On the Mechanism of Dangerous Blade Vibration due to Blade Flow Interactions on Centrifugal Compressors

Severe blade flow interactions at part load operation conditions were investigated on a centrifugal compressor with a vaned diffuser leading to material stresses beyond the allowable values. By means of a number of measurement and analysis techniques it could be found, that a stationary periodic pressure field is produced on the circumference by the vibrating blade itself, which is induced at resonance conditions by the peripheral pressure non-uniformity due to the outlet tube. This peripheral pressure field of an integer wave number intensifies the blade resonance excitation from downstream leading to an additivity effect between wave amplitude and blade displacement. The significant role in this mechanism plays the reverse flow near the corner shroud/suction side in the impeller, occurring at part load operation, which is controlled by the interaction of the tip angle of the vibrating blade and the flow angle at that location. It could be demonstrated, that this dangerous blade vibration — in addition — is the source of a shift of the surge line towards higher mass flow, reducing the compressor operating range considerably in this operating zone.

Optimum Weight Design of a Rotor Bearing System With Dynamic Behavior Constraints

An efficient design algorithm for optimum weight design of a rotor bearing system with dynamic behavior constraints is investigated. The constraints include the restrictions on stresses, unbalance response, and/or critical speeds. The system dynamic behaviors are analyzed by the finite element method. And the exterior penalty function method is used as the optimization technique to minimize the system weight. The system design variables are the cross-sectional areas of the shaft and the stiffnesses of the bearings. The sensitivity analysis of the system parameters is also investigated. The example of a single spool rotor bearing system is employeed to demonstrate the merits of the design algorithm with different combination of dynamic behavior constraints. At the optimum stage, it is shown that the weight of rotor system can be significantly reduced. Moreover, the optimum design weights are quite difference for various combinations of dynamic behavior constraints.

A New Analytical Model for Interpreting the Wear Mechanisms of Abradable Seal Systems and Verification by Testing

In order to minimize the specific fuel consumption of gas turbines it is necessary to increase the gas temperatures and pressure ratios. Therefore, new high-temperature resistant abradable seal systems must be developed, especially for the hot section. Since the required operating temperatures are above 1050°C, the use of metallic materials as abradables is out of the question. A problem commonly encountered in the selection of new (ceramic) materials for seal systems is that of insufficient knowledge of the tribological process occurring when turbine blades rub against an abradable seal. The purpose of the investigation was to find a simplified analytical model to describe the tribological process occurring in the rubbing of the blades against the seal, in order to help in the preselection of materials for abradable seals. The model was verified by testing high-temperature resistant abradable seals under simulated engine conditions, followed by metallurgical examination. The results of the examination of two abradable seals on run engine components confirmed the analytical prediction and laboratory tests. The differences in material loss from the blade and the abradable seal can be correlated to the heat flux distribution in the sliding parts. Using different materials on the blade tip and stationary seal (e.g. ceramic blade tip and ceramic or metallic abradable seal), the heat flux can be directed in such a way that the wear takes place largely on the static part of the engine. By calculating their relative abradability, material combinations with optimum performance for each seal application can be found.

Effects of Free Damping Layers on Harmonic Response of Rotating Blades

This paper studies the forced vibration of a rotating blade with free damping layers to harmonic excitation by means of the finite element method. The damping layers are made of viscoelastic material with complex elastic modulus, and the excitation may be either distributed or concentrated. Triangular elements with totally 15 d.o.f. are used to allow for a great variety of shapes and boundary conditions. The effects of various parameters, such as loss factor, storage modulus and thickness of damping layers, are investigated. The results show that the vibration amplitudes near resonances can be significantly reduced by the free damping layers.

Automated Welding of Turbine Blades

The welding of superalloys has been regarded, generally, as an art requiring the highest degree of welder skill and discipline. These highly alloyed materials are prone to micro-cracking and, in some cases, even the best welders cannot achieve satisfactory results. Now, however, advances in automation technology have made it possible to program precisely the complex airfoil shapes and the welding parameters. Consequently, turbine blades can be welded in a repeatable manner, with a minimum of heat input resulting in better metallurgical quality both in the base metal and the weld deposit. The application of this technology to the automated welding of high-pressure compressor turbine blade tips, and the refurbishment of low-pressure turbine blade shrouds are presented in this paper.

Gas Turbine Compressor Operating Environment and Material Evaluation

The reliability and performance of a gas turbine compressor is strongly dependent upon the environment in which it operates, the materials which are used, and the filtration system. Erosion and to a certain extent fouling can be controlled by the filtration system, but corrosion is largely controlled through site and material selection. The factors which determine the corrosivity of a site are humidity, the concentration of acid-forming gases, and the composition of particulates. The interrelationships of these factors are discussed with an aim of reducing their impact on compressor operation. A necessary condition for corrosion is the presence of moisture. The acidity of the moisture results from its interaction with the gases and particulates of the environment. The details of these interactions which are important to turbine operators are discussed. A considerable amount of corrosion testing of base materials and coatings has been performed and this is reviewed. A table is presented for selection of compressor materials based on the nature of the site environment and the type of compressor filtration.

Casing Vibration and Gas Turbine Operating Conditions

The results from an experimental investigation of the compressor casing vibration of an industrial Gas Turbine are presented. It is demonstrated that statistical properties of acceleration signals can be linked with engine operating conditions. The power content of such signals is dominated by contributions originating from the stages of the compressor, while the contribution of the shaft excitation is secondary. Using non-parametric identification methods, accelerometer outputs are correlated to unsteady pressure measurements taken by fast response transducers at the inner surface of the compressor casing. The transfer functions allow reconstruction of unsteady pressure signal features from the accelerometer readings. A possibility is thus provided, for “seeing” the unsteady pressure field of the rotor blades without actually penetrating through the casing, but by simply observing its external surface vibrations.

A Numerical Three-Dimensional Thermal Stress Analysis for Cooled Blades

A modern gas turbine engine subjects the turbine rotor blade to severe thermal stress conditions. Thermal stresses constitute a major part of the state of stress in the blade. Improved blade design with optimum weight, long creep life and structural integrity necessitates more refined thermal stress analysis. In this work a plane blade profile is assumed to take the form of a second degree surface with constant curvatures. The general second degree polynomial allows all the six static equilibrium equations to be considered in evaluating the strained surface. The three dimensional state of stress is determined considering the lateral deformations of the blade profile. A numerical procedure is adopted to solve the non-linear simultaneous equations arising due to the self equilibrating thermal stress system. The results are close to that of original analysis. However, this procedure employs all the necessary equilibrium conditions and relaxes the constraints imposed due to the plane surface assumption to a lower level. The method of analysis is discussed in this paper.

A Magnetic Damper for First Mode Vibration Reduction in Multimass Flexible Rotors

Many rotating machines such as compressors, turbines and pumps have long thin shafts with resulting vibration problems. They would benefit from additional damping near the center of the shaft. Magnetic dampers have the potential to be employed in these machines because they can operate in the working fluid environment unlike conventional bearings. This paper describes an experimental test rig which was set up with a long thin shaft and several masses to represent a flexible shaft machine. An active magnetic damper was placed in three locations: near the midspan, near one end disk, and close to the bearing. With typical control parameter settings, the midspan location reduced the first mode vibration 82%, the disk location reduced it 75% and the bearing location attained a 74% reduction. Magnetic damper stiffness and damping values used to obtain these reductions were only a few percent of the bearing stiffness and damping values. A theoretical model of both the rotor and the damper was developed and compared to the measured results. The agreement was good.

Repair and Life Extension of Titanium Alloy Fan Blades in Aircraft Gas Turbines

Commercial and military aircraft gas turbine fan blades can suffer various types of damage in service, such as foreign object damage (FOD), high strain low cycle fatigue (LCF), wear and fretting fatigue. In addition, cracks initiated by one or more of these types of damage may propagate by a high cycle fatigue (HCF) mechanism. The component may therefore be life limited by the dominant failure mechanism. In this paper, a new, comprehensive scheme for economical refurbishment and qualification of service damaged titanium alloy fan blades is described, along with a critical review of the merits and demerits of existing repair schemes. The metallurgical and process variables to be considered in the repair of FOD, LCF life extension, wear and fretting fatigue life improvement are considered in detail with practical examples derived from experience. A complete qualification testing program including metallography, non-destructive inspection and mechanical property testing, for the refurbished component is outlined.