Introducing H ₂ as fuel in gas turbines is a promising step towards decarbonizing the energy sector. However, the future availability of H ₂ in large quantities remains uncertain. Consequently, designing fuel flexible (CH ₄/H ₂) combustion chambers for various fuel blends is necessary. The distinct combustion characteristics of H ₂, such as high flame speeds and high adiabatic flame temperatures, pose challenges when designing systems that can operate in a stable manner and with low emissions across a wide range of fuel mixtures. This paper investigates the fuel-flexibility of an atmospheric laboratory scale, partially premixed swirl stabilized combustor. By deploying a non-rotating axial air jet (AAI) in the center-line of the swirling flow, the flashback risk for high H ₂ content fuels is minimized. This study provides detailed insights into AAI's interaction with CH ₄/H ₂ fuel blends, analyzing the resulting flow field from Particle Image Velocimetry, emissions from exhaust gas analyser measurements, and flame structures from OH* chemiluminescence and OH Planar Laser Induced Fluorescence. The results show that AAI enables flame stabilization across the full range from 100% CH ₄ to 100% H ₂ in the same injector geometry. However, a high portion of the total airflow must be injected axially to stabilize H ₂ flames. Increasing the level of AAI increases NO emissions and alters flame stabilization mechanisms. This is likely due to a decrease in mixing quality, resulting in the fuel staying close to the periphery of the mixing tube. Switching the fuel from 100% CH ₄ to 100% H ₂ leads to an increase in NO emission, despite lower adiabatic flame temperatures for the perfectly premixed case. This indicates that the mixing process and flame location within the combustion chamber are essential in controlling NO emissions. Moreover, the flow field transforms significantly from a swirl-stabilized flow field featuring an inner recirculation zone to one resembling the one of a jet flame.

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In this study, the macroscopic properties of kerosene-H₂ blended flames are investigated in a multi-phase, multi-fuel combustor, focusing on the effects of increasing H₂ blending fractions. The non-reacting flow field of the swirl-stabilized combustor is characterized using PIV, and macro-structures in the flow and spray-swirl interactions are analyzed. Kerosene atomizers are tested to estimate variations in spray quality across different fuel blends. The changes in the optical properties of the flames are recorded using broadband chemiluminescence imaging while the changes in the acoustic emissions are recorded using a microphone. Results show that H₂ addition significantly alters the flame topology, transitioning from a lobed flame for pure kerosene to a single contiguous swirling flame for blended or pure H₂ cases. The flame luminosity decreases, with the emission color shifting from bright yellow (pure kerosene case) to dull yellow (multi-fuel cases) to a red-blue hue (pure H₂ case). These changes are attributed to variations in fuel distribution, heat release patterns, combustion mode, flame speed, and soot formation tendencies. The acoustic analysis reveals that a strong tonal behavior is observed under pure fuel conditions (prominent peaks at higher harmonics of 150 Hz) while broadband characteristics are exhibited under blended fuel conditions. The overall acoustic emissions in multi-fuel cases are reduced by ~80% compared to pure H₂ and ~55% compared to pure kerosene. This study highlights the effects of high levels of H₂ blending on flame dynamics and acoustic behavior in a multi-phase, multi-fuel combustor, offering valuable insights for the development of fuel-agnostic combustion systems.@en

The jet-in-coflow is a two-stream configuration having engineering applications in combustors and gas turbine engine exhausts. In practical systems, the coflow generates a boundary layer of the outer wall of the jet pipe and may also have a certain level of turbulence. In the current work, the evolution of this flow configuration is studied using an air-air turbulent jet in a low turbulence coflow (turbulence intensity < 6%), and the 2D velocity field is measured by planar particle image velocimetry. Cases of varying coflow ratio (ratio of coflow velocity to jet velocity) of 0 (turbulent free jet), 0.09, 0.15, and 0.33 are generated by keeping a constant velocity jet (Re = 14000) and varying the coflow velocity. The trends of jet centerline properties such as velocity decay, jet spread, and jet momentum of jet-in-coflow cases, scaled to represent an equivalent free jet, show deviations from that of the turbulent free jet. The radial profile of mean velocity shows a region of velocity deficit, compared to a turbulent free jet, on the coflow side in the jet-in-coflow cases. In contrast, the turbulence intensity and Reynolds shear stress profiles show an enhanced peak near the interface for the jet-in-coflow cases. Further, conditional statistics were extracted by detecting the interface between the jet and the surroundings, wherein the same trends are observed. The low turbulence levels of the coflow have little effect on the jet/coflow interface, as seen by the conditional enstrophy diffusion and tortuosity compared to a turbulent free jet. The differences at the jet/coflow interface of a jet-in-coflow with respect to a turbulent free jet are attributed to the boundary layer initially developed by the turbulent coflow over the pipe generating the jet, and these are seen throughout the near-to-intermediate field (0≤x/D≤40).

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Low emissions and fuel flexibility are two important criteria required for gas turbine combustors to facilitate the energy transition to low-carbon fuels for propulsion and power applications. A jet-stabilized combustor, having both these characteristics, was operated with CH ₄–H ₂ fuel mixtures with H ₂ varying from 0 to 100 % and with varying equivalence ratios (ϕ). Comprehensive measurements were carried out of the velocity field using Particle Image Velocimetry (PIV), temperature and gas composition by traversing probes in the chamber, and flame topology using chemiluminescence imaging. The flow field in this combustor consists of a jet that undergoes recirculation, generating Central and Peripheral Recirculation Zones (CRZ and PRZ). The recirculation ratio in the PRZ is found to be twice that of the CRZ. Increasing H ₂ % for the same ϕ leads to higher NO _x. Ultra-low ϕ flames could be stabilized only at H ₂≥50 %, which in turn leads to low NO _x due to low adiabatic flame temperatures. The combination of temperature, gas composition (CO/NO), and chemiluminescence images is used to identify the extent and location of the reaction zone. Distributed reaction zones, stabilizing at around 30 % of the length of the chamber, are achieved at lean conditions, whereas an increase in H ₂ % makes the reaction zone more compact and shifts upstream towards the burner head. Flame kernels are extracted from the instantaneous chemiluminescence images, and probability distribution functions for their aspect ratio and axial location are constructed. It is seen that reducing ϕ leads to low aspect ratio kernels that tend to occur further downstream, whereas increasing H ₂ % leads to higher aspect ratio kernels, stabilizing upstream. These flame kernel statistics are also used to identify ignition modes (autoignition/flame propagation) for varying fuel H ₂ % and inlet ϕ based on a hypothesis of flame stabilization mechanisms.

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The jet-in-hot-coflow is a canonical combustion setup, which has been used in several studies to study Flameless/MILD combustion and auto-ignition of fuels. However, the NO_x and CO emission measurements from these combustion setups were not possible due to the entrainment of laboratory air and a lack of a well-defined physical system limit. These limitations have been overcome by a new enclosed jet-in-hot-coflow setup. The combustor was operated by injecting a mixture of CH₄-Air in the central jet, and the coflow comprised of hot products from CH₄-Air combustion in burners upstream. The coflow composition was further controlled by adding diluents such as N₂ and CO₂. Measurements were done using stereoscopic particle image velocimetry, suction probe gas analysis, thermocouples, and chemiluminescence imaging. Increasing central jet velocity and equivalence ratio led to lower NO_x and a reaction zone that enlarged and shifted downstream. The reduction in NO_x emission was attributed to the returning mechanism. Adding CO₂ and N₂ as diluents in the coflow resulted in a longer combustion zone and reduced temperatures in the combustion chamber, leading to decreased NO_x production and increased reburning. These experiments provide relevant flowfield and emissions data for modelers and help characterize combustion regimes such as Flameless/MILD.

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HYLENA will investigate, develop and optimize an innovative, highly efficient integrated hydrogen powered, electrical aircraft propulsion concept for short and medium range. It will achieve significant climate impact reduction by being completely carbon neutral with radical increase of overall efficiency. The full synergistic use of: a) an electrical motor (as the main driver for propulsion), b) a contoured hydrogen fueled SOFC stacks (geometrically optimized for nacelle integration), c) a gas turbine (to thermodynamically integrate the SOFC), will act as an enabler for hydrogen aviation and will allow for efficient and compact engine concepts. This disruptive propulsion system will be called HYLENA concept. HYLENA aims to evaluate and demonstrate the feasibility of a “game changing” engine type which integrates Solid Oxide Fuel Cells (SOFC) into a turbomachine, in order to utilize the heat generated by the fuel cells on top of its electrical energy. The combination of e-motor, turbomachine and contoured SOFCs fueled with H2 will deliver high overall efficiency and performance versus state-of-the-art turbofan engines. Indeed, HYLENA Figures of Merit consist of minimizing CO2 emission; negligible NOX and an unmatched overall efficiency versus state-of-the-art turbofans which corresponds to an outstanding performance increase. It will also enable to extend the flight range for the same fuel tank size. The HYLENA project will deliver: 1. On SOFC cell level: Experimental investigations on SOFC cell technologies and identification of the most promising one(s) for aeronautical applications; 2. On SOFC stack level: Studies and tests to determine the most compact/light/manufacturable way of stack integration; 3. On thermodynamic level: Cycles simulations of the proposed novel HYLENA concept architecture and down selection of the most performing one; 4. On engine design level: Exploration, through resilient calculation and simulation, of the best engine design, sizing and overall components integration; 5. On overall engine efficiency level: Demonstration that HYLENA concept can reach very high efficiency levels with limited weight and complexity; 6. On demonstration level: A decision dossier for a potential ground test demonstrator to prove that the HYLENA concept works in practice during a second phase in the continuity of this project.

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Due to climate change concerns, hydrogen is being considered for future aviation, but its commercial availability is limited, storage is bulky and its combustion with 100% concentration still poses numerous technical challenges. This leads to a certain interest in multi-fuel systems using both hydrogen and kerosene to facilitate the transition without completely redesigning the existing engines. Within the HOPE project, the present study focuses on an innovative multi-fuel combustion concept for aircraft propulsion, considering a laboratory-scale combustor hosted at TU Delft. Such a device, originally fueled with hydrogen and methane, is schematically composed of an axial swirler, four ducts for gaseous fuel injection, a mixing tube and a cylindrical combustion chamber. To avoid flashback, also an axial air injection duct is present that bypasses the swirler and directly reaches the air-fuel mixture in the mixing tube.
In this work, reactive CFD simulations are used to explore different spray injection configurations and assess the impact of kerosene on the flow field, the flame shape and the NO emissions of the modified system. In particular, three different injection positions are studied, featuring injection points on the backplane of the combustion chamber, inside the fuel/air mixing tube or on the axis of the burner. It is found that the most suitable position for kerosene injection is on the axis of the burner, so that the spray is surrounded by the swirling flow and undergoes a rapid mixing with the oxidising stream, limiting the maximum temperature reached by the mixture. Moreover, in this case, the addition of hydrogen leads to reduced NO emissions since it decreases the size of the hot spots generated by the combustion of kerosene.@en

Results from the APPU project, which investigated the concept of an "Auxiliary Power and Propulsion Unit" (APPU) are presented. The APPU is a hydrogen-driven boundary-layer-ingesting engine at the tail end of a passenger aircraft which replaces the conventional APU and contributes about 15% of total thrust at top of climb. The aim of the configuration is to allow the introduction of hydrogen and BLI technology by upgrading existing aircraft designs. The concept aims to benefit from the advantages of these new technologies as much possible, without requiring the same level of reliability as for conventional propulsion, during times when hydrogen infrastructure is not universally available. The investigation concerns hydrogen tank mass, engine efficiency, operational, aerodynamic and reliability aspects, and finds block CO2 emissions can be reduced by a larger amount than the thrust rating of the auxiliary hydrogen engine may suggest. One reason for this is that the additional engine permits smaller and more efficient designs for the main engines. A still larger benefit is found to arise out of the assumption that the APPU engine and associated H2 fuel systems is less reliable than the conventional underwing engines. This assumption permits different strategies to maximize the utilization of hydrogen over kerosene. CO2 emissions for the design mission are found to be reduced by 23.1% over the A321neo, and by 15.5% over an A321neo fitted with updated turbofan engines.

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HYLENA will investigate, develop and optimize an innovative, highly efficient integrated hydrogen powered, electrical aircraft propulsion concept for short and medium range. It will achieve significant climate impact reduction by being completely carbon neutral with radical increase of overall efficiency. The full synergistic use of: a) an electrical motor (as the main driver for propulsion), b) a contoured hydrogen fueled SOFC stacks (geometrically optimized for nacelle integration), c) a gas turbine (to thermodynamically integrate the SOFC), will act as an enabler for hydrogen aviation and will allow for efficient and compact engine concepts. This disruptive propulsion system will be called HYLENA concept. HYLENA aims to evaluate and demonstrate the feasibility of a “game changing” engine type which integrates Solid Oxide Fuel Cells (SOFC) into a turbomachine, in order to utilize the heat generated by the fuel cells on top of its electrical energy. The combination of e-motor, turbomachine and contoured SOFCs fueled with H2 will deliver high overall efficiency and performance versus state-of-the-art turbofan engines. Indeed, HYLENA Figures of Merit consist of minimizing CO2 emission; negligible NOX and an unmatched overall efficiency versus state-of-the-art turbofans which corresponds to an outstanding performance increase. It will also enable to extend the flight range for the same fuel tank size. The HYLENA project will deliver: 1. On SOFC cell level: Experimental investigations on SOFC cell technologies and identification of the most promising one(s) for aeronautical applications; 2. On SOFC stack level: Studies and tests to determine the most compact/light/manufacturable way of stack integration; 3. On thermodynamic level: Cycles simulations of the proposed novel HYLENA concept architecture and down selection of the most performing one; 4. On engine design level: Exploration, through resilient calculation and simulation, of the best engine design, sizing and overall components integration; 5. On overall engine efficiency level: Demonstration that HYLENA concept can reach very high efficiency levels with limited weight and complexity; 6. On demonstration level: A decision dossier for a potential ground test demonstrator to prove that the HYLENA concept works in practice during a second phase in the continuity of this project.

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Lean-premixed swirl-stabilized combustion is a successful strategy to reduce pollutant emissions. However, these combustion systems are especially prone to thermoacoustic instabilities. The precessing vortex core (PVC) plays a significant role in suppressing or exciting those instabilities. Therefore, it is necessary to predict the PVC dynamics in different operating conditions. The introduction of alternative aviation fuels like hydrogen in fuel-flexible gas turbines might require changes in the combustor geometry. However, the influence of particular geometric parameters on the PVC dynamics in less conventional combustion chamber configurations is not yet clear. To contribute to the knowledge of PVC dynamics in different combustor geometries, this paper presents an experimental study of the PVC dynamics in isothermal conditions in a counter-rotating dual swirler configuration in different confinement ratios. Additionally, a non-rotating axial air jet can be injected on the center line of the primary swirler as a provision for increased flashback resistance in the reacting case with H ₂. PVC frequencies and amplitudes are obtained by spectral proper orthogonal decomposition (SPOD) of time-resolved PIV measurements, and by time-resolved pressure measurements. The study shows that the frequency of the PVC scales with StPVC = 0.78, based on the diameter and the bulk velocity of the mixing tube. The PVC frequency is only determined by the conditions in the primary swirler and is fully independent of the amount of airflow going through the secondary counter-rotating swirler. Introducing an axial air jet on the center line decreases the PVC frequency significantly, which can be related to the change in effective swirl number. It is also shown that the smallest combustion chamber diameter results in the highest spectral energy for the PVC mode for all investigated points, hence the PVC motion is the strongest. Meanwhile, the biggest combustion chamber diameter shows the weakest pressure fluctuations. The results obtained in this study provide evidence that the periodic oscillations arising in the swirling flow field can be predicted and follow a Strouhal scaling independent of the geometry, even for more unconventional configurations.

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The climate impact of aviation is considerably different from that of other transport modes. The turbofan engine’s efficiency can be increased by increasing the Operating Pressure Ratio (OPR), bypass ratio (BPR) and Turbine Inlet Temperature (TIT), thereby reducing CO2 and H2O emissions. However, this may have an adverse effect on the secondary emissions, such as NOx, soot, etc. Taking a holistic view in evaluating the climate impact of engine development trends considering all the climate forcers is imperative for design trends in the future. This research investigates the impact of some key engine design parameters on climate. The emission changes due to design variations in the CFM56-5B are estimated using in-house engine performance and emission prediction tools. Accordingly, the changes in the species’ Average Temperature Response for 100 years (ATR100) are analyzed using a climate assessment tool, AirClim. The results show that the overall climate impact increases by 40% when increasing OPR from 25 to 40. Meanwhile, the Twin Annular Premixed Swirler (TAPS-II) combustor reduces the total ATR100 drastically, in the range of 52–58%, due to lean combustion.@en

As climate change aggravates, the aviation sector strives to minimize its climate footprint. To this end, international organizations, such as ICAO and ACARE, are promoting mitigation measures including novel technologies, operations, and energy carriers to reduce aircraft emissions significantly. Hydrogen (H2) as an alternative fuel has the advantage of eliminating CO2 and soot emissions and the potential to reduce NOx emission substantially. Nevertheless, burning H2 emits more H2O and increases the contrail formation probability. Therefore, the actual climate impact of hydrogen aircraft is still uncertain. This paper intents to evaluate the climate impact of a hydrogen powered aircraft considering the effects of H2O, NOx , and contrails . To frame the contribution of each individual climate agent, the research compares a hydrogen and a kerosene aircraft with similar mission capabilities. To assess the climate impact, a modeling chain was developed including network selection, flight routes calculation, aircraft and propulsion performance, emissions prediction, and climate impact assessment. In total, 2.24 million flights covering 1128 city pairs were analyzed. The energy consumption of hydrogen aircraft is about 10% higher than that of the kerosene aircraft due to the larger wetted area for hydrogen storage. However, the average atmospheric temperature response caused by the hydrogen aircraft is 67% lower compared to the kerosene aircraft due to the absence of CO2, the lower radiative forcing of hydrogen contrails, and the reduction in NOx emissions when assuming advanced hydrogen combustion technology. It was also observed that climate impact from hydrogen aircraft is more sensitive to flights over the tropics than to flights over the poles.@en

The power sector accounts for ∼40% of global energy-related CO₂ emissions. Its decarbonization by switching to low-carbon renewables is essential for a sustainable future. Existing electrical grids, however, have limited capacity to absorb the variability introduced by these new energy sources and rely largely on natural-gas-based power generation. For deep decarbonization, alternative solutions to increase grid flexibility are needed. Among these, energy storage is expected to have a key role. This paper proposes a unique energy storage and re-conversion system by coupling the hydrogen combustion in supercritical CO₂ (HYCOS) cycle, a zero-emission combustion cycle, with long-term/seasonal energy storage based on green H₂ production. This power cycle is expected to be highly scalable and compact and can deliver power at net electrical efficiency between 55% and 60% at distributed generation levels. Thus, it can be highly competitive with existing solutions such as fuel cells, reciprocating engines, and gas turbines.

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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.

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Boundary-layer instability on a rotating cone induces coherent spiral vortices that are linked to the onset of laminar–turbulent transition. This type of transition is relevant to several aerospace systems with rotating components, e.g., aeroengine nose cones. Because a variety of options exist for the nose-cone shapes, it is important to know how their shape affects the boundary-layer transition phenomena. This study investigates the effect of varying cone angle on the boundary-layer instability on rotating cones facing axial inflow. It is found that increasing cone angle has a stabilizing effect on the boundary layer over rotating cones in axial inflow. The parameter space of Reynolds number Re _l and local rotational speed ratio S is experimentally explored to find the spiral vortex growth on rotating cones of half angle ψ 22.5°, 45°, and 50°. The previously addressed cases of ψ 15° and 30° are also revisited. Increasing half-cone angle is found to have a stabilizing effect on the boundary layer on the rotating cones with ψ ≲ 45°; i.e., the spiral vortex growth is delayed to higher Re _l and S. This effect diminishes when the half-cone angle increases from ψ 45° to 50°. The spiral vortex angle ϵ decreases with increasing rotational speed ratio S for all the investigated cones, irrespective of the half-cone angle. However, the instability on the broader cones is found to induce shorter azimuthal wavelengths.

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Fuselage Boundary-Layer Ingestion (BLI) is a promising example of synergistic design and propulsion-airframe integration to reduce fuel burn. For a BLI configuration, the aero propulsive performance of the aircraft is a result of the complex aerodynamic interaction between the fuselage airframe and the BLI propulsor. This paper presents a design method for the aft fuselage including the propulsor shrouding to minimize the required shaft power of an aft-mounted propulsor in the conceptual design phase. First, a global aerodynamic design space exploration is carried out using Computational Fluid Dynamics (CFD) to identify the key design parameters and their influence to the aerodynamic performance of the propulsive fuselage. An optimization study is subsequently carried out to improve the aerodynamic performance of a baseline design. The optimization was performed for a turbo-electric BLI configuration and within representative design constraints. The optimization achieved a decrease of approximately 10% of the isentropic shaft power required for the aft-mounted propulsor for a constant net force acting on the propulsive fuselage. The presented methodology and the resulting design practices can be effectively applied to other advanced aircraft configurations.@en

Flameless Combustion is an interesting low NO_x combustion technology for gas turbine engines. In order to design systems for stringent performance standards, it is important to understand emission formation in this regime. To this end, the characteristics of a combustor capable of operating in the Flameless regime are studied. Particle Image Velocimetry and thermocouple measurements were performed to obtain the velocity field and gas temperatures respectively, in the combustor under reacting conditions. Results from experiments were used to generate an "informed" chemical reactor network (CRN) model from which, temperature and species distributions were obtained. As such, this paper presents measured data and a methodology to combine it with CRN modelling to obtain gas composition and temperature. The temperature, NO_x, CO and O₂ mole fractions obtained at three different operating conditions shall be validated with gas composition measurements in the future.

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The fuel efficiency of turbofan engines has improved significantly, hence reducing aviation's CO2 emissions. However, the increased operating pressure and temperature for fuel efficiency cause adverse effects on NOx emissions. Therefore, a novel engine concept, which can reduce NOx emissions without affecting the cycle efficiency, is of high interest to the aviation community. This paper investigates the potential of an intercooler and inter-turbine burner (ITB) for the future low NOx aircraft propulsion system. The study evaluates performance and NOx emissions of four engine architectures: a very high bypass ratio (VHBR) turbofan engine (baseline), a VHBR engine with intercooler, a VHBR engine with ITB, and a VHBR engine with both intercooler and ITB. The cycles are optimized for minimum cruise Thrust Specific Fuel Consumption (TSFC), considering the same design space, thrust requirements, and operational constraints. The ITB is only used during take-off to minimize cruise fuel consumption. The analysis shows that using an ITB solely, with the energy split of 75% (the first burner) / 25% (ITB), reduces the cruise NOx emission by 26%, and the cruise TSFC slightly by 0.5%. The intercooler alone reduces the NOx emissions by 16% and the cruise TSFC by 0.8%. The combination of intercooler and ITB reduces the NOx emissions further by 38%. The analysis confirms that introducing an intercooler and ITB can potentially resolve the contradicting effects of fuel efficiency and NOx emissions for the future advanced turbofan engine.

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Boundary-layer ingestion (BLI) has been proposed as one of the novel airframe–engine integration technologies to reduce aircraft fuel consumption. The current numerical analysis involves the evaluation of the effect of fuselage design on the power consumption of a boundary-layer ingesting propulsor modeled as an actuator volume without nacelle. An axisymmetric fuselage model is used as a canonical case to study BLI in transonic flight conditions. The flowfield is investigated through the power balance and the exergy analysis methods. Results show that the fuselage geometry and flight conditions only have a minor effect on the BLI power saving benefit when compared to the effect on the drag power of the fuselage. This indicates that, for the range of fuselage geometries and flight conditions studied, the isolated fuselage drag can be used for a qualitative performance assessment of different fuselage designs even for BLI configurations. Also, the power saving results obtained based on the power balance and the exergy analysis methods show similar qualitative trends for the fuselage geometries and flight conditions considered. Furthermore, the BLI propulsor has a negligible effect on the upstream anergy generation rate. Turbulence and temperature gradients within the flow are the important reasons for the deterioration of the BLI propulsor performance as expected.

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The concept of an "Auxiliary Power and Propulsion Unit" (APPU) is introduced, which consists of a Boundarylayer ingesting (BLI) propulsor with an engine mounted at the rear of an passenger aircraft fuselage, replacing the Auxiliary Power Unit (APU) and contributing around 10% of total cruise thrust, as well as auxiliary power. This APPU unit is using hydrogen provided by an additional tank installed in the tailcone of the aircraft. The concept is aimed at lowering the threshold to installing both hydrogen-driven propulsion and BLI propulsors on aircraft in the short term, while minimizing resulting operational risk. The concept has been investigated using a preliminary aircraft synthesis tool and further component-level mass estimates. Operational aspects,
sensitivities and limits to the design have been investigated. Estimates of mission fuel burn find that CO2 emissions emissions reduce roughly proportionally to the APPU thrust share, with additional savings due to improved overall efficiency. Further improvements are deemed feasible and are the topic of ongoing research.@en