fundamental physical questions concerning SAF combustion operability in terms of stabilization, dynamics, and pollutant formation. A series of reactive experimental configurations of increasing complexity in operating pressure and injection systems will be introduced to address each scientific question separately. The relevance of plasma-assisted combustion for stabilizing and preventing instabilities in lean two-phase flames will be assessed. The project will conclude with four applicative challenges identified to characterize, under real aero-engine conditions, 1) the behavior of high-pressure flames, 2) pollutant emissions, 3) chamber operability, and 4) combustion control by plasma-assisted combustion, respectively.

Plasma reprocessing of CO2 is a technique used to convert CO2 to CO. The CO produced may then be used to create a fuel known as syngas. This process has the potential for turning CO2 into value-added products, and it has been proposed as part of several carbon-neutral energy systems. However, successful implementation of this reprocessing technique requires that it be done in an energy efficient manner. This, in turn, requires a detailed understanding of CO2 chemical kinetics: in particular, state-specific recombination kinetics. We propose using our plasma torch facility at CNRS laboratory EM2C to study relevant CO/CO2 recombination kinetics. Our facility can produce an equilibrium CO2 plasma with a temperature between 7000 and 9000 K. This plasma is then passed through a water-cooled tube which forces rapid recombination. We have successfully used this approach to study recombination kinetics in nitrogen plasmas. The relevant recombination dynamics are studied using optical diagnostics and the measurements are compared to models in order to extract state-specific recombination rates. We propose here measurements of species concentrations and temperature in a recombining CO2 plasma. This will be done using both optical emission spectroscopy and laser-based techniques (LIF). The state-specific data provided will be valuable for model validation and contribute to an understanding of the kinetic mechanisms behind plasma reprocessing of CO2.

Aviation is currently responsible for 2% of global CO2 emissions but its impact is growing. For the moment, and for the coming years too, the energy density of battery storage is still too low for commercial aviation applications; it will therefore be necessary to keep on using combustion in gas turbines, which have an excellent power-to-weight ratio. Consequently, the fuel will have to be decarbonized. Among the different candidates, hydrogen gas H2 has interesting properties: it can be produced by water electrolysis without CO2 emissions, has a high calorific value by mass and does not contain carbon, which prevents the emission of pollutants such as carbon monoxide or soot particles. However, the combustion of hydrogen in an aeronautical type chamber presents several challenges. On the one hand, because of its high flame speed, combustion in premixed mode becomes sensitive to the flashback phenomenon, which can raise safety issues. On the other hand, its relatively high combustion temperature may enhance the production of nitrogen oxides (NOx), a toxic pollutant. In addition, flames can be subject to thermo-acoustic instabilities during which the flame heat release couples with the acoustics of the combustion chamber, creating a resonance phenomenon that can lead to the extinction or destruction of the engine. The FlyHy project aims to provide the building blocks to overcome these obstacles by studying the stabilization of hydrogen-air flames in industrially-relevant injection configurations and geometries. For this, it relies on an existing experimental platform, named X-ICCA, located at EM2C laboratory. This platform consists of three experimental rigs of increasing complexity allowing the same injection system to operate at different scales. The first one, SICCA, is based on a single injector and allows to study the behavior of an isolated flame. TICCA has three aligned injectors to provide a more realistic environment for the central flame and to study the interactions between flames. Finally, MICCA presents the annular shape of aeronautical combustion chambers and sixteen injectors are used. These three rigs have been used to study premixed propane-air flames as well as spray flames from kerosene-like liquid fuels. The FlyHy project is split in 4 scientific tasks. The first one consists in designing an injector based on the principle of direct hydrogen injection and to characterize it in SICCA. Resistance to flashback, NOx production and response to acoustic solicitations (through the concept of flame describing function, FDF) will be investigated. The second task aims at studying the possible differences in stabilization and FDF due to the presence of surrounding flames. It will be based on the TICCA rig equipped with the injector developed in task 1. Tasks 3 and 4 are based on MICCA and can potentially be carried out in parallel. The third task aims at analyzing azimuthal thermo-acoustic instabilities associated with the circumference of the combustion chamber. These are known to be particularly problematic because their relatively low frequencies make the flames very sensitive to them. FDFs measured during tasks 1 and 2 will be used to design low-order models able to predict the onset of such instabilities. Finally, task 4 focuses on the phenomenon of light-round which corresponds to the flame propagation from injector to injector during the ignition of an annular combustion chamber.

The aim of the project is to develop modelling and numerical methods for the accurate prediction of the heterogeneous spray-flow coupling, in the scope of Large Eddy Simulation of turbulent spray flames. Today's simulations of realistic systems such as aeronautical combustors are relying on point-droplet assumption, which avoids the flow resolution at the droplet scale, and the description of spray-flow coupling must rely on modelling. A proper description requires a multi-scale approach, from the droplet interface to the interactions with the flame. As the main process to be accounted for is the vaporization of the droplets, the first goal will be the development and validation of accurate vaporization and heating models, using droplet-resolved simulations to properly characterize the heat and mass transfers between the droplets and the gaseous phase. These models will be used for spray simulations, which can either be performed using Lagrangian or Eulerian methods. For the Lagrangian approach, spray-flow coupling strategies will be developed using the droplet-resolved simulations, in order to incorporate the macroscopic effects of the flow structure at the droplet scale in the Lagrangian simulations. For the Eulerian method, which considers the spray through its statistics and generally see it as a continuum, the heterogeneous character of the spray, i.e. the fact that droplets are local inclusions and not a continuum, will be incorporated in the statistical description. This aspect can have a great impact on spray flames, for instance when large droplets cross the flame. These new modelling approaches will be extended to Large Eddy Simulation. The modelling of the subgrid scale effects will not only account for the unresolved dynamics, but also for the subgrid scale spray-flow coupling and spray heterogeneity. The developed LES approaches will be finally applied to the simulation of turbulent spray flames, and to a realistic aeronautical-type burner.