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Editorial

J. Eng. Gas Turbines Power. 2005;127(2):229-230. doi:10.1115/1.1870018.
FREE TO VIEW
Commentary by Dr. Valentin Fuster

TECHNICAL PAPERS: Gas Turbines: Aircraft Engine

J. Eng. Gas Turbines Power. 2005;127(2):231-239. doi:10.1115/1.1807412.

This paper documents the pioneering work of Dr. Max Bentele during his long and distinguished career in Germany, the UK, and the United States. His early work on turbojets at the Heinkel-Hirth Corporation in conjunction with his life-long friend, Dr. Hans von Ohain, culminated in the development of the advanced HeS 011 turbojet. Dr. Bentele’s pioneering work in the area of blade vibration is documented along with details of his spectacular solution of the turbine blade vibration problem of the Junkers 004B engine which propelled the world’s first operational jet fighter—the Me-262. Also covered are his pioneering contributions to turbine blade cooling and blade manufacturing and his important work at Curtiss—Wright and Avco Lycoming prior to his retirement.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2005;127(2):240-248. doi:10.1115/1.1806453.

Bolted joints are used at numerous locations in the rotors and carcass structure of modern aircraft turbine engines. This application makes the design criteria and process substantially different from that used for other types of machinery. Specifically, in addition to providing engine alignment and high-pressure gas sealing, aircraft engine structural joints can operate at high temperatures and may be required to survive very large applied loads which can result from structural failures within the engine, such as the loss of a fan blade. As engine bypass ratios have increased in order to improve specific fuel consumption, these so-called “Ultimate” loads increasingly dominate the design of bolted joints in aircraft engines. This paper deals with the sizing and design of both bolts and lever flanges to meet these demanding requirements. Novel empirical methods, derived from both component test results and correlated analysis have been developed to perform strength evaluation of both flanges and bolts. Discussion of analytical techniques in use includes application of the LS-DYNA™ code for modeling of high-speed blade impact events as related to bolted joint behavior.

Topics: Engines , Stress , Flanges , Design
Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2005;127(2):249-256. doi:10.1115/1.1806454.

This paper presents detailed heat flux measurements on a flat plate subjected to the ISO2685 [The International Organization for Standardization (ISO), 1992, “Aircraft—Environmental Conditions and Test Procedures for Airborne Equipment—Resistance to Fire in Designated Fire Zones,” ISO2685:1992(E)] standard, propane fueled burner used throughout the industry in aero-engine fire-certification. The authors have developed a custom-built heat transfer gauge to measure the heat flux from the burner under isothermal wall conditions. The heat flux from the standard burner is normally calibrated using either a water-cooled copper tube or a Gardon gauge, each sited at a single position in the flame. There are no reports in the literature of a detailed survey of heat flux distribution for the burner and the results are of considerable interest to engineers involved in fire-certification. The reported measurements constitute the first, detailed distribution of heat flux from the actual burner flame during a fire test. These measurements provided benchmark data which allowed the heat flux distribution from the ISO burner to be compared to levels derived from the low-temperature analog burner developed by the authors. The analog burner uses liquid crystals to measure heat transfer coefficient and adiabatic wall temperature on scale models of engine components and provides key data to facilitate the successful design of components used in fire zones. The objective of this paper is to further validate the low-temperature analog burner technique developed by the authors which simulates the standard large propane-air burner for fire-certification in aero engine.

Commentary by Dr. Valentin Fuster

TECHNICAL PAPERS: Gas Turbines: Combustion and Fuel

J. Eng. Gas Turbines Power. 2005;127(2):257-267. doi:10.1115/1.1806455.

The current demands for high performance gas turbine engines can be reached by raising combustion temperatures to increase power output. Predicting the performance of a combustor is quite challenging, particularly the turbulence levels that are generated as a result of injection from high momentum dilution jets. Prior to predicting reactions in a combustor, it is imperative that these turbulence levels can be accurately predicted. The measurements presented in this paper are of flow and thermal fields produced in a large-scale combustor simulator, which is representative of an aeroengine. Three-component laser Doppler velocimeter measurements were made to quantify the velocity field while a rake of thermocouples was used to quantify the thermal field. The results indicate large penetration depths for the high momentum dilution jets, which result in a highly turbulent flow field. As these dilution jets interact with the mainstream flow, kidney-shaped thermal fields result due to counter-rotating vortices that develop.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2005;127(2):268-275. doi:10.1115/1.1806838.

The drive to reduce emissions has led to the development of lean premixed combustors. However, lean premixed combustion is often associated with combustion oscillations which can be so severe that they can cause structural damage to the engine. Since the associated frequencies are typically of the order of hundreds of Hertz, there is a need for a compact device to absorb the noise which drives the oscillation. Helmholtz resonators are commonly used as absorbers of incident acoustic power. In addition they are relatively compact. However, their use in combustors creates practical issues, such as placement within the chamber, neck length, and cooling, which need to be addressed. In this paper we consider these practical problems and describe how to overcome them in a real combustor.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2005;127(2):276-285. doi:10.1115/1.1839920.

ALZETA Corporation has developed surface-stabilized fuel injectors for use with lean premixed combustors which provide extended turndown and ultralow NOx emission performance. These injectors use a patented technique to form interacting radiant and blue-flame zones immediately above a selectively perforated porous metal surface. This allows stable operation at low reaction temperatures. A previous ASME paper (IJPGC2002-26088) described the development of this technology from the proof-of-concept stage to prototype testing. In 2002 development of these fuel injectors for the 5.5 MW turbine accelerated. Additional single-injector rig tests were performed which also demonstrated ultralow emissions of NOx and CO at pressures up to 1.68 MPa (16.6 atm) and inlet temperatures up to 670°K (750°F). A pressurized multi-injector “sector rig” test was conducted in which two injectors were operated simultaneously in the same geometric configuration as that expected in the engine combustor liner. The multi-injector package was operated with various combinations of fired and unfired injectors, which resulted in low emissions performance and no adverse affects due to injector proximity. To date sub-3 ppm NOx emissions with sub-10 ppm CO emissions have been obtained over an operating range of 0.18–1.68 MPa (1.8–16.6 atm), inlet temperatures from 340 to 670K (186–750°F), and adiabatic flame temperatures from 1740 to 1840K (2670–2850°F). A full scale multi-injector engine simulation is scheduled for the beginning of 2003, with engine tests beginning later that year.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2005;127(2):286-294. doi:10.1115/1.1839921.

The objective of the work described in this paper was to identify a method of making measurements of the smoke particle size distribution within the sector of a gas turbine combustor, using a scanning mobility particle sizing (SMPS) analyzer. As well as gaining a better understanding of the combustion process, the principal reasons for gathering these data was so that they could be used as validation for computational fluid dynamic and chemical kinetic models. Smoke mass and gaseous emission measurements were also made simultaneously. A “water cooled,” gas sampling probe was utilized to perform the measurements at realistic operating conditions within a generic gas turbine combustor sector. Such measurements had not been previously performed and consequently initial work was undertaken to gain confidence in the experimental configuration. During this investigation, a limited amount of data were acquired from three axial planes within the combustor. The total number of test points measured were 45. Plots of the data are presented in two-dimensional contour format at specific axial locations in addition to axial plots to show trends from the primary zone to the exit of the combustor. Contour plots of smoke particle size show that regions of high smoke number concentration once formed in zones close to the fuel injector persist in a similar spatial location further downstream. Axial trends indicate that the average smoke particle size and number concentration diminishes as a function of distance from the fuel injector. From a technical perspective, the analytical techniques used proved to be robust. As expected, making measurements close to the fuel injector proved to be difficult. This was because the quantity of smoke in the region was greater than 1000 mg/m3. It was found necessary to dilute the sample prior to the determination of the particle number concentration using SMPS. The issues associated with SMPS dilution are discussed.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2005;127(2):295-300. doi:10.1115/1.1789512.

In order to reduce the dimensions of the combustor, swirl stabilized flames are used in heavy duty gas turbines. In our recent investigation of the swirling flow at a single heavy duty gas turbine burner under nonreacting conditions typical instabilities like precessing vortex cores and vortex shedding have been found (Schildmacher et al., Proceedings of the 6th European Conference on Industrial Furnaces and Boilers). In the present paper the experimental investigations will be discussed. Combustion instabilities have been analyzed by phase-locked laser doppler anemometer measurements. For the reacting flow, also combustion instabilities could be detected. The amplitude increases strongly with the equivalence ratio. The frequency of the oscillations for reacting conditions has been found to be slightly shifted towards lower frequencies compared to those of the corresponding nonreacting flow. In addition, for the reacting flow a linear and nonlinear range of oscillations could be discriminated.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2005;127(2):301-306. doi:10.1115/1.1789515.

The low-frequency response of the spray from a generic airblast diffusion burner with a design typical of an engine system has been investigated as part of an experimental study to describe the combustion oscillations of aeroengine combustors called rumble. The atomization process was separated from the complex instability mechanism of rumble by using sinusoidal forcing of the air mass flow rate without combustion. Pressure drop across the burner and the velocity on the burner exit were found to follow the steady Bernoulli equation. Phase-locked particle image velocimetry measurements of the forced velocity field of the burner show quasisteady behavior of the air flow field. The phase-locked spray characteristics were measured for different fuel flow rates. Here again quasi-steady behavior of the atomization process was observed. With combustion, the phase-locked Mie-scattering intensity of the spray cone was found to follow the spray behavior measured in the noncombusting tests. These findings lead to the conclusion that the unsteady droplet Sauter mean diameter mean and amplitude of the airblast atomizer can be calculated using the steady-state atomization correlations with the unsteady burner air velocity.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2005;127(2):307-313. doi:10.1115/1.1789516.

Combustion using silicon carbide coated, carbon–carbon composite porous inert media (PIM) was investigated. Two combustion modes, surface and interior, depending upon the location of flame stabilization, were considered. Combustion performance was evaluated by measurements of pressure drop across the PIM, emissions of NOx and CO, and the lean blow-off limit. Data were obtained for the two combustion modes at identical conditions for a range of reactant flowrates, equivalence ratios, and pore sizes of the PIM. Results affirm PIM combustion as an effective method to extend the blow-off limit in lean premixed combustion.

Commentary by Dr. Valentin Fuster

TECHNICAL PAPERS: Gas Turbines: Controls, Diagnostics, and Instrumentation

J. Eng. Gas Turbines Power. 2005;127(2):314-322. doi:10.1115/1.1772406.

The application of wavelet analysis to diagnose loose blades condition in gas turbines is examined in this paper. Experimental studies were undertaken to simulate loose blades condition occurring in gas turbines in an attempt to understand vibration response associated with loose blades under different operating conditions. Results showed that loose blades were undetectable under steady state operating condition. During turbine coast down, a loose blade could be detected based on the impactic signals induced by the loose blades on the rotor and thus excited the natural frequencies of the rotor assembly. Results from the coast down condition showed that wavelet analysis was more sensitive and effective than Fourier analysis for loose blade diagnosis. The severity, the number, and the configuration of the loose blades could be potentially estimated based on the pattern of the coast down wavelet map.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2005;127(2):323-328. doi:10.1115/1.1789153.

Kalman filters are often used to estimate the state variables of a dynamic system. However, in the application of Kalman filters some known signal information is often either ignored or dealt with heuristically. For instance, state-variable constraints (which may be based on physical considerations) are often neglected because they do not fit easily into the structure of the Kalman filter. This paper develops an analytic method of incorporating state-variable inequality constraints in the Kalman filter. The resultant filter is a combination of a standard Kalman filter and a quadratic programming problem. The incorporation of state-variable constraints increases the computational effort of the filter but significantly improves its estimation accuracy. The improvement is proven theoretically and shown via simulation results obtained from application to a turbofan engine model. This model contains 16 state variables, 12 measurements, and 8 component health parameters. It is shown that the new algorithms provide improved performance in this example over unconstrained Kalman filtering.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2005;127(2):329-339. doi:10.1115/1.1850491.

The removal of noise and outliers from measurement signals is a major problem in jet engine health monitoring. In this study, we look at the myriad filter as a substitute for the moving average filter that is widely used in the gas turbine industry. The three ideal test signals used in this study are the step signal that simulates a single fault in the gas turbine, while ramp and quadratic signals simulate long term deterioration. Results show that the myriad filter performs better in noise reduction and outlier removal when compared to the moving average filter. Further, an adaptive weighted myriad filter algorithm that adapts to the quality of incoming data is studied. The filters are demonstrated on simulated clean and deteriorated engine data obtained from an acceleration process from idle to maximum thrust condition. This data was obtained from published literature and was simulated using a transient performance prediction code. The deteriorated engine had single component faults in the low pressure turbine and intermediate pressure compressor. The signals are obtained from T2 (IPC total outlet temperature) and T6 (LPT total outlet temperature) engine sensors with their nonrepeatability values that were used as noise levels. The weighted myriad filter shows even greater noise reduction and outlier removal when compared to the sample myriad and a FIR filter in the gas turbine diagnosis. Adaptive filters such as those considered in this study are also useful for online health monitoring, as they can adapt to changes in quality of incoming data.

Commentary by Dr. Valentin Fuster

TECHNICAL PAPERS: Gas Turbines: Cycle Innovations

J. Eng. Gas Turbines Power. 2005;127(2):340-347. doi:10.1115/1.1839918.

Microturbines have a less complex mechanical design than large-size gas turbines that should make it possible to fit them with a more straightforward control system. However, these systems have very low shaft mechanical inertia and a fast response to external disturbances, such as load trip, that make this very difficult to do. Furthermore, the presence of the recuperator requires smooth variations to the Turbine Outlet Temperature (TOT), when possible, to ensure reduced thermal stresses to the metallic matrix. This paper, after a brief overview of microturbine control systems and typical transients, presents the expected transient behavior of two advanced cycles: the Externally Fired micro Gas Turbine (EFmGT) cycle, where the aim is to develop a proper control system set-up to manage safe part-load operations at constant rotational speed, and a solar Closed Brayton Cycle (CBC), whose control system has to ensure the maximum efficiency at constant rotational speed and constant Turbine Inlet Temperature (TIT).

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2005;127(2):348-357. doi:10.1115/1.1789513.

A massive effort towards sustainability is necessary to prevent global warming and energy sources impoverishment: both biomass and waste to energy conversion may represent key actions to reach this goal. At the present, state of the art available technologies for biomass and waste to energy conversion are similar and include low to mid efficiency grate incineration or fluidized bed combustion with steam power cycles or mid to high efficiency gas turbine based cycles through integrated gasification technology. Nevertheless, these plants are all available from mid-to-high scale range that can be highly intrusive on protected areas and socially unacceptable. This paper proposes an innovative, low cost, high efficiency plant in which the residue is gasified in the absence of oxygen (pyrolysis), in a rotary kiln, by means of a highly regenerative gas turbine based cycle. Pyrolysis is preferred to gasification, because the syngas obtained has a higher low heating value and produces char or tar as a by-product with an interesting energy content to be re-utilized inside the cycle. Different plant configurations are proposed and discussed through principal thermodynamic variables parametric analysis. Results show that very interesting efficiencies are obtainable in the 30–40% range for every plant scale. This fact shows how IPRP technology can provide an interesting alternative to traditional technologies, especially for the small size (below 5MW). Moreover, the IPRP technology provides a unique solution for microscale (below 500 kW) power plants, opening a new and competitive possibility for distributed biomass or waste to energy conversion systems where low environmental and social impact turns into higher interest and positive dissemination effect.

Commentary by Dr. Valentin Fuster

TECHNICAL PAPERS: Gas Turbines: Electric Power

J. Eng. Gas Turbines Power. 2005;127(2):358-368. doi:10.1115/1.1806456.

In this paper we describe the conceptual design and cooling blade development of a 1700°C-class high-temperature gas turbine in the ACRO-GT-2000 (Advanced Carbon Dioxide Recovery System of Closed-Cycle Gas Turbine Aiming 2000 K) project. In the ACRO-GT closed cycle power plant system, the thermal efficiency aimed at is more than 60% of the higher heating value of fuel (HHV). Because of the high thermal efficiency requirement, the 1700°C-class high-temperature gas turbine must be designed with the minimum amount of cooling and seal steam consumption. The hybrid cooling scheme, which is a combination of closed loop internal cooling and film ejection cooling, was chosen from among several cooling schemes. The elemental experiments and numerical studies, such as those on blade surface heat transfer, internal cooling channel heat transfer, and pressure loss and rotor coolant passage distribution flow phenomena, were conducted and the results were applied to the conceptual design advancement. As a result, the cooling steam consumption in the first stage nozzle and blade was reduced by about 40% compared with the previous design that was performed in the WE-NET (World Energy Network) Phase-I.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2005;127(2):369-374. doi:10.1115/1.1850490.

Mitsubishi completed design development and verification load testing of a steam-cooled M501H gas turbine at a combined cycle power plant at Takasago, Japan in 2001. Several advanced technologies were specifically developed in addition to the steam-cooled components consisting of the combustor, turbine blades, vanes, and the rotor. Some of the other key technologies consisted of an advanced compressor with a pressure ratio of 25:1, active clearance control, and advanced seal technology. Prior to the M501H, Mitsubishi introduced cooling-steam in “G series” gas turbines in 1997 to cool combustor liners. Recently, some of the advanced design technologies from the M501H gas turbine were applied to the G series gas turbine resulting in significant improvement in output and thermal efficiency. A noteworthy aspect of the technology transfer is that the upgraded G series M701G2 gas turbine has an almost equivalent output and thermal efficiency as H class gas turbines while continuing to rely on conventional air cooling of turbine blades and vanes, and time-proven materials from industrial gas turbine experience. In this paper we describe the key design features of the M701G2 gas turbine that make this possible such as the advanced 21:1 compressor with 14 stages, an advanced premix DLN combustor, etc., as well as shop load test results that were completed in 2002 at Mitsubishi’s in-house facility.

Commentary by Dr. Valentin Fuster

TECHNICAL PAPERS: Gas Turbines: Heat Transfer and Turbomachinery

J. Eng. Gas Turbines Power. 2005;127(2):375-382. doi:10.1115/1.1787509.

Preswirl nozzles are often used in gas turbines to deliver the cooling air to the turbine blades through receiver holes in a rotating disk. The distribution of the local Nusselt number, Nu, on the rotating disk is governed by three nondimensional fluid-dynamic parameters: preswirl ratio, βp, rotational Reynolds number, Reϕ, and turbulent flow parameter, λT. A scaled model of a gas turbine rotor–stator cavity, based on the geometry of current engine designs, has been used to create appropriate flow conditions. This paper describes how a thermochromic liquid crystal, in conjunction with a stroboscopic light and digital camera, is used in a transient experiment to obtain contour maps of Nu on the rotating disk. The thermal boundary conditions for the transient technique are such that an exponential-series solution to Fourier’s one-dimensional conduction equation is necessary. A method to assess the uncertainty in the measurements is discussed and these uncertainties are quantified. The experiments reveal that Nu on the rotating disk is axisymmetric except in the region of the receiver holes, where significant two-dimensional variations have been measured. At the higher coolant flow rates studied, there is a peak in heat transfer at the radius of the preswirl nozzles. The heat transfer is governed by two flow regimes: one dominated by inertial effects associated with the impinging jets from the preswirl nozzles, and another dominated by viscous effects at lower flow rates. The Nusselt number is observed to increase as either Reϕ or λT increases.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2005;127(2):383-388. doi:10.1115/1.1787514.

In high-efficiency gas turbine engines, the cooling air for the high-pressure turbine stage is expanded through stationary preswirl nozzles, transferred through the preswirl chamber, and delivered to the blade feed holes of the rotor. By accelerating the cooling air in the direction of rotation, the total temperature relative to the rotor disk and the pressure losses occurring at the receiver hole inlet can be reduced. The discharge behavior of a direct-transfer preswirl system has been investigated experimentally for different number of receiver holes and different inlet geometries, varying axial gap widths between stator and rotor and for rotational Reynolds numbers up to Reϕ=2.3×10 6. The discharge coefficients of the preswirl nozzles are given in the absolute frame of reference while the definition of the discharge coefficients of the receiver holes is applied to the rotating system in order to consider the work done by the rotor. A momentum balance is used to evaluate the deflection of the preswirled air entering the receiver holes. The flow in the preswirl chamber is characterized by introducing an effective velocity of the cooling air upstream of the rotor disk. The influences of geometrical parameters and operating points are reported and discussed in this paper.

Commentary by Dr. Valentin Fuster

TECHNICAL PAPERS: Gas Turbines: Industrial and Cogeneration

J. Eng. Gas Turbines Power. 2005;127(2):389-396. doi:10.1115/1.1806836.

Economic feasibility of microturbine cogeneration systems is investigated by analyzing relationships between the optimal number of microturbine units and the maximum energy demands under various conditions. For this purpose, a method to obtain the maximum energy demand at which the optimal number changes is proposed by combining a nonlinear equation problem and an optimal unit sizing problem hierarchically. Based on the proposed method, a map expressing the aforementioned relationships can be illustrated. Through numerical studies carried out on systems installed in hotels by changing the electrical generating efficiency and the capital unit cost of the microturbine cogeneration unit as parameters, the influence of the parameters on the economic feasibility of the microturbine cogeneration system is clarified.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2005;127(2):397-403. doi:10.1115/1.1839924.

This paper is a guideline to selecting the most appropriate technology for the power plant heat sink based on water availability, site location, and wastewater disposal requirements. The paper discusses wet as well as dry cooling systems and evaluates the impact of the heat sink technology on the performance and cost of combined cycle power plants. Cogeneration applications and cycling plant operations are also considered. For each proposed option, the performance, relative costs, and noise issues will be presented.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2005;127(2):404-409. doi:10.1115/1.1789993.

Gas turbines are projected to meet increasing power demand throughout the world. Cogeneration plants hold the promise of increased efficiency at acceptable cost. In a general case, a cogen plant could be able to meet power, heating and cooling demands. Yet those demands are normally uncoupled. Control and storage strategies need to be explored to ensure that each independent demand will be met continuously. A dynamic model of a mid-capacity system is developed, including gas and steam turbines, two heat recovery steam generators (HRSG) and an absorption-cooling machine. Controllers are designed using linear quadratic regulators (LQR) to control two turbines and a HRSG with some novelty. It is found that the power required could be generated exclusively with exhaust gases, without a duct burner in the high-pressure HRSG. The strategy calls for fuel and steam flow rate modulation for each turbine. The stability of the controlled system and its performance are studied and simulations for different demand cases are performed.

Commentary by Dr. Valentin Fuster

TECHNICAL PAPERS: Gas Turbines: Oil and Gas Applications

J. Eng. Gas Turbines Power. 2005;127(2):410-417. doi:10.1115/1.1789511.

The paper describes the effects of 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 (LES); 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 scales 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

TECHNICAL PAPERS: Gas Turbines: Structures and Dynamics

J. Eng. Gas Turbines Power. 2005;127(2):418-424. doi:10.1115/1.1789492.

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
J. Eng. Gas Turbines Power. 2005;127(2):425-436. doi:10.1115/1.1789514.

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
J. Eng. Gas Turbines Power. 2005;127(2):437-444. doi:10.1115/1.1807413.

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 22A 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 one-dimensional 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 than 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 are after bearing operation for eleven total hours at 538°C and six thermal cycles.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2005;127(2):445-451. doi:10.1115/1.1807415.

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 the 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

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