https://cronfa.swan.ac.uk/Record/cronfa38838/Download/0038838-22022018084913.pdf

Effect of gravity-induced asymmetry on the nonlinear vibration of an overhung rotor

In modern turbomachine designs, the nominal clearances between rotating bladed-disks and their surrounding casing are reduced to improve aerodynamic performances of the engine. This clearance reduction increases the risk of contacts between components and may lead to hazardous interaction phenomena. A common technical solution to mitigate such interactions consists in the deposition of an abradable coating along the casing inner surface. This enhances the engine efficiency while ensuring operational safety. However, contact interactions between blade-tips and an abradable layer may yield unexpected wear removal phenomena. The aim of this work is to investigate the numerical modeling of thermal effects within the abradable layer during contact interactions and compare it with experimental data. A dedicated thermal finite element mesh is employed. At each time step, a weak thermo-mechanical coupling is assumed: thermal effects affect the mechanics of the system, but the mechanical deformation of the elements has no effect on temperatures. Weak coupling is well appropriated in the case of rapid dynamics using small time step and explicit resolution schemes. Moreover, only heat transfer by conduction is considered in this work. To reduce computational times, a coarser spatial discretization is used for the thermal mesh comparing to the mechanical one. The time step used to compute the temperature evolution is larger than the one used for the mechanical iterations since the time constant of thermal effect is larger than contact events. The proposed numerical modeling strategy is applied on an industrial blade to analyze the impact of thermal effects on the blade's dynamics.

/pdf/thermo-mechanical-modeling-of-abradable-coating-wear-in-1a9yqcu007.pdf

Thermo-Mechanical Modeling of Abradable Coating Wear in Aircraft Engines

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.

https://tel.archives-ouvertes.fr/tel-01259136/document

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

The common technique frequently employed in dynamic stress analysis of vibration systems is time-consuming and difficult to solve when dealing with the dynamic systems with nonlinear contacts, such as turbine blades subjected to rubbing faults. To address this deficiency, an approach combining the incremental harmonic balance (IHB) method with the finite element method (FEM) was proposed for predicting the dynamic stress of rubbing blades. First, the finite element model of a warp three-dimensional (3D) entity blade was developed, and its dynamic equation was established by considering the effect of centrifugal force under high speed revolution. Then, as the rubbing fault is highly nonlinear and strong coupling, the IHB method was applied to solve the periodic solutions of the system, and the deformation of each node was obtained. Third, taking the deformation response obtained as initial displacement constraints and imposing it on relevant points, the dynamic stresses were then obtained by using the static analysis in ANSYS. Since employing the IHB method, the transient nonlinear finite element analysis was transformed into the static analysis in the present method, so it significantly raised the solution efficiency. To show the effectiveness of the method, dynamic stress prediction and parameter analysis of a rubbing blade were studied as an example.

/pdf/dynamic-stress-prediction-method-for-rubbing-blades-27m6i6hbgj.pdf

Dynamic stress prediction method for rubbing blades

In the current design of turbomachines, engine performance is improved by reducing the clearances between the rotating components and the stator, which allows for loss decrease. Due to these clearance reductions, contact events may occur between the rotor and the stator. An abradable coating is deposited along the stator circumference as a sacrificial material to lower the contact severity. However, experiments highlighted the occurrence of rotor/stator interactions with high wear depth on the abradable coating as well as high temperature increases within the abradable coating following contacts. This work focuses on the sensitivity analysis of rotor/stator interactions with respect to the rotor angular speed and the initial clearances between the rotor and the stator, taking into account thermal effects within the abradable coating. Convergence analyses are first conducted to validate the numerical model. Then, after a calibration of the thermal model of the abradable coating based on two experimental test cases, the numerical model is used to investigate the cross effects of the angular speed and the initial clearances on the obtained rotor/stator interactions.

https://hal.archives-ouvertes.fr/hal-02454895/document

Sensitivity Analysis of Rotor/Stator Interactions Accounting for Wear and Thermal Effects within Low- and High-Pressure Compressor Stages

Rotor dynamic behavior depends considerably on how much the specific physical phenomena accompanying rotor rubbing against the stator is involved The experimental results of rotor-to-stator rubbing contact are analyzed The computer code is described for obtaining numerical calculations of rotor-to-stator rubbing system dynamic responses Computer generated results are provided The reduced dynamic data from High Pressure Fuel Turbo Pump (HPFTP) hot fire test are given The results provide some significant conclusions Information is provided on the electronic instrumentation used in the experimental testing

Influence of rubbing on rotor dynamics, part 2

The results of analytical and experimental research on rotor-to-stationary element rubbing in rotating machines are presented. A characterization of physical phenomena associated with rubbing, as well as a literature survey on the subject of rub is given. The experimental results were obtained from two rubbing rotor rigs: one, which dynamically simulates the space shuttle main engine high pressure fuel turbopump (HPFTP), and the second one, much simpler, a two-mode rotor rig, designed for more generic studies on rotor-to-stator rubbing. Two areas were studied: generic rotor-to-stator rub-related dynamic phenomena affecting rotating machine behavior and applications to the space shuttle HPFTP. An outline of application of dynamic stiffness methodology for identification of rotor/bearing system modal parameters is given. The mathematical model of rotor/bearing/seal system under rub condition is given. The computer program was developed to calculate rotor responses. Compared with experimental results the computed results prove an adequacy of the model.

Influence of rubbing on rotor dynamics, part 1

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Influence of rubbing on rotor dynamics, part 2

Influence of rubbing on rotor dynamics, part 1