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Institute
BTU
Reduced Order Analyses of Multi-stage Coupled Structures with Main Focus on Disk-Dominated Modes
(2016)
Rotors manufactured as blisk (Blade Integrated Disk) has become state-of-the-art in turbomachinery. This integral design saves a lot of mass and enables higher rotational speeds allowing for higher pressure ratios and hence an increased efficiency. The advantage comes along with the drawback that the structural damping level is extremely low. Nowadays, the dynamics of single-stage rotors is well-investigated, as dynamical analysis of cyclic structures is one of the most important subjects in applied research in turbomachinery. However, the stage-to-stage coupling effect is neglected in most cases. The importance of proper interstage treatment is obvious for adequate multi-stage analyses. Hence the structural dynamics of a multi-stage assembly has recently become an important area of research. In this paper, some multi-stage effects are discussed and three different reduced order techniques are summarized and demonstrated on a multi-stage assembly of academic blisks. The findings are compared to a FE-solution. Particular attention must be paid to disk-dominated modes, which are highly affected by multi-stage behavior. Mistuning modeling is not considered, because it mainly influences blade-dominated vibrations.
Rotors manufactured as blisk (Blade Integrated Disk) are manifoldly used in state-of-the-art turbomachinery.
Commonly, the endurance is evaluated by a numerical analysis of as designed rotor. Since small deviations due to the manufacturing cause slightly different blades, mistuning in turbomachinery is unavoidable. Mistuning causes increased vibration amplitudes - higher than those to be expected in case of the ideal design intention. Nowadays, there are various model updating procedures to ensure a more realistic modelling of blisks. Within Monte Carlo simulations, one obtains a well-approximated maximum vibration amplitude at
tolerable costs, the effect of fractional alterations of eigenmodes due to geometric imperfections is normally
neglected. Value and location of maximum stress are sensitive to geometric deviations but also decisive for an adequate calculation of the High Cycle Fatigue, which itself is one of the main causes of blisk failure.
As-manufactured impellers behave significantly different from nominal impellers. There are no identical blades due to geometric and material deviations. In this paper three model updating procedures are discussed with the objective to achieve realistic models of as-manufactured impellers. The techniques are applied to radial inflow turbine wheel of an exhaust gas turbocharger. The first approach creates a model through optical measurement and mesh morphing. The second approach is based on a contactless measurement of blade individual vibration responses. An iterative update process gains the corresponding mistuning pattern and thus the associated model. Third, a model is found by an optimisation, that identified a mistuning pattern, that fits modal characteristics, which are evaluated during experimental modal analysis at vacuum. In-depth analyses of these models are realised to determine advantages and drawbacks of the procedures.
Blade vibrations are one of the main cost drivers in turbo-machinery. Computational blade vibration analysis facilitates an enormous potential to increase the productivity in the design of bladed components. Increasing computing power as well as improved modeling and simulation methods lead to comprehensive calculation results. This allows for a more precise prediction and assessment of experimental data.
Usually, in the field of turbomachinery, identical blades are assumed to lower the required computational resources. However, mistuning is unavoidable, since small deviations due to the manufacturing process will lead to slightly different blade behavior. Potential effects such as mode localization and amplification can be treated statistically and have been thoroughly studied in the past. Since then, several reduced order models (ROMs) have been invented in order to calculate the maximum vibration amplitude of a fleet of mistuned blisks. Most commonly, mistuning is thereby modeled by small material deviations from blade to blade, e.g. Young’s modulus or density.
Nowadays, it is common knowledge that the level of manufacturing imperfection (referred as level of mistuning) significantly influence mode localization as well as vibration amplification effects. Optical measurements of the geometric deviations of manufactured blades and converting to a high-fidelity finite element model make huge progress. However, to the knowledge of the authors, there is no reliable method, that derives a characteristic quantity from the geometric mistuning, that fits into the mentioned statistically approaches.
Therefore, experimental data is needed to quantify the level of mistuning. Several approaches, which isolate blade individual parameters, are used to identify the dynamic behavior of axial compressors and turbines. These methods can be applied to medium-speed centrifugal turbine wheels but tend to fail to evaluate high-speed compressor with splitter blades. This paper briefly presents the original approach and discusses the reasons for failure. Thereafter, a new approach is proposed. Finally the level of mistuning and important quantities to perform a statistical evaluation of a high-speed compressor is shown.
Copyright © 2018 by ASME
Radiale Turbinen- und Verdichterräder bilden hinsichtlich Wirtschaftlichkeit, Effizienz und insbesondere Umweltverträglichkeit essenzielle Eckpfeiler moderner Verbrennungskraftmaschinen. Sie repräsentieren in vielfältigen technischen Anwendungen den Stand der Technik. Umweltrelevante Aspekte beeinflussen die Entwicklung der Verbrennungsmotoren und treiben somit die Entwicklung der Abgasaufladung voran. Im Allgemeinen sind hierfür besonders hohe Druckverhältnisse erforderlich, die die Struktur erheblich belasten.
Die hohen aerodynamischen Ansprüche erfordern filigrane Schaufelgeometrien. Immer dünnere Schaufeln, in Verbindung mit der geringen mechanischen Dämpfung infolge der Integralbauweise erschweren die schwingungssichere Auslegung von Turbomaschinen. Kleinste Imperfektionen, die bereits während der Fertigung entstehen, dominieren das dynamische Verhalten realer Komponenten. Die Abweichung vom ursprünglichen Design wird als Verstimmung (engl. Mistuning) bezeichnet. Die weitgehend zufällige Verstimmung reduziert die Lebensdauer der Komponenten, die durch Fliehkraft, Strömungsumlenkung, instationäre Druckschwankungen der Strömung sowie Temperaturgradienten dauerhaft hoch belastet sind.
Die vorliegende Arbeit widmet sich der Beschreibung des Strukturverhaltens radialer Laufräder. Der Fokus der Arbeit liegt auf der Beschreibung der Radialturbine des Typs MAN TCR 18. Zusätzlich werden die Erkenntnisse dieses Laufrades mit weiteren radialen Laufrädern verglichen. Auf Grundlage von Messdaten, numerischen Berechnungsmodellen und Simulationsergebnissen gelingt es schließlich einen Leitfaden zur schwingungssicheren Auslegung von radialen Turbomaschinen mit dem Fokus auf Mistuning und Dämpfung bereitzustellen.
Die Arbeit kommt zu dem Schluss, dass die erzwungene Schwingungsantwort mithilfe der gezielten Schaufelverstimmung signifikant gesenkt werden kann.
Turbolader tragen erheblich zur Steigerung des Motorenwirkungsgrads bei. Rotierende Komponenten sind infolge der Fliehkraft, der zur Aufladung notwendigen Strömungsumlenkungen, der instationären Druckschwankungen der Strömung sowie von Temperaturgradienten als hochbelastete Laufräder einzustufen, die unter erheblicher Schwingungsanfälligkeit leiden. Am Lehrstuhl Strukturmechanik und Fahrzeugschwingungen der BTU Cottbus-Senftenberg wurde im Rahmen eines FVV-Forschungsvorhabens der Einfluss der fertigungsbedingten Toleranzen auf eben jenes Schwingungsverhalten untersucht. Es wird nachgewiesen, dass Intentional Mistuning zu signifikant niedrigeren Belastungen führen kann.
The wear and damage of High-Pressure Compressor (HPC) blades due to erosion or Foreign Object Damage (FOD) have a significant influence on HPC aerodynamic performance, vibration resistance against High-Cycle Fatigue (HCF) and thus component lifetime. The changes in airfoil geometry reduce the overall engine efficiency. Furthermore extended off-wing engine maintenances due to blade failures are increasing the cost of ownership. The safe operation of every engine within a reduced number of shop visits requires a reliable prediction of future deterioration. This enables the optimization of services and off-wing time. One contribution to this is a better understanding of the component’s dynamics and based on this providing an improved wear modeling to reliably predict the remaining lifetime and the decreased efficiency.
This contribution determines the material removal of HPC blades due to sand erosion. Originally, this stage was built as a blisk (Blade Integrated Disk). After sand erosion test completion, the blisk was cut into segments containing one airfoil only. First, the material removal is determined for ten blades of one exemplary rotor. A blue light fringe projector is employed to identify the geometrical differences between the eroded blades and the nominal design. Second, realistic finite element models are generated to enable comparable modal analyses of eroded blades. This procedure suffers from unavoidable and mostly random imperfections due to the manufacturing process, which significantly affects the blade surface before the erosion test can be conducted. Therefore, an already published approach is implemented in the third step to predict the blade surface after erosion based on nominal blade design. The investigation is completed by comparing measured and predicted surfaces. Finally, the aforementioned tool is employed to predict the locations and intensities of the material losses and the accompanying change in modal properties of this compressor blade concerning operational time.