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

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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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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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

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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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

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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Decades of improvements of engine efficiency on internal engine components through better materials, design methods and novel fabrication techniques have resulted in fuel consumption reductions. Another major factor for improving engine fuel consumption has been the increase of bypass ratio. However, this has a significant impact on engine dimensions and weight, and, consequently, the installation of the engine on the airframe. Evaluation of engine installation penalties is not a new topic; literature provides various studies on aerodynamic effects. These primarily studied the effects of drag increase and the impact on drag of engine location and nacelle shape. This article investigates the performance impact of installation penalties from an increase in bypass ratio on narrow body aircraft, specifically the fuel consumption, weight and stability. Additionally, an analysis is made comparing aircraft retrofit and redesign for increased bypass ratio engines. It can be concluded from retrofit analyses that engine size is more significant than its location. Changes in aerodynamic center, CLα , and CMAC cause stability/controllability criteria to shift to the left. Heavier engines at the same spanwise location cause a more forward CG location, which may become limiting. With the engine increasing in size (thus increasing the drag and increasing the weight), the overall increase in fuel burn is 5.9%. However, the decrease in fuel burn due when the SFC and engine effects are considered together, the fuel burn drops by 50%. The reduction in fuel burn thereby negating the increase in engine weight, drag, and integration issues. From BPR 10 onwards, the decreasing trend in tail size stagnates and actually reverses, indicating that larger tail sizes might be required for even larger BPR engines.@en

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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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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The AeroCity is a new form of transportation concept that has been developed to provide high-speed ground transportation at a much lower cost than the existing high-speed railway. Utilizing the Wing-in-Ground (WIG) effect, the AeroCity vehicle does not require complex infrastructures like other contemporary concepts, such as the Hyperloop or Maglev trains. In the current work, the aerodynamic characteristics of the AeroCity vehicle are examined through a Computational Fluid Dynamics (CFD) analysis. The results from the CFD analysis qualitatively match with the findings of wind tunnel experiments. Surface streamlines and boundary layer measurements correspond well with the numerical data. However, the force measurements show a discrepancy. It is found that the separation bubble over the side plates is not captured by the CFD, and this is responsible for an under-prediction of the drag at higher free-stream velocities. The Transition SST model improved the matching between the experiments and numerical simulations. The influence of the moving ground is numerically investigated, and the effect of non-moving ground on the vehicle aerodynamics was found not to be significant. Finally, the inclusion of the track wall is examined. It is found that the merging of the wingtip vortices is responsible for a significant drag increase and, therefore, an alternative track geometry should be investigated. @en

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

Boundary-layer ingestion (BLI) is a propulsor–airframe integration technology that promises substantial fuel consumption benefits for future civil aircraft. This paper discusses an experimental study, conducted within the European Union–funded Horizon 2020 CENTRELINE project, on the aerodynamic performance of an aircraft with a BLI propulsor integrated at the aft-fuselage section (known as the Propulsive Fuselage Concept). The low-speed wind-tunnel experiments were carried out at Reynolds and Mach numbers of 460,000 and 0.12, whereas the Reynolds and Mach numbers are 40,000,000 and 0.82 at full-flight scale. Aerodynamic loads measurements show that the BLI propulsor affects the longitudinal and lateral-directional equilibrium of the aircraft in off-cruise conditions. Moreover, velocity and total pressure measurements characterize the flowfield around the BLI propulsor in cruise and off-cruise conditions. The analysis of the momentum and power fluxes in the flowfield shows that, while around 20% of the total aircraft drag is due to the fuselage body, only less than 5% of the total aircraft drag power is dissipated in the fuselage wake. Furthermore, the BLI propulsor recovers around 50% the axial kinetic energy flux in the fuselage boundary layer (the so-called wake-filling effect), suggesting an increased propulsive efficiency.@en

This work shows the behaviour of an unstable boundary-layer on rotating cones in high-speed flow conditions: high Reynolds number Rel > 106, low rotational speed ratio S < 1–1.5, and inflow Mach number M = 0.5. These conditions are most-commonly encountered on rotating aero-engine-nose-cones of transonic cruise aircraft. Although it has been addressed in several past studies, the boundary-layer instability on rotating cones remained to be explored in high-speed inflow regime. This work uses infrared-thermography with POD approach to detect instability-induced flow structures by measuring their thermal footprints on rotating cones in high-speed inflow. Observed surface temperature patterns show that the boundary-layer instability induces spiral modes on rotating cones, which closely resemble the thermal footprints of the spiral vortices observed in the past studies at low-speed flow conditions: Rel < 105, S > 1, and M ≈ 0. Three cones with half-cone angles y = 15◦, 30◦, and 40◦ are tested. For a given cone, the Reynolds number relating to the maximum amplification of the spiral vortices is found to follow an exponential relation with the rotational speed ratio S, extending from low- to high-speed regime. At a given rotational speed ratio S, the spiral vortex angle appears to be as expected from the low-speed studies, irrespective of the half-cone angle.@en

Centrifugal instability of the boundary layer is known to induce spiral vortices over a rotating slender cone that is facing an axial inflow. This paper shows how a deviation from the symmetry of such axial inflow affects the boundary layer instability over a rotating slender cone with half-angle. The spiral vortices are experimentally detected using their thermal footprint on the cone surface for both axial and non-axial inflow conditions. In axial inflow, the onset and growth of the spiral vortices are governed by the local rotational speed ratio and Reynolds number in agreement with the literature. During their growth, the spiral vortices significantly affect the mean velocity field as they entrain and bring high-momentum flow closer to the wall. It is found that the centrifugal instability induces these spiral vortices in non-axial inflow as well; however, the asymmetry of the non-axial inflow inhibits the initial growth of the spiral vortices, and they appear at higher local rotational speed ratio and Reynolds number, where the azimuthal variations in the instability characteristics (azimuthal number and vortex angle) are low.

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Boundary Layer Ingestion (BLI) is a promising propulsion integration technology capable of enhancing aircraft propulsive efficiency. The Propulsive Fuselage Concept (PFC), a tube-and-wing configuration with an aft-fuselage-mounted BLI propulsor, is particularly suited for BLI. Although extensively studied on a system level, the aerodynamic performance of the PFC, resulting from the complex interaction between the airframe and the propulsor, is still largely uncharted. In this paper, the results of wind-tunnel tests on a simplified PFC model are presented. The model featured an axisymmetric fuselage body with an integrated BLI shrouded fan. Flowfield measurements were performed through particle image velocimetry to analyze the key aerodynamic phenomena and to assess the distribution of momentum and mechanical energy around the aft-fuselage propulsor. Results show that the BLI fan alters the surrounding flowfield by increasing the mass flow in the inner part of the fuselage boundary layer and by reducing the boundary-layer thickness. Moreover, the power analysis indicates that the potential benefit of BLI is strongly dependent on the fan setting. Increasing the fan shaft power leads to a higher amount of power dissipated in the near wake. However, an increasing share of the energy flux is associated with the momentum excess contained in the wake.

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The fuel efficiency of civil aviation has improved significantly by more than 70%, hence reducing aviation’s CO2 emissions. Among the 70% fuel reduction, more than half has been achieved due to engine technology development, e.g., advanced engine cycles and technologies. To reduce fuel consumption, engine designer and manufacturers are striving to increase operating pressure and turbine inlet temperature; however, this increase causes adverse effects on NOx emissions for a given combustion technique. Especially at higher operating temperature and pressure, this trade-off between fuel consumption and NOx emission is highly nonlinear. This study tends to quantify the aviation’s climate impact from CO2 and NOx emissions by considering the nonlinear variations of CO2 and NOx caused by the nature of engine cycles. The analysis starts with engine modelling combined with emission calculations to understand the trade-off between CO2 and NOx emissions when varying the design variables, like overall pressure ratio and turbine inlet temperature. A typical turbofan engine with a high bypass ratio is considered. We then apply the exchange rate of CO2 and NOx emissions to the baseline emission inventory, the well-known AERO2K dataset. The state-of-the-art climate assessment tool, AirClim, is used to calculate the changes of climate impact concerning the exchange rate of CO2 and NOx emissions. Accordingly, we quantify the development trend of aviation's climate impact from CO2 and NOx concerning the intrinsic feature of an engine cycle. This research can provide us with insights into the engine development strategy that could be adapted to optimize the engine not only to reduce fuel consumption but also for climate impact reduction.@en