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Title: Towards reliable and efficient calibration of blade tip timing measurements against finite element model predictions
Author: Mohamed, Mohamed
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2019
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Over the last decade, Blade tip timing (BTT) has been replacing strain gauge (SG) systems for the measurement of the vibration of turbomachinery blades, reducing development times and costs of new engine programmes and enabling more cost-effective engine health monitoring systems. A BTT system comprises two parts: (a) the measurement system for blade tip arrival times; and (b) the analysis algorithms. The determination of blade stress from BTT data is accomplished through a Finite Element model (FEM) by relating the measured blade tip deflection to the stress at the failure location. However, the literature review of the present work shows that the link between BTT data and FEM predictions is not well established or standardised. Furthermore, many uncertainties are associated with the entire process of BTT technology. One of the main sources of uncertainty is the shift in the equilibrium position of the blade tip due to steady (non-oscillatory) bending and/or twisting of the blade, and axial movement of the bladed disk (blisk)-shaft system. This results in a shift in the effective measurement position of the probe relative to the blade chord, resulting in errors in the tip vibration as well as stress estimates. All uncertainties have to be quantified and controlled. However, this requires a highly experienced analyst to manage the process. Thus, there is a need for a standard method of quantifying the uncertainty of the whole of this technology and perhaps automating the process, and also to set guidelines as to how BTT processing and analysis algorithms should be applied to different scenarios. This would allow moderately skilled users to manage the process. The overall aim of this thesis is therefore to establish a methodology that enables a standardized approach towards the following issues (a) the link between BTT data and FEM; (b) the applicability of the data analysis techniques (with a view towards providing guidance); (c) uncertainties associated with these processes. Three main contributions to the literature are presented as progress towards the achievement of the main goals. 1) A novel realistic BTT data simulator based on the experimentally validated FEM of a blisk. Transformation to modal space enables the efficient generation of simulated BTT data regardless of the complexity of the blisk geometry. The simulator is used in a first-time comparative study of three principal BTT algorithm methods (autoregressive; sine fitting; two-parameter plot). The study clarifies obscure aspects appearing in the methods and demonstrates the applicability of each method to different BTT scenarios. 2) A novel method for the determination of three basic components of blade tip steady movements: axial; lean; untwist. The method relies on linking the shift in the averages of the BTT data to a number of geometrical relations. Development of the method is presented such as to provide a solution for the case of simultaneous movement components. The validity of the method has been examined by simulated data, laboratory data, and real engine measurements. 3) A methodology to include rotation-induced inertia effects in a generic shaft-blisk system, using zero-speed FE modal data, without the need for additional FE analysis at each speed or the derivation of equations from first principles. Rotational effects are added as discretised “external” excitations to the modal equations, which remain based on the zero-speed modes. The inclusion of rotation-induced inertia effects is motivated by the need to upgrade the BTT simulator for more accurate FEM and simulated data. All these contributions will form the basis for a proposed BTT calibration method that will be developed and validated directly after the completion of the current work through an EU funded project.
Supervisor: Bonello, Philip Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Vibration measurements ; Bladed Assembly Mechanics ; Blade tip timing