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- Gao, Yang, 1988-
(author)
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Aerodynamic Design and Aeromechanical Analysis of Mixed and Radial Flow Turbines : A study on meanline method, stator tilting endwall design and forced response analysis
- 2021
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Doctoral thesis (other academic/artistic)abstract
- In this energy transition era, turbocharging is still an important technology for the automotive industry to reduce fuel consumption and lower emissions in its vehicles. This importance can be seen from both conventional fossil-fuel powertrains, and emerging applications, such as increased utilization of biofuels along with hydrogen fuel cells. For automotive turbochargers, the turbine has mainly two alternative types, i.e., mixed flow turbines (MFTs) and radial flow turbines (RFTs). These devices are mature and commercially available yet still have significant potential for improvement towards ensuring higher performance, more robust operation, and lower cost. With this in mind the overall aim of this study is to improve the aerodynamic design and the aeromechanical analysis methods for MFTs and RFTs. Specifically, the investigation covers three research topics: meanline method, stator tilting endwall design, and forced response analysis. A meanline method tool is newly developed to predict the performance curves. For RFTs, the results present a generally good agreement between the predicted performance and experimental data. However, for MFTs, two limitations of loss models used in the meanline method have been identified: spanwise variation of incidence at the rotor inlet is neglected; and performance variations at different speeds cannot be captured by the investigated passage loss models. To overcome the first limitation, a multi-section incidence loss model is proposed. For the second limitation, more research work is suggested to investigate the effect of mixed-flow features at the MFT rotor inlet. As a contribution to investigate the mixed-flow feature at the MFT rotor inlet, different stator tilting endwall designs are numerically evaluated with computational fluid dynamics (CFD) tools. An MFT with well-documented experimental data is selected as the baseline and used to validate the CFD method. Performance improvement has been seen from those designs with a sharp turning on the shroud-side endwall just before the rotor leading edge. The optimal design in this study has a -45° tilting angle of the shroud-side stator endwall. It achieves approximate 1%-point higher efficiency than the baseline design over the 100% and 50% speed lines. Detailed aerodynamic analyses of the internal flow field contribute to the understanding of the performance change. After the aerodynamic design, aeromechanical analyses are necessary steps to achieve the mechanical robustness. In this part of the study the accuracy and the computational cost of different CFD methods are compared in the forced response analysis of an open-geometry RFT using three CFD methods – full annular, phase-lag, and non-linear harmonic (NLH)– for forcing predictions paired with time-domain and harmonic balance (HB) methods for the aero-damping predictions. It is found that for the stator-induced forcing, all three CFD methods predict the same pattern of forcing distribution. Taking the full annular method as the reference, the maximum blade displacement predicted by the other two methods has less than 15% deviation. However, for the volute-induced forcing, the NLH method is excluded due to increased computational cost. The phase-lag method predicts a distinct forcing distribution to the reference full annular method, leading to approximate 50% difference of the maximum blade displacement. When predicting the aero-damping, the reference time-domain and the HB methods predict similar log decrement values with less than 4.6% deviation. In terms of computational effort, the harmonic methods, namely NLH and HB, reduce the effort by a factor of 42 and 6 respectively for the forcing and aero-damping predictions compared with the reference method.
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| 2. |
- Fruth, Florian, 1981-
(author)
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Reduction of Aerodynamic Forcing inTransonic Turbomachinery : Numerical Studies on Forcing Reduction Techniques
- 2013
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Doctoral thesis (other academic/artistic)abstract
- Due to more and more aggressive designs in turbomachinery, assuring the structural integrity of its components has become challenging. Also influenced by this trend is blade design, where lighter and slimmer blades, in combination with higher loading, lead to an increased risk of failure, e.g. in the form of blade vibration. Methods have been proposed to reduce vibration amplitudes for subsonic engines, but cannot directly be applied to transonic regimes due to the additional physical phenomena involved. Therefore the present work investigates numerically the influence of two methods for reducing blade vibration amplitudes in transonic turbomachines, namely varying the blade count ratio and clocking. As it is known that clocking affects the efficiency, the concurrent effects on vibration amplitudes and efficiency are analyzed and discussed in detail.For the computational investigations, the proprietary Fortran-based non-linear, viscous 3D-CFD solver VolSol is applied on two transonic compressor cases and one transonic turbine case. In order to reduce calculation time and to generate the different blade count ratios a scaling technique is applied.The first and main part of this work focuses on the influence of the reduction techniques on aerodynamic forcing. Both the change in blade count ratio and clocking position are found to have significant potential for reducing aerodynamic force amplitudes. Manipulation of the phasing of excitation sources is found herein to be a major contributor to the amplitude variation. The lowest stimulus results are achieved for de-phased excitation sources and results in multiple blade force peaks per blade passing. In the case of blade count ratio variation it was found that blockage for high blade count ratios and the change in potential field size have significant impacts on the blade forcing. For the clocking investigation, three additional operating points and blade count ratios are analyzed and prove to have an impact on the force reduction achievable by clocking.The second part of the work evaluates the influence of clocking on the efficiency of a transonic compressor. It is found that the efficiency can be increased, but the magnitude of the change and the optimal wake impingement location depend on the operating point. Moreover it is shown that optimal efficiency and aerodynamic forcing settings are not directly related. In order to approximate the range of changes of both parameters, an ellipse approximation is suggested.
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| 3. |
- Glodic, Nenad, 1980-
(author)
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Sensitivity of Aeroelastic Properties of an Oscillating LPT Cascade
- 2013
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Licentiate thesis (other academic/artistic)abstract
- Modern turbomachinery design is characterized by a tendency towards thinner, lighter and highly loaded blades, which in turn gives rise to increased sensitivity to flow induced vibration such as flutter. Flutter is a self-excited and self-sustained instability phenomenon that may lead to structural failure due to High Cycle Fatigue (HCF) or material overload. In order to be able to predict potential flutter situations, it is necessary to accurately assess the unsteady aerodynamics during flutter and to understand the physics behind its driving mechanisms. Current numerical tools used for predicting unsteady aerodynamics of vibrating turbomachinery components are capable of modeling the flow field at high level of detail, but may fail in predicting the correct unsteady aerodynamics under certain conditions. Continuous validation of numerical models against experimental data therefore plays significant role in improving the prediction accuracy and reliability of the models. In flutter investigations, it is common to consider aerodynamically symmetric (tuned) setups. Due to manufacturing tolerances, assembly inaccuracies as well as in-service wear, the aerodynamic properties in a blade row may become asymmetric. Such asymmetries can be observed both in terms of steady as well as unsteady aerodynamic properties, and it is of great interest to understand the effects this may have on the aeroelastic stability of the system. Under certain conditions vibratory modes of realistic blade profiles tend to be coupled i.e. the contents of a given mode of vibration include displacements perpendicular and parallel to the chord as well as torsion of the profile. Current design trends for compressor blades that are resulting in low aspect ratio blades potentially reduce the frequency spacing between certain modes (i.e. 2F & 1T). Combined modes are also likely to occur in case of the vibration of a bladed disk with a comparatively soft disk and rigid blades or due to tying blades together in sectors (e.g. in turbines). The present investigation focuses on two areas that are of importance for improving the understanding of aeroelastic behavior of oscillating blade rows. Firstly, aeroelastic properties of combined mode shapes in an oscillating Low Pressure Turbine (LPT) cascade were studied and validity of the mode superposition principle was assessed. Secondly, the effects of aerodynamic mistuning on the aeroelastic properties of the cascade were addressed. The aerodynamic mistuning considered here is caused by blade-to-blade stagger angle variations The work has been carried out as compound experimental and numerical investigation, where numerical results are validated against test data. On the experimental side a test facility comprising an annular sector of seven free-standing LPT blades is used. The aeroelastic response phenomena were studied in the influence coefficient domain where one of the blades is made to oscillate in three-dimensional pure or combined modes, while the unsteady blade surface pressure is acquired on the oscillating blade itself and on the non-oscillating neighbor blades. On the numerical side, a series of numerical simulations were carried out using a commercial CFD code on a full-scale time-marching 3D viscous model. In accordance with the experimental part the simulations are performed using the influence coefficient approach, with only one blade oscillating. The results of combined modes studies suggest the validity of combining the aeroelastic properties of two modes over the investigated range of operating parameters. Quality parameters, indicating differences in mean absolute and imaginary values of the unsteady response between combined mode data and superposed data, feature values that are well below measurement accuracy of the setup. The findings of aerodynamic mistuning investigations indicate that the effect of de-staggering a single blade on steady aerodynamics in the cascade seem to be predominantly an effect of the change in passage throat. The changes in steady aerodynamics are thereby observed on the unsteady aerodynamics where distinctive effects on flow velocity lead to changes in the local unsteady pressure coefficients. In order to assess the overall aeroelastic stability of a randomly mistuned blade row, a Reduced Order Model (ROM) model is introduced, allowing for probabilistic analyses. From the analyses, an effect of destabilization due to aero-asymmetries was observed. However the observed effect was of moderate magnitude.
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