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Modeling of shaft precessional motions induced by unilateral and frictional blade/casing contacts in aircraft engines

Abstract : In modern aircraft turbomachinery, undesirable unilateral and frictional contact occurrences between rotating and stationary components, favored by always tighter blade-tip clearances, are expected to arise during standard operating conditions. It has been shown that potentially harmful interactions may threaten the engine structural integrity. In particular, among various interaction mechanisms, shaft whirl motions may be a major ingredient in the blade-to-casing structural interactions at the fan stage. The present work targets the appropriate modeling of such shaft precessional motions and contact dynamics in order to enhance existing predictive numerical solution methods. A qualitative study is first presented, employing a simplified two-dimensional in-plane finite element model representative of a bladed-disk/casing system, accounting for the shaft flexibility as well as the structural coupling provided by the fan frame and bearings. A previously established time-marching strategy is implemented to solve the equations of motion and unilateral contact constraints are enforced via Lagrange multiplier method, allowing to exactly satisfy non-penetration conditions. The interactions between these flexible structures are initiated via two distinct mechanisms: (1) a prescribed casing distortion and (2) a mass imbalance on the bladed-disk. The shaft dynamics proved to have a key role in producing potentially harmful regimes, in particular in the former scenario, which can lead to divergent modal interactions. Further, a novel modal analysis of a fully coupled industrial bladed-disk/casing model is proposed in a three-dimensional cyclic-symmetric framework. This analysis considers the entire stage (rotating shaft+bladed-disk assembly+casing+frame+bearings) as a single global structural component. Parametric instabilities are revealed and critical rotational velocities emanate from the linear modal coincidence interaction speeds. As these fully coupled models exclude reasonable computational time in a nonlinear framework, the casing is then assumed rigid in a time-domain exploratory investigation of bladed-disk/abradable interactions. Abradable coatings are disposed along the casing circumference in order to mitigate direct structural contacts between rotating blades and the surrounding stator while allowing for a self-tuning of operating blade-tip clearances. They should be sufficiently resilient to endure severe thermal conditions and hostile constraints but also adequately soft not to alter the structural integrity of the blades. However, divergent interactions are found within the engine operating speed range, characterized by high-amplitude backward whirling motions which could result in catastrophic structural failures. A time-stepping prediction-correction procedure is implemented part of which the wear of the abradable layer is accounted through a plastic constitutive law. A sensitivity analysis of the detected interactions to the shaft dynamics and associated gyroscopic terms shows that divergent backward whirling motions may emerge within the targeted speed range, generating high-stress levels on the bladed-disk/shaft assembly and significant material removal over the entire casing circumference. It is also shown that all the blades should be included in the modeling as simplified systems (single blade or contact handled on one sector) would lead to incorrect predictions. An alternative method is developed based on the analogy between the abradable coating wear caused by the incursion of the blades and the material removal process in traditional machining operations. A set of Delay Differential Equations (DDE) is derived through an explicit yet simplified expression of the contact forces. The stability of the equilibrium solution of the DDE is then assessed through the semi-discretization method. An industrial compressor blade is employed to demonstrate the advantages of the proposed approach which features reduced computational costs and consistency with existing time-marching solution methods. Potentially dangerous interaction regimes are accurately predicted and instability lobes match both the flexural and torsional modal responses. A comparison of exploratory blade profiles illustrates how engine manufacturers may benefit from the presented methodology; in this last study, shaft motions should be ignored.
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https://tel.archives-ouvertes.fr/tel-01259136
Contributor : Nicolas Salvat Connect in order to contact the contributor
Submitted on : Tuesday, January 19, 2016 - 9:41:43 PM
Last modification on : Thursday, June 18, 2020 - 12:32:05 PM
Long-term archiving on: : Friday, November 11, 2016 - 1:21:42 PM

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Distributed under a Creative Commons Attribution - NonCommercial - ShareAlike 4.0 International License

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  • HAL Id : tel-01259136, version 1

Citation

Nicolas Salvat. Modeling of shaft precessional motions induced by unilateral and frictional blade/casing contacts in aircraft engines. Vibrations [physics.class-ph]. McGill University, 2015. English. ⟨tel-01259136⟩

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