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Abstract Proton exchange membrane fuel cells (PEMFCs) are seen as one possible future means of driving the change towards a zero-emission society. In a civil aircraft, fuel cell systems can have multiple potential benefits, such as reduced noise, lowered emissions and higher fuel economy compared to jet aircraft. For controlling the fuel cell temperature, thermal management systems are required which can be optimized for aircraft applications regarding their weight and reliability. In this work, a simplified and light-weight thermal management system relying on hydrogen cooling is presented and analysed. To investigate the feasibility, a test rig and a three-dimensional, singular channel model in ANSYS Fluent were designed. Fuel cell temperature could be maintained within the set threshold in the model and the test rig, thus showing that controlling the fuel cell temperature via the hydrogen reactant flow is a viable alternative thermal management system. Results from the model indicate that both the hydrogen mass flow and hydrogen inlet temperature should be used to control the fuel cell temperature. Furthermore, operating the fuel cell at medium to low current densities is favourable for hydrogen cooling. Future studies will explore alternate flow field designs to facilitate thermal management system relying on hydrogen.

For the sizing of an electric motor, the diameter and length of the installation space are important parameters that significantly influence and limit the characteristics of the electric motor. From an aerodynamic point of view, it is not technically advantageous to choose a propeller hub diameter that is larger than mechanically necessary. The hub diameter defines the installation space ans thus the outer diameter for the electric motor driving the propeller. The research question that arises is this: Is there an optimum diameter for the hub and thus the electric motor and how is it limited? The present work should help to establish a better understanding of the system combination of electric motor and propeller, especially the choice of the diameter and further geometrical parameters of the electric motor. A parameter study investigating aeromechanical and electromagnetic aspects has been performed and the results are combined to achieve the best solution in terms of efficiency and power density. The investigations show that an increase in the diameter causes little disadvantage from an aerodynamic point of view, but the improvements to the electric motor are significant only up to the point where the diameter-to-length ratio becomes too high and end-region effects become more significant. The installation space for the electric motor could be determined using a hub-to-tip ratio of 0.16 to 0.20, which is applicable to any propeller. In addition, it could be shown that a co-optimisation of the speed throughout a mission profile with regard to the efficiency is affected by the propeller alone.

Laminar flow offers significant potential for increasing the energy-efficiency of future transport aircraft. The German Cluster of Excellence SE2A is developing a new approach for hybrid laminar flow control. The concept aims to maintain laminar flow up to 80% of the chord length by integrating suction panels at the rear part of the wing, which consist of a thin suction skin and a supporting core structure. This study examines effects of various suction panel configurations on wing mass and load transfer for an all-electric short-range aircraft. Suction panel material, as well as thickness and relative density of the suction panel core are modified in meaningful boundaries. Suction panels made from Ti6Al4V offer the most robust design resulting in a significant increase in wing mass. For the studied configurations, they represent up to 33.8% of the mass of the wingbox. In contrast, panels made from Nylon11CF or PU1000 do not significantly increase the wing mass. However, the use of these materials raises questions about their robustness under operational conditions. The results demonstrate that the choice of material strongly influences the load path within the wing structure. Ti6Al4V suction panels provide sufficient mechanical properties to significantly contribute to load transfer and buckling stiffness. Locally, the share of load transfer attributed to the suction panel exceeds 50%. In contrast, compliant materials such as Nylon11CF or PU1000 are inherently decoupled from load transfer. Unlike the thickness of the suction skin, the relative density of the core structure strongly affects the wrinkling stiffness. However, wrinkling failure did not appear critical for the examined suction panel configurations. In the present study, the mechanical properties of Ti6Al4V cannot fully be exploited. Therefore, compliant suction panels made from Nylon11CF are preferred in order to achieve a lightweight solution, provided that they meet operational requirements.

Typically installed in a rotor-only configuration, low-pressure axial fans discharge directly into a free atmosphere and the discharge shows a strong swirl component. Since such designs, without guide vanes, cannot convert the dynamic pressure in the swirl component back into static pressure, the dynamic pressure is usually considered a loss. However, the radial equilibrium shows that a significant part of the kinetic energy contained in this swirl component is recovered as static pressure in the free atmosphere. This additional pressure increase has been sparsely researched. A comparison between two configurations with and without outlet guide vanes allows for the formulation of an evaluation criterion of the rotor-only configuration. Utilizing this evaluation criterion, the investigation of velocity profiles corresponding to generic rotor designs shows promise in terms of pressure recovery for new designs.

The design of flexible and efficient aircraft engines and propulsion systems plays a crucial role in the development of future low-emission aircraft. Implementing shape-variable blades to compressor front stage rotors presents a high potential for increasing efficiency, since through adaptation, the blades are capable of optimizing their shape for different flight phases and aerodynamic conditions. Modifying the shape of the blades by using structurally integrated actuators allows this adaptation and therefore helps enhance their aerodynamic behavior for different flight regimes. Since up to now no morphing compressor or any other aircraft engine blades exist, here a multidisciplinary method for their design is introduced. This new method brings together existing structural and aerodynamic design methodologies, couples them together already at the earliest stages of the design process, while addressing the challenges that arise with a tightly coupled multidisciplinary design. As a result, first performance gain evaluations applied to the NASA 67 rotor test case are presented, showing the potential of morphing compressor blades and the potential of the introduced design methodology.

Myelodysplastic syndromes (MDS) comprise a heterogeneous group of hematologic malignancies characterized by clonal hematopoiesis, one or more cytopenias such as anemia, neutropenia, or thrombocytopenia, abnormal cellular maturation, and a high risk of progression to acute myeloid leukemia. The bone marrow microenvironment (BMME) in general and mesenchymal stromal cells (MSCs) in particular contribute to both the initiation and progression of MDS. However, little is known about the role of MSC-derived extracellular matrix (ECM) in this context. Therefore, we performed a comparative analysis of in vitro deposited MSC-derived ECM of different MDS subtypes and healthy controls. Atomic force microscopy analyses demonstrated that MDS ECM was significantly thicker and more compliant than those from healthy MSCs. Scanning electron microscopy showed a dense meshwork of fibrillar bundles connected by numerous smaller structures that span the distance between fibers in MDS ECM. Glycosaminoglycan (GAG) structures were detectable at high abundance in MDS ECM as white, sponge-like arrays on top of the fibrillar network. Quantification by Blyscan assay confirmed these observations, with higher concentrations of sulfated GAGs in MDS ECM. Fluorescent lectin staining with wheat germ agglutinin and peanut agglutinin demonstrated increased deposition of N-acetyl-glucosamine GAGs (hyaluronan (HA) and heparan sulfate) in low risk (LR) MDS ECM. Differential expression of N-acetyl-galactosamine GAGs (chondroitin sulfate, dermatan sulfate) was observed between LR- and high risk (HR)-MDS. Moreover, increased amounts of HA in the matrix of MSCs from LR-MDS patients were found to correlate with enhanced HA synthase 1 mRNA expression in these cells. Stimulation of mononuclear cells from healthy donors with low molecular weight HA resulted in an increased expression of various pro-inflammatory cytokines suggesting a contribution of the ECM to the inflammatory BMME typical of LR-MDS. CD34+ hematopoietic stem and progenitor cells (HSPCs) displayed an impaired differentiation potential after cultivation on MDS ECM and modified morphology accompanied by decreased integrin expression which mediate cell-matrix interaction. In summary, we provide evidence for structural alterations of the MSC-derived ECM in both LR- and HR-MDS. GAGs may play an important role in this remodeling processes during the malignant transformation which leads to the observed disturbance in the support of normal hematopoiesis.

The detailed mechanisms of action of bactericidal antibiotics remain unclear. Here, Wong et al. show that these antibiotics induce cytoplasmic condensation through membrane damage and outflow of cytoplasmic contents, as well as accumulation of reactive metabolic by-products and lipid peroxidation, as part of their lethality.

The air-management system of a proton exchange membrane fuel cell (PEMFC) is responsible for supplying the fuel cell stack with ambient air at appropriate conditions. The compressor of the air-management system can be partly driven by utilizing the fuel cell exhaust gas in a turbine. The fuel cell exhaust is partially or fully saturated with water vapor. When the exhaust gas is expanded in the turbine, supersaturation occurs. This leads to the nucleation of droplets and their subsequent growth by condensation. This study provides an overview and understanding of the various phenomena caused by condensation and liquid water in the turbine of a PEMFC air-management system. The basis for this work is previously published numerical simulations that focused on individual aspects of the above phenomena. The present work revisits these results and puts them in context to provide a comprehensive understanding. Important phenomena are the effects of condensation on turbine performance through phase change losses, release of latent heat and thermal throttling. In addition, the released latent heat offers a power potential for downstream turbine stages. Through these effects, condensation can also impact the entire air-management system. However, condensation may occur unevenly, causing a circumferential asymmetry of the turbine outflow. Liquid water in the turbine can lead to droplet erosion, corrosion, and water-induced damage. In summary, it is essential to consider condensation and liquid water when developing turbines for PEMFC air-management systems.

A mobile fuel cell systems power output can be increased by pressure amplification using an electric turbocharger. These devices are subject to frequent transient manoeuvres due to a multitude of load changes during the mission in automotive applications. In this paper, the authors describe a simulation approach for an electric turbocharger, considering the impact of moist air and condensation within the cathode gas supply system. Therefore, two simulation approaches are used: an iterative simulation method and one based on a set of ordinary differential equations. Additional information is included from turbine performance maps taking into account condensation using Euler–Lagrange CFD simulations, which are presented. The iterative calculation approach is well suited to show the impact of condensation and moist air on the steady state thermodynamic cycle and yields a significant shift of the steady state operating line towards the surge line. It is shown that a substantial risk of surge occurs during transient deceleration manoeuvres triggered by a load step.

The present methodological study aims to assess boundary layer ingestion (BLI) as a promising method to improve propulsion efficiency. BLI utilizes the low momentum inflow of the wing or fuselage boundary layer for thrust generation in order to minimize the required propulsive power for a given amount of thrust for wing or fuselage-embedded engines. A multi-segment parallel compressor model (PCM) is developed to calculate the power saving from full annular BLI as occurring at a fuselage tail center-mounted aircraft engine, employing radially subdivided fan characteristics. Applying this methodology, adverse effects on the fan performance due to varying inlet distortions depending on flight operating point as well as upstream boundary layer suction can be taken into account. This marks one step onto a further segmented PCM model for general cases of BLI-induced inlet distortion and allows the evaluation of synergies between combined BLI and active laminar flow control as a drag reduction measure. This study, therefore, presents one further step towards lower fuel consumption and, hence, a lower environmental impact of future transport aircraft.

Radial turbines used in automotive fuel cell turbochargers operate with humid air. The gas expansion in the turbine causes droplets to form, which then grow through condensation. The associated release of latent heat and decrease in the gaseous mass flow strongly influence the thermodynamics of the turbine. This study aims to investigate these phenomena. For this purpose, the classical nucleation theory and Young’s growth law are integrated into a Euler–Lagrange approach. The main advantages of this approach are the calculation of individual droplet trajectories and a full resolution of the droplet spectrum. The results indicate an onset of nucleation at the blade tip and in the tip gap, followed by nucleation over the entire blade span, depending on the humidity at the turbine inlet. With a saturated turbine inflow, condensation causes the outlet temperature to rise to almost the same level as at the inlet. In addition, condensation losses reduce the efficiency and the latent heat released by condensation leads to significant thermal throttling.

High aspect ratio aircraft have a significantly reduced induced drag, but have only limited installation space for control surfaces near the wingtip. This paper describes a multidisciplinary design methodology for a morphing aileron that is based on pressure-actuated cellular structures (PACS). The focus of this work is on the transient dynamic system behavior of the multi-functional aileron. Decisive design aspects are the actuation speed, the resistance against external loads, and constraints preparing for a future wind tunnel test. The structural stiffness under varying aerodynamic loads is examined while using a reduced-order truss model and a high-fidelity finite element analysis. The simulations of the internal flow investigate the transient pressurization process that limits the dynamic actuator response. The authors present a reduced-order model based on the Pseudo Bond Graph methodology enabling time-efficient flow simulation and compare the results to computational fluid dynamic simulations. The findings of this work demonstrate high structural resistance against external forces and the feasibility of high actuation speeds over the entire operating envelope. Future research will incorporate the fluid–structure interaction and the assessment of load alleviation capability.

In modern aircraft engines, the low-pressure compressor (LPC) is subjected to a flow characterized by strong wakes and secondary flows from the upstream fan. This concerns ultra-high bypass ratio (UHBR) turbofan engines, in particular. This paper presents the aerodynamic and aeroelastic sensitivities of parametric variations on the LPC, driven by the design considerations in the upstream fan. The goal of this investigation was to determine the influence of design-based geometry parameter variations on the LPC performance under realistic inlet flow distributions and the presence of an s-duct. Aerodynamic simulations are conducted at the design and off-design operating points with the fan outflow as the inlet boundary conditions. Based on the aerodynamic results, time-linearized unsteady simulations are conducted to evaluate the vibration amplitude at the resonance operating points. First, the bypass ratio is varied by reducing the channel height of the LPC. The LPC efficiency decreases by up to 1.7% due to the increase in blockage of the core flow. The forced response amplitude of the rotor decreases with increasing bypass ratio due to increased aerodynamic damping. Secondly, the fan cavity leakage flow is considered as it directly affects the near hub fan flow and thus the inflow of the LPC. This results in an increased total-pressure loss for the s-duct due to mixing losses. The additional mixing redistributes the flow at the s-duct exit leading to a total-pressure loss reduction of 4.3% in the first rotor at design point. This effect is altered at off-design conditions. The vibration amplitude at low speed resonance points is increased by 19% for the first torsion and 26% for second bending. Thirdly, sweep and lean are applied to the inlet guide vane (IGV) upstream of the LPC. Despite the s-duct and the variable inlet guide vane (VIGV) affecting the flow, the three-dimensional blade design achieves aerodynamic and aeroelastic improvements of rotor 1 at off-design. The total-pressure loss reduces by up to 18% and the resonance amplitude more than 10%. Only negligible improvements for rotor 1 are present at the design point. In a fourth step, the influence of axial gap size between the stator and the rotor rows in the LPC is examined in the range of small variations which shows no distinct aerodynamic and aeroelastic sensitivities. This finding not only supports previous studies, but it also suggests a correlation between mode shapes and locally increased excitaion with increasing axial gap size. As a result, potential design improvements in future fan-compressor design are suggested.

Performance degradation due to wear of high pressure compressors (HPC) is a major concern in aero-engine operation and maintenance. Among other effects especially erosion of airfoils leads to changed aerodynamic behavior and therefore to deterioration. These affects engine performance parameter like thrust specific fuel consumption (TSFC) and exhaust gas temperature (EGT). Reaching EGT-limit, the engine typically has to be overhauled during a shop visit to restore safety standards and performance. During state of the art shop visits, engines are repaired based on EGT-specifications. To further enhance the maintenance, tailored repairs for each jet engine based on engine history and operation conditions are necessary to take TSFC into account. To ensure such an effective maintenance, the aerodynamic behavior of deteriorated and repaired airfoils is the key factors. Therefore, geometric properties with high influence on aerodynamic performance have to be known. For blisks (BLade-Integrated-diSK) the approach of tailored maintenance will be even more complicated because the airfoil arrangement cannot be changed or individual airfoils cannot be replaced. Thus, the effects of coupled misshaped airfoils have a high significance. This study will present a Design of Experiments (DoE) for circumferential coupled HPC-airfoils to identify the geometric properties which lead to a reduction of performance. To focus on geometric variations, quasi3D (Q3D) simulations are taken out. Based on a sensitivity analysis, the thickness related parameters, the stagger angle as well as the max. profile camber thickness are identified as the most important parameters which are influencing adjacent airfoils and reduce the aerodynamic performance.

Today, main hub airports are already at their capacity limit and hence, smaller airports have become more interesting for providing point-to-point connections. Unfortunately, the use of regional airports induces an increased environmental footprint for the population living around it. In an attempt to solve the related problems, the research project Coordinated Research Centre 880 aims to examine the fundamentals of a single-aisle aircraft with active high-lift configuration powered by two geared ultra-high bypass turbofan engines mounted over the wing. Low direct operating costs, noise shielding due to the over-wing configuration, and short runway lengths are the main advantages. Highlighting the performance, economical and noise benefits of a geared ultra-high bypass engine is the key aim of this paper. This assessment includes a correspondingly adjusted aircraft. Open literature values are applied to design the two investigated bypass ratios; a reference engine with a bypass ratio of 5 and 17 respectively. This study shows that a careful selection of engine mass flow, turbine entry temperature and overall pressure ratio determines the desirable bypass ratio. The aircraft direct operating costs drop by 5.7% when comparing the designed conventional with a future ultra-high bypass ratio engine. Furthermore, the sound at source for a selected mission and operating condition can be reduced by 7 dB. A variable bypass nozzle area for the ultra-high bypass ratio engine is analysed in terms of performance and operability. An increase of safety margin is shown for the turbofan engine with a variable bypass nozzle. It is concluded that this unconventional aircraft configuration with ultra-high bypass ratio engines mounted over the wing has the potential to relieve main hub airports and reduce the environmental impact.

For the numerical prediction of turbomachinery flows, a two-equation turbulence model in combination with a proper transition model to account for laminar boundary layers and their transition to turbulence is state of the art. This paper presents the ability of such a method (k-ω + γ-ReΘ) for turbulence prediction and the effect on three-dimensional boundary layer behavior. For this purpose, both applied models (turbulence and transition) are improved to better account for turbulence length scale effects and three-dimensional transition prediction (Bode et al., 2014 and 2016), since these are the main deficiencies in predicting such kinds of flows. The improved numerical method is validated and tested on existing turbine cascades with detailed experimental data for the viscous regions and additionally on a low-speed axial compressor rig where wake-induced transition takes place.

Reducing losses in the secondary air system of gas and steam turbines can significantly increase the efficiency of such machines. Meanwhile, brush seals are a widely used alternative to labyrinth seals. Their most valuable advantage over other sealing concepts is the very small gap between the sealing package and the rotor and thus reduced leakage mass flow. This small gap can be achieved due to the great radial flexibility without running the risk of severe detrimental deterioration in case of rubbing. Rubbing between rotor and seal during operation might occur as a result of e.g., an unequal thermal expansion of the rotor and stator or a rotor elongation due to centrifugal forces or manoeuvre forces. Thanks to the flexible structure of the brush seal, the contact forces during a rubbing event are reduced; however, the frictional heat input can still be considerable. Particularly in aircraft engines with their thin and lightweight rotor structures, the permissible material stresses can easily be exceeded by an increased heat input and thus harm the engine’s integrity. The geometry of the seal has a decisive influence on the resulting contact forces and consequently the heat input. This paper is a contribution to further understand the influence of the geometrical parameters of the brush seal on the heat input and the leakage during the rubbing of the seal on the rotor. In this paper, a total of three seals with varied back plate inner diameter are examined in more detail. The experimental tests were carried out on the brush seal test rig of the Institute of Thermal Turbomachinery (ITS) under machine-relevant conditions. These are represented by pressure differences of 1 to 7 bar, surface speeds of 30 to 180 m/s and radial interferences of 0.1 to 0.4 mm. For a better interpretation, the results were compared with those obtained at the static test rig of the Institute of Jet Propulsion and Turbomachinery (IFAS) at the Technical University of Braunschweig. The stiffness, the blow-down and the axial behaviour of the seals as a function of the differential pressure can be examined at this test rig. It could be shown that the back plate inner diameter has a decisive influence on the overall operating behaviour of a brush seal.

Current civil aviation is characterized by rising cost and competitive pressure, which is partly passed to the MRO (Maintenance, Repair and Overhaul) companies. To improve the maintenance, condition-based maintenance is established, which is characterized by tailored maintenance actions for each part of the jet engine, depending on the individual engine history and operating conditions. Thereby, prediction models help engineers to authorize maintenance actions as effectively as possible. This paper will help to improve these prediction models. Therefore, the influence of specific deterioration of a high pressure compressor (HPC) to jet engine performance parameters such as exhaust gas temperature (EGT) and specific fuel consumption (SFC) will be investigated. For this purpose, computational fluid dynamic (CFD) calculations of deteriorated HPC geometries are carried out and serve as a basis to scale the reference HPC performance characteristics to deteriorated ones. To evaluate the changes in performance parameters, a modular performance synthesis model is set up. In this model, the HPC map is exchanged with deteriorated ones. As a result, the influence of geometric deviations to the design intent can be determined, and the MRO companies are able to focus on the most relevant sections of the compressor blading.

The decreasing performance of jet engines during operation is a major concern for airlines and maintenance companies. Among other effects, the erosion of high-pressure compressor (HPC) blades is a critical one and leads to a changed aerodynamic behavior, and therefore to a change in performance. The maintenance of BLISKs (blade-integrated-disks) is especially challenging because the blade arrangement cannot be changed and individual blades cannot be replaced. Thus, coupled deteriorated blades have a complex aerodynamic behavior which can have a stronger influence on compressor performance than a conventional HPC. To ensure effective maintenance for BLISKs, the impact of coupled misshaped blades are the key factor. The present study addresses these effects on the aerodynamic performance of a first-stage BLISK of a high-pressure compressor. Therefore, a design of experiments (DoE) is done to identify the geometric properties which lead to a reduction in performance. It is shown that the effect of coupled variances is dependent on the operating point. Based on the DoE analysis, the thickness-related parameters, the stagger angle, and the max. profile camber as coupled parameters are identified as the most important parameters for all operating points.

Boundary layer ingestion is a promising method to decrease the propulsive power consumption of an aircraft, and therefore the fuel consumption. This leads to a reduced environmental impact and an improved cost-efficiency. To get a better understanding of this method and to estimate its benefits, the modelling of a propulsor located at the upper rear centerbody of a blended wing body aircraft is presented in this paper. A parallel compressor model approach is used to analyse the impact of the ingested low velocity fluid which leads to a non-uniform inflow. The required boundary layer data are generated with an analysis tool for 2D subsonic airfoils. Some parameter variations are conducted with the developed programme to study their impact on the power saving potential. In addition, a simple estimation for the benefit of embedded aeroengines is given. Despite the drawback from fan efficiency due to the inflow distortion, the results show a significant decrease in required propulsive power for boundary layer ingestion in combination with integrated engines.

With the growth of aviation traffic and the demand for emission reduction, alternative fuels like the so-called electrofuels could comprise a sustainable solution. Electrofuels are understood as those that use renewable energy for fuel synthesis and that are carbon-neutral with respect to greenhouse gas emission. In this study, five potential electrofuels are discussed with respect to the potential application as aviation fuels, being n-octane, methanol, methane, hydrogen and ammonia, and compared to conventional Jet A-1 fuel. Three important aspects are illuminated. Firstly, the synthesis process of the electrofuel is described with its technological paths, its energy efficiency and the maturity or research need of the production. Secondly, the physico-chemical properties are compared with respect to specific energy, energy density, as well as those properties relevant to the combustion of the fuels, i.e., autoignition delay time, adiabatic flame temperature, laminar flame speed and extinction strain rate. Results show that the physical and combustion properties significantly differ from jet fuel, except for n-octane. The results describe how the different electrofuels perform with respect to important aspects such as fuel and air mass flow rates. In addition, the results help determine mixture properties of the exhaust gas for each electrofuel. Thirdly, a turbine configuration is investigated at a constant operating point to further analyze the drop-in potential of electrofuels in aircraft engines. It is found that electrofuels can generally substitute conventional kerosene-based fuels, but have some downsides in the form of higher structural loads and potentially lower efficiencies. Finally, a preliminary comparative evaluation matrix is developed. It contains specifically those fields for the different proposed electrofuels where special challenges and problematic points are seen that need more research for potential application. Synthetically-produced n-octane is seen as a potential candidate for a future electrofuel where even a drop-in capability is given. For the other fuels, more issues need further research to allow the application as electrofuels in aviation. Specifically interesting could be the combination of hydrogen with ammonia in the far future; however, the research is just at the beginning stage.

Ambitious targets to reduce emissions caused by aviation in the light of an expected ongoing rise of the air transport demand in the future drive the research of propulsion systems with lower CO2 emissions. Regional hybrid electric aircraft (HEA) powered by conventional gas turbines and battery powered electric motors are investigated to test hybrid propulsion operation strategies. Especially the role of the battery within environmentally friendly concepts with significantly reduced carbon footprint is analyzed. Thus, a new simulation approach for HEA is introduced. The main findings underline the importance of choosing the right power-to-energy-ratio of a battery according to the flight mission. The gravimetric energy and power density of the electric storages determine the technologically feasibility of hybrid concepts. Cost competitive HEA configurations are found, but do not promise the targeted CO2 emission savings, when the well-to-wheel system is regarded with its actual costs. Sensitivity studies are used to determine external levers that favor the profitability of HEA.