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Institute
BTU
Modal and Aeroelastic Analysis of a Mistuned Compressor Blisk Using an Equivalent Blisk Model
(2011)
Simulation and Investigation of an Intentionally Mistuned Blisk Rotor in a High Pressure Compressor
(2022)
In modern aircraft engines, blade integrated disk (blisk) is widely implemented. While blisk rotor design brings numerous advantages including weight reduction, aerodynamic efficiency improvement, and manufacturing simplification, its low mechanical damping due to the absence of friction between disk and blades makes the rotor more susceptible to vibration. Given that damage to blisk rotor sometimes requires the whole assembly to be replaced, effort has been made to alleviate the unexpected vibration amplitude within operating range, among which intentional mistuning is regarded as one of the commonly used technique. Mistuning refers to blade-to-blade deviation of mechanical properties, which is inevitable in practice due to manufacturing tolerances or wear. Through the application of intentional mistuning, it is expected that the amplitude of synchronous or nonsynchronous vibration (NSV) will be reduced without severely losing aerodynamic performance.
In this paper, the effect of intentional mistuning has been investigated for the blisk rotor of a 1.5-stage transonic research compressor at Technical University of Darmstadt. According to the previous test campaign, the baseline rotor has shown its susceptibility to NSV due to first torsion mode in the near stall region. The rotor was then intentionally mistuned. Subsequent tests have proven a successful suppression of flutter problem. In order to have a comprehensive understanding of the effect of the applied mistuning pattern, simulations are performed using a FVM based CFD solver to produce comparable results as shown in the test campaign. In the simulation, mistuned systems are modelled in comparison with the nominal tuned reference. Geometrical disturbance and frequency disturbance are introduced to the tuned model first separately and then simultaneously. In this way, contribution of aerodynamic and structural mistuning to the suppression of NSV is identified based on the CFD results. Later, system eigenvalues of the mistuned aeromechanical model are determined by making use of the blade individual response in time domain. The obtained results are compared with mistuned eigenvalues calculated by a reduced order model (ROM), which utilizes the idea of subset of nominal modes (SNM). This makes it possible to demonstrate the feasibility of using SNM to carry out stability analysis when designing mistuning pattern for vibration of NSV type. It also allows a compare between the linear structural model of the SNM and the non-linear aeromechanic model of the CFD solver on capturing the non-linear nature of the flow, especially in the context of NSV.
Application of Intentional Mistuning to Reduce the Vibration Susceptibility of a Steam Turbine Wheel
(2022)
Intentional mistuning (IM) is employed on a last stage turbine wheel to alleviate both the flutter susceptibility and maximum forced response. Primarily, operations at nominal speed under part-load conditions may cause unfavorable flow conditions facilitating flow separation. As a consequence, the original design intention with identical blades features negative aerodynamic damping ratios with respect to the first bending mode family. In order to prevent any self-excited vibration phenomena, intentional alternate mistuning is utilized to increase the least aerodynamic damping ratio as far as it takes a positive value and hence, to contribute to a stabilization of the rotor. For the purpose of numerically analyzing the vibration behavior, reduced order models are built up, which are based on modal reduction techniques, namely the subset of nominal system modes (SNM) [1] and the fundamental mistuning model (FMM) [2]. These types of models conveniently allow for considering both, different mistuning distributions in terms of probabilistic analyses and the aeroelastic interaction by means of prescribing aerodynamic damping ratios and aeroelastic natural frequencies of the tuned counterpart or aerodynamic influence coefficients, respectively. A detailed study is presented regarding the correction of frequency mistuning magnitudes in terms of considering the impact of centrifugal stiffening, which plays a significant role in case of long low pressure turbine blades featuring high aspect ratios.
Since alternate IM cannot be implemented perfectly, every bladed wheel as manufactured will exhibit small but unavoidable structural deviations from the design intention, which are known as random mistuning. To ensure the robustness of the IM solution in terms of positive aerodynamic damping ratios at any time, comprehensive probabilistic analyses are conducted with respect to superimposing random structural mistuning at first. Secondly, the impact of varying mistuning magnitude is analyzed. Thirdly, the robustness towards aerodynamic mistuning is investigated by means of small variations of aeroelastic influence coefficients and consequently, the inter blade phase angle dependent aerodynamic damping curves. Moreover, it becomes apparent that alternate IM superimposed with both, random structural and aerodynamic mistuning also mitigates the maximum forced response at part-speed conditions.
Radial turbine wheels of exhaust gas turbochargers are permanently exposed to centrifugal, thermal, and aerodynamic loading. However, since these wheels are commonly designed as integral structures featuring relatively little mechanical damping, they are prone to the impact of unavoidable structural random mistuning, which may evoke severe magnifications of the forced response. Nonetheless, the safe operation of turbochargers has to be ensured at any time so that the contribution of aerodynamic damping is of particular importance. Moreover, the application of intentional mistuning is known to be a suitable measure to limit or even reduce the forced response by means of increasing the resulting aerodynamic damping.
In this paper, two turbine wheels of the same type are considered, one manufactured with and another one without intentional mistuning. Experimental determinations of the mistuning patterns actually reveal deviations from the design intentions, which are considered in updated numerical models. Forced response simulations demonstrate that the targeted response reduction affected by intentional mistuning is achieved anyhow. Furthermore, the general robustness of the solution is proved with respect to the maximum forced response by means of comprehensive probabilistic numerical analyses addressing the impact of additional random structural mistuning, the magnitude of intentional mistuning, and aerodynamic mistuning.
Grundlegende Untersuchungen zum Schwingungsverhalten von Verdichterlaufrädern in Integralbauweise
(2009)
The manufacturing geometrical variability is a source of uncertainty, which cannot be avoided in the realization of machinery components. Deviations of a part geometry from its nominal design are inevitably present due to the manufacturing process. In the case of the aeroelastic forced response problem within axial compressors, these uncertainties may affect the vibration characteristics. For this reason, the impact of geometrical uncertainties due to the manufacturing process onto the modal forcing of axial compressor blades is investigated in this study. The research focuses on the vibrational behavior of an axial compressor rotor blisk. In particular, the amplitude of the forces acting as a source of excitation on the vibrating blades is studied. The geometrical variability of the upstream stator is investigated as input uncertainty. The variability is modeled starting from a series of optical surface scans. A stochastic model is created to represent the measured manufacturing geometrical deviations from the nominal model. A data reduction methodology is proposed in order to represent the uncertainty with a minimal set of variables. The manufacturing geometrical variability model allows to represent the input uncertainty and probabilistically evaluate its impact on the aeroelastic problem. An uncertainty quantification is performed in order to evaluate the resulting variability on the modal forcing acting on the vibrating rotor blades. Of particular interest is the possible rise of low engine orders due to the mistuned flow field along the annulus. A reconstruction algorithm allows the representation of the variability during one rotor revolution. The uncertainty on low harmonics of the modal rotor forcing can be therefore identified and quantified.