This thesis introduces a newly-developed turbulent combustion model, as a next step towards modelling hydrogen combustion in aircraft engines. The proposed model (FGM-ESF) merges the Flamelet Generated Manifold approach's tabulated chemistry with the Eulerian Stochastic Field method's statistical treatment of flame-turbulence interactions at the subgrid scales, which are not resolved in LES. This hybrid model excels in managing complex combustor dynamics, high turbulence, and both premixed and non-premixed combustion modes, all while maintaining computational efficiency. Validated with a lifted turbulent H2/N2 jet flame in vitiated coflow, reflecting typical combustor conditions, the FGM-ESF model produces accurate predictions of mean velocity, temperature, and mixture in close agreement with the experiments. Comparatively, its performance matches the more costly, fully transported chemistry ESF model, showing limited sensitivity to the number of stochastic fields. The balance between computational efficiency and precision in the FGM-ESF model highlights its importance in the advancement of hydrogen-powered aircraft engines.

Recent years have seen an increase in studies focusing on data-driven techniques to enhance modelling approaches like the two-equation turbulence models of Reynolds-averaged Navier-Stokes (RANS). Different techniques have been implemented to improve the results from these simulations. In particular, the main focus has been on overcoming the limitations implied by the Boussinesq assumption. This has been approached by using machine learning techniques as a way of discovering new formulations that could overperform when compared to traditional models. Despite promising results for steady RANS simulations, little has yet been investigated in URANS applications. In this dissertation, this lack of research will be addressed. The main ideas are then, first, to see how the available information in URANS simulations can be used to improve the anisotropic Reynolds stress tensor prediction, and second if and how this can be done by using a sparse regression technique, whose framework is known as SpaRTA. A procedure involving the triple decomposition of High-Fidelity velocity fields is applied, aiming at finding a model exclusively for the stochastic component of the anisotropy. The test case which is considered is the flow around a cylinder at Re=3900. The High-Fidelity data was collected by running Large Eddy Simulations in OpenFOAM, after which the velocity was split into its different components through Proper Orthogonal Decomposition. A priori results have shown good performance of the trained models, outperforming the Boussinesq assumption both in the prediction of turbulence componentiality and also on the values of the single anisotropy components.

With climate change posing increasing risks, the Advisory Council for Aviation Research and Innovation in Europe (ACARE) aims to reduce CO2 emissions by 75% and NOx emissions by 90% per passenger kilometer by 2050, compared to a baseline aircraft from 2000. The ARENA project addresses this by developing a waste heat recovery system using aircraft engine exhaust gases, improving fuel efficiency. This thesis focuses on integrating the condenser of such a system into the propulsion unit, aiming to minimize drag from the ram air cooling duct by evaluating different heat exchanger topologies and duct designs. The study uses the IMOTHEP Distributed fans Research Aircraft with electric Generators by ONERA (DRAGON) concept, a hybrid electric aircraft with two tail-mounted turbogenerators. The research proceeds in several stages. Initially, a multipass-condenser's potential to reduce pressure drop was examined by adjusting the heat exchanger blockage factor per pass, but results showed no reduction in pressure drop. Next, a lumped parameter model was developed to analyze drag, pressure drop, temperature increase, and ram air duct length. This model evaluated the sensitivity of duct geometrical parameters on the drag recovery factor—a dimensionless number indicating net thrust. Findings revealed that inclining the heat exchanger efficiently increases the drag recovery factor, while the diffuser area ratio has a similar effect but is less space-efficient. The mass flow rate ratio showed less sensitivity, and fin height or pitch had the smallest effect on drag recovery. Using the lumped parameter model, an optimal preliminary ram air duct design was identified for different spatial constraints. The study found that inline plain tube bundle and flat tube offset strip fin heat exchangers provided more compact solutions with higher drag recovery factors. For large diffuser-blocked area fractions, optimal duct geometry remained independent of heat exchanger type, characterized by a 70-degree maximum inclination angle, a 0.7 mass flow rate ratio, and maximized diffuser area ratio within spatial constraints. A verification study of the lumped parameter model was conducted using a two-dimensional Reynolds Averaging Navier Stokes (RANS) computational fluid dynamics (CFD) analysis with k-ω SST turbulence and a porous zone to mimic the heat exchanger. The CFD analysis confirmed the lumped parameter model's predictions, with a 0.8% difference in drag recovery factor for optimal duct geometry. Across various geometries, the mean absolute difference in drag recovery factor between models was 1.082%, with a standard deviation of 0.584%. In conclusion, integrating the condenser in the propulsion unit within spatial constraints results in positive thrust, thus supporting Meredith's 1935[2] claim. This integration reduces net drag and improves overall efficiency, contributing to significant emission reductions in line with ACARE's targets.

To meet the goals of the Paris agreement,sustainable aviation fuels, such as hydrogen, need be adopted on a large scale within the coming decades. Hydrogen-assisted combustion of kerosene is investigated as an intermediate step in the transition towards sustainable aviation. This study addresses the gaps in the current understanding of hydrogen blending into kerosene with respect to chemical kinetics and emission of NOx, CO and un burnt hydrocarbons.  A perfectly-stirred chemical reactor model is developed to study the fundamental effects of hydrogen blending into kerosene. Additionally, to focus on a more practical application the single-reactor model is extended to a chemical reactor network which represents a lab-scale lean premixed prevaporized combustor setup supporting multi-fuel combustion. Combustion characteristics and emission profiles are carefully studied for different hydrogen blending fractions using the developed models, as well as the reaction pathways in oxidation of hydrocarbons.  Hydrogen blending is observed to increase the reactivity of kerosene, mainly due to increased availability of OH and H radicals, as well as O to a lesser extent. For 20% of kerosene mass substituted with H2, ignition delay time at 1200 K and 1 bard ecreases by 55%. Peak laminar flame speed increases by up to a factor of 2.2,while peak adiabatic flame temperature rises by 74 K. In rich burning conditions, H2 additions remove hydrocarbon reaction loops, which causes a strong decrease in un burnt hydrocarbon emissions and potentially soot. CO emissions can be greatly reduced from H2 additions at very lean conditions as overall burning rate is increased, which is corresponding to a reduction in lean extinction limit. This limit is reduced in the chemical reactor network combustor model by ∆φ=0.09 already for substitution of kerosene mass by 20%.  For φ>0.6, dissociation of CO2 is enhanced from thermal effects and causes emission of CO to rise with hydrogen blending when normalized to carbon in fuel. This effect is reflected in the combustion efficiency, which is improved with H2 blending only for very lean conditions,φ<0.6. The reaction pathways of aromatics are modified with H2 addition,which results in a chemical inhibition of the mechanism responsible for thema jority of un burnt hydrocarbon emissions at very lean conditions. Furthermore,formation of NOx is increased via the thermal path since flame temperatures rise. The prompt NOx route becomes more efficient due to promotion of CH formation, while the N2O and NNH paths are enhanced from increased radical availability. Despite the increased emission of NOx at constant φ, blending H2under constant combustion power and combustor mass flow leads to decreasedflame temperatures and reduces total emissions of NOx, CO and unburnt hydrocarbons.

Large eddy simulation paradigm is employed to analyse the internal flow field of a lean premixed swirlstabilized 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 with the kinetic mechanism GRI 3.0 and the detailed chemical kinetics solver SAGE available in ConvergeCFD 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 are first validated against in-house experimental velocity measurements obtained via particle image velocimetry, and then leveraged to provide further insights on the flow behaviour. Significant reductions in CO2, CO and NOx emissions are observed when changing the fuel to the CH4/H2 mixture. From a POD analysis is observed that a Precessing Vortex Core is present in both the reacting and non-reacting conditions. Flashback and severe local extinctions are not observed during the simulated time.

This thesis covers the experimental characterisation of a multi-nozzle research combustor capable of achieving flameless combustion (FC). The combustor is operated with methane as fuel and air as the oxidiser, under an overall lean equivalence ratio ranging from 0.6-0.9. The work focused on designing and applying different experimental techniques under varying operating conditions. Particle Image Velocimetry (PIV) is used to characterise the flow field for reacting and non-reacting conditions. Local gas temperature and gas composition measurements are performed to characterise the formation of NOx in the combustor. Emission data and local gas temperatures are further reported as a function of the equivalence ratio, thermal power input, and oxidiser's oxygen concentration. PIV revealed the flow field structures to be very similar between non-reacting and reacting flow conditions; the location of the recirculation zones remained unaffected. Significant differences exist in the axial flow velocities and turbulence levels in the central recirculation zone (CRZ). Radial temperature measurements near the reaction zone revealed the temperature profiles to flatten for a decrease in equivalence ratio, indicative of distributed combustion. Gas composition measurements are performed near the exhaust of the combustor. The measurements showed the combustor to perform very well from an emission standpoint. Measurements showed the NOx on the centreline to be ultra-low (<10ppm) for all cases investigated. The N2O and NNH mechanisms are identified as the major contributors to the production of NOx and not the thermal pathway, as the gas temperature is consistently below the thermal NOx threshold of 1800K. A reduction in the equivalence ratio and a reduction in the oxygen concentration both reduce NOx emissions. The measured levels of CO and CH4 were negligible near the exhaust for all operating conditions investigated, confirming that combustion is fully completed at this location. Measurements of CO and CH4 near the reaction zone indicate distributed combustion, where a reduction in the equivalence ratio below 0.7 resulted in elongation and widening of the reaction zone. Oxygen measurements near the exhaust and close to the reaction zone indicate the entrainment of cooling air into the combustion chamber, which would locally influence the equivalence ratio. It is recommended to redesign the exhaust duct connection to eliminate the entrainment of cooling air.

There is a need to examine the potential of alternative methods such as Variational Multiscale Method (VMM) for the ability to improve the efficiency of combustion simulations. However the computational expense of a LES run of a 3D turbulent non-premixed combustion problem is high due to the large variety of length and time scales in turbulent non-premixed combustion. Secondly, VMM is a sophisticated model and the models used to simulate turbulent non-premixed combustion are complex. And finally, the results of a full 3D turbulent non-premixed combustion simulation are difficult to interpret due to their complexity. Therefore the objectives as follows: Firstly to define a simplified model problem which has the important aspects of turbulent non-premixed combustion for comparing numerical methods. Secondly, to propose and to investigate the potential of a framework based on VMM formulation for non-premixed combustion computations. The results of the simplified model problem in a VMM framework are compared to the results when using a standard Smagorinsky model. Results show that in general the VMM model with the dynamic subscale assumption outperforms the Smagorinsky model when looking at the mean and RMS value of the velocity, mixture fraction and progress variable.

Although significant strides have been made with regards to increasing the fuel economy of commercial passenger aircraft, the reduction of the environmental footprint remains of the utmost importance. The flow development over an aero-engine spinner will affect the velocity distribution over the fan of the engine and this affects the estimation of the losses generated in the fan and consequently the performance of the engine itself. Previous experimental studies have shown the existence of spiral vortices formed due to boundary layer instabilities over a rotating cone under axial and non-axial inflow. To study the effects of hub-corner separation in detail, it is first important to check the efficacy of existing commercial numerical simulation tools in the prediction of this boundary layer transition development of flow over a rotating nose-cone. The objective of the thesis was thus to simulate using URANS and LES, the formation of the counter-rotating vortices over a rotating cone. To this end. ANSYS DesignModeler and ICEM CFD were used to generate a meshed computational domain and ANSYS CFX was used as the solver. With axis-symmetric cones considered as an idealised geometry for the spinner of an aircraft engine, a 15° half-angle cone with a diameter of 47 mm was chosen. The tip of this cone was blunted by a factor of 1/100th of the cone-diameter. The domain diameter was set to 10 times the cone diameter to reduce the impact of the flow over the side walls. Two structured hexahedral meshes were generated. The BSL EARSM was chosen for the URANS simulation and the WALE Model for the LES run. To check if the simulations were able to capture the vortices, the footprint left behind by them were visualised using the non-dimensionalised instantaneous wall shear values.  The magnitude of the inlet flow velocity given was 2.46 m/s. A 2° incidence is given to the flow for the non-axial case. The effect of mesh refinement was studied for the axial case using both meshes and the finer mesh was used for the non-axial case. The axial case was studied using both the URANS and LES runs, while the non-axial case was studied using only the LES run. The variation of local Reynolds number (defined using boundary layer edge velocity) with local rotation ratio (defined using local radius, boundary layer edge velocity, and rotation velocity of the cone) was also studied and compared with available experimental data. The wall parallel velocity profiles over the length of the cone were also studied. Using the contours of the wall parallel velocity, the momentum mixing in the boundary layer due to the vortices was visualised and compared with the results from the experiments. To characterise the spatial development of the spiral vortices, a critical point was defined using the wall friction coefficient. This critical point was then compared to the one obtained through the experiments. The number of counter-rotating vortices formed over the length of the cone was also checked and the trend between the simulations and experiments were compared.

The energy transition from fossil fuels to renewable energy sources is crucial for climate resilience. In particular, hydrogen combustion in gas turbine combustors is expected to play an essential role in the energy transition. Hence, a fuel-flexible gas turbine combustor operating with a wide range of natural gas and hydrogen fuels is necessary. Low Swirl Burner(LSB), developed at Lawrence Berkeley National Laboratory (LBNL), is one of the promising solutions for fuel-flexible gas turbine combustors. However, hydrogen combustion in LSB at lean conditions is accompanied by several challenges, especially flame flashback. Flashback results in damage and shutdown of gas turbines. Recent experimental studies performed on LSB revealed the occurrence of flame flashback at a high volume percentage of hydrogen, but the exact cause of the flashback in LSB is yet to be discovered. In this study, numerical modelling of low swirl premixed burner with turbulent flow and high hydrogen content is performed. RANS simulations are performed initially for the non-reacting flow and are validated with the experimental measurements. Flamelet combustion model with Zimont Turbulent Flame speed correlation is employed to model the reacting flow. The flame front position obtained from the Zimont correlation closely agrees with the experiments. Further, Algebraic Flame Surface Wrinkling (AFSW) correlation is compared with the Zimont correlation for hydrogen-enriched flames. Based on the comparison, it is concluded that the AFSW correlation did not lead to a significant difference. However, both the correlations failed to model the flashback process. Therefore, the potential mechanism responsible for the flame flashback in LSB is examined. It has been found that none of the standard flashback mechanisms is accountable for the flame flashback in LSB. Furthermore, the effect of different flame shapes reported in the experiments is evaluated. It has been observed that ’M’ shape flame has more propensity to flashback than ’V’ shape flame. Then, a shear layer and turbulent wakes in the premix nozzle of the LSB were noticed. Hence, an increase in flame speed due to the local enrichment of thermo-diffusive unstable hydrogen flame and all the aforementioned effects might lead to a sudden flashback in LSB.

During a severe accident in a water­-cooled nuclear power plant, large quantities of hydrogen can be generated in the reactor core. The hydrogen mixed with air presents a potential risk of combustion when the hydrogen concentration reaches flammability limits. If combustion occurs, pressure loads can damage safety systems and even compromise the structural integrity of the nuclear reactor walls. Thus, predicting the pressure loads is an important safety issue to ensure the reliability of critical structures in the event of a severe accident. Computational Fluid Dynamics (CFD) codes can be used as numerical tools to assess the risks of hy­drogen combustion and predict detailed transients of the pressure loads. The scenarios simulated in nuclear safety engineering can be analyzed with low fidelity models based on reduction assumptions of the physical phenomena. Specifically, the flame gradient models are chosen to model the com­bustion phenomena based on a turbulent flame speed to which the combustion propagates. Different approaches to compute the turbulent flame speed are implemented and compared in this study, in­cluding the Turbulent Flame Closure (TFC) and the Extended Turbulent Flame Closure (ETFC). New models are proposed based on new experimental correlations specifically derived for lean mixtures of hydrogen. Other required libraries as flame radius calculation, radiative heat transfer properties determination, and axisymmetric Adaptive Mesh Refinement are implemented and evaluated. A study on the influence of the ignition source term is also carried out. The implementation of the solver and the models is verified with the analytical solution of a planar one­-dimensional premixed flame moving into frozen turbulence. Moreover, three different experimen­tal cases, relevant to nuclear safety, are selected to perform the validation of the solver. The first one is a spherical combustion chamber which was experimentally tested with lean hydrogen mixtures and controlled measured homogeneous isotropic turbulence. The second one is the Thermal-hydraulics Hydrogen Aerosol and Iodine (THAI) facility, which is a larger vessel where effects as buoyancy be­ come more relevant. Finally, the third one is a flame acceleration enclosure called ENACCEF­2, where obstacles are placed along a cylindrical tube to promote flame acceleration and the transition from de­flagration to detonation. Numerical results are presented in terms of the flame front location, the flame front velocity, and the pressure rise. The sensitivity of the results to mesh resolution, time discretization, and initial turbu­lence levels is assessed. The implemented combustion models are simulated with multiple turbulence models, analyzing the results and the influence on one each other. In general, the proposed mod­els represent an approach that provides good predictions in both flame development and pressure dynamics, being more robust than other models previously available

In reacting flows, detailed chemistry computations are usually avoided precomputing the thermochemical quantities as functions of a reduced set of variables such as the Flamelet Generated Manifold (FGM) approach[34]. Although it mitigates the calculations of detailed chemical mechanism, the memory requirement associated to store the lookup table and retrieve the information during numerical simulations is usually large (order of Gigabytes). Thereby, extending the FGM approach in order to include other conditions requires to add other independent variables which will inevitable lead to increase the size of the lookup table. This will generate that Large Eddy Simulations (LES) cannot be performed such as is the case of the Diluted Air FGM (DA-FGM) approach developed by Xu Huang[9], limiting the simulations to Reynolds Average Navier-Stokes (RANS) approach. In this master thesis, the goal is to use Artificial Intelligence (AI)-Machine Learning (ML) techniques in order to reduce substantially the computational cost of storing lookup tables. In order to achieve this, the Artificial Neural Network (ANN) technique is used. First, a 4D FGM lookup table for hydrogen flames is simplified using the aforementioned AI technique. Then, this technique is used to replace a 6D lookup table generated using the DA-FGM approach. The accuracy and stability of the models provided by ANNs is measured by statistical indicators, providing high accurate and stable AI models. Finally, in the middle of this project, unexpected issues regarding the 4D lookup table were encountered, which lead to recreate the 4D lookup table. After studying carefully how the 4D lookup table was created, a new 4D lookup table is generated, providing excellent results and improving the AI models obtained.

TOP contoured nozzles, with large area-ratios, are commonly employed in rocket propulsion systems as they feature an excellent thrust-to-weight ratio. A significant shortcoming to this design is that, during the startup and shutdown transients of a LRE, the internal nozzle flow progresses through a series of overexpanded flow states - Free Shock Separation (FSS) and Restricted Shock Separation (RSS), which produce critical loads associated with SWBLI and asymmetric flow separation. Exacerbated by FSI, this operational phase is known to generate the highest vibroacoustic loads at which payload and vehicle structures are subjected to when the engine is operated at off-design conditions. Motivated by the importance of understanding how the interaction between the developing flow and the vibrating nozzle walls has an effect on supersonic noise generation and propagation, this work has studied the effect that wall compliancy has on the vibroacoustic loading of TOP contoured nozzles. This is demonstrated by means of cold flow tests carried out in the High Speed Laboratories of the Delft University of Technology on a stiff-walled aluminum nozzle, which serves as a baseline test case, and on a urethane-based compliant walled nozzle. Tests are conducted under comparable flow conditions and test parameters are measured by means of acoustic and optical techniques. Simultaneous recordings are performed and include the nozzle-wall deformation, by means of stereoscopic tracking of tracers on the nozzle lip, the imprint of the near-field acoustic signature, by means of arrays of pressure-microphones, and Schlieren imaging of the jet plume. Measurement data allows for a Fourier decomposition of the nozzle lip displacement and of the acoustic pressure field in azimuth. Reconstruction of the instantaneous plume development enables the identification of the main flow structures responsible for noise generation. Comparison of results between the two test articles highlights a different spectral content and directivity pattern. Correlation between the structural displacements and the acoustic signal, together with the use of DMD, quantitatively aids the investigation of how FSI has an impact on the generation of an aeroelastic tone at 180 Hz. Findings suggest that its production is the result of the periodic thickening and thinning of the shear layer owing to the heightened flapping motion of the nozzle lip preceding RSS transition, driven by an intensified shock foot instability.

In the future, fossil fuels will be replaced by renewable sources. Hydrogen seems to be a promising energy carrier (fuel). A lot of research has been conducted on the combustion of hydrogen, but the effect of the high diffusivity of hydrogen compared to other species (differential diffusion) was often not included in simulations of turbulent flames because turbulent mixing is expected to suppress strong influence of differential diffusion. Nevertheless effects of differential diffusion have been reported in experiments. The main purpose of this thesis is to include the effect of differential diffusion in a CFD model of turbulent non-premixed hydrogen flames and to validate the models with available experimental data. The validation data set used in this study is a non-premixed turbulent jet flame of hydrogen diluted with nitrogen on 50/50 volume ratio. This flame is interesting because the role of differential diffusion in this flame has been a matter of discussion in the literature. A review is given of previous work on turbulent non-premixed hydrogen flames and effects of differential diffusion. It is concluded that the Flamelet Generated Manifold (FGM) model and the Transported Probability Density Function (PDF) model are both promising turbulent combustion models for these flames. Next, a new comparative study is made of several turbulence and combustion models using Reynolds-averaged Navier Stokes simulations in Ansys Fluent, and focusing on FGM as turbulent combustion model. It is concluded that the standard k-e with a correction as suggested by Pope is the best choice for the turbulence model. The ANSYS Fluent implementation of FGM does not have the option to include differential diffusion. Therefore it is added in a separate way. To do this, flamelets are created with the help of CHEM1D, a tool developed by TU Eindhoven. These flamelets are combined to an FGM table also including the effect of turbulence via a PDF and this table is imported into Fluent with the help of a user-defined function overwriting the default Fluent FGM table. In order to clearly see the effects of differential diffusion, simulations with and without differential diffusion are made. Good agreement is obtained between experimental results and the numerical simulation for the models without differential diffusion. This confirms that turbulence can suppress strong influence of differential diffusion. In experiments effects of differential diffusion have been observed at the base of the flame, close to the burner nozzle. The model simulations with differential diffusion included, provide a slightly more accurate prediction of the mean temperature close to the nozzle but still large discrepancies remain.