Browsing by Author "Vogt, Damian (Prof. Tekn. Dr.)"

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Open Access
Experimental and numerical investigation on aero-thermodynamics in a low-pressure industrial steam turbine with part-span connectors
(Düren : Shaker Verlag, 2020) Häfele, Markus; Vogt, Damian (Prof. Tekn. Dr.)
Industrial steam turbines are designed for flexible, reliable and robust operation with the ability for short start-up times. Basically assigned to the power class up to 250 MW, this type of turbine is normally operated over an extremely wide range of speed, loading and backpressure. In order to ensure safe turbine operation, even in blade resonance condition, part-span connectors (PSC) are mounted between adjacent blades. However, additional losses are generated, affecting the turbine performance. The main focus of the present work is put on the loss due to PSCs in low-pressure (LP) steam turbine blading. Turbine test rig measurements under real steam conditions and three-dimensional computational fluid dynamics (CFD) utilizing a non-equilibrium steam model are conducted. Both reveal strongly pronounced aero-thermodynamic effects of the PSCs on the wet steam flow, whereby the PSC in the last stage LP blading results in a reduction of stage efficiency by almost 4% at the best efficiency point. Based on the acquired experimental data, the developed CFD models are validated successfully over a wide range of operating conditions. Overall, the applied models are suitable for an industrial design process. Within a PSC parameter study, a performance assessment of PSC designs is presented. Moving a cylindrical PSC down to mid-span and further lowering its diameter provides the largest leverage in terms of efficiency gain. Additional benefits can be achieved by switching to a more streamlined shape. The PSC study is complemented by a validation of analytical loss correlations widely used in industry. Overall, a good agreement is found between the correlations and CFD. In conclusion, using the present results a substantial improvement of turbine efficiency can be achieved.
Open Access
Investigation of the operating point influence on the low-engine-order excitation in vaneless radial turbines
(2024) Kovachev, Nikola; Vogt, Damian (Prof. Tekn. Dr.)
Aerodynamic excitation in turbomachines can induce synchronous forced response vibrations, which may pose a high risk due to High-Cycle-Fatigue (HCF) failure of the rotor when excited at resonance. In the case of vaneless radial turbines, which are often used in turbochargers, the excitation mainly occurs at the so-called Low Engine Orders (LEO) stemming from asymmetrical geometry of the volute. These machines are operated dynamically over a wide range of operating conditions, constantly altering the harmonic blade forces and, hence, the aerodynamic excitation. The current work aims at providing a thorough understanding of the mechanisms for the variation of the LEO excitation as a function of operating point. Therefore, numerical and experimental investigations are performed on three vaneless radial turbines of similar size. The numerical investigations are performed at numerous operating points, employing transient Computational Fluid Dynamics (CFD) simulations. An analysis of local excitation variations on the blade surface at changed operating conditions has shown that these are governed by the wheel’s inflow velocity triangle and the dynamic pressure. Incidence variations, in particular, have a crucial impact on the harmonic pressure distribution and may cause a non-linear forcing behavior between operating points. Besides, an increase of the dynamic pressure ideally corresponds to a proportional rise in the excitation. A deviation to this proportionality occurs due to flow phenomena, such as the tip clearance flow, that modulate the forcing field originating from the volute. These observations are found to be valid but with a distinctive effect at the different LEOs and test objects. Consequently, the shape of an excitation map, describing the forcing under different operating conditions, is unique for each resonance crossing. However, it can be related to the excitation variation mechanisms of the local forcing. The numerical results are largely supported by the experimental vibration data, measured by means of blade tip timing. This confirms that significant differences in the aerodynamic excitation, sometimes at a factor of more than two, are present due to a change of operating conditions as well as between the mistuned response of the blades at a single resonance crossing.
Open Access
A Lanczos-filtered harmonic balance method for aeroelastic applications of turbomachinery resolving unsteady turbulence effects
(2022) Heners, Jan Philipp; Vogt, Damian (Prof. Tekn. Dr.)
Open Access
Numerical analysis of loss and performance optimization efforts for LP steam turbine exhaust hoods
(2024) Munyoki, Dickson; Vogt, Damian (Prof. Tekn. Dr.)
Most of the world’s power is produced by large steam turbines using fossil fuel, nuclear and geothermal energy. The LP exhaust hoods of these turbines are known to contribute significantly to the losses within the turbine, hence a minor improvement in their performance, which results in a lower back-pressure and hence higher enthalpy drop for the steam turbine, will give a considerable benefit in terms of fuel efficiency. This thesis is divided in two parts: In the first part, a detailed numerical analysis of sources of loss in LP exhaust hoods is carried out. The methodology used in doing this starts with a well known approach from literature where the diffuser inflow is divided into sectors and the streamlines originating from these sectors are used for flow field visualization. In the new approach, this existing procedure is developed further such that the flow properties of the various streamlines are analysed at predetermined evaluation surfaces within the flow domain. In so doing, this more advanced methodology shows clearly where most losses occur within the flow domain and makes it easier to make decisions for improving the exhaust hood performance. Using this approach, most losses are found to occur at the upper hood and are associated with the swirling flows resulting from the difficulty experienced by the flow in turning towards the condenser. At the lower side of the diffuser, the initial flow direction is more or less towards the condenser hence these flows contribute less to exhaust hood losses. The numerical results of the reference configuration based on a scaled axial-radial diffuser test rig operated by ITSM are thoroughly validated at both the design and overload operating conditions and at three tip jet Mach numbers (0, 0.4 and 1.2). The second part of the thesis focuses on possible modifications of LP exhaust hoods to achieve better performance. Having identified that most losses occur at the upper hood and the reason for it well understood, the influence of changing the hood height above the diffuser is extensively investigated at design load. It is found that the hood height has huge impact on performance and that an optimum hood exists for a given tip jet Mach number. Deflector configurations at the upper hood are also investigated. They are found to redirect the flow at the upper hood and minimise the intensity of the swirling flows hence leading to improvement in performance of LP steam turbine exhaust hoods. The best performing deflector configuration, the double wall deflector, is found to give a considerable improvement in performance amounting to 20% at design load and 40% at overload both at tip jet Mach number of 0.4 (corresponding to shrouded last stage blades).
Open Access
Vorhersage strömungsinduzierter Turbinenschaufelschwingungen und Beeinflussung dieser durch Maßnahmen am Turbinengehäuse
(2020) Netzhammer, Stephan; Vogt, Damian (Prof. Tekn. Dr.)
Die vorliegende Forschungsarbeit befasst sich mit der Beeinflussung von aerodynamisch induzierten resonanten Turbinenschaufelschwingungen an der Radialturbine eines Abgasturboladers. Die Anregung der Schaufelschwingungen ist auf die Asymmetrie des turbineneintrittseitigen Strömungsfeldes zurückzuführen, welche im Fall der untersuchten leitgitterlosen doppelflutigen Spiralgeometrie hauptsächlich durch die Gehäusezungen verursacht wird. Kennzeichnend für die Anregung sind niedrige drehzahlsynchrone Erregerordnungen, welche im vorliegenden Falle durch Resonanzanregung der ersten Schaufelbiegemode zu hohen Schwingungsbelastungen führen. Ziel der vorliegenden Arbeit ist es, unterschiedliche Maßnahmen zu erarbeiten, die zur Reduzierung der Schwingungsbelastung eingesetzt werden können. Hierfür wird in einem ersten Schritt eine Simulationsmethodik erarbeitet, welche das Anregungsfeld mit Hilfe instationärer Strömungsfeldberechnungen ermittelt. Das für die Schwingungsanregung benötigte modale Strukturverhalten wird mittels FE-Simulation bestimmt. Als Bewertungskriterium für die Intensität der Schwingungsanregung wird die als generalisierte Kraft bezeichnete Berechnungsgröße verwendet, welche nach einer Validierung mittels Messdaten als relatives Maß für die Schwingungsamplitude verwendet werden kann. Die Arbeit ist unterteilt in zwei thematische Schwerpunkte, wobei sich der erste Teil mit der Beeinflussung der Schaufelschwingungen durch die Gehäusegeometrie, insbesondere des sogenannten Zungenbereichs befasst. Hierbei kann gezeigt werden, dass durch den Versatz der beiden Gehäusezungen in Umfangsrichtung die Schaufelschwingungen effektiver reduziert werden können als durch die Vergrößerung des Zungenabstandes zum Rotor. Durch eine optimale Nutzung des Zungenversatzes von 30 kann die Schwingungsamplitude experimentell um 75% reduziert werden, was sich mit der Vorhersage der Berechnung deckt. Der Turbinenwirkungsgrad bleibt beim Vergleich mit dem Basisdesign auf gleichem Niveau. Im zweiten Themenschwerpunkt werden Turbinengehäusemaßnahmen untersucht, mit welchen das Strömungsfeld lokal verändert werden kann, um das Schaufelschwingungsverhalten zu reduzieren, ohne die eintrittsseitige Gehäusegeometrie zu verändern. Mehrheitlich werden hierbei aktive Zusatzmaßnahmen in Form einer Einblasdüse (?1,5mm) untersucht, welche am Turbinenaustritt einen Sekundärmassenstrom in den Laufschaufelbereich injiziert, der etwa 0,5% des Hauptpassagenstroms entspricht. Mit Hilfe dieses Konzeptes kann eine Schwingungsreduzierung um bis zu 70% erreicht werden wobei kein messbarer Einfluss auf den Wirkungsgrad festgestellt wird. Das Simulationsmodell kann hierbei für die Identifikation der besten Anordnung verwendet werden. Neben der Reduktion kann mittels aktiver Maßnahmen ebenfalls eine kontrollierte Erhöhung der Schwingungsamplituden erreicht werden, was für Untersuchungen der Betriebsfestigkeit eingesetzt werden kann. In einem letzten Schritt werden passive Zusatzmaßnahmen untersucht, welche in Form von Kavitäten in der Turbinengehäusewandung dargestellt sind. Experimentelle Untersuchungen von insgesamt 14 verschiedenen Maßnahmen zeigen ein Reduktionspotenzial von bis zu 50% bei einer zu vernachlässigenden Wirkungsgradreduktion von unter 0,3 %. Im Rahmen der Arbeit kann gezeigt werden, dass eine zuverlässige Berechnung von Schaufelschwingungen möglich ist. Durch die aufgezeigte Möglichkeit Schaufelschwingungen gezielt zu beeinflussen, entstehen neue Wege zur Optimierung von Turbomaschinen.