A large share of aircraft emissions occur at cruising altitude, where they significantly affect climate. While models are being developed to study these impacts, their validation is limited by scarce measurements at high altitudes. Existing efforts to use instrumented aircraft are constrained by fleet availability and limited endurance, resulting in poor temporal coverage, while measurements from satellites have poor spatial resolution. To address these limitations, this project focuses on designing a medium to high altitude, long-endurance observatory aircraft for continuous measurement of atmospheric composition, cruise emissions, and radiation for climate research. SORA (Stratospheric Observatory Research Aircraft) was developed to fulfil this mission. SORA will be capable of missions up to a duration of 60 days, and fly at cruise altitudes between 15 km and 25 km. It will generate its own power through solar panels installed on top of the wing, which also charge batteries that support night operations. The propulsion system consists of 4 electrically-powered propellers attached to the wing, which ensure zero operational emissions while fulfilling all operational requirements. Operations depend heavily on seasonal and geographical solar availability, so the aircraft was then designed to maximize its operational coverage around the planet. The final design allows full year-round operation between latitudes of -30 °N and 30 °N, with an operational map having been developed for other latitude ranges, thus allowing to know the full geographical and seasonal operational range of the aircraft. Other operational considerations include taking-off from dry aerodromes in sunny conditions, avoiding high-turbulence areas, among others. SORA will carry a series of scientific payload instruments, from spectrometers, to a LiDAR instrument and several in-situ sensors that allow it to fulfil the scientific objective of this mission. ... Read more

Aviation is responsible for a significant share of global greenhouse gas emissions and remains one of the more challenging industries to transition to carbon-neutral operation. Research into alternative fuels and reductions in fuel consumption is therefore critical. This thesis investigates the extent to which integrating a hydrogen-fuelled Solid Oxide Fuel Cell into a CFM56-5B1 turbofan can reduce the Thrust Specific Fuel Consumption during cruise conditions. The SOFC was modelled as a 0D electrochemical component developed in OpenMDAO and integrated into pyCycle for cycle-level steady-state analysis. To assess feasibility, four heat exchanger placements were evaluated across a range of Bypass Ratio (BPR) and Jet Velocity Ratio (JVR) values. Maintaining the BPR and JVR equal to those of the baseline hydrogen-fuelled CFM56-5B1 yielded fuel consumption reductions of 12.2-16.2%, depending on heat exchanger placement. Allowing both parameters to vary produced reductions of up to 27.6% relative to the hydrogen-fuelled baseline. Total engine efficiency increased from approximately 33% for the baseline to approximately 43% for the most efficient hybrid configuration. The inter-turbine heat exchanger placement was found to be thermodynamically optimal, while positioning the heat exchanger downstream of the cathode exhaust achieved the second-highest efficiency with potential benefits in terms of system weight and compactness. The maximum power output of the fuel cell stack is fundamentally limited by the available cooling capacity of the cathode airflow in this type of SOFC integrated type. Several hybrid architectures also operated at lower combustion chamber temperatures than the baseline engine, offering the additional benefit of reduced NOx emissions. ... Read more

Liquid hydrogen fuel is considered to be the most promising alternative to conventional kerosene based fuels for eliminating CO2 emissions in aviation. However, liquid hydrogen carries many challenges that need to be addressed before proper introduction into the market can be established. One of the main challenges is the extremely low temperature that is associated with the storage method. This requires a complete reconsideration of the design of fuel systems for liquid aircraft, along with their components.
This thesis provides three concepts for aircraft gas turbine LH2 fuel systems, based on existing and proposed fuel systems that have been encountered in literature for LH2 aircraft. Numerical fluid dynamic models have been generated in MATLAB & Simulink for each fuel system concept, using the Simscape Fluids library. Models have been generated for the storage tanks including pressure control systems, lowpressure boost pumps, transfer lines, high pressure fuel pumps, heat exchangers and controlled fuel metering valves. The first model, the basic fuel system, feeds all discharge fuel from the fuel pump directly to the heat exchanger. The second concept, the bypass flow system, includes a bypass flow separation downstream of the fuel pump, splitting into ametered flow stream to the injectors, and a bypass flow stream which is recirculated back to the fuel pump inlet. The third concept, the boost-pump-fed system, feeds the fuel from the boost pumps to the heat exchanger directly, allowing for the omission of the engine pump. This configuration was found to require three scaled boost pumps in series, to generate sufficient pressure gain upstream of the engine fuel system.
The results showed that the basic fuel system and the boost-pump-fed system provided feasible designs in terms of the power requirements of the pumps, the pressure drops in the pipes, and the performance of the heat exchanger. The bypass flow system provided a slight increase in engine pump efficiency at lower power settings, providing a possible longer lifetime of the pump. The basic system configuration benefitted from a lower total power requirement, and a higher net positive suction pressure at the pump inlet. The third configuration revealed very high power requirements due to the inefficiency of the scaling of the pumps. Finally, the metering response and accuracy was found to be highly satisfactory.
Finally, pressure control systems of the tank provided satisfactory control of fluid pressure within its boundaries for all configurations. Furthermore, the heat exchanger provided the desired target fluid temperature rise, required for efficient combustion.
Further in-depth modelling of components within the fuel systems is recommended, along with more in-depth validation when experimental data becomes available. In the future, the model can be used for further exploration for designing innovative concepts for fuel distribution, thermal management and metering systems in the LH2 fuel system. ... Read more

Hydroisomerization of alkane isomers is an important step in the manufacture of current kerosene and sustainable aviation fuels. Zeolites are used as acid catalysts in the process. It is therefore important to have predictions of the maximum loading of hydrocarbons in zeolites. Here, a cascade model using machine learning models is used to predict the maximum loading of alkane isomers in zeolites. The cascade is composed of a gradient-boosted tree classifier stage that predicts whether adsorption occurs or not, and a regressor predicting the value of the maximum loading. The final dataset consists of 45 different molecules (both linear and branched alkanes up to C16) and 97 different zeolites structures, resulting in 4365 datapoints. Descriptors include information on the geometry and topology of zeolite channels, as well as shape and size of molecules. Extra composite descriptors are also present to provide the models a physical basis for predictions. Multiple regressors of different nature are considered: Support Vector Regressors, Gradient-Boosted Trees, extreme Gradient-Boosted Trees, and the TabPFN pretrained model. Out of all the models, TabPFN yields the highest generalization performance and lowest error. An interpretability analysis is conducted to assess whether the decisions abide by the governing physics of adposition. It is confirmed that the top descriptor choices abided by the necessary physical constraints, but also that secondary properties such as shape-based selectivity are also accounted for. It is shown that despite both classifier and regressor being insensitive to random splits in data, the regressor is prone to overfitting at low fractions of data withheld for testing. The cascade model is compared with an Artificial Neural Network for training and deployability. Despite training taking more resources for the neural network, the latter is lighter both in memory and storage when compared to the cascade. This work builds on previous research in predicting the Henry coefficient at zero loading. Using this previous model and the findings of this work, one can draw the full adsorption isotherm for any alkane, thus enabling the analysis of adsorption behaviour of alkane mixtures using IAST. ... Read more

Sustainability of civil aviation in the future must be achieved through a drastic reduction of aircraft emissions. To accomplish this, new generation aircraft must be employed that exploit innovative technologies, architectures and energy sources. Unconventional aircraft–propulsion integration promises a leap in aircraft efficiency by leveraging synergistic aerodynamic interactions. In particular, Boundary Layer Ingestion (BLI) has raised interest due to the expected benefits on the aircraft power consumption. Unlike in conventional aircraft, in BLI the propulsive system (consisting of one or more distributed propulsors) is tightly coupled and integrated with the aircraft. In particular, the BLI propulsor operates within the low-momentum, low-energy fluid in close proximity to the aircraft surface. By transferring energy to this flow region, viscous dissipation that would otherwise occur in the aircraft wake is avoided, reducing the amount of power needed to sustain flight. In recent years, different BLI configurations have been proposed, of which one of the most promising designs is the so-called propulsive fuselage, featuring a BLI propulsor at the aft-fuselage section. This particular design is the focus of the work presented in this thesis.... ... Read more

In the urgent need to decarbonise energy systems, hydrogen combustion is set to play a key role in hard-to-electrify sectors such as aviation, power generation, and heavy industry. However, the practical deployment of hydrogen combustion faces critical challenges, including high reactivity, high NOx emissions, and thermodiffusive instabilities, which compromise flame stability and control. While hydrogen premixed flames show a distinctive response to strain compared to other fuels, the fundamental effects of strain on hydrogen flame dynamics and emissions remain poorly understood. Furthermore, lean premixed hydrogen flames feature differential and preferential diffusion effects which lead to the onset of thermodiffusive instabilities. These instabilities, in turn, interact with turbulence and strain. Existing computational fluid dynamics (CFD) models struggle to accurately and affordably predict these distinctive features, thereby limiting the development of safe and low-emissions hydrogen combustion devices in industrial design frameworks. Addressing these gaps is essential for advancing hydrogen combustion technologies, particularly within the aviation and transportation sectors where they are still at a low Technology Readiness Level (TRL).

This thesis aims to contribute to the development of more accurate and affordable CFD tabulated-chemistry large eddy simulation (LES) models of lean premixed hydrogen flames subjected to intensive strain, thereby advancing the capabilities to optimally design hydrogen combustor leveraging strained regimes. First, the fundamental hydrogen flame response to strain is investigated extensively from the point of view of emissions, flame structure, and flame stability through high-fidelity detailed chemistry simulations in simplified laminar settings. Hence, with the help of the insights gathered in the previous phase, novel tabulated chemistry modelling approaches are proposed for LES of strained and turbulent hydrogen flames. The proposed models are tested a priori at unfiltered and filtered grids in a turbulent counterflow setup, where strain is established both by shear-driven turbulence and by the configuration.... ... Read more

Combustion technology currently supplies a large share of global energy demand, but it is also the main source of anthropogenic carbon emissions, driving climate change. While transitioning to renewable energy is essential for achieving a net-zero-carbon economy, this shift is progressing slowly. Global energy demand continues to grow, renewable sources can be intermittent, and certain sectors, such as heavy industry and aviation, are difficult to electrify due to their need for high energy density or thermal power. As a result, the development of cleaner and more efficient combustion technologies remains crucial for enabling a gradual and non-disruptive energy transition.

Hydrogen is considered a promising alternative fuel because it produces no carbon emissions during combustion and can be generated from renewable energy sources. However, hydrogen combustion introduces significant challenges due to the complex behaviour of turbulent flames. Accurately predicting these behaviours requires advanced numerical methods, such as Large Eddy Simulations (LES), which capture unsteady flow dynamics at relatively affordable computational cost. Flamelet-based LES models are particularly attractive because they simplify combustion chemistry by representing turbulent flames as collections of laminar flame structures. While effective for hydrocarbon fuels, applying these models to hydrogen requires additional considerations, especially regarding differential diffusion effects that strongly influence flame stability and structure.

This thesis advances the modelling of turbulent hydrogen combustion by developing and validating flamelet-based LES approaches. It introduces improved modelling techniques, including dynamic closures and methods to account for non-unity Lewis number effects, which are essential for capturing hydrogen-specific behaviour. The models are tested across various flame configurations and subsequently applied to a hydrogen-capable combustor developed at TU Delft. Through simulation, the research provides insights into fuel-air mixing, flame stabilization, and nitrogen oxide (NOx) formation during the transition from methane to hydrogen operation. Overall, the work contributes to the development of reliable simulation tools that support the design of cleaner combustion systems and facilitate the integration of hydrogen into future energy and aviation applications. ... Read more

The aviation sector has set goals to reduce its carbon footprint, as the industry continues to grow. To capture the climate impact of millions of flights, climate models play a crucial role in this task. However, these climate models depend on information about the concentration and distribution of emissions around the globe. To provide such data, bottom-up emission inventories are used. These inventories model aircraft flight performance and engine emission characteristics to provide the best estimate of the geographical distribution of emissions due to aviation.

The research presented in this thesis aims to critically evaluate and improve existing implementations of global aviation emission inventories, with the ultimate goal of achieving more representative estimations of aircraft emissions and their distribution. The goal should be achieved without compromising on computational efficiency and the flexibility of the model. This work builds upon an existing emission inventory.

In order to achieve the goal of this study, revisions to the information, performance and emission models are incorporated. The first revision involves a more accurate representation of the actual engine equipped for each flight analysed. Next, flight trajectory correction factors (lateral inefficiency) are improved by using data provided in literature, derived from a large set of ADS-B trajectory data. Furthermore, the latest version of EUROCONTROL’s Base of Aircraft Data (BADA) performance model is implemented for all the aircraft which it covers (89% of total flown distance). For emission modelling, the Boeing Fuel Flow Method 2 (BFFM2) is kept for gaseous emissions; however, an updated method (MEEM), validated on a large set of engine manufacturer data and more recent measurement campaigns, is utilised for nvPM estimates.

Compared to existing global inventories for 2019, the updated model estimates total fuel burn at 250 Tg, slightly lower than the earlier estimate (254 Tg) and significantly below estimates by Teoh et al. (283 Tg) and Quadros et al. (297 Tg). On a per-kilometre basis, however, the fuel burn estimate is 2% lower than Rik Kroon’s but 9.6% higher than Teoh et al. The nitrogen oxides (NO𝑥) emission index closely aligns with benchmarks by Teoh et al. and Quadros et al., differing by less than 2.5%, yet is 11% lower than Rik Kroon’s due to performance model corrections. For nvPM emissions, notable discrepancies arise: mass emission estimates are higher by 62% and 39% compared to Rik Kroon and Quadros et al., respectively, but 43% lower than Teoh et al. Conversely, nvPM number emission estimates exceed those by Teoh et al. and Quadros et al. by approximately 60% and 54%, respectively. Geographically, emission hotspots align with previous studies, though data limitations cause under-representation in certain southern hemisphere routes, highlighting areas for future improvement.

Sensitivity analysis revealed that the performance model results are highly sensitive to the parameters used to determine cruise altitude and initial fuel mass estimate, with up to ±4% changes in nvPM emissions and fuel burn observed for heavy-weight aircraft. The uncertainty analysis using Monte Carlo simulations showed total uncertainties of ±9% for fuel consumption and emission indices uncertainties ranging from ±8% forNO𝑥 to ±40% for nvPM number and ±95% for nvPM mass, reflecting the significant impact of methodological assumptions and limited validation data on nvPM emissions.

The thesis concludes by confirming that the updates provided lead to an improvement in the estimation of the quantity and distribution of aviation emissions. Limitations related to the coverage of annual flights, the estimation of the take-off mass, and the uncertainty related to nvPM emissions are also identified. This work can serve as a baseline for future work in those aspects. ... Read more

Increasing air traffic demand and new environmental sustainability requirements incentivise research into technologies that both increase fuel efficiency and reduce emissions, with the hydrogen steam injection and water recovery cycle showing potential to improve both. How to model such an engine in a numerical simulation tool and what the performance of the engine is at design and off-design conditions is investigated. Previous applications of steam injection are limited to ground-based turbines and hydrogen is not in use on aircraft engines, with their combination being novel and with limited public research. An engine model is built with the Numerical Propulsion System Simulation software and engine parameters studied in order to design a fully water self-sufficient engine at the top of climb design point. A detailed heat exchanger model is developed in order to accurately size and predict the performance of evaporators and condensers which are identified as critical components in the cycle. Water self-sufficiency is reached in both top of climb and cruise while a significant deficit at take off necessitates carrying water for take off and part of climb. An improvement in fuel efficiency is found over a baseline hydrogen engine but the required heat transfer area of the air-cooled exhaust condenser is significantly large, rendering the cycle infeasible. Harnessing the potential of the cycle may be possible with drastic redesign of the heat exchanger or alterations to the cycle architecture. ... Read more

The composite cycle engine (CCE) is a radically new aero engine architecture that shows the potential of high fuel burn reductions by integrating the high efficiency potential of the Seiliger cycle that is used in piston engines into a conventional turbofan engine cycle. To minimise the weight penalty and increased NOx emissions from the integration of such a piston engine, a CCE concept is proposed that incorporates a free-piston engine (FPE) architecture and homogeneous charge compression ignition (HCCI) combustion. Little research has been conducted on the combination of these three cutting-edge technologies. In this study, the design space of a hydrogen HCCI opposed free-piston linear alternator (OFPLA) engine was explored through two sensitivity analyses on a validated model, showing that scavenging efficiency is critical for OFPLA performance. The bore diameter is the most influential design parameter when the scavenging efficiency is fixed. The OFPLA design is also optimised for integration into a CCE. To maximise total CCE efficiency, the inlet temperature and pressure drop over the OFPLA core should be minimised. Compared to a state-of-the-art conventional turbofan engine, the CCE with an optimally integrated OFPLA core did not achieve the efficiency gains that were reported in the literature. However, higher efficiency gains may be achieved when a holistic CCE optimisation approach is used that also includes the turbofan components. ... Read more

Hydrogen technologies show promise in reducing the effects of aviation on anthropogenic climate forcing. This report aims to develop the design of a robust, low-maintenance, low NOx emission hybrid hydrogen-electric powertrain for retrofitting the Beechcraft 1900D by 2035. An optimal sizing of the two powerplants: high-temperature proton-exchange membrane fuel cells and a gas turbine with a rich-burn, quick-mix, lean-burn combustor, was performed, minimizing both the cost and climate performance of the design. In this optimization, the storage system, electrical system and thermal management system were sized utilizing technology projections for 2035. Additionally, exhaust water from the fuel cell stack is injected into the combustion chamber to further reduce NOx emissions. The resulting design has a passenger capacity of 15 with a range of 707 km, at a ticket price of €221 and a climate impact of 29.29 kgCO2,eq per passenger per flight, which is four times more sustainable than the current Beechcraft 1900D at a similar price point. The aircraft produces 1.1 grams of NOx on a typical flight, and never emits more than 8 ppm at any stage. The fuel cell provides all of the aircraft power during cruise and other low power flight phases, whereas the combustion chamber provides additional power during takeoff and climb. It is recommended to monitor the development of hydrogen technologies so they may be implemented in this retrofit by 2035. ... Read more

As green transition in aviation continues to be pushed, SAFs becomes increasingly important. Hydrotreated Pyrolysis Oils (HPO) could potentially expand the ways SAFs could be produced. This thesis looks at the flame speed as well as NOx and CO emissions of this potential SAF when blended with Jet-A1. Experiment was performed in a bunsen burner setup, with flame speed determined from chemiluminescence images and emission data obtained from a gas analyzer. The results showed higher NOx emission, especially when equivalence ratio is close to stoichiometric condition. Higher CO was also observed for rich conditions, while no significant change in flame speed was seen. Experiment was also performed for Jet-A1 & hydrogen blend, which saw higher NO and CO emission at stoichiometric condition, and much higher flame speed. ... Read more

This thesis develops a chemical reactor network (CRN) model for an RQL aero-engine combustor and uses it to study water injection and NOx formation across realistic operating conditions. The model includes parallel primary-zone subzones to represent mixture inhomogeneity, detailed gas-phase chemistry and residence-time based zone sizing. It is validated against ICAO LTO emissions for the CF6-80C2B1F and Trent XWB-84 and reproduces the expected NOx and CO trends, including the idle CO peak.

For each operating point, the combustor geometry is kept fixed while the primary-zone equivalence ratios and the mixing parameter are optimised. This same methodology is applied at cruise using cruise-specific inlet conditions. The CRN predicts NOx within the measured in-flight range; BFFM2 also falls within this band, while P3T3 remains close to it. Only the CRN resolves the internal mixture structure and reaction pathways.

Water-to-fuel sweeps show that water injection consistently reduces NOx, with the strongest effect at high thrust where baseline temperatures are highest. CO increases mainly at idle and approach due to lower burnout temperatures. Reaction-pathway analysis confirms that thermal NO remains the dominant mechanism and that water suppresses existing pathways by reducing temperature and radicals.

Overall, the CRN provides an accurate and computationally efficient framework for analysing water injection and predicting emissions at both LTO and cruise, while resolving the internal combustor processes that are inaccessible to simpler correlation-based methods. ... Read more

The increase in air traffic creates the need for more efficient aircraft. Current design trends of increasing efficiency and reducing fuel consumption also lead to an increase in NOx emissions. Environmental considerations and regulations create a demand for aircraft engine technology with NOx emissions lower than that of current engines. The Steam Injected Water Recovering Turbofan engine is proposed; it injects steam before the combustion chamber and recovers it through condensation in a heat exchanger. Moreover, the cycle recovers more heat from the exhaust gasses, using them as a heat source in a Rankine cycle. The SIWRT was modeled in NPSS in collaboration with GKN Aerospace Sweden AB, to better understand this configuration. A new method was devised to model heat exchanger performance, referred to as a 'variable Cp NTU', capable of taking into account phase change. This new method was verified and partially validated. The engine was studied in a range of water to air of 1% to 7%. It was determined that it is capable of reducing thrust specific fuel consumption by up to 8\% and NOx emissions by up to 66% compared to a conventional future turbofan. Increasing WAR leads to an increase in the fan bypass ratio (FBPR) mostly by reducing the core size. The SIWRT displays design trends similar to conventional turbofan when varying overall pressure ratio, combustor outlet temeperature, and fan pressure ratio. At top of climb the condenser was not able to condense enough water for the engine to self sustain, thus extra water was supplied through an on board reservoir. The research questions were answered, and concluding remarks with suggestions for future work were provided. ... Read more

In the search for renewable, alternative propulsion technologies for use in aviation, hybrid SOFC-jet-engine architectures are seen as very promising. The critical performance characteristics for a fuel cell in such an application differ greatly from those currently in use, however: Volumetric power density, fuel utilisation, and operating temperature are key criteria. One way to increase these characteristics is to use novel stack architectures, such as monolithic or tubular arrays. There is a large variation in exact implementations of such architectures, primarily geometrically-- the size of fluid channels, skewedness, etc. As a first venture into this design- and optimisation space, a numerical model is developed that enables analysis for arbitrary three-dimensional geometries of SOFC stacks at steady-state conditions, implemented with ease of integration in mind. It is released as open source software for anyone to use. This model was extensively validated and verified, after which it was applied by performing sensitivity analyses on geometry parametrisations for three characteristic geometry classes: planar, tubular and corrugated monolithic. It was found that the stack geometry has only minor effects on the thermal performance, but strong implications on power density and fuel efficiency. Strong variations in the effects of similar changes exist between the different geometries. % Drawing % general conclusions for 'all' fuel cells is inadvisable, as it skews % expectations. Instead, individual studies around specific geometries % should be performed. The core design consideration is also more complicated than a mere trade-off of power density and fuel efficiency, with certain parameters yielding improvements in both factors due to strong coupling of disparate processes. Miniaturisation increases power density for all geometries considered, but it is seen to come with diminishing returns. The results also give insight into the process of stack design itself: as an indicator of volumetric power density, current density alone has proven to be an unreliable indicator, despite being used analogously as such in existing literature. ... Read more

Aviation is an important contributor to worldwide greenhouse gas emissions. This thesis aims to help reduce NOx emissions by proposing a gradient-based optimisation framework that allows to implement NOx emission constraints from the preliminary design phase. Simple chemical reactor networks built on Cantera are used to predict NOx emissions of Jet A and hydrogen combustors. The models are used to train Kriging surrogate models with the Surrogate Modelling Toolbox, which provides analytical gradient information. The gradient-enabled surrogate models are integrated into pyCycle, a modular, gradient-based engine cycle analysis tool built on OpenMDAO. The models are used to implement NOx constraints during the optimisation of conventional turbofans, and turbofans with water recirculation. This work shows that it is possible to implement NOx emissions constraints during the preliminary design phase, and that a 40% reduction in NOx can be achieved with a maximum fuel penalty of 1.2% across the analysed cases. ... Read more

With the aviation industry ever growing the climate impact of the aviation industry grows too. Although most innovations focus on reducing the CO2 emissions, non-CO2 emissions play a large role in shaping the climate impact of flying. Contrail formation due to aviation is the primary non-CO2 factor, but there is a large amount of uncertainty in determining the impact of contrails. Currently most contrail formation theory is based on using the Schmidt-Appleman criteria (SAC), but the underlying thermodynamic assumptions are being increasingly challenged by new turbofan engines with ever higher and higher bypass ratio’s. This thesis aims to evaluate the impact of the bypass ratio on the mixing physics and contrail formation in the near field, and see if a relation between bypass ratio and contrail occurrence can be found. To evaluate the relation between bypass ratio and contrail formation, 4 different modern and future turbofan engines were simulated in the Fluent CFD solver, all with a thrust rating of about 120kN and designed for short-to-medium range aircraft such as the A320. 2D Axisymmetrical meshes were made in ICEM of around 300.000 cells for each of the engines. Grid independence was evaluated using the Grid Convergence Index method and the results were proven to be independent of the grid. To evaluate which turbulence model to use two experimental cases were used from which the κ − ω − SST-model was chosen as the most accurate one. A validation case for the CFM56-3 based on research by Cantin et al. was used to check the validity of the flow field results obtained using Fluent. Contrail modeling was performed based on thresholds for Gibbs free energy, temperature and relative humidity over ice. These thresholds were used to determine the area of possible contrail formation, and again checked against the same case from Cantin et al. . The contrail analysis model presented here showed good agreement with the verification case, although it does overestimate the contrail a bit The contrail analysis model showed that there was a relation between bypass ratio and contrail formation, with higher bypass ratio engines having a higher absolute plume intensity. The presence and intensity of a shock in the nozzle seems to correlate better with the possible contrail formation volume and plume intensity. The SAC diagrams showed that the SAC underestimated the slope of the mixing lines as calculated from the flow field. Since the plume did not return to ambient conditions at the end of the domain, no statement could be made on the persistency of the contrail. The impact of H2 was also researched, as well as the impact of relative humidity on contrail characteristics. ... Read more

To meet the climate goals and reduce the negative effects of anthropogenic industrial activity, the human civilization must move toward sustainable energy sources. However, in hard-to-abate applications and to compensate for the intermittency of renewable energy sources, combustion will continue to be a crucial energy conversion mechanism. This requires modern combustion devices to have highly reduced emissions not only to abide by stringent regulations but also as a collective social responsibility. Usually, a reduction of NOx requires a lowering of flame temperatures, which leads to an increase in CO. However, Flameless/MILD combustion is a technology that has the potential for low NOx while attaining complete combustion. This is because of its unique requirements of having very high reactant temperatures (typically above autoignition temperatures) and having low oxygen concentration. The current work investigates the phenomenology of the combustion process in a jet-stabilized combustor, where this regime can be produced with fuel and oxidizer injected in a premixed manner. The work has three main components; the first deals with the autoignition chemistry of methane-air mixtures under exhaust gas vitiation conditions, the second investigates the flow field and turbulent interface of a turbulent-jet-in-coflow which is a canonical version of the flow conditions in a jet stabilized combustor and finally, experiments were done on a jet-stabilized combustor with methane-hydrogen blends to classify the flame stabilization mechanism and quantify emissions.

Autoignition characteristics of vitiated methane-air mixtures were studied by simulations in 0D reactor setup. Unlike most studies in literature, vitiation, in the context of flameless combustion, was generated as the hot combustion product of fresh reactants that also contained radicals that existed in equilibrium. Particularly, the effect of varying levels of vitiation and heat loss was studied on properties such as ignition delay time, reaction time scale, and the NOx and CO emissions. This revealed the most suitable conditions to achieve low emissions and distributed reaction zones for premixed reactants that are vitiated by exhaust gases. Further, a regime of multi-ignition was discovered where prior to the main ignition event, there is a pre-ignition event attributed to the initial pool of radicals in a vitiated mixture. The conditions of occurrence were mapped out, as well as the mechanism behind it was explained.

The mixing at the interface of the jet and the recirculation zone in a jet-stabilized combustor has an important role in determining the composition of the hot-diluted mixtures. Thus, the fluid mechanics of the turbulent-turbulent interface were studied in a canonical configuration of a turbulent-jet-in-turbulent-coflow. This is also a common configuration used to produce Flameless/MILD combustion under laboratory conditions. Although there is vast research on free turbulent jets, combustors operating in the Flameless Combustion regime would reach flow conditions where the ratio of coflow to jet velocity would increase. This work elucidates the evolution of such a flow field through Particle Image Velocimetry (PIV) measurements done along the axis of the jet in the range 0<x/D<42. Further, the interface is detected using an algorithm developed based on other image processing algorithms using vorticity as a criterion. This enables the assembly of conditional statistics with respect to the interface. The results show that the cases with higher coflow have a lower jet centerline velocity decay rate and reduced jet spreading. The mean axial velocity shows a region of deficit compared to the free jet near the interface region. Further, the case with higher coflow shows higher turbulence intensity and Reynolds Shear Stress close to the interface. The detailed results are presented as both unconditional and conditional statistics and the mechanism behind this effect is deduced.

Experiments were done on a jet-stabilized combustor capable of producing the Flameless Combustion regime. It was operated using methane-hydrogen fuel admixtures at varying equivalence ratios. The combustor performance was analyzed based on the stabilization of the flame zone and the emissions. This work presents a unique, comprehensive measurement of temperature, gas composition, velocity field, and chemiluminescence signal in a jet-stabilized combustor. The recirculation regions are visualized through PIV measurements and the recirculation ratio is quantified. The instantaneous flame images are used to identify flame kernels and construct probability density functions of the aspect ratio, rotation angle, and location along the combustor axis. An increase in hydrogen content in the fuel mixture shifts the stabilization mechanism from autoignition to flame propagation. There is also an increase in NO emissions. A similar effect is seen with the increase in equivalence ratio from lean to stoichiometric condition. Distributed reaction regimes with ultra-low NO and moderate flame temperatures are achieved at very low equivalence ratios. Such mixtures are stabilized better with the addition of hydrogen to the fuel mixture.

This thesis provides fundamental information on the chemistry and flow physics of the phenomenon in a jet-stabilized combustor followed by measurements from the operation of one. The data and conclusions are a suitable reference for future engineers designing jet-stabilized combustors for low NOx emissions and high combustion efficiency. ... Read more

Human civilization must transition tomore sustainable energy sources to meet the goals of the Paris Agreement, which aims to limit the global temperature increase to well below 2 ◦C above pre-industrial levels. However, hard to abate sectors such as aviation and heavy industries will continue to rely on combustion for the foreseeable future. For these industries, the development and deployment of alternative fuels are essential. One of the most promising alternative fuels is hydrogen (H2), primarily because it enables carbonfree combustion. Nevertheless, significant challenges remain regarding its production, storage, and transportation, leading to uncertainties in its large-scale availability. As a result, there is growing interest in fuel-flexible combustion systems that can operate efficiently on traditional carbon-based fuels, hydrogen, or any mixture of the two, while maintaining combustion stability and lowemissions across the full fuel range. Hydrogen differs significantly from carbon-based fuels such as methane (CH4) in its combustion characteristics. It has a much higher flame speed and higher adiabatic flame temperature at the same equivalence ratio. These properties can pose serious design challenges such as increased risk of flashback and elevated NOx emissions.

In swirl-stabilized combustion, injecting non-swirled air axially on the centreline can be a very efficient way to stabilize flames with high hydrogen content. This work investigates the emissions and flame stability of a fuel flexible swirl-stabilized combustor that can operate on fuel mixtures ranging from 100% CH4 to 100% H2. In this set-up, fuel is injected in a jet in cross-flow configuration just downstream of the swirler exit. A mixing tube is placed between the injection point and the combustion chamber to allow for fuel-air mixing. The objective of this thesis is to identify the dominant parameters that govern emissions and stability in fuel-flexible combustion systems. To support this aim, several research questions are formulated and addressed in dedicated chapters…
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