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Oil and Gas Applications

2003;():1-10. doi:10.1115/GT2003-38019.

This paper discusses issues that influence the decision on the arrangement of compressors and the type of equipment in gas pipeline compressor stations. Different concepts such as multiple small units versus single large units are considered, both regarding their impact on the individual station and the overall pipeline. The necessity of standby units is discussed. Various concepts for drivers (gas turbine, gas motor and electric motor) and compressors (centrifugal and reciprocating) are analyzed. The importance of considering all possible operating conditions is stressed. With the wide range of possible operating conditions for the pipeline in mind, the discussion will be brought into the general context of operational flexibility, availability, reliability, installation issues, remote control, and operability of gas turbine driven centrifugal compressors compared to other solutions such as electric motor driven compressors or gas engine driven reciprocating compressors. The impact of different concepts on emissions and fuel cost is discussed. Among the assumptions in this paper are the performance characteristics of the compressor. It will be outlined how these performance characteristics influence the conclusions.

Topics: Compressors
Commentary by Dr. Valentin Fuster
2003;():11-16. doi:10.1115/GT2003-38035.

A review of regenerative gas turbines operating at natural gas pipeline compressor stations across the Russia has been performed. Main performance characteristics, first of all, the power and efficiency of the recuperated gas turbines, many of those have been used up a design service life, can be recovered. Since a large negative effect on the performances is contributed by defective plate-type regenerators, their change seems to be essential when updating gas-pumping units. A tubular regenerator is developed and incorporated into the gas turbines for driving natural gas blowers. The regenerator being installed in place of the plate-type heat exchanger joints a rather simple fabrication process and high durability. Its design features are presented and discussed. Ways to enhance efficiency and decrease the weight of the regenerator are considered.

Commentary by Dr. Valentin Fuster
2003;():17-27. doi:10.1115/GT2003-38071.

On-line compressor wash is discussed for a RB211 compressor driver running at peak load at the Statoil Heidrun offshore platform. The oil field’s economy is directly linked to oil production; however, the production rate is limited by driver and gas compressor capacity. From this perspective, the power output and gas turbine uptime become decisive economic factors. The economic potentials related to successful on-line washing are given. This work is based on a series of trials with on-line compressor washing over a two-year period. Results include effect of different on-line washing procedures and washing fluids. The field test campaign has shown no significant improvements with on-line compressor washing at peak load. Understanding the gas turbine performance deterioration is of vital importance. Trending of its deviation from the engine baseline (datum maps) facilitates load-independent monitoring of the gas turbine’s condition. Peak load turbine response to compressor deterioration is analyzed. Instrument resolution and repeatability are key factors that sometimes are more important than absolute accuracy in condition trending. As a result of these analyses, a set of monitoring parameters is suggested for effective diagnostics of compressor degradation in peak load operation. Avenues for further research and development are suggested as our understanding of the deterioration mechanisms at peak load remains incomplete.

Commentary by Dr. Valentin Fuster
2003;():29-41. doi:10.1115/GT2003-38108.

Concerns about the effects of greenhouse gas emissions on the Earth’s climate have lead to a considerable focus by the public and governments on the levels of emissions that are generated by industrial activities. In Canada, it has been recognized that gas transmission systems are rated second in overall CO2 production in the Natural Gas Industry (next to gas processing). Most of the gas transmission systems are powered by gas turbines at compressor stations resulting in significant CO2 emissions (at the rate of ∼ 6 kilo tonnes/ per MW-year). This can be reduced if the CO2 can be separated from the gas turbine exhaust stream and directed for reuse or sequestration. This paper presents results of techno-thermodynamic assessment of two power cycle adjustments to increase CO2 concentrations in the exhaust gas from turbines. The working fluid in the two semi-closed cycles are made rich in CO2 , thus making it easy to capture the CO2 from the flue gas by means of physical absorption techniques rather than by the conventional expensive amine adsorption methods. Additionally, the CO2 rich working fluid is shown to give rise to a higher exhaust gas temperature from the gas turbine semi-closed cycles, allowing a steam bottom cycle to be effective in augmenting the power delivered by the entire system by 50%, hence contributing to reducing emission by increasing the overall thermal efficiency of the system.

Commentary by Dr. Valentin Fuster
2003;():43-51. doi:10.1115/GT2003-38191.

Gas turbine operating state determination can be performed using Gas Path Analysis (GPA) techniques, which use measurements taken on the machine to calculate the characteristic parameters that are indices of the machine health state. The number and type of characteristic parameters that can be evaluated depend on the number and type of the available measured variables. Thus, when there are not enough measured variables to determine all the characteristic parameters, some of them have to be estimated independently of the actual gas turbine health state. In this way, variations due to aging or deterioration which, in the actual machine, may occur on these last characteristic parameters, cause estimation errors on the characteristic parameters assumed as problem unknowns. The available instrumentation in field applications is often inadequate to ensure reliable operating state analysis when GPA-based techniques are used. This problem may be partially overcome using a multiple operating point minimization technique. This consists of the determination of the characteristic parameters that minimize the sum of the square differences between measured and computed values of the measurable variables in multiple operating points. In this way the lack of data is overcome by data obtained in different operating points. This paper describes a procedure for gas turbine operating state determination based on a multiple operating point minimization technique and presents a study aimed at selecting the best set and number of operating points that should be used.

Topics: Gas turbines
Commentary by Dr. Valentin Fuster
2003;():53-61. doi:10.1115/GT2003-38397.

A multi-stage compressor test-facility, fully instrumented with its dedicated data acquisition and processing system, has been developed to conduct experimental research work at the University of Ferrara. This paper provides a systematic description of the uncertainty analysis procedures required for compressor testing, including preliminary performance test results, in addition to a brief description of the test facility and its capabilities.

Commentary by Dr. Valentin Fuster
2003;():63-70. doi:10.1115/GT2003-38533.

Electrical energy storage might become a strategic topic if distributed generation will be matched with stochastic sources as wind or sun. Compressed Air Energy Storage (CAES) is one of the most promising options today: energy is stored as pressurized air in a cavern. Transient phenomena, occurring during the charging process, are analyzed in this paper. Two kinds of systems are considered with or without pressure compensation; in fact a water column can be used to link the cavern to a pond in order to compensate the pressure oscillations. A lumped parameter model has been adjusted by the authors to simulate the initial charging and the subsequent surge. The obtained results supply some insight about the safe working conditions and also the surge operation.

Commentary by Dr. Valentin Fuster
2003;():71-82. doi:10.1115/GT2003-38630.

The paper describes the effects of a forced harmonic oscillations of fixed frequency and amplitudes in the range Λ = Um /Ub = 1 ÷ 11 on the characteristics of a turbulent pipe flow with a bulk Reynolds number of 5900. The resulting Stokes layer δ is a fraction of the pipe radius (χ = R/δ = 53) so that the vorticity associated to the oscillating motion is generated in a small near wall region. The analysis is carried out processing a set of statistically independent samples obtained from wall resolved Large Eddy Simulations; time and space averaged global quantities, extracted for the sake of comparison with recent experimental data, confirm the presence of a non negligible drag reduction phenomenon. Phase averaged profiles of the Reynolds stress tensor components provide valuable material for the comprehension of the effects of the time varying mean shear upon the near wall turbulent flow structures. The large scale of motion are directly computed through numerical integration of the space filtered three dimensional Navier-Stokes equations with a spectrally accurate code; the subgrid scale terms are parametrized with a dynamic procedure.

Commentary by Dr. Valentin Fuster
2003;():83-89. doi:10.1115/GT2003-38676.

ALSTOM’s new gas turbine, the GT10C, is a 30 MW industrial gas turbine for mechanical drive and power generation, which has been upgraded from the 25 MW GT10B. The thermal efficiency of the new gas turbine is 37.3% at ISO inlet conditions with no losses. The GT10C features a dual-fuel dry low emission gas turbine, with emissions values of 15 ppm NOx on gaseous fuel and 42 ppm NOX on liquid fuel (also dry). The GT10C was first started and operated on load in November 2001 and the test program is ongoing until the fall of 2002. The program covers a complete package test, including gas turbine, auxiliaries and control system, to ensure package availability. For the tests, a new test rig has been built in Finspong, Sweden, for testing on both natural gas and liquid fuels. The tests have been very successful, achieving the product targets, for example below 15 ppm NOx, without combustor pulsations. This paper discusses operation experience from the test rig, where the engine has been tested on both natural gas and liquid fuel over the whole load range. The engine has been equipped with over 1200 measuring points, covering the complete gas turbine. All critical parameters have been carefully verified in the test, such as turbine blade temperature and stresses, combustor temperatures and dynamics and engine performance. Results from the tests and measurements will be discussed in this paper. Performance and emissions will also be evaluated.

Topics: Gas turbines , Testing
Commentary by Dr. Valentin Fuster
2003;():91-98. doi:10.1115/GT2003-38745.

Determining the airflow through a gas turbine’s axial compressor is not a simple or one step process as many factors affect flow and there is seldom a flow meter or a means to directly measure airflow rate. Speed of the compressor, inlet pressure and temperature, and resistance or backpressure at the compressor’s outlet all affect the amount of airflow. The type of gas turbine, single or twin spool, the magnitude of power produced, the use of bleed or bypass valves, the power turbine speed, and operating conditions all have influences on the amount of airflow. Despite this, there are several reasons why an estimate of airflow is useful for understanding and describing the behavior and performance of gas turbines. The amount of airflow compared to fuel flow determines the composition and condition of the exhaust gases and is directly related to the turbine’s power output, heat rate, and waste heat recovery potential. A predicted airflow rate and the corresponding axial compressor discharge pressure can be used to identify deterioration in performance and to estimate emission characteristics of a unit. This paper presents an approach based on easily obtained gas turbine data, such as the design point data, test stand data, or manufacturer’s curves for the compressor. Compressor performance curves may be obtained from the manufacturer or by mapping compressor output during normal operations. A great deal of information has been presented in the literature about the performance of gas turbines and axial compressors but this paper focuses on methods that are sufficiently simple and direct that users can obtain an estimate of their unit’s airflow, References 1, 2, and 3. Some manufacturers provide computer data bases or on-line control panel estimates of gas turbine airflow but in these cases, the user has no idea what causes a change. Detailed performance curves for axial compressors are usually not available, however, through the methods presented in this paper, a reasonable approximation of the operating curves can be developed and used to estimate axial compressor airflow over the full range of normal operations. The methods described are based on tracking and mapping a compressor’s operations over a period of time and relating compressor output to other performance parameters and known conditions (design point) in order to establish a normally expected airflow rate.

Commentary by Dr. Valentin Fuster
2003;():99-106. doi:10.1115/GT2003-38785.

The trend towards more gas turbine power and thermal efficiency, derived through high firing temperatures and air compression ratios, has been translated into increased uncontrolled emissions of NOx . Most of these can be readily reduced by 70–90% with reliable Dry Low Emissions combustion. However, legal and regulatory pressures are requiring NOx emission reductions in the order of 98% in some regions. A key point in these traditional methods is that engine system efficiency and CO2 emissions, as well as other impacts, are not directly considered. More efficient units with higher pressure ratios have difficulty meeting ultra-low ppm concentration based standards, especially under transient conditions. This paper examines the permitting approach on overall plant emissions prevention versus controls, and engine operating parameters for several types of units. This can establish a relationship between the mass quantity of NOx emissions, the engine airflows, and the system operational power output. The intent is to explore whether a different approach to emissions standards would enhance engine system reliability and emission prevention effectiveness for already clean energy facilities.

Commentary by Dr. Valentin Fuster
2003;():107-111. doi:10.1115/GT2003-38816.

Most Oil&Gas companies that currently outsource both maintenance and related engineering activities, by establishing Long Terms Service Agreements (LTSA) with engine manufacturers, base their contracts on several clauses. A minimum level of overall Plant Availability is usually to be guaranteed by the maintenance services supplier. Both parts agree upon this availability threshold and Bonus/Penalty clauses are based on this value. The Availability of a system is a non-linear function of: • Reliability, in terms of components life, system functional configuration and scheduled maintenance frequencies; • Maintainability, in terms of time to repair/restore, duration of scheduled maintenance tasks and special maintenance tools on site; • Logistics. Concerning Logistics, the problem is to define both the set and levels of spare parts in a warehouse for one or more installations (pooling) and the location of the warehouse itself. In this paper a solution for this problem is presented, based on RBD (Reliability Block Diagram) Monte Carlo simulation techniques and an associated What-If analysis. One of the deliverables of this optimisation process is the ranking of all the system components in terms of their influence on the availability of the whole system. This spare part list optimisation is one of the major deliverables of the discipline called ACM “Availability Centered Maintenance”, currently developed in Nuovo Pignone.

Commentary by Dr. Valentin Fuster
2003;():113-117. doi:10.1115/GT2003-38868.

Hydrocarbon dew point in fuel gas has become a recent concern for Saudi Aramco and Saudi Electricity Company (SEC). Together they operate more than 150 land based industrial and aeroderivative combustion gas turbines (CGTs) on natural gas produced in the Eastern Province of Saudi Arabia. Automated on-line hydrocarbon dew point monitoring conducted over the past two years has revealed wide variations in the hydrocarbon dew points of the fuel gas that supply these turbines. During winter months at some locations, hydrocarbon dew points are now known to rise to, and slightly above ambient gas temperatures. Daily variations in dew point temperature average about 12–14 °F (7–8 °C) in winter months with highs occuring in late morning hours. More dramatic changes (60 °F, 33 °C) were recorded and attributed to operational changes in two major gas plants. At a major power generation facility, hydrocarbon dew point data was referenced in time with inlet fuel temperature and inlet fuel pressure recorded from a turbine control system. This data reveals evidence that liquid hydrocarbon was entering the CGT fuel intake. Damage to hot gas path components from liquids in natural gas fuel has cost Saudi Aramco several million dollars over a 10 year period (1985–1995). The estimates are much higher now with expanded use of natural gas into the Central and Western Provinces of Saudi Arabia. The new sources of gas, new processing facilities and longer transmission distances all contribute to greater potential for liquids formation, while upgrades to higher firing temperatures also increases the sensitivity of some CGTs to liquids in the fuel. With dew point monitoring, we will be able to recommend the necessary fuel conditioning equipment and fuel preheating temperatures that will be needed to prevent costly distress to CGTs due to the passage of liquids in the fuel gas.

Topics: Gaseous fuels
Commentary by Dr. Valentin Fuster
2003;():119-127. doi:10.1115/GT2003-38965.

A study of casing distortion in General Electric MS3002 gas turbines used in the oil and gas industry revealed significant distortion for MS3002 Models C through G. The primary distortion problem was ovalization of the turbine casing, which could occur in either the horizontal or vertical directions. Malfunctioning of the water cooling system, or improper disassembly and assembly procedures can cause casing distortion. The MS3002 Models A-G gas turbines have water cooled turbine casings, and malfunctioning of their cooling water systems, regardless of distortion, is also a significant problem.

Commentary by Dr. Valentin Fuster

Structures and Dynamics: General

2003;():129-138. doi:10.1115/GT2003-38158.

A unique structural component known as the thermally free axial support or TFA has been developed to restrain a foil substrate catalyst within a gas turbine combustor. Designing a device for this application has been extremely challenging due to competing requirements. Functionally, this support must restrain a catalyst against significant pressure load at temperatures as high as 940°Celsius (C). Enough contact area with the catalyst must exist to avoid foil deformation while at the same time the device must not overly restrict or disturb combustion fluid flow. The aerodynamic performance of the TFA has been validated in actual turbine operation. The TFA is also expected to survive in base loaded gas turbine operation for at least 8000 hours. Long-term durability, crucial to this component since it is upstream of the turbine has been proven with significant analytical and testing efforts. Manufacturing quality assurance and control has been utilized to achieve a consistent product.

Topics: Catalysts
Commentary by Dr. Valentin Fuster
2003;():139-146. doi:10.1115/GT2003-38285.

Models of oxidation, low-cycle fatigue and thermomechanical fatigue are studied in order to establish a life model of APS-TBC. An experimental programme to pinpoint the relevant mechanisms is run in parallel with macro- and micromechanical modelling, including also fracture-mechanical analysis of the BC/TC interface. Finally, the models are calibrated by further experiments.

Commentary by Dr. Valentin Fuster
2003;():147-153. doi:10.1115/GT2003-38351.

A system designed to control and predict the length of cracks that generate in the first-stage nozzles of E and F class gas turbines was developed. This system consists of three programs for (1) inputting cracks, (2) displaying cracks, and (3) predicting cracks, and a database consisting of approximately 350,000 cracks generated in first-stage nozzles taken from past repair records of five power plants operating in Japan. The database also contains data on operating time and number of starts of gas turbines. The distinctive features of this system are described below. 1) The crack data can be entered on the nozzle drawing as a picture by using the mouse. 2) The accumulated data allows the sections of nozzles in which cracks have generated most frequently to be identified. 3) The correlation formula of cracks and operating time or number of starts can be obtained simply. 4) By entering the scheduled operating time or number of start-ups to the time of the next scheduled inspection in the correlation formula, the length of cracks in optional sections and propagating in optional directions can be predicted. Using this system, the statuses of cracks generated in nonrepaired and repaired nozzles of E class gas turbines were compared. The comparison focused on 11 patterns with comparatively long cracks selected from the cracks propagating together with the increase in operating time or number of starts. The propagation of cracks covering a period of approximately two years, which corresponds to the inspection interval of power plants in Japan, was also compared. The results showed that the extent of crack propagation tends to increase with the increase in the number of repairs. Furthermore, the propagation of cracks in repair nozzles is about two times greater than that in non-repair nozzles. It was also found that the system could identify the sections in which the longest cracks are generated.

Commentary by Dr. Valentin Fuster
2003;():155-162. doi:10.1115/GT2003-38510.

Various sources of uncertainty greatly affect the life of structural components of gas turbines. Probabilistic approaches provide a means to evaluate these uncertainties; however, the accuracy of these approaches often remains unknown. Published quantitative studies of the effectiveness of various uncertainty quantification techniques are usually based on very simple examples. This is contrasted by the large-size finite element models that are used for complex geometries of critical structural parts such as turbine blades or nozzles. In such real-life applications the expenses of the “function calls” (runs of these models) preclude systematic studies of probabilistic methods. These expenses are attributed not only to the actual runs of the model, but to the difficulties in parametrically changing the model as well. Such a “complexity gap” leads to a justifiable concern over whether the trends identified in academic studies are relevant to these industrial applications. As a result, structural engineers end up with the number of function calls that they can afford rather than what would be needed for the required level of accuracy. The present effort intends to bridge this gap by studying a mid level problem: a simplified notional finite element model of a gas turbine component is presented. Despite its simplicity, the model is designed to reflect the major features of more realistic models. The parametric changes of the model are fully automated, which allows for performing an extensive set of benchmark tests that help to determine the relative merits of various existing probabilistic techniques for component life assessment. Several meta-modeling techniques are investigated and their performance compared based on direct sampling methods. In this context, various Design of Experiments (DoE) methods are studied. The results are used to construct the Response Surface Equations (RSE) as well as the kriging models. It is emphasized that changes in the relative locations of the critical points induced by variation of independent parameters can critically affect the overall fidelity of the modeling; the means of remedying such a degradation in precision are discussed. Finally, it is shown that when the ranges of independent variables are large, kriging generally provides precision that is an order of magnitude better than RSE for the same DoE.

Topics: Gas turbines
Commentary by Dr. Valentin Fuster
2003;():163-170. doi:10.1115/GT2003-38827.

Generator Rotor Tooth Cracks have been recently observed on some long service generator rotors. The purpose of this paper is to document the root cause analysis as well as a repair methodology that was developed in support of one of these cracked units. The paper discusses the development of a finite element model to understand the operating modes and failure mechanisms that caused the initiation of these cracks. The results of the analysis agreed very well with field observations on cracked units and with some limited repair options that have been successfully used in the past. An experimental fretting fatigue testing program is currently underway to substantiate and extend the results reported in this paper.

Commentary by Dr. Valentin Fuster

Structures and Dynamics: Structural Mechanics and Vibration

2003;():171-182. doi:10.1115/GT2003-38007.

The influence of blade-to-blade coupling and rotation speed on the sensitivity of bladed disks to mistuning is studied. A transonic fan is considered with part span shrouds and without shrouds, respectively, constituting a high and a low blade-to-blade coupling case. For both cases, computations are performed at rest as well as at rotation speeds corresponding to resonant crossings in the Campbell diagram. Mistuning sensitivity is modeled as the dependence of amplitude magnification on the standard deviation of blade stiffnesses. A state of the art finite element reduction technique, based on classical modal analysis, is employed for the structural analysis. This reduced order model is solved for sets of random blade stiffnesses with various standard deviations, i.e. Monte Carlo simulations. In order to reduce the sample size, the statistical data is fitted to a Weibull (type III) parameter model. Three different parameter estimation techniques are applied and compared. The key role of blade-to-blade coupling, as well as the ratio of mistuning to coupling, is demonstrated for the two cases. The mistuning sensitivity behavior of the fan without shrouds is observed to be unaffected by rotation speed at its resonant condition, due to insignificant changes in coupling strength at this speed. The fan with shrouds, on the other hand, shows a significantly different behavior at rest and resonant speed, due to increased coupling under rotation. Comparing the two cases at resonant rotor speeds, the fan without shrouds is less or equally sensitive to mistuning than the fan with shrouds in the entire range of mistuning strengths considered.

Topics: Disks
Commentary by Dr. Valentin Fuster
2003;():183-194. doi:10.1115/GT2003-38050.

It is known that the forced response of mistuned bladed disks can strongly be amplified in comparison with the forced response of the tuned system. The random character of mistuning thus requires the construction of probabilistics models of random uncertainties. This paper presents a nonparametric probabilistic model of random uncertainties which is adapted to the problematics of the blade mistuning. This nonparametric approach allows all the uncertainties yielding mistuning (manufacturing tolerances, dispersion of materials) to be taken into account and includes also the uncertainties due to the modeling errors. This new probabilistic model takes into account both the mistuning of the blade eigenfrequencies and the blade modal shapes. The first point concerns the construction of this nonparametric approach in order to perform a mistuning analysis. The second part is devoted to the inverse problem associated with the manufacturing tolerances. A relationship between the manufacturing tolerances and the level of mistuning is also constructed.

Topics: Manufacturing
Commentary by Dr. Valentin Fuster
2003;():195-205. doi:10.1115/GT2003-38060.

A theory was previously developed for predicting robust maximum forced response in mistuned bladed disks from distortion of a structural mode. This paper describes an experiment to demonstrate the theory. A bladed disk is designed to be sufficiently sensitive to mistuning to obtain maximum response. The maximum amplitude magnification from mistuning is predicted using the theory, 1.918. The bladed disk is intentionally mistuned to obtain the maximum response, and the response to an engine order traveling wave excitation is measured. The measured amplitude magnification is in close agreement with the theory. The robustness of the maximum response is demonstrated.

Topics: Disks
Commentary by Dr. Valentin Fuster
2003;():207-214. doi:10.1115/GT2003-38327.

Strain measurement on blade and the calculation of blade stress during test are critical to determine the actual stress and the life of blades. Since the dynamic loads acting on blades, such as gas pressures and load changes during operation, might not be known, the concept of strain amplification factor (SAF) can be used to estimate the maximum strain/stress of blade during resonance. The SAF is defined as a ratio between the maximum modal strain on blade from FE analysis and the measured strain or computed strain of a particular place on blade. In this paper, this concept is reviewed and further analysis is carried out. To verify the theoretical analysis, the experimental tests of a radial turbine (blisk) are performed in detail. However, only experiments in static condition are considered in this work, since a better experimental condition can be achieved. Moreover, the verification of rotating blades could be accomplished with the similar procedure. The FE method is chosen as a tool to provide theoretical results. Two computations by using FE method are performed to obtain the SAFs. The first computation considers the use of sector model of turbine, which is usually practiced in industry. In the second calculation, the complete tuned turbine is taken into account. The obtained results for both computations and the effect of mistuning on SAF are discussed. Furthermore, the computed SAFs are compared to the experimental results.

Topics: Turbines
Commentary by Dr. Valentin Fuster
2003;():215-222. doi:10.1115/GT2003-38447.

It is now increasingly necessary to predict accurately, at the design stage and without excessive computer costs, the dynamic behavior of rotating parts of turbomachines, in order to be able to avoid resonant conditions at operating speeds. Classical approaches are based on different uncoupled models. For example, rotordynamics deals with the shaft behavior while bladed assemblies dynamics deals with wheels, and the possibility of interaction between those elements is generally not analyzed. In this study, the global non-rotating mode shapes of flexible bladed disc–shaft assemblies are used in a modal analysis method for calculating the dynamic characteristics (frequencies and mode shapes) of the corresponding rotating system. The non rotating mode shapes are computed using a finite element cyclic symmetry approach. Rotational effects, such as centrifugal stiffening and gyroscopic effects are accounted for. All the possible couplings between the flexible parts and every kind of deformations are allowed. The proposed model is applied to a thin-walled composite shaft and to a turbomolecular pump rotating assembly. The results obtained illustrates clearly some of the limitations of classical approaches.

Commentary by Dr. Valentin Fuster
2003;():223-233. doi:10.1115/GT2003-38475.

New efficient models have been developed to describe dynamic friction effects in order to facilitate analysis of the vibration of bladed discs in the time domain. These friction models describe friction forces occurring at contact interfaces under time-varying normal load variations, including cases of separation. The friction models developed allow one to take into account time-varying friction contact parameters, such as friction coefficient and contact stiffness coefficients. Anisotropy and variation of the friction characteristics over the contact surfaces are included in the proposed models. The capabilities of the new friction models are demonstrated. Analysis of the friction forces is performed for different motion trajectories and different time variations of the normal load, and the effects of anisotropy, variation in time of the friction characteristics and normal load variation are discussed. A numerical analysis of transient vibrations of shrouded blades using the new models is presented.

Commentary by Dr. Valentin Fuster
2003;():235-245. doi:10.1115/GT2003-38480.

An effective method for analysis of periodic forced response of nonlinear cyclically symmetric structures has been developed. The method allows multiharmonic forced response to be calculated for a whole bladed disc using a periodic sector model without any loss of accuracy in calculations and modelling. A rigorous proof of the validity of the reduction of the whole nonlinear structure to a sector is provided. Types of bladed disc forcing for which the method may be applied are formulated. A multiharmonic formulation and a solution technique for equations of motion have been derived for two cases of description for a linear part of the bladed disc model: (i) using sector finite element matrices; (ii) using sector mode shapes and frequencies. Calculations validating the developed method and a numerical investigation of a realistic high-pressure turbine bladed disc with shrouds have demonstrated the high efficiency of the method.

Topics: Vibration , Disks
Commentary by Dr. Valentin Fuster
2003;():247-255. doi:10.1115/GT2003-38557.

This paper investigates the effect of manufacturing variations on the modal response of a transonic low aspect ratio fan. A simulated set of coordinate measurement machine measurements from a single rotor, representative of actual manufacturing variations, are used to investigate geometric effects. Principal component analysis is used to statistically model spatial geometry variations and reduce variable space dimensionality. Statistics from this analysis are used with Monte Carlo sampling to generate random blades realizations that are used to predict response distributions for a simulated fleet of 1000 blades. An existing approach to approximate blade frequency response is extended to include modal displacement and stress. These approximations are based on eigensensitivity analysis and first order Taylor series approximations. An approximation error analysis is conducted to quantify accuracy. The effect of small geometry variations on blade natural frequency, mode shape, and modal stress is investigated with results showing that small variations on the order of mils can cause significant variations in both scale and location of free and forced response.

Topics: Blades , Uncertainty
Commentary by Dr. Valentin Fuster
2003;():257-266. doi:10.1115/GT2003-38808.

Numerical predictions of the forced vibration of a disc assembly including frictional effects between the shrouds are presented concerning engineering needs for the blade design process. Assuming a tuned disc assembly, numerical static, free and then forced vibration analyses of a shrouded turbine blade measured in the spin pit are performed systematically. For the excitation forces of an air jet evaluated from the fairly linear behavior of the experimental blade resonance peaks, the reliability of the proposed approach is validated through the very close agreement of the computed and measured resonant peaks. These resonant peaks demonstrate either a fairly linear behavior or a non-linear one like the jump effect of blade resonance amplitudes, or elastic impacts between the shrouds. Also, the damping performance for different contact configurations between the shrouds is numerically analyzed. These numerical results indicate that the shrouds generate higher frictional damping for small angles (0–30 deg) between the circumferential direction and the normal vector to the contact surface.

Commentary by Dr. Valentin Fuster
2003;():267-277. doi:10.1115/GT2003-38952.

This paper is the first in a two-part study of identifying mistuning in bladed disks. It develops a new method of mistuning identification based on measurements of the vibratory response of the system as a whole. As a system-based method, this approach is particularly suited to integrally bladed rotors, whose blades cannot be removed for individual measurements. The method is based on a recently developed reduced order model of mistuning called the Fundamental Mistuning Model, FMM, and is applicable to isolated families of modes. Two versions of FMM system identification are presented: a basic version that requires some prior knowledge of the system’s properties, and a somewhat more complex version that determines the mistuning completely from experimental data.

Topics: Disks
Commentary by Dr. Valentin Fuster
2003;():279-286. doi:10.1115/GT2003-38953.

This paper is the second in a two-part study of identifying mistuning in bladed disks. It presents experimental validation of a new method of mistuning identification based on measurements of the vibratory response of the system as a whole. As a system based method, this approach is particularly suited to integrally bladed rotors, whose blades cannot be removed for individual measurements. The method is based on a recently developed reduced order model of mistuning called the Fundamental Mistuning Model and is applicable to isolated families of modes. Two versions of FMM system identification are applied to the experimental data: a basic version that requires some prior knowledge of the system’s properties, and a somewhat more complex version that determines the mistuning completely from experimental data.

Topics: Disks
Commentary by Dr. Valentin Fuster
2003;():287-297. doi:10.1115/GT2003-38961.

The method of polynomial chaos has been used to analytically compute the statistics of forced response of a mistuned bladed disk assembly. The model of the bladed disk assembly considers only one mode of vibration of each blade. Mistuning phenomenon has been simulated by treating the modal stiffness of each blade as a random variable. The validity of the polynomial chaos method has been corroborated by comparison with the results from numerical simulations.

Commentary by Dr. Valentin Fuster
2003;():299-309. doi:10.1115/GT2003-38966.

This paper focuses on the identification/prediction of the blade exhibiting the largest response in mistuned bladed disks. This information is very important in experimental/testing efforts as it permits the most effective positioning of a few gages to capture the maximum response on the disk. In computational statistical analyses, knowing the highest responding blade is also quite valuable as it may lead to computational savings in the determination of the maximum response. Different strategies are proposed here for the experimental and computational contexts. In the former situation, mistuning is typically unknown but only one or a few disks must be considered. The proposed solution is then to estimate the mistuned blade properties and to rely on this identified bladed disk model to predict the blades that are likely to exhibit the largest responses through exact, full disk solutions. On the contrary, in computational statistical analyses, mistuning is specified but a potentially large number of disks must be analyzed and it is desired to bypass the ensemble of full disk solutions. Accordingly, a novel, computationally very efficient algorithm is proposed for a preliminary estimation of the forced response of mistuned disks from which the blades that are likely to exhibit the largest responses can be predicted. Examples of application on single- and two-degree-of-freedom per blade models and a reduced order model of a blisk demonstrate the reliability of the proposed strategies.

Topics: Disks , Blades
Commentary by Dr. Valentin Fuster

Structures and Dynamics: Unsteady Aerodynamics and Aeromechanics

2003;():311-320. doi:10.1115/GT2003-38182.

Pulse charged turbocharger turbines are exposed to highly unsteady aerodynamic forces, which cause substantial blade vibrations. With respect to high cycle fatigue, the determination of the resonant vibration responses of the turbocharger blades rotating with variable speed is of great importance for a reliable and safe design. The first step to a numerical prediction of the blade’s loading by oscillations in low orders is the calculation of the aerodynamic excitation. This paper contains results of a study, which has been carried out at ABB Turbo Systems Ltd., to define the necessary numerical effort for the calculation of the low order blade excitation in pulse charged axial turbocharger turbines by means of computational fluid dynamics. The paper presents comparisons between experimental and numerical results concerning the unsteady loading of pulse charged axial turbocharger turbines. It shows the potential of quasi-three-dimensional computational fluid dynamics to predict the dynamic blade loading under real engine operation.

Topics: Turbines , Blades
Commentary by Dr. Valentin Fuster
2003;():321-330. doi:10.1115/GT2003-38199.

One of the outstanding issues in turbomachinery aeromechanic analysis is the intra-row interaction effects. The present work is aimed at a systematic examination of rotorstator gap effects on blade aerodynamic damping by using a 3D time-domain single-passage Navier-Stokes solver. The method is based on the upwind finite volume discretization (AUSMD/V) and the single-passage Shape-Correction approach with enhanced accuracy and efficiency for unsteady transonic flows prediction. A significant speed up (by a factor of 20) over to a conventional whole annulus solution has been achieved. A parametric study with different rotor-stator gaps (56%–216% chord) for a 3D transonic compressor stage illustrates that the reflection from an adjacent stator row can change rotor aerodynamic damping by up to 100%. It is shown that this intra-row interference effect on the rotor aero-damping can be qualitatively altered by changing the number of stator blades. Thus, the stator blade count could be considered as a useful aeromechanical control/design parameter. Furthermore, the predicted non-monotonic relationship between the rotor blade aerodynamic damping and the gap distance suggests the existence of an optimum gap regarding rotor flutter stability and/or forced response stress levels.

Topics: Damping , Rotors , Blades , Stators
Commentary by Dr. Valentin Fuster
2003;():331-338. doi:10.1115/GT2003-38311.

Turbomachinery shrouded rotor blade design has been widely used in fans, compressors, and turbines. By using shroud design, the blade structural damping can be increased to prevent blade flutter. However, the shrouded rotor blade design will cause the blade mode shapes to be complex, and in some cases both bending and torsion mode components can be present at the same time in a single mode. Therefore, a complex mode analysis was developed to predict shrouded rotor blade flutter with these bending and torsion combined system modes. Using the blade natural frequencies and mode shapes from a finite element model, and the blade aerodynamic flow-field, the unsteady aerodynamic forces of the system mode can be calculated. A complex mode flutter analysis was then performed using a modal solution to determine the stability of the system. The analysis system was applied to two shrouded rotor blade applications. The bending and torsion combined system mode was decomposed into a real mode component and an imaginary mode component. Bending-dominated or torsion-dominated mode shapes can be analyzed using single mode approach to obtain consistent flutter stability results. However, for the bending and torsion combined mode shape cases, the single mode analysis can be misleading, and the complex mode analysis can be a useful tool.

Commentary by Dr. Valentin Fuster
2003;():339-348. doi:10.1115/GT2003-38353.

Experiments are performed on a modern design transonic shroudless low-aspect ratio fan blisk that experienced both subsonic/transonic and supersonic stall-side flutter. High-response flush mounted miniature pressure transducers are utilized to measure the unsteady aerodynamic loading distribution in the tip region of the fan for both flutter regimes, with strain gages utilized to measure the vibratory response at incipient and deep flutter operating conditions. Numerical simulations are performed and compared with the benchmark data using an unsteady three-dimensional nonlinear viscous computational fluid dynamic (CFD) analysis, with the effects of tip clearance, vibration amplitude, and the number of time steps-per-cycle investigated. The benchmark data are used to guide the validation of the code and establish best practices that ensure accurate flutter predictions.

Commentary by Dr. Valentin Fuster
2003;():349-356. doi:10.1115/GT2003-38425.

The unsteady aerodynamic characteristics of an oscillating compressor cascade composed of Double-Circular-Arc airfoil blades were both experimentally and numerically studied under transonic flow conditions. The study aimed at clarifying the role of shock waves and boundary layer separation due to the shock boundary layer interaction on the vibration characteristics of the blades. The measurement of the unsteady aerodynamic moment on the blades was conducted in a transonic linear cascade tunnel using an influence coefficient method. The cascade was composed of seven DCA blades, the central one of which was an oscillating blade in a pitching mode. The unsteady moment was measured on the central blade as well as the two neighboring blades. The behavior of the shock waves was visualized through a schlieren technique. A quasi-three dimensional Navier-Stokes code was developed for the present numerical simulation of the unsteady flow fields around the oscillating blades. A k-ε turbulence model was utilized to adequately simulate the flow separation phenomena caused by the shock-boundary layer interaction. The experimental and numerical results complemented each other and enabled a detailed understanding of the unsteady aerodynamic behavior of the cascade. It was found that the surface pressure fluctuations induced by the shock oscillation were the governing factor for the unsteady aerodynamic moment acting on the blades. Such pressure fluctuations were primarily induced by the movement of impingement point of the shock on the blade surface. During the shock oscillation the separated region caused by the shock boundary layer interaction also oscillated along the blade surface, and induced additional pressure fluctuations. The shock oscillation and the movement of the separated region were found to play the principal role in the unsteady aerodynamic and vibration characteristics of the transonic compressor cascade.

Commentary by Dr. Valentin Fuster
2003;():357-364. doi:10.1115/GT2003-38454.

This paper reports the results of an ongoing research effort to explain the underlying mechanisms for aeroacoustic fan blade flutter. Using a 3D integrated aeroelasticity method and a single passage blade model that included a representation of the intake duct, the pressure rise vs. mass flow characteristic of a fan assembly was obtained for the 60%–80% speed range. A novel feature was the use of a downstream variable-area nozzle, an approach that allowed the determination of the stall boundary with good accuracy. The flutter stability was predicted for the 2 nodal diameter assembly mode arising from the first blade flap mode. The flutter margin at 64% speed was predicted to drop sharply and the instability was found to be independent of stall effects. On the other hand, the flutter instability at 74% speed was found to be driven by flow separation. Further post-processing of the results at 64% speed indicated significant unsteady pressure amplitude build-up inside the intake at the flutter condition, thus highlighting the link between the acoustic properties of the intake duct and fan blade flutter.

Commentary by Dr. Valentin Fuster
2003;():365-377. doi:10.1115/GT2003-38484.

An experiment has been carried out to enhance the understanding of 3D blade aeroelastic mechanisms and to produce test data of realistic configurations for validation of advanced 3D aeromechanical methods. A low speed rig with a compressor cascade consisting of seven prismatic blades of controlled diffusion profile has been commissioned. The middle blade is mechanically driven to oscillate in a 3D bending/flapping mode. At a nominal steady flow condition unsteady pressure measurements were performed at six spanwise sections for three different reduced frequencies and two different tip-clearance gaps. Off-board pressure transducers were utilized in conjunction with a transfer-function method to correct tubing distortion errors. The linearity of aerodynamic response is confirmed by the tests with different blade oscillation amplitudes, which enables the tuned cascade results to be constructed by using the Influence Coefficient Method. The measured results illustrate fully three-dimensional unsteady behaviour. Strong spanwise unsteady interaction leads to a non-proportional distribution of pressure amplitude at different spanwise locations. The tests with different tip-clearance gaps (1–2% span) show that the near tip region is destabilised as the tip gap is increased. This may be attributed to the local unloading of the corresponding steady flow. The destabilised region is seen to extend to approximately 20% of the blade span. The total aerodynamic damping at the least stable inter-blade phase angle has been reduced by 27%, when the tip gap is increased from nearly zero to 2% span.

Commentary by Dr. Valentin Fuster
2003;():379-388. doi:10.1115/GT2003-38560.

A computationally efficient time-accurate vortex method for unsteady incompressible flows through multiple blade row systems is presented. The method represents the boundary surfaces using vortex systems. A local coordinate system is assigned to each independently moving blade row. Blade shed vorticity is determined from two generating mechanisms and convected using the Euler equation. The first mechanism of vorticity generation is a potential mechanism from a nonlinear unsteady pressure-type Kutta condition applied at the blade trailing edges. The second mechanism is a viscous mechanism from a viscous wake vorticity (VWV) model implemented to simulate the viscous shear layers on the blade pressure and suction sides. Two different two-blade-row compressor systems, a rotor/stator (R/S) system and a stator/rotor (S/R) system, were used to investigate the interaction forces on each blade row. Computational results of the potential and viscous interaction forces are presented and compared to measurements. The comparison suggests that the viscous wake interaction accounts for 25–30% of the peak loading for an axial spacing of 10% chord length between the blade rows. The efficient computational method is particularly attractive for blade indexing study. Therefore a three-blade-row rotor/stator/rotor (R1/S/R2) compressor system is used to demonstrate the indexing calculations between the two rotor positions. Resultant forces on each blade row are presented for ten rotor indexing positions and three axial gap sizes for the gaps between R1 and S and between S and R2. The unsteady peak-to-peak force can reach 10–15% of inflow dynamic head for the gap spacing investigated. The minimum-to-maximum variation of the unsteady force can account for 40–50% of averaged unsteady force.

Topics: Compressors , Blades
Commentary by Dr. Valentin Fuster
2003;():389-398. doi:10.1115/GT2003-38632.

The paper presents a method to investigate the flutter appearance in a cascade, where blades are connected together in a number of identical sectors. The key parameters of the method are vibration amplitudes and mode shapes of the blades belonging to the same sector. The aerodynamic response from a sectored vane cascade is calculated based on the aerodynamic work influence coefficients of freestanding blades performed with two-dimensional inviscid linearized flow solver. A case study based upon the presented methodology shows that, despite stabilizing effect of tying blades together into sectors, a sectored vane consisting of six low-pressure turbine blades vibrating with real single modes, and identical amplitudes can be unstable at realistic design conditions.

Commentary by Dr. Valentin Fuster
2003;():399-406. doi:10.1115/GT2003-38634.

In this paper, we investigate non-synchronous vibrations (NSV) in turbomachinery, an aeromechanic phenomenon in which rotor blades are driven by a fluid dynamic instability. Unlike flutter, a self-excited vibration in which vibrating rotor blades and the resulting unsteady aerodynamic forces are mutually reinforcing, NSV is primarily a fluid dynamic instability that can cause large amplitude vibrations if the natural frequency of the instability is near the natural frequency of the rotor blade. In this paper, we present both experimental and computational data. Experimental data was obtained from a full size compressor rig where the instrumentation consisted of blade-mounted strain gages and case-mounted unsteady pressure transducers. The computational simulation used a three-dimensional Reynolds averaged Navier-Stokes (RANS) time accurate flow solver. The computational results suggest that the primary flow features of NSV are a coupled suction side vortex shedding and a tip flow instability. The simulation predicts a fluid dynamic instability frequency that is in reasonable agreement with the experimentally measured value.

Topics: Compressors , Blades
Commentary by Dr. Valentin Fuster
2003;():407-414. doi:10.1115/GT2003-38640.

Forming the first part of a two-part paper, the methodology of an efficient frequency-domain approach for predicting the forced response of turbomachinery blades is presented. The capability and computational efficiency of the method are demonstrated in Part Two with a three-stage transonic compressor case. Interaction between fluid and structure is dealt with in a loosely coupled manner, based on the assumption of linear aerodynamic damping and negligible frequency shift. The Finite Element (FE) package ANSYS is used to provide the mode shape and natural frequency of a particular mode, which is interpolated onto the CFD mesh. The linearised unsteady Navier-Stokes equations are solved in the frequency domain using a single-passage approach to provide aerodynamic excitation and damping forces. Two methods of obtaining the single degree-of-freedom forced response solution are demonstrated: the Modal Reduction Technique, solving the modal forced response equation in modal space; and a new Energy Method, an alternative method allowing calculations to be performed directly and simply in physical space. Both methods are demonstrated in a preliminary case study of the NASA R67 transonic fan blade with excitation of the 1st torsion mode due to a hypothetical inlet distortion.

Commentary by Dr. Valentin Fuster
2003;():415-422. doi:10.1115/GT2003-38642.

This is part two of a two-part paper. Part One describes the methodologies of a blade forced response prediction system. The emphasis of this part is to demonstrate the capability and computational efficiency of the system for predicting blade forced response. Part two firstly presents verification of the multistage time-linearized unsteady flow solver through comparison of predicted blade surface pressure distributions with data measured on a VKI transonic turbine stage. It concludes with presentation of the results of an analysis carried out on the last stage rotor blade of an ALSTOM three-stage transonic test compressor. In the analysis, strain gauge results together with Finite Element (FE) modal analysis identify the resonant crossings. The mode shape of the blade vibration is used in the CFD code to predict the blade aerodynamic damping. The aerodynamic damping is compared with the blade system damping obtained from the strain gauge tests. The variation is shown of aerodynamic and mechanical damping with blade mode shape. The blade unsteady modal forces induced by the upstream stators are derived from the calculated unsteady flows. The blade vibration at three resonant crossings is compared with those given by strain gauge measurements. Good comparisons and high computational efficiency demonstrate that the forced response methodologies described in Part One can be used in the blade design process to tackle blade aeromechanical issues.

Commentary by Dr. Valentin Fuster
2003;():423-428. doi:10.1115/GT2003-38694.

A current preliminary design method for flutter of low pressure turbine blades and vanes only requires knowledge of the reduced frequency and mode shape (real). However, many low pressure turbine (LPT) blade designs include a tip shroud, that mechanically connects the blades together in a structure exhibiting cyclic symmetry. A proper vibration analysis produces a frequency and complex mode shape that represents two real modes phase shifted by 90 degrees. This paper describes an extension to the current design method to consider these complex mode shapes. As in the current method, baseline unsteady aerodynamic analyses must be performed for the 3 fundamental motions, two translations and a rotation. Unlike the current method work matrices must be saved for a range of reduced frequencies and interblade phase angles. These work matrices are used to generate the total work for the complex mode shape. Since it still only requires knowledge of the reduced frequency and mode shape (complex), this new method is still very quick and easy to use. Theory and an example application are presented.

Commentary by Dr. Valentin Fuster
2003;():429-437. doi:10.1115/GT2003-38904.

This paper presents an adjoint analysis for 3d unsteady viscous flows aimed at the calculation of linear worksum sensitivities involved in turbomachinery forced response predictions. The worksum values are normally obtained from linear harmonic flow calculations but can also be computed using the solution to the adjoint of the linear harmonic flow equations. The adjoint method has a clear advantage over the linear approach if used within a rotor forced vibration minimization procedure which requires the structural response to a large number of different flow excitation sources characterized by a unique frequency and inter-blade phase angle. Whereas the linear approach requires a number of linear flow calculations at least equal to the number of excitation sources, the adjoint method reduces this cost to a single adjoint solution for each structural mode of rotor response. A practical example is given to illustrate the dramatic computational saving associated with the adjoint approach.

Commentary by Dr. Valentin Fuster

Structures and Dynamics: Rotor Dynamics and Magnetic Bearings

2003;():439-446. doi:10.1115/GT2003-38024.

Currently, the use of magnetic levitation systems has incremented in lieu of the many advantages they present respect to conventional systems. They provide frictionless operation and thus, a wearless life, eliminate the need for lubricants and allow for active vibration control. However, there are some limitations to their use, the dynamic load capacity is restricted by the magnetic properties of the materials used in their constructions and, therefore, their tolerance to large dynamic loads, such as in the case of blade loss or similar sudden failures, is small. For these cases, as well as for the case of bearing power loss, all commercial magnetic suspensions contain a safety backup system, usually consisting of roller bearings that avoid contact between stationary and rotating parts. The present work analyses the behavior of a rotor supported by a magnetic radial bearing on the non-drive end, which is operated in an overload regime. In this regime, a series of impacts occurs between the rotor and the backup bearing, which results on a highly non-linear system that might become unstable depending on the geometry, the control algorithm, the speed and excitation conditions. A non-linear model is proposed. The equations are separated into two regimes, one when the rotor is levitated and one during contact with the backup bearings; the contact is modeled by kinematic conditions. The magnetic bearing forces are estimated using a non-linear model and a PID algorithm is considered as a system’s control strategy. Rigid body theory for planar collision is considered for description of impacts between the backup bearing and the rotor.

Topics: Rotors
Commentary by Dr. Valentin Fuster
2003;():447-458. doi:10.1115/GT2003-38038.

A new approach has been developed and utilized to determine the flow field perturbations (i.e. disturbances due to rotor whirl) upstream of a non-contacting seal. The results are proposed for use with bulk-flow perturbation and CFD-perturbation seal rotordynamic models, as well as with fully 3-D CFD models, to specify the approximate inlet boundary flow disturbance values at the computational domain inlet. The radially bulk-averaged disturbance quantities were evaluated in the upstream chamber from nearly 40 cases of geometry/operating conditions. The proposed upstream chamber boundary conditions are applicable for liquid as well as gas seals. For each of the measurement test cases considered, improved agreement with measurements was obtained when using the new boundary conditions, even though there was generally little room for improvement when not using the new boundary conditions. Based on the findings in this study it is recommended that the first-order correlations developed here be used to specify approximate boundary conditions at the domain inlet to be located in the upstream chamber.

Commentary by Dr. Valentin Fuster
2003;():459-472. doi:10.1115/GT2003-38041.

The vibration of cracked rotor is investigated by numerical method. The FEM is used to model the rotor with cracks. Six degrees of freedom are considered in each elemental node. Full 6×6 flexibility matrix is deduced by Papadopoulos and Dimarogonas’ method, and 12×12 stiffness matrix of cracked element is derived. The influence of one or more cracks on the natural frequencies and different modals (including bending modal, torsion modal and longitudinal modal) of cracked rotor is explored. Vibration responses of rotor with open cracks or breathing crack loading by eccentric force and rotor gravity force are obtained and analyzed by numerical integer method and spectral technology. The coupling of lateral, longitudinal and torsion vibrations due to transverse surface crack is studied. It is concluded that the above research is useful in detecting crack in rotor.

Commentary by Dr. Valentin Fuster
2003;():473-480. doi:10.1115/GT2003-38146.

A general method is presented for obtaining the unbalance response orbit of a gear-coupled two-shaft rotor-bearing system, based on the finite element approach. Specifically, analytical solutions of the maximum and minimum radii of the orbit are proposed. The method has been applied to the unbalance response analysis of a 600 kW turbo-chiller rotor-bearing system, having a bull-pinion speed increasing gear. Bumps in the unbalance responses have been observed at the first torsional natural frequency because of the coupling between the lateral and torsional dynamics due to gear meshing. In addition, the analytical solutions have been validated with results obtained by a full numerical approach. The proposed method can be generally applied to an analysis of the unbalance response orbits of dual-shaft rotor-bearing systems coupled by bearings as well, which are often found in aerospace gas turbine engines.

Topics: Bearings , Gears , Rotors
Commentary by Dr. Valentin Fuster
2003;():481-489. doi:10.1115/GT2003-38225.

In this paper the complete set of modified Reynolds’ equations for the active lubrication is presented. The solution of such a set of equations allows the determination of stiffness and damping coefficients of actively lubricated bearings. These coefficients are not just dependent on Sommerfeld number, as it would be the case of conventional hydrodynamic bearings, but they are also dependent on the excitation frequencies and gains of the control loop. Stiffness as well as damping coefficients can be strongly influenced by the choice of the control strategy, servo valve dynamics and geometry of the orifices distributed over the sliding surface. The dynamic coefficients of tilting-pad bearings with and without active lubrication and their influence on an industrial compressor of 391 Kg, which operates with a maximum speed of 10,200 rpm, are analyzed. In the original compressor design, the bearing housings are mounted on squeeze-film dampers in order to ensure reasonable stability margins during full load condition (high maximum continuous speed). Instead of having a combination of tilting-pad bearings and squeeze-film dampers, another design solution is proposed and theoretically investigated in the present paper, i.e. using actively lubricated bearings. By choosing a suitable set of control gains, it is possible not only to increase the stability of the rotor-bearing system, but also enlarge its operational frequency range.

Commentary by Dr. Valentin Fuster
2003;():491-498. doi:10.1115/GT2003-38453.

This paper provides an overview of the current available technologies for automated machinery condition evaluation and fault diagnosis within an overall plant asset management system. The paper presents a basic overview of an integrated plant asset management system, and focuses on the available technologies for automated diagnostics including statistical analysis of data, parametric model diagnosis, non-parametric model diagnosis (artificial neural networks), and rule-based diagnostics including expert systems and fuzzy logic. The current state-of-the-art and the expected realistic future developments are discussed.

Topics: Machinery
Commentary by Dr. Valentin Fuster
2003;():499-507. doi:10.1115/GT2003-38457.

A recently developed finite length model of squeeze film dampers is extended and used in predicting the behavior of a rigid rotor supported by squeeze film dampers (SFDs). The model is based on a perturbation solution of Reynolds’ equation. The finite length SFD damping coefficients are presented for various L/R ratios. The effect of damper finite length is studied. Simulations of the behavior of a rigid rotor with the finite length SFDs illustrate the response of the roto-rbearing system. The accuracy of the finite damper model is shown for cases comparable to short and long dampers models. The short damper and long damper models are generally accepted to be valid for L/D < 1/4, and for L/D > 4, respectively. The capability of the finite length damper model to capture the main essence of the L/R ratio on the rotor response at resonance is illustrated. Analytical formulae for damping estimates are provided for finite length dampers. It is shown that the finite length damper actually provides less damping than either the short or the long damper models, which means that current design practices actually overestimate the SFD damping capabilities.

Topics: Dampers
Commentary by Dr. Valentin Fuster
2003;():509-517. doi:10.1115/GT2003-38583.

A simple procedure, with potential as a field resource, for identification of bearing support parameter from recorded transient rotor responses due to impact loads follows. The method is applied to a test rotor supported on a pair of mechanically complex bearing supports, each comprising a tilting pad bearing in series with an integral squeeze film damper. Identification of frequency dependent bearing force coefficients is good at a rotor speed of 2,000 rpm. Stiffness coefficients are best identified in the low frequency range (below 25 Hz) while damping coefficients are best identified in the vicinity of the first natural frequency (48 Hz) of the rotor bearing system. The procedure shows that using multiple-impact frequency averaged rotor responses reduces the variability in the identified parameters. The identification of frequency-dependent force coefficients at a constant rotor speed is useful to assess rotor-bearing system stability.

Commentary by Dr. Valentin Fuster
2003;():519-526. doi:10.1115/GT2003-38585.

This paper describes a procedure suitable for field implementation that allows identification of synchronous bearing support parameters (force coefficients) from recorded rotor responses to imbalance. The experimental validation is conducted on a test rotor supported on two dissimilar bearing supports, both mechanically complex, each comprising a hydrodynamic film bearing in series with a squeeze film damper and elastic support structure. The identification procedure requires a minimum of two different imbalance distributions for identification of force coefficients from the two bearing supports. Presently, the test rotor responses show minimal cross-coupling effects, as also predicted by analysis, and the identification procedure disregards cross-coupled force coefficients thereby reducing its sensitivity to small variations in the measured response. The procedure renders satisfactory force coefficients in the speed range between 1,500 and 3,500 rpm, enclosing the rotor-bearing system first critical speed. The identified direct force coefficients are in accordance with those derived from the impact load excitations presented in a companion paper [1].

Commentary by Dr. Valentin Fuster
2003;():527-534. doi:10.1115/GT2003-38595.

A method called model reconciliation (MR) has previously been reported which modifies analytic rotordynamic models to make them match experimental data. An identified model derived from the experimental data and a nominal model from the engineering analysis are needed; an auxiliary dynamic system is then synthesized to modify the dynamics of the nominal model to match the identified model. The combination of the engineering model and the auxiliary dynamic system is the reconciled model. Due to numerous experimental uncertainties, the identified model may contain significant errors. These errors are then embedded by the reconciliation process so that the reconciled model is incorrect, and sometimes quite poor. A robust control synthesis method is presented here which accounts for model error bounds in generating the auxiliary dynamics to minimize their impact on the final model. To examine the validity of the process, extra data as a reference are used to compare with the data produced by the reconciled model. Both simulated data and experimental data are used to demonstrate the results of the method. The error model permits the synthesis to ignore spurious features of the identified model, thereby producing more reasonable results.

Commentary by Dr. Valentin Fuster
2003;():535-542. doi:10.1115/GT2003-38596.

The rotordynamic performance of API 617 standards provides two primary requirements. First, the standard stipulates system damping near the expected operating speed range. Second, the standard requires a specific bound of the worst case unbalance response. The problem this poses for machine designers is 1) feasibility: can bearings be designed for a given rotor to meet API 617 and 2) if so, how can these bearings be designed? Our primary effort in this research is to convert the API requirements to a control design objective for a bearing. This permits direct assessment of the feasible design problem as well as providing a means to synthesize optimal bearing dynamics. In addition to providing synthesis of magnetic bearings, the resulting bearing transfer functions give direct guidance to selection of more conventional fluid film or rolling element bearings.

Commentary by Dr. Valentin Fuster
2003;():543-550. doi:10.1115/GT2003-38597.

This paper discusses frequency domain identification for a MIMO active magnetic bearing (AMB) rigid rotor system based on measured open-loop frequency responses. The frequency responses measured by a digital controller include the effects of sensor, amplifier, and discretization dynamics. These dynamics affect the identification of poles of the AMB system as well as the achievable performance of the controller. A new reduced order frequency domain identification of a multi-input/multi-output (MIMO) AMB rigid rotor system is proposed, which implicitly includes these dynamics along with the rest of the identified plant. The proposed identification scheme consists of three distinct steps: i) sensor, amplifier and delay dynamics; ii) rigid modes; and iii) flexible modes. The proposed scheme directly tackles the open-loop MIMO frequency data and is based on an explicit MIMO parameterization. An accurate reduced order model, suitable for advanced control synthesis, can be obtained through the proposed identification scheme.

Commentary by Dr. Valentin Fuster
2003;():551-560. doi:10.1115/GT2003-38612.

Large vibration of a rotor-bearing system excited by unbalance of rotor shaft or external forces can deteriorate the performance and shorten the lifetime of the system. The hydrodynamic bearing can provide desirable damping for a rotor-bearing system. In order to fully utilize the function of the hydrodynamic bearing for vibration reduction, a state-space technique is developed to identify the parameters (stiffness and damping) of the linearized hydrodynamic bearing. The eigensystem realization algorithm (ERA) is adopted to find the discrete state space model of system. It is shown that the ERA approach can be a very effective way for identification of the rotor-bearing system. The discrete state space model is further transformed to the continuous model that can be utilized to obtain the coefficients for the hydrodynamic bearing system. By comparing the output signal of the identified system and the nonlinear rotor-bearing dynamic model, the identification accuracy is verified. More simulation results on different values of eccentricity are also plotted to show the characteristic of a hydrodynamic bearing.

Commentary by Dr. Valentin Fuster
2003;():561-567. doi:10.1115/GT2003-38641.

The purpose of this study is to investigate the dynamics of a nonlinear hydrodynamic thrust bearing-mounted rigid rotor subjected to an unbalance force and parametric excitations. The parametric excitations include hydrodynamic forces and momenta of the thrust bearing when the rotor is subjected to an axial harmonic excitation. The effect of an eccentric axial excitation is also considered and the associated equations of motion are derived. Nonlinear phenomena including periodic response, hysteresis cycle, chaotic motion, etc. are investigated. The bifurcation values of parametric and unbalance excitation for steady state response are determined. The solutions of lateral response are dominated by the magnitude and frequency of axial excitation under light unbalance. The results show that the effect of axial excitation on bifurcation phenomenon of lateral vibration is significant as the eccentricity of axial force is enlarged.

Commentary by Dr. Valentin Fuster
2003;():569-582. doi:10.1115/GT2003-38659.

Many practical rotor dynamic systems contain shaft/rotor elements that are highly susceptible to transverse cross-sectional cracks due to fatigue. The early detection of mechanical malfunction that can be provided by an effective vibration monitoring system is essential. Two theoretical analyses, global and local asymmetry crack models, are utilized to identify characteristics of the system response that may be directly attributed to the presence of a transverse crack in a rotating shaft. A model consisting of an overhung whirling rotor is utilized to match an experimental test rig. A 2X harmonic component of the system response is shown to be the primary response characteristic resulting from the introduction of a crack. Once the unique characteristics of the system response are identified, they serve then as target observations for the monitoring system.

Commentary by Dr. Valentin Fuster
2003;():583-591. doi:10.1115/GT2003-38750.

In previous work the authors presented a Lorentz self-bearing motor design targeted for precision pointing and smooth angular slewing applications. The motor also offers potential advantages when operated as a synchronous machine at high speed including larger power densities and shorter shafts. In this paper, the closed loop performance of the motor at low transient speeds (0–588 rpm) is presented. Using these results, several challenges to achieving high-speed rotation are identified and discussed. The most significant is the heavy cross coupling within the actuator which limits bearing stiffness and stability, and is amplified at rotor natural frequencies resulting in potential loss of levitation when passing through critical speeds. Of particular interest is the discovery of a significant cross coupling effect between the radial and tangential directions. A theory is put forth explaining this effect.

Topics: Engines , Bearings
Commentary by Dr. Valentin Fuster
2003;():593-600. doi:10.1115/GT2003-38753.

An experimental method is proposed to obtain simplified mathematical models of rotating equipment systems. The Instrumental Variable Filter (IVF) method is applied to estimate mass, damping and stiffness force coefficients within a frequency range, through experimental measurements. This method is based on the least squares approximation technique and it uses analytical weight functions to reduce the effect of noise in the measurements. The experimental data is obtained for different configurations of rotating equipments, which consist of rigid wheels, a flexible shaft supported by bushing bearings, an electrical motor, a base-plate, and a concrete foundation. Frequency response functions (FRF) were obtained by impact excitation techniques. In the tests, the unbalanced response measurements were compared with the ones predicted by the IVF model. The method allows the study of mass, damping and stiffness force coefficients as a function of excitation frequency. Linearities and non-linearities of phenomena are identified, and the method sums up all the individual components into a definition for the system. The tests were conducted by operating, or not, the motor, in order to evaluate the IVF method in both cases. The high correlation between the IVF (FRF, and unbalance responses) and the actual measurements of the FRF and unbalance responses, shows that the method generates useful mathematical models of dynamic systems, that can have industrial applications. Modal analysis methods were used to compare the natural frequencies and the damping ratios, obtained by dynamic coefficients estimation.

Commentary by Dr. Valentin Fuster
2003;():601-606. doi:10.1115/GT2003-38783.

This paper discusses the interaction of support structures on the dynamics of a dual rotor system. The system considered is a dual rotor, supported on flexible bearings, which are in turn mounted in a flexible casing. ANSYS® is used for modeling and meshing the dual rotors and the casing. The rotors are modeled using solid elements. The bearings are simulated as springs, wherein the direct and cross coupled stiffness and damping coefficients are applied. The casing is also modeled and meshed in ANSYS® using solid elements. Different spin speeds are applied to the dual rotor system. The casing is also rotated at a zero spin speed. The Stress stiffening and spin softening options are also set on for the dual rotor system. The system natural frequencies are obtained for different spin speeds and the Campbell diagram of the system is plotted. The critical speeds due to per revolution excitations are then extracted from the Campbell diagram.

Commentary by Dr. Valentin Fuster
2003;():607-617. doi:10.1115/GT2003-38833.

Gas film bearings offer unique advantages enabling successful deployment of high-speed micro-turbomachinery (< 0.4 MW). Current applications encompass micro power generators, air cycle machines and turbo expanders. Mechanically complex gas foil bearings are in use; however, their excessive cost and lack of calibrated predictive tools deter their application to mass-produced systems. The present investigation provides experimental results for the rotordynamic performance of a small rotor supported on simple and inexpensive hybrid gas bearings with static and dynamic force characteristics desirable in high-speed turbomachinery. These characteristics are adequate load support, stiffness and damping coefficients, low friction and wear during rotor startup and shutdown, and most importantly, enhanced rotordynamic stability. The test results evidence the paramount effect of feed pressure on early rotor lift off and substantially higher threshold speeds of rotordynamic instability. Higher supply pressures also determine larger bearing direct stiffnesses, and thus bring an increase in the rotor-bearing system critical speed albeit with a reduction in damping ratio.

Commentary by Dr. Valentin Fuster
2003;():619-632. doi:10.1115/GT2003-38859.

Current applications of gas film bearings in high-speed oil-free micro-turbomachinery (<0.4 MW) require calibrated predictive tools to successfully deploy their application to mass-produced systems, for example oil-free turbochargers. The present investigation details the linear rotordynamic analysis of a test rotor supported on externally pressurized gas bearings. Model predictions are compared with the test rotordynamic response determined through comprehensive experiments conducted on a small rotor supported on three lobed hybrid (hydrostatic/hydrodynamic) rigid gas bearings. Predictions for the rotor-bearing system synchronous response to imbalance show good agreement with measurements during rotor coast downs, and manifest a decrease in damping ratio as the level of external pressurization increases. The rotor-bearing eigenvalue analysis forwards natural frequencies in accordance with the measurements, and null damping ratios evidence the threshold speeds of rotordynamic instability. Estimated whirl frequency ratios are typically 50% of rotor speed, thus predicting sub synchronous instabilities at lower rotor speeds than found experimentally when increasing the magnitude of feed pressurization. Rationale asserting the nature of the discrepancies calls for further analysis.

Commentary by Dr. Valentin Fuster
2003;():633-642. doi:10.1115/GT2003-38870.

Open loop, experimental force and power measurements of a radial, redundant-axis, magnetic bearing at temperatures to 1000°F (538°C) and rotor speeds to 15,000 RPM along with theoretical temperature and force models are presented in this paper. The experimentally measured force produced by a single C-core circuit using 22 A was 600 lb. (2.67 kN) at room temperature and 380 lb. (1.69 kN) at 538°C. These values were compared with force predictions based on a 1D magnetic circuit analysis and a thermal analysis of gap growth as a function of temperature. The analysis showed that the reduction of force at high temperature is mostly due to an increase in radial gap due to test conditions, rather that to reduced core permeability. Tests under rotating conditions showed that rotor speed has a negligible effect on the bearing’s static force capacity. One C-core required approximately 340 W of power to generate 190 lb. (845 N) of magnetic force at 538°C, however the magnetic air gap was much larger than at room temperature. The data presented is after bearing operation for eleven total hours at 538°C and six thermal cycles.

Commentary by Dr. Valentin Fuster
2003;():643-650. doi:10.1115/GT2003-38912.

The NASA Glenn Research Center (GRC) has developed a magnetic bearing system for the Dynamic Spin Rig (DSR) with a fully suspended shaft that is used to perform vibration tests of turbomachinery blades and components under spinning conditions in a vacuum. Two heteroplolar radial magnetic bearings and a thrust magnetic bearing and the associated control system were integrated into the DSR to provide magnetic excitation as well as non-contact magnetic suspension of a 15.88 kg (35 lb) vertical rotor with blades to induce turbomachinery blade vibration. For rotor levitation, a proportional-integral-derivative (PID) controller with a special feature for multidirectional radial excitation worked well to both support and shake the shaft with blades. However, more advanced controllers were developed and successfully tested to determine the optimal controller in terms of sensor and processing noise reduction, smaller rotor orbits, more blade vibration amplitude, and energy savings for the system. The test results of a variety of controllers that were demonstrated up to 10,000 rpm are shown. Furthermore, rotor excitation operation andconceptual study of active blade vibration control are addressed.

Commentary by Dr. Valentin Fuster
2003;():651-659. doi:10.1115/GT2003-38931.

The traditional method for bearing and damper analysis usually involves a development of rather complicated numerical calculation programs that may just focus on a simplified and specific physical model. The application of the general CFD codes may make this analysis available and effective where complex flow geometries are involved or when more detailed solutions are needed. In this study, CFX-TASCflow is employed to simulate various fixed geometry fluid-film bearing and damper designs. Some of the capabilities in CFX-TASCflow are applied to simulate the pressure field and calculate the static and dynamic characteristics of hydrodynamic, hydrostatic and hybrid bearings as well as squeeze film dampers. The comparison between the CFD analysis and current computer programs used in industry has been made. The results show reasonable agreement in general. Some of possible reasons for the differences are discussed. It leaves room for further investigation and improvement on the methods of computation.

Commentary by Dr. Valentin Fuster
2003;():661-667. doi:10.1115/GT2003-38984.

Labyrinth seals are used in various kinds of turbo machines to reduce internal leakage flow. The working fluid or, the gas passing through the rotor shaft labyrinth seals, often generates driving force components that may increase the unstable vibration of the rotor. It is important to know the accurate rotordynamic force components for predicting the instability of the rotor-bearing-seal system. The major goals of this research was to calculate the rotordynamic force of a labyrinth seals utilizing a commercial CFD program and to further compare those results to an existing bulk flow computer program currently used by major US machinery manufacturers. The labyrinth seals of a steam turbine and a compressor eye seal are taken as objects of analysis. For each case, a 3D model with eccentric rotor was solved to obtain the rotordynamic force components. The leakage flow and rotor dynamics force predicted by CFX TASCFlow are compared with the results the existing bulk flow analysis program DYNLAB. The results show that the bulk flow program gives a pessimistic prediction of the destabilizing forces for the conditions under investigation. Further research work will be required to fully understand the complex leakage flows in turbo machinery.

Commentary by Dr. Valentin Fuster

Scholar Lecture

2003;():669-696. doi:10.1115/GT2003-38866.

The confluence of market demand for greatly improved compact power sources for portable electronics with the rapidly expanding capability of micromachining technology has made feasible the development of gas turbines in the millimeter-size range. With airfoil spans measured in 100’s of microns rather than meters, these “microengines” have about 1 millionth the air flow of large gas turbines and thus should produce about 1 millionth the power, 10–100 W. Based on semiconductor industry-derived processing of materials such as silicon and silicon carbide to submicron accuracy, such devices are known as micro-electro-mechanical systems (MEMS). Current millimeter-scale designs use centrifugal turbomachinery with pressure ratios in the range of 2:1 to 4:1 and turbine inlet temperatures of 1200–1600 K. The projected performance of these engines are on a par with gas turbines of the 1940’s. The thermodynamics of MEMS gas turbines are the same as those for large engines but the mechanics differ due to scaling considerations and manufacturing constraints. The principal challenge is to arrive at a design which meets the thermodynamic and component functional requirements while staying within the realm of realizable micromachining technology. This paper reviews the state-of-the-art of millimeter-size gas turbine engines, including system design and integration, manufacturing, materials, component design, accessories, applications, and economics. It discusses the underlying technical issues, reviews current design approaches, and discusses future development and applications.

Commentary by Dr. Valentin Fuster

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