A Blade by Blade Tip Clearance Measurement System for Gas Turbine Applications

It is difficult to make a reliable measurement of running clearance in the hostile environment over the blading of a modern gas turbine. When engine manufacturers require the measurement to be made over every blade during live engine tests, system reliability, ruggedness and ease of operation are of primary importance. This paper describes a tip clearance measurement system that can measure clearance over every blade around a rotor. The measurement system concept is presented, and the system design described in detail. Commissioning of the measurement system on a compressor test facility, and the results obtained are discussed. An analysis of system performance during the commissioning trials concludes the paper.

A Spinning Finite Beam Element of General Orientation Analyzed With Rayleigh/Timoshenko/Saint-Venant Theory

Linear vibrations are studied for a straight uniform finite beam element of general orientation spinning at a constant angular speed about a fixed axis in the inertial space. The gyroscopic and circulatory matrices and also the geometric stiffness matrix of the beam element are presented. The effect of the centrifugal static axial load on the bending and torsional dynamic stiffnesses is thereby accounted for. The Rayleigh/Timoshenko/Saint-Venant theory is applied, and polynomial shape functions are used in the construction of the deformation fields. Non-zero off-diagonal elements in the gyroscopic and circulatory matrices indicate coupled bending/shearing/torsional/tensional free and forced modes of a generally oriented spinning beam. Two numerical examples demonstrate the use and performance of the beam element.

Active Control of Dynamic Bearing Loads in Rotating Machinery Using Non-Invasive Measurements

A new technique for measuring and actively controlling dynamic bearing loads in rotating machinery is presented. Bearing loads are estimated using the Deflection-Coefficient Method, a technique which does not rely on a full system model, and which applies commonly-used shaft-deflection measurement equipment to obtain estimates of bearing loads. The estimated bearing load is used as an error signal in an adaptive feedforward disturbance rejection controller. The result is a control system which can selectively minimize dynamic bearing loads in real time in rotating machinery systems. The method is applied to a numerical model of a typical rotating machinery system to suppress dynamic reaction forces at bearing supports.

Optimization of Rotor-Stator-Strut Potential Flow Interaction Including Rotor Feedback Effects

A method for the optimization of stator blade stagger angles and of strut circumferential position in order to shield a rotor cascade from downstream strut pressure disturbances is presented. Potential flow interaction between rotor, stator and strut rows is analyzed by a boundary integral method for unsteady incompressible 2-D flows, based on singularity superposition over every blade profile. The flow field is solved in terms of velocity, and the pressure field is computed using the unsteady Bernoulli equation. An optimization technique based on a constrained minimization problem is applied; objective functions related to pressure coefficient and lift coefficients are considered. Force coefficients in the tangential and axial directions, and the momentum coefficient acting on the blades are calculated by integrating the pressure distribution over the blade profile. Results of the stagger angle and strut position optimization are presented and discussed. An analysis in the frequency domain is also performed.

Dynamic Response of a Cracked Rotor With Some Comments on Crack Detection

By considering time dependent terms as external excitation forces, the approximate dynamic response of a cracked horizontal rotor is analysed theoretically and numerically. The solution is good for small cracks and small vibrations in the stable operating range. For each steady state harmonic component the forward and backward whirl amplitudes, the shape and orientation of the elliptic orbit and the amplitude and phase of the response signals arc analysed, taking into account the effect of crack size, crack location, rotor speed and unbalance. It is found that the crack causes backward whirl, the amplitude of which increases with the crack. For a cracked rotor, the response orbit for each harmonic component is an ellipse, the shape and orientation of which depends on the crack size. The influence of the crack on the synchronous response of the system can be regarded as an additional unbalance whereupon, depending on the speed and the crack location, the response amplitude differs from that of the uncracked rotor. The nonsynchronous response provides evidence of crack in the sub-critical range, but is too small to be detected in the supercritical range. Possibilities for crack detection over the full speed range include the additional average (the constant) response component, the backward whirl of the response, the ellipticity of the orbit, the angle between the major axis and the vertical axis and the phase angle difference between vertical and horizontal vibration signals.

Comparison of NDE Results and Correlation With Microstructural Characteristics of NiFeAl/Wf

Metal matrix composites (MMC) and intermetallic matrix composites (IMC) are materials of complex structure. Nominally defect-free, as-manufactured MMC requires nondestructive evaluation (NDE) for quality assurance and process monitoring purposes. In this work, three NDE techniques — ultrasonics, eddy current, and X-ray radiography — were applied to un-damaged NiFeAI/Wf coupons. Images of the coupons were obtained using the three techniques. The NDE results were compared among themselves, and correlations were sought between these results and microstructural features of the specimen. Consistencies were found among the NDE results and a strong correlation was found between the spatial variation of fiber density and the NDE signals.

A Reduced Order Model of Unsteady Flows in Turbomachinery

A novel technique for computing unsteady flows about turbomachinery cascades is presented. Starting with a frequency domain CFD description of unsteady aerodynamic flows, we form a large, sparse, generalized, non-Hermitian eigenvalue problem which describes the natural modes and frequencies of fluid motion about the cascade. We compute the dominant left and right eigenmodes and corresponding eigenfrequencies using a Lanczos algorithm. Then, using just a few of the resulting eigenmodes, we construct a reduced order model of the unsteady flow field. With this model, one can rapidly and accurately predict the unsteady aerodynamic loads acting on the cascade over a wide range of reduced frequencies and arbitrary modes of vibration. Moreover, the eigenmode information provides insights into the physics of unsteady flows. Finally we note that the form of the reduced order model is well suited for use in active control of aeroelastic and aeroacoustic phenomena.

The Development of Fast Response Aerodynamic Probes for Flow Measurements in Turbomachinery

The advent of a new generation of transient rotating turbine simulation facilities, where engine values of Reynolds and Mach number are matched simultaneously together with the relevant rotational parameters for dimensional similitude (Dunn et al [1988], Epstein et al [1984]. Ainsworth et al [1988]), has provided the stimulus for developing improved instrumentation for investigating the aerodynamic flows in these stages. Much useful work has been conducted in the past using hot-wire and laser anemometers. However, hot-wire anemometers are prone to breakage in the high pressure flows required for correct Reynolds numbers, Furthermore some laser techniques require a longer runtime than these transient facilites permit, and generally yield velocity information only, giving no data on loss production. Advances in semiconductor aerodynamic probes are beginning to fulfil this perceived need. This paper describes advances made in the design, construction, and testing of two and three dimensional fast response aerodynamic probes, where semiconductor pressure sensors are mounted directly on the surface of the probes, using techniques which have previously been successfully used on the surface of rotor blades (Ainsworth, Dietz and Nunn [1991]). These are to be used to measure Mach number and flow direction in compressible unsteady flow regimes. In the first section, a brief review is made of the sensor and associated technology which has been developed to permit a flexible design of fast response aerodynamic probe. Following this, an extensive programme of testing large scale aerodynamic models of candidate geometries for suitable semiconductor scale probes is described, and the results of these discussed. The conclusions of these experiments, conducted for turbine representative mean and unsteady flows, yielded new information for optimising the design of the small scale semiconductor probes, in terms of probe geometry, sensor placement, and aerodynamic performance. Details are given of a range of wedge and pyramid semiconductor probes constructed, and the procedures used in calibrating and making measurements with them. Differences in performance are discussed, allowing the experimenter to choose an appropriate probe for the particular measurement required. Finally, the application of prototype semiconductor probes in a transient rotor experiment at HP turbine representative conditions is described, and the data so obtained is compared with (PD solutions of the unsteady viscous flow-field.

A Finite Element Approach to the Analysis of Rotating Bladed-Disk Assemblies Coupled With Flexible Shaft

A method for dynamic analysis of flexible bladed-disk/shaft coupled systems is presented in this paper. Being independant substructures first, the rigid-disk/shaft and each of the bladed-disk assemblies are analyzed separately in a centrifugal force field by means of the finite element method. Then through a modal synthesis approach the equation of motion for the integral system is derived. In the vibration analysis of the rotating bladed-disk substructure, the geometrically nonlinear deformation is taken into account and the rotationally periodic symmetry is utilized to condense the degrees of freedom into one sector. The final equation of motion for the coupled system involves the degrees of freedom of the shaft and those of only one sector of each of the bladed-disks, thereby reducing the computer storage. Some computational and experimental results are given.

Diagnosis of Turbine Engine Transient Performance With Model-Based Parameter Estimation Techniques

This review surveys knowledge needed to develop an improved method of modelling the dynamics of gas turbine performance for fault diagnosis applications. Aerothermodynamic and control models of gas turbine processes are examined as complementary to models derived directly from test data. Extensive, often proprietary data are required for physical models of components, while system identification (SI) methods need data from specially-designed tests. Current methods are limited in: tuning models to test data, non-linear effects, component descriptions in SI models, robustness to noise, and inclusion of control systems and actuators. Conclusions are drawn that SI models could be formulated, with parameters which describe explicitly the functions of key engine components, to offer improved diagnostic capabilities.