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.

Gas Turbine and Turbomachinery Education at Carleton University

Although a relatively small industrial nation, Canada has a very well developed gas turbine industry with both an original design and manufacturing capability and a large industrial user base. Research and teaching at Carleton University has focused on the needs of the Canadian industry over many years. Particular strengths have been established in the areas of experimental aerodynamics for turbomachinery and the use of mathematical modelling for engine performance investigations. Strong links are maintained with both manufacturers and users, and the well-trained engineers produced by the program readily find employment in the industry.

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.

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.

Gas Turbine Research at the University of British Columbia

This paper describes two gas turbine related research projects in the department of Mechanical Engineering at the University of British Columbia. Of the two projects described, one involves fundamental turbomachinery research while the second is a more applied project concerned with gas turbine based cogeneration systems in process industries. In the fundamental research area, both an experimental and computational study of unsteady boundary layer development on turbomachinery blading is described. The applied research program involves an engineering and economic assessment of a gas turbine based cogeneration system for sawmills. The system is designed to use wood-waste generated during the saw-milling process as a source of heat for an indirectly fired gas turbine. Studies to date indicate that such a system could result in many sawmills becoming completely energy self-sufficient.

Probabilistic Structural Analysis Methods of Hot Engine Structures

Development of probabilistic structural analysis methods for hot engine structures is a major activity at Lewis Research Center. It consists of three program elements: (1) composite load spectra methodology, (2) probabilistic structural analysis methodology, and (3) probabilistic structural analysis application. Recent progress includes: (1) quantification of the effects of uncertainties for several variables on High Pressure Fuel Turbopump (HPFT) turbine blade temperature, pressure, and torque of the Space Shuttle Main Engine (SSME), (2) the evaluation of the cumulative distribution function for various structural response variables based on assumed uncertainties in primitive structural variables, and (3) evaluation of the failure probability. Collectively, the results demonstrate that the structural durability of hot engine structural components can be effectively evaluated in a formal probabilistic/reliability framework.

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.

Determining Redundancy Requirements for a Fault Tolerant Control System

The key element in all fault tolerant controls is redundancy [1], but it can be difficult to determine how much redundancy is necessary to achieve the desired level of control system availability. Through examples, this paper examines reliability effects of various elements used in real turbo control systems, including the input sensors, control channels and output stages. The paper then shows that with module reliability and repair rate information, we can determine the redundancy necessary to achieve the desired level of control system availability.

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.