06 Fakultät Luft- und Raumfahrttechnik und Geodäsie
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/7
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Item Open Access Implementierung eines Verfahrens höherer Ordnung zur numerischen Simulation reaktiver Strömungen auf unstrukturierten Rechengittern(Stuttgart : Deutsches Zentrum für Luft- und Raumfahrt, Institut für Verbrennungstechnik, 2024) Setzwein, Florian; Gerlinger, Peter (apl. Prof. Dr.-Ing.)Diskretisierungsverfahren hoher Ordnung, die sich auf unstrukturierten Rechengittern einsetzen lassen, bieten ein großes Potential zur Reduzierung der Rechenzeiten von detaillierten Grobstruktursimulationen. Gleichzeitig lässt sich gegenüber strukturierten Diskretisierungsansätzen eine hohe geometrische Flexibilität für die Generierung der Rechengitter realisieren. Viele Verfahren, die eine höhere Rekonstruktionsordnung auf unstrukturierten Gittern ermöglichen, beruhen auf der Einführung von zusätzlichen Freiheitsgraden innerhalb der Berechnungselemente. Ihre Implementierung in etablierte Finite-Volumen Strömungslöser ist jedoch aufgrund großer Unterschiede in den Datenstrukturen mit einem hohen Aufwand verbunden. Doch auch unstrukturierte Finite-Volumen Verfahren, welche eine höhere räumliche Fehlerordnung durch eine nicht-kompakte Rekonstruktion ermöglichen, verlangen einen hohen Implementierungsaufwand, um eine parallele Skalierbarkeit zu realisieren. Ein vielversprechender Ansatz zur Erhöhung der räumlichen Genauigkeit von etablierten unstrukturierten Finite-Volumen-Lösern stellt das k-exakte Multi-Korrekturverfahren dar. Der Schlüssel der Methode ist eine sukzessive Korrektur von approximativen Green-Gauss-Ableitungen, die eine Rekonstruktion hoher Ordnung mit guten Parallelisierungseigenschaften und einem moderatem Implementierungsaufwand ermöglicht. In dieser Arbeit wird der k-exakte Multi-Korrekturansatz, welcher ursprünglich für kompressible Strömungsprobleme und für zellzentrierte Rechengitter entwickelt wurde, für die Anwendung auf einer knotenzentrierten Gitterrepräsentation erweitert und für die Exaktheiten k = 1 und k = 2 in den DLR Strömungslöser ThetaCOM implementiert. Des Weiteren wird die Methode mit einem Druckkorrektur-Verfahren für die zeitgenaue Diskretisierung der Erhaltungsgleichungen reaktiver Fluide bei niedrigen Mach-Zahlen kombiniert. Hierfür werden entsprechende Korrekturterme hergeleitet. Des Weiteren wird die in ThetaCOM implementierte Approximation der konvektiven und diffusiven Flüsse mit dem k-exakten Rekonstruktionsansatz vereint. Für die Berechnung der konvektiven Flüsse wird außerdem ein Ansatz vorgestellt, mit dem sich die Bestimmung der numerischen Dissipation zur Stabilisierung des Verfahrens auf ein Minimum reduzieren lässt. Dieser beruht auf der Herleitung einer Stabilitätsgleichung, welche aus einer Von-Neumann-Stabilitätsanalyse für eine lineare Advektions-Diffusion-Gleichung hervorgeht und deren Lösung zur Beschleunigung des Verfahrens indirekt in einem Verbund aus kompakten neuronalen Netzwerk-Modellen tabelliert wird. Dieser Ansatz wird mit einem Verfahren zur Gradientenlimitierung gekoppelt, um mit dem Diskretisierungsverfahren eine akkurate Auflösung von steilen Lösungsgradienten zu ermöglichen, welche in Verbrennungssimulationen in unmittelbarer Nähe zur Flammenfront auftreten. Für das implementierte Multi-Korrekturverfahren wird die räumliche Genauigkeit der verschiedenen numerischen Operatoren durch zahlreiche kanonische Testfälle verifiziert. Es wird gezeigt, dass sich die räumlichen Gradienten der Feldgrößen infolge der k-exakten Korrekturen mit einer wesentlich höheren Genauigkeit approximieren lassen. Des Weiteren lässt sich der diffusive Transport durch beide Schemata mit einer zweiten räumlichen Fehlerordnung und der konvektive Transport für k = 1 und k = 2 mit jeweils einer zweiten beziehungsweise dritten Fehlerordnung approximieren. Durch die Simulation zahlreicher laminarer und turbulenter Strömungsprobleme werden die beiden k-exakten Diskretisierungsverfahren mit experimentellen und numerischen Referenzdaten aus der Literatur validiert. Dabei wird der Einfluss der höheren Ordnung auf die räumliche Genauigkeit im Vergleich zu einem konventionellen Diskretisierungsverfahren beleuchtet. Hierbei wird insbesondere das Potential der beiden k-exakten Verfahren hinsichtlich der Einsparung von Rechenzeit und Freiheitsgraden dargestellt, sowie deren Fähigkeit zur Erhaltung der parallelen Skalierungseigenschaften von ThetaCOM. Ein weiterer Fokus liegt auf dem neuen Ansatz zur adaptiven Bestimmung der numerischen Dissipation und dessen Kopplung mit der implementierten Methode zur Gradientenlimitierung. Im Vergleich zur Rekonstruktion hoher Ordnung mit einer konstanten numerischen Dissipation liefert die vorgestellte adaptive Methode konsistente und genaue Ergebnisse, unabhängig vom Strömungsproblem und ohne eine Feinjustierung von empirischen Parametern. Abschließend wird für den Testfall einer turbulenten Wasserstoff-Luft-Diffusionsflamme demonstriert, dass sich beide Verfahren zur Simulation von turbulenten, reaktiven Strömungen auf vollständig unstrukturierten Rechengittern einsetzen lassen und eine deutliche Verbesserung des Simulationsergebnisses im Vergleich zu einem konventionellen Diskretisierungsansatzes bewirken.Item Open Access Data-based methods for the screening and design of jet fuels(Stuttgart : Deutsches Zentrum für Luft- und Raumfahrt, Institut für Verbrennungstechnik, 2024) Hall, Clemens Alexander; Aigner, Manfred (Prof. Dr.-Ing.)To achieve climate neutrality in the aviation sector, research on new sustainable aviation fuels (SAF) is needed as the growing demand will exceed the production potential of established sustainable pathways. The focus is thereby not only on the exploration of sustainable feedstocks and the development of new production processes but also on the facilitation and acceleration of the whole fuel development process, from its conceptualization to its approval. The critical evaluation of a new production pathway guarantees the safe application and performance of a new fuel. The approval poses a major challenge for fuel producers, requiring a tremendous commitment of time, fuel volume and cost. Concepts that allow a fast-iterative, low-cost screening and design of new candidate fuels, to assess and optimize their chances for approval are thereby seen as key enablers. Established fuel screening concepts rely on model-based prediction, which, together with state- of-the-art compositional analytics, allow the fast assessment of SAF candidates from volumes as low as 5 mL. The design of new fuels, on the other hand, requires a comprehensive understanding of the composition of a jet fuel and properties considered critical for the fuel approval. This work describes the research and development of tools for the screening and design of jet fuels. Focusing on data-based methods, the tools are built from a database composed of both jet fuels and fuel components. It is thereby investigated whether and how data-based tools are able to support the screening and design of new SAF candidates and what their limitations are. For the jet fuel screening, three different modeling methods to predict physicochemical properties from compositional measurements are adapted and investigated: Direct correlation (DC), Mean Quantitative Structure-Property Relationship Modeling (M-QSPR) and Quantitative Structure-Property Relationship Modeling (QSPR) with sampling. All developed models are probabilistic, since the safety-relevant use case of jet fuel screening makes the consideration of uncertainties necessary. Rather than estimating one deterministic property value, probabilistic models estimate a distribution of values and with it the associated uncertainty. The predictive capabilities of the developed models are assessed using specially developed metrics and compared on the prediction of conventional and synthetic jet fuels. To put the developed models into reference, they are compared to established deterministic models from the literature. Identifying strengths and limitations of the different approaches, the models are applied to jet fuel screening to test theiradequacy for the assessment of new SAF candidates. To support the design of new SAF candidates, the relationships between the fuel composition and critical physicochemical properties are investigated. The relationships are investigated on the basis of fuel components and the influence of their chemical families as well as the structural aspects size and the branching. Trends and relations are characterized with graphs and quantitative metrics that illustrate correlation and state the average value for a change in composition. Both the developed models and design tools are applied to the use case of screening and then optimizing a real SAF candidate to maximize its chances for successful fuel approval. The SAF candidate and three optimized fuel variants with reformulated compositions are thereby screened to assess the most suitable production route. Afterwards, a blending analysis of the SAF candidate and the variants is conducted to estimate their maximum volume fraction in the mixture with representative conventional jet fuels, considering both the safety requirements as well as the potential reduction of CO2 and soot emissions. As potential next steps, this work identifies the need for advancements in the analytics of the fuel composition as well as the extension of the existing fuel property databases. The former would reduce the uncertainty in the property modeling, while the latter would increase both the predictive capability of the models and the understanding of the fuel property relations.Item Open Access Experimental investigation of a low-NOx swirl-assisted and jet-stabilized gas turbine combustor concept(2025) Izadi, Saeed; Aigner, Manfred (Prof. Dr.-Ing.)Today's aircraft engine emission standards regulate, among other aspects, the emissions of nitrogen oxides (NOx), carbon monoxide (CO) and unburned hydrocarbons (UHC) at low altitudes, i.e. during the take-off and landing cycle. It is expected that international aviation regulatory bodies will extend the standards to include high-altitude emissions. This will reduce the global impact of these pollutants. In particular, NOx emissions will need to be reduced due to their role in the greenhouse effect as one of the major non-CO2 factors at higher altitudes. Therefore, in order to meet the upcoming stricter emission standards while maintaining optimal combustor reliability, affordability and efficiency, innovative combustor concepts are required. As a low-NOx combustion technology for future gas turbine engines, a low-swirl, lean premixed prevaporized concept can be an alternative to current conventional combustor systems. The concept is characterized by a lean-fuel and a high degree of mixing of the fuel with air prior to the reaction zone. This results in minimized hot spots and a significant reduction in thermal NOx levels. This work aims to investigate an innovative jet-stabilized concept. Initially, a single-nozzle jet-stabilized gas turbine combustor as a reference combustor was tested using both spray and superheated injection (flash atomization) of Jet A-1 at atmospheric pressure. Non-reactive tests using Mie scattering showed that as the fuel temperature increased, the fuel spray gradually vanished and was replaced by a rapidly evaporating fuel plume. The primary effect was a re-duction in the size of the fuel droplets, but also a rapid axial acceleration of the fuel vapor. As a result of the superheated injection, the Jet A-1's radial penetration was significantly reduced. This resulted in poorer mixing of the fuel with the air and led to shifting flame downstream of the flow. Additionally, the high temperatures caused carbon deposits to form within the fuel lines and the injector, which limited the operation of the combustor. These initial tests showed that fundamental changes to the combustor design are required to utilize superheated fuel injection with low emissions and a wide operating range in the jet-stabilized single-nozzle com-bustor. Due to the narrow operating range of the single-nozzle jet-stabilized combustor under spray conditions and the extremely unstable flame under superheated conditions, the combustor was iteratively developed to incorporate additional components. This was followed by a thorough study of how each component affected fuel vaporization and emissions. The results showed that, the additional components allowed for improved fuel-air mixing, fuel atomization, and evaporation prior to the reaction zone. The axial swirler slowed the rapidly expanding, high-velocity, superheated fuel by providing moderate swirling motion. The swirler hub proved to be an effective baffle, allowing the expanding and superheated fuel to mix better with the air. In addition, a prefilmer channel was installed around the axial swirler to increase the velocity through the swirler vanes, which allowed for improved secondary atomization of the fuel by means of an air-blast effect. As a result, a systematic variation of combustor operational and geometric design parameters was experimentally performed to study their effects on a newly developed swirl-assisted jet-stabilized combustor. The operational parameters included the adiabatic flame temperature, the thermal power, and the air and fuel temperatures, while the geometric parameters were the type of fuel injector, swirl number, the flame tube and the air nozzle diameters. In addition, to evaluate their behavior under sprayed and superheated injection regimes, four different liquid fuels with different thermochemical properties were tested. Finally, water vapor was added to the fuel-air mixture for evaluation of flame resistance to perturbations such as dilution and combustion inhibitors. For the characterization of the physical phenomena, established methods of combustion diag-nostics have been applied. Mie scattering was used in non-reactive and reactive tests for quali-tative analysis of fuel spray angle, penetration depth and degree of evaporation in the flame tube. Flame length (FL) and height above burner (HAB) of the heat release zone were deter-mined using OH* chemiluminescence. Furthermore, an emission analyzer was used to evaluate the pollutants emitted from the flames. These pollutants include NOx, CO, UHC and particu-late matter (PM). The mean residence time, bulk velocity, and recirculation rate and shape in the flame tube were primarily affected by variation of the flame tube diameter (DFT). This led to a change in reaction zone’s HAB and FL. The lowest NOx and CO levels were consistently observed with the smallest air nozzle diameter (DAN). This could be attributed to improved fuel-air mixing resulting from increased air dispersion at the nozzle exit, which led to increased turbulence at higher jet velocities. For both Jet A-1 and natural gas combustion, the injection of steam re-duced NOx emissions by lowering the adiabatic flame temperature. The characterized combustor concept features very low-emission combustion of a variety of liquid fuels over a wide operating range. The combustor concept is insensitive to spray quality so that injectors with poorer spray characteristics can be used. For the presented concept it was also shown that the injection of superheated fuel does not offer significant advantages due to the fuel preparation in the combustor.Item Open Access Carrier-phase DNS of ignition and combustion of iron particles in a turbulent mixing layer(2024) Luu, Tien Duc; Shamooni, Ali; Kronenburg, Andreas; Braig, Daniel; Mich, Johannes; Nguyen, Bich-Diep; Scholtissek, Arne; Hasse, Christian; Thäter, Gabriel; Carbone, Maurizio; Frohnapfel, Bettina; Stein, Oliver ThomasThree-dimensional carrier-phase direct numerical simulations (CP-DNS) of reacting iron particle dust clouds in a turbulent mixing layer are conducted. The simulation approach considers the Eulerian transport equations for the reacting gas phase and resolves all scales of turbulence, whereas the particle boundary layers are modelled employing the Lagrangian point-particle framework for the dispersed phase. The CP-DNS employs an existing sub-model for iron particle combustion that considers the oxidation of iron to FeO and that accounts for both diffusion- and kinetically-limited combustion. At first, the particle sub-model is validated against experimental results for single iron particle combustion considering various particle diameters and ambient oxygen concentrations. Subsequently, the CP-DNS approach is employed to predict iron particle cloud ignition and combustion in a turbulent mixing layer. The upper stream of the mixing layer is initialised with cold particles in air, while the lower stream consists of hot air flowing in the opposite direction. Simulation results show that turbulent mixing induces heating, ignition and combustion of the iron particles. Significant increases in gas temperature and oxygen consumption occur mainly in regions where clusters of iron particles are formed. Over the course of the oxidation, the particles are subjected to different rate-limiting processes. While initially particle oxidation is kinetically-limited it becomes diffusion-limited for higher particle temperatures and peak particle temperatures are observed near the fully-oxidised particle state. Comparing the present non-volatile iron dust flames to general trends in volatile-containing solid fuel flames, non-vanishing particles at late simulation times and a stronger limiting effect of the local oxygen concentration on particle conversion is found for the present iron dust flames in shear-driven turbulence.