Multi-row Aeromechanical and Aeroelastic Aspects of Embedded Gas Turbine Compressor Rotors

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2021

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This research helps address one of the grand challenges of turbomachinery i.e., the accurate prediction of the forced response in multi-row compressors subjected to various crossings and operating points. Specifically, this focuses on understanding the impact of multi-row interaction on the unsteady aerodynamics and mistuned forced response behavior of a subsonic axial compressor. The phenomena of forced response remain one of the most challenging areas of turbomachinery aeromechanics. This thesis helps address some of the shortcomings in current literature related to unsteady aerodynamics and mistuned forced response predictions. The flow is inherently unsteady due to the complex flow field, blade row interactions, and secondary flows. Predicting the forced response behavior is a challenging task. Blade failures due to aeromechanical problems have resulted in fatalities and severe engine/aircraft damage, with some of the recent incidents being on Air France Flight 66 and Southwest Airlines Flight 1380. The experimental compressor studied herein is the Purdue 3.5 stage compressor, representing the rear stages of a modern high-pressure compressor (HPC). The focus of this research is on the vibratory response of rotor 2 (R2). One interesting feature of this configuration is that three rows have the same vane count i.e., the inlet guide vanes (IGV), stator 1, and stator 2. All contribute to the forcing function simultaneously. Also, the difference in blade count between the embedded R2 and the other rotors is the same. Computational data obtained using a commercial computational fluid dynamics (CFD) code, CFX, and an in-house mistuning response code MISER are compared against experimental data to understand the physical phenomena, determine the predictions' accuracy, and develop methods to improve predictions further. The first part of this research presents results from the torsional mode (1T) and a higher-order mode (1CWB) for the case where the stator count (44) of both neighboring stators is the same. Since both contribute to the forcing simultaneously, wake and potential field effects cannot be easily distinguished. The impact of physical wave reflection from downstream (Rotor 3) and the upstream influence from the IGV is also determined. The influence of operating conditions on the forcing function is also investigated. This is further fed into an in-house mistuning code, which predicts the response of all blades. The computational results are compared with experimental data. Finally, the effect of sideband traveling wave excitations (both amplitude and phase) on the blade response prediction was determined. The second part of the thesis deals with the study extended to a more realistic case in which the stator count of the embedded stators is different. Since the upstream and downstream influences are at different frequencies, we can separate the effects. This creates two torsional mode crossings (1T/44 and 1T/38) at different rotational speeds. Once again, the impact of operating conditions on the forced response behavior and the individual blade responses are determined. Further, this research contributes to the future development of model reduction methods and quantifies the error induced by utilizing model reduction techniques under different circumstances. The third section of the thesis deals with a configuration in which the stator is asymmetric i.e., has a different stator count on either side of the “split line.” The idea of having an asymmetric configuration originated in a NASA report [45] but has received little attention in the literature. Although current literature provides an insight into the steady aerodynamic performance of such configurations, no work to date explains the complex unsteady blade row interactions occurring in such configurations. This research describes the forcing function reduction phenomena due to asymmetry, provides general guidance on modeling techniques for such cases, and investigates possible scenarios and outcomes. The thesis then dives into determining the impact of stator hub cavities on the forced response prediction. Currently, the research on stator hub cavities only involves determining their influence on steady aerodynamics. The current work helps fill up the gap in the literature by determining its influence on unsteady aerodynamics and mistuned blade predictions. The fourth section discusses the impact of hub cavities on the steady flow in multiple locations around the blade passage and the impact of hub cavity flow on the unsteady aerodynamics, which determines the magnitude of the forcing function. The last chapter of the thesis quantifies the individual blade responses for all multi-row cases described in the previous sections. This section also discusses the impact of veering region modes and mode localization on the mistuned response prediction. The idea of perturbing the system mode frequency in a probabilistic manner was introduced in this thesis for the first time. Physical responses and dependencies have never been seen in the literature. The concept of strain energy-based mistuning models was expanded. For the first time in two decades, two new mistuning models were introduced, which were developed under the framework of the FMM. Also, the idea of perturbing structural damping in a probabilistic manner was introduced for the first time in this thesis. This thesis contributes extensively to understanding the various steady and unsteady aerodynamic interactions of multi-row configurations and some of the key findings are: 1. The impact of a downstream rotor (R3) cannot be neglected in forced response computations. The modal force prediction was within 10% accuracy, which was achieved by adding the downstream row. 2. The work also highlights the significance of having a downstream row that does not contribute to the forcing function at the same frequency but acts as a wall to reflect waves, contributing to the forcing. 3. The impact of spurious wave reflections on the forcing function was also quantified. In the absence of non-reflecting boundary conditions, these spurious waves can have a tremendous influence on the forcing function 4. The fidelity of model reductions techniques, particularly the time transformation method, is highlighted. This can not only serve as a guiding tool for the development of methods in the future but also reduce computational time significantly (3-4X reduction) 5. The impact of having an asymmetric stator exciting an embedded rotor was determined at multiple operating conditions. The benefit of asymmetry was limited to how the asymmetric stator excited the embedded rotor and not when any other rows excited the rotor. The asymmetry also results in the creation of sideband excitation responses, the magnitude of which is comparable to the dominant response. Also, the influence of stator hub cavities on the unsteady aerodynamic flow field was quantified, and the modal force prediction was found to improve by 10% for a 3-row case 6. Finally, the mistuned blade response was predicted using the modal forces obtained earlier, system modes obtained computationally, and blade frequencies obtained experimentally. This work contains several new insights into mistuned predictions. The mistuning work described here provides guidance, including sideband traveling wave excitations in the mistuning model. The thesis also introduced the concept of system mode and structural damping perturbations in a probabilistic manner, and the result was found to be deterministic. Several new plotting methods were introduced to represent data in a novel manner. Two new high fidelity strain energy-based mistuning models helped improve the blade response prediction and provided the most accurate date under the FMM framework. This work guides mistuning computations, including the effect of sideband excitations on mistuning parameters.

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hegde, shreyas (2021). Multi-row Aeromechanical and Aeroelastic Aspects of Embedded Gas Turbine Compressor Rotors. Dissertation, Duke University. Retrieved from https://hdl.handle.net/10161/24353.

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