Abstract: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.

A triangular array of nanoscaled artificial pinning centers ͑APCs͒ for magnetic flux lines is prepared into a Nb thin film. The APCs are formed by deposition of Nb onto a Si substrate covered with nanopillars with diameters of 20 nm and lattice constant a = 122 nm. The production of pillars is based on arrays of gold nanoparticles used as etching masks during reactively ion etching a Si substrate. In this way, the pattern of the original nanoparticle array is transferred onto the substrate. The Au nanoparticles in turn were prepared using the self-organization of inverse micelles formed by diblock copolymers, whose core is loaded with a gold precursor. The resulting lattice of APCs formed by the Si pillars perforating the Nb film mirrors the order of the micellar array which has triangular, short-range order, but loses its directional order for distances larger than about 4-8 lattice constants. Similar to the Little-Parks experiments with external magnetic fields perpendicular to the superconducting film, the resulting T c ͑B͒ curves show ⌬T c deviations. Those vanish for B larger than the first matching field B 1 indicating that no more than one single flux quantum can be captured at each APC, even at low temperatures. Moreover, integer and fractional matching effects in the critical current are observed within a wide temperature range. Two critical currents can be distinguished indicating different types of pinning mechanisms: Strong pinning at APCs and weaker pinning at interstitial sites accompanied by strong caging effects. A unique feature of such prepared samples is the surprising temperature dependence of the matching field for which a value of B 1 is observed close to T c , which, however, is shifted towards smaller values at lower temperatures. This effect is traced back to a temperature dependent averaging over differently ordered domains within the array of APCs.

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In this paper, we have investigated numerically the interaction between secondary flow and a normal compression shock in a transonic turbine cascade. We observe that the sudden deceleration caused by the compression shock triggers an instability in weaker regions of the vortex system. This instability forms a pattern of concentric rings of elevated vorticity, which, in turn, cause the shock to deform into the same concentric wave-like pattern. The same pattern appears in the entropy distribution just downstream of the shock as a result of the uneven shock-intensity distribution.

Defects in the hot-gas path of aero engines have been shown to leave typical signatures in the density distribution of the exhaust jet. These signatures are superposed when several defects are present. For improved maintenance and monitoring applications, it is important to not only detect that there are defects present but to also identify the individual classes of defects. This diagnostic approach benefits both, the analysis of prototype or acceptance test and the preparation of Maintenance, Repair, and Overhaul. Recent advances in the analysis of tomographic Background-Oriented Schlieren (BOS) data have enabled the technique to be automated such that typical defects in the hot-gas path of gas turbines can be detected and distinguished automatically. This automation is achieved by using Support Vector Machine (SVM) algorithms. Choosing suitable identification parameters is critical and can enable SVM algorithms to distinguish between different defect types. The results show that the SVM can be trained such that almost no defects are missed and that false attributions of defect classes can be minimized.

This paper presents those flow parameters at which coherent structures appear in the blade tip cavities of shrouded turbine blades. To the authors' knowledge, this is reported for the first time in the open literature. The unsteady flow in a shroud cavity is analysed based on experimental data recorded in a labyrinth seal test rig. The unsteady static wall pressure in the shroud cavity inlet and outlet is measured using time-resolving pressure sensors. Sensors are located at staggered circumferential positions to allow cross-correlation between signals. The unsteady pressure signals are reduced using Fourier analysis and cross-correlation in combination with digital filters. Based on the data, a theory is formulated explaining the phenomena reflected in the measurements. The results suggest that pressure fluctuations with distinct numbers of nodes are rotating in the shroud cavity outlet. Moreover, modes with different node numbers appear to be superimposed, rotating at a common speed in circumferential direction. The pressure fluctuations are not found at all operating points. Further analysis indicates that the pressure fluctuations are present at operating points matching distinct parameters correlating with the cavity flow coefficient. Unsteady RANS simulations predict similar flow structures for the design operating point of the test rig.

The aim of this work is the decomposition, quantification, and analysis of losses related to the axial gap size effect. Both experimental data and unsteady RANS calculations are investigated for axial gaps equal to 20 %, 50 % and 80 % of the stator axial chord. A framework for identifying sources of loss typical in turbomachinery is derived and utilized for the low-pressure turbine presented. The analysis focuses on the dependency of these losses on the axial-gap variation. It is found that two-dimensional profile losses increase for smaller gaps due to higher wake-mixing losses and unsteady wake-blade interaction. Losses in the end-wall regions, however, decrease for smaller gaps. The total system efficiency can be described by a superposition of individual loss contributions, the optimum of which is found for the smallest gap investigated. It is concluded that these loss contributions are characteristic for the medium aspect-ratio airfoils and operating conditions investigated. This establishes a deeper physical understanding for future investigations into the axial-gap size effect and its interdependency with other design parameters.