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Manufacturing Materials and Metallurgy

1982;():V005T11A001. doi:10.1115/82-GT-87.
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Burning residual oil in utility combustion turbines and the consequent deposition on blades and vanes may adversely affect reliability and operation. Corrosion and deposition data for combustion turbine materials have been obtained through dynamic testing in pressurized passages. The deposition produced by the 1900°F (1038°C) combustion gases from a simulated and a real residual oil on cooled Udimet 500 surfaces is described. Higher deposition rates for the doped fuel than for the real residual oil raised questions of whether true simulation with this approach can be achieved. Particles 4–8 μ m in dia predominated in the gas stream, with some fraction in the 0.1–12 μ m range. Deposition rates seemed to be influenced by thermophoretic delivery of small molten particles, tentatively identified as magnesium pyro and metavanadates and free vanadium pentoxide, which may act to bond the larger, solid particles arriving by inertial impaction to turbine surfaces. Estimated maintenance intervals for current utility turbines operating with washed and treated residual oil agreed well with field experience.

Topics: Combustion , Turbines
Commentary by Dr. Valentin Fuster
1982;():V005T11A002. doi:10.1115/82-GT-88.
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Experimental data obtained in heavy fuels operation of a gas turbine simulator with a water-cooled, transonic turbine nozzle cascade are presented. The ash fouling is characterized by the rate of decrease of the aerodynamic throat area. Particular attention is given to the cleanability of the ash deposits. A simple heat transfer analysis was performed to assist in evaluating the data. The rate of ash fouling in the water-cooled nozzle was found to be of the same order of magnitude as for conventional air-cooled designs. Cleanability, both on and off-line, was found to be significantly enhanced, thus making the water-cooled gas turbine an attractive alternative for heavy fuels applications.

Commentary by Dr. Valentin Fuster
1982;():V005T11A005. doi:10.1115/82-GT-96.
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Small gas turbine engine manufacturers, with Armed Services support, are utilizing advanced manufacturing technology programs to incorporate net shaped components into both the current and the next generation of small high performance gas turbine engines. A coordinated program is underway to provide near-net shaped components which will minimize machining costs, save critical materials, and in some cases provide increased component capabilities. These programs include isothermal forging of titanium, casting of titanium, HIP’ed titanium powder metal techniques, diffusion bonded dual property approaches, and a diffuser case fabrication process. The manufacturing technologies presented in this paper are currently being targeted for small gas turbine engines.

Topics: Gas turbines , Shapes
Commentary by Dr. Valentin Fuster
1982;():V005T11A006. doi:10.1115/82-GT-97.
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Combustion turbines have been in service in Saudi Arabia for many years. Over this period, it has become apparent that the unique conditions of high sulfur gas fuel (sometimes with liquid carryover) and wind blown sand and salt particles result in a serious turbine hot corrosion problem. Metallurgical study of service blades and vanes, together with site analyses, served as background for a detailed laboratory coating evaluation. As a result, two corrosion resistant coatings fan electron beam physical vapor deposited CoCrAlY overlay and a pack diffusion Rh-Pt aluminide) were selected for application to turbine blades and vanes installed in Westinghouse W501D units for service in Saudi Arabia. State-of-the-art inlet air filtration was also installed to further reduce the possibility of hot corrosion. Comparisons were made between the two coatings after as much as 20,000 hr of service, and results were correlated with laboratory pressurized rig tests conducted under simulated conditions. In general, the performance of the CoCrAlY overlay coating was superior to that of the Rh-Pt aluminide diffusion coating.

Commentary by Dr. Valentin Fuster
1982;():V005T11A007. doi:10.1115/82-GT-98.
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Encotech was commissioned by the Electric Power Research Institute (EPR1) to survey the owners of gas turbines burning residual fuel or crude oil. The survey’s purpose was to identify operating problems, costs, and general owner reaction to burning residual fuel and crude oil. This paper presents an excerpt of the report related to corrosion and fuel treatment as well as additional data.

Commentary by Dr. Valentin Fuster
1982;():V005T11A008. doi:10.1115/82-GT-99.
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This paper describes the ongoing results being obtained from a joint General Electric/ARAMCO program to improve gas turbine bucket corrosion lives in desert environments involving high concentrations of alkali salts and sulfur-containing fuels. The program has involved the insertion of buckets coated with a variety of experimental or developmental coatings into several General Electric MS-5002 gas turbines being operated by ARAMCO in the Eastern Province of Saudi Arabia. Buckets coated by a variety of process techniques, such as pack cementation, physical vapor deposition, plasma spray and sheet cladding, have been evaluated during this program. Service life on some of these coated buckets has now reached approximately 35,000 hr. In general, Pt-Cr-Al pack cementation coatings have been observed to perform well, with the Pt being critical for superior long-time service.

Commentary by Dr. Valentin Fuster
1982;():V005T11A009. doi:10.1115/82-GT-104.
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The methods used to repair turbine components from Industrial gas turbines are reviewed and explained. Particular reference is made to cleaning, heat treatments, welding, brazing, machining, coatings and inspection.

Commentary by Dr. Valentin Fuster
1982;():V005T11A010. doi:10.1115/82-GT-105.
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Fiber metal thermal barriers are used where low thermal conductivity and mechanical integrity are required. Fiber metal materials have good insulating characteristics at low densities; product thermal conductivity is influenced more by numerous air filled pores than by heat flow through the metal fiber skeleton. Fiber metal thermal barriers are produced as randomly oriented fiber sheets in both sintered and unsintered form. Product properties are controlled by choice of alloy, fiber diameter, and porosity (or density). Fiber metal thermal barriers combine insulating properties, mechanical strength and elasticity, oxidation resistance, and temperature capability to 2000°F. Applications include ceramic coating substrates, insulating abradable seals, space shuttle fuel lines and engine components.

Topics: Metals , Fibers
Commentary by Dr. Valentin Fuster
1982;():V005T11A011. doi:10.1115/82-GT-106.
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Sodium chloride can be present in gas turbine hot sections in the vapor as well as the condensed state. Particles containing sodium chloride may randomly break from compressor components, then impinge upon and stick to turbine components further down the gas path. However, NaCl vapor can deleteriously affect processes involving the formation and maintenance of protective oxide layers on turbine components. The relative importance of these two distinctly different mechanisms involving NaCl in corrosion processes is currently unknown. Numerous studies have dealt with the effect on corrosion behavior of Na2SO4-NaCl condensed mixtures with appreciable amounts of NaCl. However, the possibility that low NaCl vapor activities effect major changes in oxide scale formation and retention is equally as probable as that involving condensed NaCl. Therefore, the results presented here will deal with effects of low activities of NaCl vapor (0.1–100 ppm) upon scales formed by selected high temperature materials.

Commentary by Dr. Valentin Fuster
1982;():V005T11A012. doi:10.1115/82-GT-114.
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This paper presents an account of the mechanical design and the fabrication development required to produce the functional nozzle segment of a gas turbine generator operated with coal-derived fuels at a firing temperature of 2600°F (1426°C). The nozzle is water cooled by a closed loop system. It is constructed of a composite of materials which have been totally diffusion bonded by means of the Hot Isostatic Pressing (HIP) process. Several segments have been made. These segments will be tested late in 1980.

Commentary by Dr. Valentin Fuster
1982;():V005T11A013. doi:10.1115/82-GT-115.
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Crude oils are favorable gas turbine fuels, particularly in areas where light crudes are available and distillates in sufficient quantities are difficult to obtain. In Riyadh, Saudiarabia, local Khurais crude oil is therefore certainly the most reasonable gas turbine fuel. This paper shows the long time experience with this type of fuel gathered in ten modern BBC type 11 turbines with a total of over 100,000 operating hours. The main problems and the measures taken to overcome these problems are described in detail. The operational record of the Riyadh 5 power plant of the last three years demonstrates that it is possible to run a powerplant without any diesel oil for blending or start up, e.g., and still to obtain availability and reliability numbers which are as good or better than for a diesel or gas fired plant.

Commentary by Dr. Valentin Fuster
1982;():V005T11A014. doi:10.1115/82-GT-244.
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Engine experience must be the final arbiter of the most suitable materials and coatings to minimize turbine corrosion. Facts and examples obtained on Rolls-Royce industrial and marine gas turbines are presented. Rig tests provide useful advance information by filtering out unsatisfactory materials and coatings before fixing engine standard for service trials. However, for definitive answers, most rigs are limited in one form or another. Running results show that different engine types under similar environmental conditions give different rates of corrosion. Cooling configuration has highlighted sulphidation in two or more forms and has defined their positions on the hardware. Extended cyclic running, under aggressive conditions, with various material and coating combinations gives results which suggest the need for two layer coating systems.

Commentary by Dr. Valentin Fuster

Ceramics

1982;():V005T12A002. doi:10.1115/82-GT-182.
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Utilizing dirty fuels such as coal in gas turbine engines requires that heat input to the cycle working fluid occur through a heat exchanger. For high cycle efficiencies such a heat exchanger must operate in the 700–1400 KPA, 1100–1200°C (100–200 psi, 2000–2200°F) range. In this temperature range, ceramic heat exchangers are required. Ceramic heat exchangers that can operate in this regime have been under development for several years on a very modest scale. These programs are briefly reviewed. Major material issues are reviewed and the status of each is presented. Mechanical reliability and joining technology have been successfully demonstrated in short term tests. Long-term durability and the manufacturing technology to produce large scale components reproducibly remains to be demonstrated in the future.

Commentary by Dr. Valentin Fuster
1982;():V005T12A003. doi:10.1115/82-GT-184.
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Ceramic hot-section turbine components allow higher temperature for up-rated power and improved fuel economy. The methodology used in arriving at a successful 200-hr engine demonstration of an all ceramic nozzle is presented along with current progress in demonstrating the remaining ceramic hot-section static components.

Commentary by Dr. Valentin Fuster
1982;():V005T12A004. doi:10.1115/82-GT-202.
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Corning became actively involved with vehicular gas turbine programs in 1952 during the development of a ceramic rotary regenerator core for the Chrysler automotive gas turbine. Material and process development research in this program led to efforts to apply very low thermal expansion, modest strength, sintered glass-ceramic materials to some of the static gas turbine engine components. One early program involved the first stage turbine plenum for the Ford Model 707 industrial gas turbine. Several process/material iterations ultimately led to the development of a glass-ceramic turbine housing for the Ford Motor Company Model 820 ceramic gas turbine. The housing required a sophisticated slip cast process and a special lithium aluminosilicate (LAS) glass-ceramic composition (Corning Code 9458) with a total thermal expansion between room temperature and 1200C of about 700 ppm. This paper reviews this project and indicates some possible directions for future developments. The design concept and application of the glass-ceramic housing to the Ford Model 820 ceramic gas turbine is discussed in an associated ASME paper by Mr. A. F. McLean of Ford Motor Company.

Topics: Ceramics , Glass , Turbines
Commentary by Dr. Valentin Fuster
1982;():V005T12A005. doi:10.1115/82-GT-216.
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Development of Alpha Silicon Carbide components by net shape fabrication techniques has continued in conjunction with several turbine engine programs. Progress in injection molding of simple parts has been extended to much larger components. Turbine rotors fabricated by a one piece molding have been successfully spin tested above design speeds. Static components weighing up to 4.5 Kg (10 lb) and 33 cc (13 in.) in diameter have also been produced using this technique. Use of sintering fixtures significantly improves dimensional control. A new Si-SiC composite material has also been developed with average strengths up to 1000 MPa (150 ksi) at 1200°C.

Commentary by Dr. Valentin Fuster
1982;():V005T12A006. doi:10.1115/82-GT-252.
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This paper presents a status report on the injection molding of sinterable silicon nitride at GTE Laboratories. The effort involves fabrication of single axial turbine blades and monolithic radial turbine rotors. The injection molding process is reviewed and the fabrication of the turbine components discussed. Oxidation resistance and strength results of current injection molded sintered silicon nitride as well as dimensional checks on sintered turbine blades demonstrate that this material is a viable candidate for high temperature structural applications.

Commentary by Dr. Valentin Fuster
1982;():V005T12A007. doi:10.1115/82-GT-253.
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Detroit Diesel Allison is actively applying advanced ceramic materials to components in gas turbine engines. Silicon carbide, silicon nitride, aluminum silicate, lithium aluminum silicate, and mullite are materials being used in various components in both the DDA GT 404-4 and AGT 100 engines. Approximately 9400 hr of ceramic component operating time in the GT 404 engine has been accumulated, and design, component processing, proof testing, and engine testing experience have begun to show the applicability of ceramic materials in production engines. Material variability, processing procedures, strength characterization, and nondestructive evaluations are emerging as critical but controllable factors. Ceramic components offer the potential of significant fuel consumption improvements in gas turbine engines for vehicles and other applications.

Commentary by Dr. Valentin Fuster
1982;():V005T12A008. doi:10.1115/82-GT-264.
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The use of ceramic coatings on gas turbine engine hot section airfoils has the potential for major improvements in engine performance, durability, and fuel economy, for both industrial/utility and flight engine applications. Major improvements in the durability of ceramic thermal barrier coatings for turbine engine application have been made in both hot corrosion resistance and ceramic thermal cycle fatigue. This paper describes the early history, as well as, recent success in the development of mechanically segmented or “microcolumnar” structured coatings that are produced by electro-beam, physical vapor deposition (P. V.D.) of ceramic materials on metallic turbine airfoil components. Due to their columnar structure, these coatings accommodate the thermal cycle stresses that develop between the ceramic coating and the metal substrate due to their very different coefficients of thermal expansion. The hot corrosion, erosion, and thermal fatigue protection properties are also discussed.

Commentary by Dr. Valentin Fuster
1982;():V005T12A009. doi:10.1115/82-GT-265.
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Ceramic thermal barrier coatings are currently under active development in the U.S. for both aircraft and industrial/Utility gas turbine operation. These coating systems generally consist of an oxidation-corrosion resistant metal bond coat of the MCrAlY type and either a single thick layer ceramic overcoat or a graded ceramic/MCrAlY overcoat. This paper summarizes studies conducted on the high-temperature corrosion resistance of ZrO2 · Y2O3, ZrO2 · MgO and Ca2SiO4 plasma sprayed coatings that are candidates for use as thermal barrier coatings in gas turbine engines. Coatings were evaluated in both atmospheric burner rig and pressurized passage tests using GT No. 2 fuel and that doped with corrosive impurities such as sodium, sulfur and vanadium. The test results showed that the coatings perform very well in the clean fuel pressurized passage tests as well as burner rig tests. With impure fuels, it was found that chemical reactions between the ceramic coatings and combustion gases/condensates played the critical role in coating degradation. This work was conducted for NASA and EPRI under contract NAS3-21377. Advanced coating development studies have also been conducted for NASA and DOE under contract DEN3-110.

Commentary by Dr. Valentin Fuster
1982;():V005T12A010. doi:10.1115/82-GT-266.
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Plasma and detonation gun coatings of ceramic and cermet materials have achieved widespread application throughout gas turbine engines, and their use continues to grow. The coatings are used in increasingly demanding environments, thus successful utilization requires a thorough understanding of their structure/property relationships and close control of the parameters of deposition. The areas of application within the engine can be roughly separated into low and high temperature regimes. At low temperature, impact/fretting wear, erosion resistance, gas path seals, and various seals and bearings are of concern. Each area requires one or more coating types to satisfy the specific requirements of a given engine. The most common coating types include tungsten carbide-cobalt, alumina, chromium carbide-nickel chromium, a new family of cobalt-base alloys with alumina or chromia dispersions, and magnesium zirconate or stabilized zirconia. The characteristics of major significance of these coatings and their applications will be discussed.

Commentary by Dr. Valentin Fuster
1982;():V005T12A011. doi:10.1115/82-GT-293.
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Ford’s research and development of ceramic flowpath components for uncooled, 1370°C (2500°F), vehicular turbine engines has become well known over the past decade. This paper, for the first time, describes parallel and associated developments to apply ceramics to engine inner housings. Design concepts are reviewed and compared to conventional sheet metal or cast iron structures to provide internal hot gas transition ducts. Candidate ceramic material properties are compared, leading to selection and development of lithium-aluminum-silicate. Forming processes such as slip casting and glass forming are reviewed along with problems experienced with such a relatively large ceramic component. Evaluation methods are presented and salient engine and rig testing is reviewed to establish the potential for ceramic turbine housings.

Topics: Ceramics , Turbines
Commentary by Dr. Valentin Fuster
1982;():V005T12A012. doi:10.1115/82-GT-322.
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The durability of plasma sprayed ceramic thermal barrier coatings subjected to cyclic thermal environments has been improved substantially by improving the strain tolerance of the ceramic structure and also by controlling the substrate temperature during the application of the coating. Improved strain tolerance was achieved by using ceramic structures with increased porosity, microcracking or segmentation. Plasma spraying on a controlled-temperature substrate also has been shown to improve durability by reducing harmful residual stresses. The most promising of the strain tolerant ceramic coatings have survived up to 6000 cycles of engine endurance testing with no coating or vane platform damage. In side-by-side engine tests, thermal barrier coatings have shown that they greatly reduce platform distress compared to conventionally coated vanes in addition to permitting reductions in cooling air and attendant increases in engine efficiency.

Commentary by Dr. Valentin Fuster

Structures and Dynamics

1982;():V005T13A002. doi:10.1115/82-GT-148.
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A perturbation method is described which predicts the changes in eigenfrequencies resulting from geometrical changes of a structure. This dependence is represented by dimensionless functions, one for each eigenfrequency, which vary over the surface of the structure. The functions are presented for each eigenfrequency as isoline plots. An easily estimated integration of these functions allows one to predict a geometrical change which results in a desired change in the resonance frequencies. The method was applied to a turbine blade and a rectangular beam. For the turbine blade isoline plots are presented for the first five eigenfrequencies. Eigenfrequency changes up to 8 percent were modeled accurately.

Topics: Turbine blades
Commentary by Dr. Valentin Fuster
1982;():V005T13A003. doi:10.1115/82-GT-162.
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In the context of jet engines, significant vibration damping due to dry friction can occur at (a) shroud interfaces of fans and (b) the platform of turbine blades fitted with “platform dampers.” Analytical and experimental studies in regard to this important source of nonaerodynamic damping of blade vibration are presented in this paper. Comparisons between results from analytical models and laboratory test data are made and discussed.

Commentary by Dr. Valentin Fuster
1982;():V005T13A005. doi:10.1115/82-GT-173.
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Optimum stepped shaft designs are obtained through an application of Pontryagin’s Minimum Principle. Optimum designs are obtained for a given critical speed of specified order. Indexes of Performance to be minimized include mass and rotating inertia. A general problem formulation illustrates how constraints on stress, deflections, and geometric design are taken in account. Numerical solutions are obtained to nonlinear multi-point-boundary-value-problems. A Newton-Raphson algorithm was developed to determine step locations precisely in order to facilitate the convergence of the shooting method used to solve the boundary value problem. Numerical solutions are determined with an assumed critical speed; a Rayleigh quotient calculation is used to verify that the optimum design possesses the assumed value.

Topics: Design , Optimization
Commentary by Dr. Valentin Fuster
1982;():V005T13A006. doi:10.1115/82-GT-218.
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Previous investigations have shown that the uncentralized type of squeeze-film damper is an effective means of reducing the transmission of unbalance forces into the supporting structure. In this theoretical study a more complex model, which includes an overhung “fan” disc and a noncentral “turbine” disc, has been employed. This model represents the conventional gas turbine somewhat closer than does the previously studied single disc system. This investigation has shown that it is possible to minimize the force transmitted into the surrounding structure by a careful selection of squeeze-film damper characteristics, although it may be found that some larger amplitudes of motion accompany the minimized transmissibility.

Topics: Bearings , Dampers , Rotors
Commentary by Dr. Valentin Fuster
1982;():V005T13A007. doi:10.1115/82-GT-223.
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A great deal of published literature exists which analyzes the free vibrations of turbomachinery blades by means of one-dimensional beam theories. Recently, a more accurate, two-dimensional analysis method has been developed based upon shallow shell theory. The present paper summarizes the two types of theories and makes quantitative comparisons of frequencies obtained by them. Numerical results are presented for cambered and/or twisted blades of uniform thickness. Significant differences between the theories are found to occur, especially for low aspect ratio blades. The causes of these differences are discussed.

Commentary by Dr. Valentin Fuster
1982;():V005T13A008. doi:10.1115/82-GT-231.
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In the field of rotor dynamics, increased attention is being given to the transient response analysis of rotor, since the effects of impact loading and vibrations of rotor arising from blade loss can be studied by a time transient solution of the rotor system. As recent trends in rotating machinery have been directed towards light-weight, high-speed flexible rotors, the effect of flexibility on transient response analysis is becoming of increasing importance. In the present paper, a transient vibration analysis is carried out on a flexible disc-flexible shaft system or rigid disc-flexible shaft system subjected to a sudden imbalance that is assumed to represent the effect of blade loss. To solve the basic equation governing a rotating flexible disc the Galerkin’s method is used, and the equation of motion of the rotor system is numerically solved by employing the Runge-Kutta-Gill’s method. Experiments were conducted on a model rotor having a blade loss simulator; the shaft vibrations were also measured. The validity of the analytical results was demonstrated by comparison with the experimental results.

Commentary by Dr. Valentin Fuster
1982;():V005T13A009. doi:10.1115/82-GT-232.
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Assuming central preloading operation below the second bending critical speed and full film lubrication, this paper presents a theoretical model which allows one, with minimum computation, to design squeeze film damped rotors under conditions of high unbalance loading. Closed form expressions are derived for the maximum vibration amplitudes pertaining to optimally damped conditions. The resulting vibration amplitude and transmissibility data of design interest are presented for a wide range of practical operating conditions on a single chart. It can be seen that for a given rotor, the lighter the bearing the more easily one can satisfy design constraints relating to allowable rotor vibration levels and lubricant supply pressure requirements. The data presented are shown to be applicable to a wide variety of rotors, and a recommended procedure for optimal design is outlined.

Topics: Design , Rotors
Commentary by Dr. Valentin Fuster
1982;():V005T13A010. doi:10.1115/82-GT-233.
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This paper describes the experimental investigations which were conducted to verify existing theoretical vibration amplitude predictions for centrally preloaded, squeeze film supported flexible rotors. The influence of measurement errors and operating condition uncertainties are quantified. The agreement between theory and experiment was excellent, and it is shown that any discrepancy can be explained in terms of errors in determining the mean lubricant viscosity and the orbit magnitudes. Hence, for the range of parameters investigated, the theoretical model and predictions therefrom are validated.

Topics: Rotors
Commentary by Dr. Valentin Fuster
1982;():V005T13A011. doi:10.1115/82-GT-240.
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The trend towards higher speeds and lighter weight rotor designs, particularly in gas turbine applications, has brought about the need to consider the flexibility of the total shaft-disk-blade system in the calculation of system frequencies and whirl modes. It has been traditional to assume that blades and disks are rigid, although recent work has been aimed at evaluating the assumption of rigid disks. These works have used Laplace transform and Liapunov methods and have been limited to simple geometries. This paper describes the development of a special shaft finite element which incorporates analytical solutions for disk dynamics and approximations of blade effects. It is shown that the addition of disk and blade dynamics does not increase the size of system matrix and that this approach can consider multiple disks. Because of the versatility of the finite element approach, complex configurations and boundary conditions can be considered.

Commentary by Dr. Valentin Fuster
1982;():V005T13A012. doi:10.1115/82-GT-241.
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This paper describes the following roles of a squeeze-film damper when used in gas turbine applications as a means of reducing vibration and transmitted force due to unbalance. (a) as an element in parallel with a soft spring in a vibration isolator and (b) as an element in series with the stiffness of the engine pedestal. The effects of cavitation on performance are elucidated and the dangers of jump phenomena and subsynchronous response are discussed. Experimental work is described in which both roles of the squeeze-film damper are investigated and the results are compared with theoretical predictions.

Topics: Engines , Dampers , Vibration
Commentary by Dr. Valentin Fuster
1982;():V005T13A013. doi:10.1115/82-GT-246.
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A mathematical model representing the torsional characteristics of a machine train including a gear has been developed incorporating a number of features which are usually neglected, namely, damping and flexibility in the bearings and damping, backlash and pitch error in the gear teeth. This model has been used in conjunction with a computer simulation language to predict the performance of a geared compressor in which there is a torsional resonant frequency close to twice the motor speed.

Commentary by Dr. Valentin Fuster
1982;():V005T13A014. doi:10.1115/82-GT-262.
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A method is presented for efficiently calculating the damped natural frequencies of complex rotor bearings systems. The procedure is applicable to the repeated reanalysis of rotor systems during the search for an optimal design. The generalized receptances used in the method are calculated with a series of formulas that improves the convergence characteristics when only an incomplete set of modes is available. A nonsynchronous gyroscopic rotor example is examined to illustrate the reanalysis procedure.

Commentary by Dr. Valentin Fuster
1982;():V005T13A016. doi:10.1115/82-GT-267.
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In this paper, a procedure is presented to determine the unbalance distribution in a multi-mass flexible rotor system without requiring that trial weights be placed upon the shaft to first determine the influence coefficient matrix of the various balance planes. A modified Nyquist plotting procedure is presented to generate a polar plot of proximity probe measurements for determination of the 90-deg phase shift position of the modal eccentricity. By knowing the rotor modal mass and mode shape, a modal balancing distribution can be calculated. This relationship provides a quick procedure in estimating a first mode balance correction weight in both magnitude and angular location. An application is presented for a steam turbine during the startup of a hydrogen gas compression train. Higher order modal unbalance corrections are shown to be calculable in a similar manner.

Topics: Rotors
Commentary by Dr. Valentin Fuster
1982;():V005T13A017. doi:10.1115/82-GT-269.
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The purpose of this paper is two-fold: first, to discuss the origin and symptoms of the non-independent balance plane problem and thus alert the balancing engineer to its existence; and, second, to provide a systematic method for identifying and eliminating this problem which is consistent with the general influence coefficient balancing procedure.

Topics: Engineers
Commentary by Dr. Valentin Fuster
1982;():V005T13A018. doi:10.1115/82-GT-281.
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During service introduction of an uprated turbojet engine, which has a three-bearing rotor support system with an overhung turbine, several problems related to system vibration were encountered — cabin noise and high engine vibration levels. These problems led to a factory and field investigation which showed that the source of the problem was a nonlinear interaction between rotor and casing modes coupled through bearing clearance. This interaction led to a bistable vibration of the system. This paper documents the results of this investigation and demonstrates that the use of a plain, uncentralized, squeeze film damper to support the turbine rotor solves all vibration problems by reducing the turbine critical speed and separating it from the casing mode. Also included are effects of exhaust system weight on engine vibration and cabin noise levels.

Topics: Bearings , Dampers , Vibration
Commentary by Dr. Valentin Fuster
1982;():V005T13A019. doi:10.1115/82-GT-284.
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Structural and dynamic analyses are presented for a shroudless, hollow, titanium fan blade proposed for future use in aircraft turbine engines. The blade was modeled using the Lewis Research Center’s Blade Structural Analysis Computer Program (COBSTRAN). COBSTRAN is an integrated program consisting of mesh generators, composite mechanics, NASTRAN, and pre- and post-processors. Vibration, and impact analyses are presented. The vibration analysis was conducted with COBSTRAN. Results show the effect of the centrifugal force field on frequencies, twist, and blade camber. This program uses the geometric model and modal analysis from the COBSTRAN vibration analysis automatically to determine the gross impact response of the fan blades to bird strikes. The structural performance of this blade is compared to a blade of similar design but with composite in-lays on the outer surface. Results show that the composite in-lays can be selected (designed) to substantially modify the mechanical performance of the shroudless, hollow, fan blade.

Commentary by Dr. Valentin Fuster
1982;():V005T13A021. doi:10.1115/82-GT-286.
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Tests were conducted on a linear cascade of airfoils oscillating in pitch to measure the unsteady pressure response on selected blades along the leading edge plane of the cascade and over the chord of the center blade. The pressure data were reduced to Fourier coefficient form for direct comparison, and were also processed to yield integrated loads and, particularly, the aerodynamic damping coefficient. In addition, results from two unsteady theories for cascaded blades with nonzero thickness and camber were compared with the experimental measurements. The three primary results that emerged from this investigation were: (a) from the leading edge plane blade data, the cascade was judged to be periodic in unsteady flow over the range of parameters tested, (b) as before, the interblade phase angle was found to be the single most important parameter affecting the stability of the oscillating cascade blades, and (c) the real blade theory and the experiment were in excellent agreement for the several cases chosen for comparison.

Topics: Airfoils
Commentary by Dr. Valentin Fuster
1982;():V005T13A022. doi:10.1115/82-GT-287.
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A method has been developed and demonstrated for the direct measurement of aerodynamic damping in a transonic compressor. The method is based on the inverse solution of the structural dynamic equations of motion of the blade-disk system. The equations are solved inversely to determine the forces acting on the system. If the structural dynamic equations are transformed to multiblade or modal coordinates, the damping can be measured for blade-disk modes, and related to a reduced frequency and interblade phase angle. This method of damping determination was demonstrated using a specially instrumented version of the MIT Transonic Compressor run in the MIT Blowdown Compressor Test Facility. No regions of aeroelastic instability were found. In runs at the operating point, the rotor was aerodynamically excited by a controlled two-per-revolution fixed upstream disturbance. The disturbance was sharply terminated midway through the test. Analysis of the data in terms of multiblade modes led to a direct measurement of aerodynamic damping for three interblade phase angles.

Topics: Compressors , Damping
Commentary by Dr. Valentin Fuster
1982;():V005T13A023. doi:10.1115/82-GT-288.
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A model has been developed and verified for blade-disk-shaft coupling in rotors due to the in-plane rigid body modes of the disk. An analytic model has been developed which couples the in-plane rigid body modes of the disk on an elastic shaft with the blade bending modes. Bench resonance tests were carried out on the M.I.T. Compressor Rotor, typical of research rotors with flexible blades and a thick rigid disk. When the rotor was carefully tuned, the structural coupling of the blades by the disks was confined to zero and one nodal diameter modes, whose modal frequencies were greater than the blade cantilever frequency. In the case of the tuned rotor, and in two cases where severe mistuning was intentionally introduced, agreement between the predicted and observed natural frequencies is excellent. The analytic model was then extended to include the effects of constant angular rotation of the disk.

Topics: Disks
Commentary by Dr. Valentin Fuster
1982;():V005T13A024. doi:10.1115/82-GT-291.
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Nonlinear effects due to fluid film bearings become significant when vibratory amplitudes are large. To include these effects in rotor dynamic analysis requires conducting time-transient response analysis, where the fluid film forces are estimated at each time step. The present paper describes an approach where a unique treatment of bearing forces results in an efficient computational scheme for performing time transient analysis. The method developed is applicable to a flexible-rotor system with multi-degrees of freedom. A study conducted on a canned annulus motor pump using tilting pad bearings for the rotor support is described in this paper. The results showed severe loading at the bearings due to the canned annulus forces. Nonlinear effects due to the bearings were found only at large rotor unbalance loads.

Commentary by Dr. Valentin Fuster
1982;():V005T13A025. doi:10.1115/82-GT-292.
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In an attempt to increase jet engine efficiences, typically smaller rotor-stator-running clearances are being employed. Because of this and the ever present need to improve maintenance, reliability and structural integrity under various modes of operation, more sophisticated analyses tools are required to model the engine. Due to the wide-spread usage of general purpose finite element codes, this report seeks to adapt the procedure to use in modeling turbine rotor-bearing-stator structure. The work outlined in this report covers the second phase of work on a three-phase NASA Lewis sponsored research grant on engine dynamic simulation by developing strategies which enable the use of available finite element codes.

Commentary by Dr. Valentin Fuster
1982;():V005T13A027. doi:10.1115/82-GT-303.
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The method of component mode synthesis is developed to determine the forced response of nonlinear, multishaft, rotor-bearing systems. The formulation allows for simulation of system response due to blade loss, distributed unbalance, base shock, maneuver loads, and specified fixed frame forces. The motion of each rotating component of the system is described by superposing constraint modes associated with boundary coordinates and constrained precessional modes associated with internal coordinates. The precessional modes are truncated for each component and the reduced component equations are assembled with the nonlinear supports and interconnections to form a set of nonlinear system equations of reduced order. These equations are then numerically integrated to obtain the system response. A computer program, which is presently restricted to single shaft systems, has been written and results are presented for transient system response associated with blade loss dynamics with squeeze film dampers, and with interference rubs.

Topics: Bearings , Rotors
Commentary by Dr. Valentin Fuster
1982;():V005T13A028. doi:10.1115/82-GT-310.
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The thermal stresses generated in a cylindrical shell due to axisymmetric temperature fields, which vary in the longitudinal direction, are examined by using an influence function formulation. Closed form solutions for the longitudinal, hoop and shear stresses are derived for any axially varying temperature distribution expressible by a Fourier expansion. The thermal stresses generated in a typical cylindrical gas turbine combustor cooled by periodically spaced circumferential bands of cooling holes are investigated using the derived solutions. It is shown that a critical pitch in the cooling hole spacing can create high bending stresses at the cooling holes which could contribute to thermal fatigue failure.

Commentary by Dr. Valentin Fuster
1982;():V005T13A029. doi:10.1115/82-GT-311.
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This paper describes a USAF sponsored effort to develop, apply, test, and evaluate Pratt & Whitney’s Damage Tolerant Design System for cold-section gas turbine engine disks. The design system includes a Damage Tolerance Specification proposed for new USAF engine programs, material characterization for crack-growth behavior, design procedures, and analytical life prediction methodology for consideration of large flaws. To evaluate and refine the design system, a current engine fan disk was redesigned to operate safely for a specified time after the occurrence of 0.030-inch (0.76 mm) surface length fatigue cracks. The redesigned disk was tested to failure while monitoring crack growth and correlating observed measurements with analytical prediction. Test results were used to refine the design system. Current work involves extending Damage Tolerant Design capability to hot-section powder-metallurgy disks. The impact of these efforts is twofold; current designs will benefit from improved life prediction capability in applying Retirement-for-Cause philosophy, and future designs can take advantage of the Life-Cycle-Cost benefit of designing for damage tolerance.

Commentary by Dr. Valentin Fuster

Controls, Diagnostics and Instrumentation

1982;():V005T14A001. doi:10.1115/82-GT-150.
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The requirement for gas turbine engines to operate over wider ranges has resulted in today’s more sophisticated engines with multiple controls. Current control philosophy for such powerplants is to adopt separate control systems for each of the major components with the minimum of communication between them. Interactions between the various control parameters can, however, seriously degrade the performance, and a design strategy aimed to take account of these interactions — the multivariable approach — may be better. Although several multivariable design methods have been proposed, most are highly theoretical and a need exists for a more practical straightforward procedure. This paper aims to demonstrate such a procedure, based on the characteristic locus technique, for the design of a gas turbine multivariable controller.

Topics: Design , Gas turbines
Commentary by Dr. Valentin Fuster
1982;():V005T14A002. doi:10.1115/82-GT-165.
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The paper describes a joint British Aerospace/Rolls Royce study on the role of the Powerplant Control System in a high technology combat aircraft. The need to reduce pilot workload and the trend towards the use of Integrated Avionic Systems and Serial Digital Data Transmission on modern aircraft projects has forced a change in the traditional approach to powerplant control system design. The principles of Fault Tolerance with the ability of the Powerplant Control System to survive combinations of faults is described. An ideal system is presented which will provide benefits for the airframe constructor. The utilization of cheap, accurate and reliable microprocessor devices with compatible L.S.I. MIL-STD-1553 Chip Sets offers an attractive solution to aircraft avionic data transmission systems including Powerplant Control and Management for future aircraft.

Commentary by Dr. Valentin Fuster
1982;():V005T14A003. doi:10.1115/82-GT-186.
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Reliability is a performance parameter that must be addressed within the context of systems engineering as applied to turbine engine controls. Understanding the parameters which control electronic system reliability is necessary in order to perform the cost/benefit analyses required for proper system engineering. Factors imposing special consideration in the requirements for turbine controls are addressed, system design alternatives which affect performance are considered, and parts selection criteria and production efforts which enhance reliability are described.

Topics: Reliability
Commentary by Dr. Valentin Fuster
1982;():V005T14A004. doi:10.1115/82-GT-200.
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A generalized modular digital computer code — TURBODYNE — is shown to be a flexible easily coded high-level programming language for the steady-state and transient performance simulation of arbitrary gas turbine engines with arbitrary control systems. The configuration of the engine and control system can be simply described by using “CODEWORDS” corresponding to the various components and processes of both engine and control systems. TURBOTRANS is an analytical tool for performance simulation of gas turbine engines with their control systems. Its modular structure facilitates additional features when required.

Commentary by Dr. Valentin Fuster
1982;():V005T14A005. doi:10.1115/82-GT-250.
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This paper describes the design and development of a full authority Digital Engine Control and intelligent cockpit Command Box by Dowty and Smiths Industries Controls Limited (DSIC) under a Ministry of Defence and Private Venture demonstrator program. The aims of this program are to: (1) demonstrate the recent and continuing improvements in IC technology and allied computing power; (2) employ control laws more relevant to electronic control, thus absolving the hydromechanical unit from precise fuel metering; (3) improve, by a quantum leap, the methods of monitoring engine/system performance. This control system has undergone successful engine running on a Rolls-Royce GEM, and will shortly be tested, in prototype form, in a Lynx.

Commentary by Dr. Valentin Fuster
1982;():V005T14A006. doi:10.1115/82-GT-251.
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This paper outlines one approach taken in designing a software system for the production of high quality software for use in gas turbine control applications. Central to the approach is a special control language with its inherent features of visibility, reliability and testability, leading to a software system which can be applied to applications in which the integrity of the units is of prime importance. The structure of the language is described together with the method of application in the field of aircraft gas turbine control. The provision of documentation automatically is an integral part of the system together with the testing procedures and test documentation. A description of how these features are combined into the total software system is also given.

Commentary by Dr. Valentin Fuster
1982;():V005T14A007. doi:10.1115/82-GT-318.
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Two Data-Tronic remote control-monitor systems are now in service on multiple gas turbine power plants. In each system eight gas turbine generators are controlled from one station operator’s console, via redundant twisted-pair party line cables. The station operator can also assign control of individual units to a remote dispatching center. In addition to control and display information, the party lines carry comprehensive data from the turbines back to an optional minicomputer for storage. The computer data base can then be analysed to present diagnostic and maintenance information. This paper is a sequel to the author’s 1980 ASME paper on the design of the Data-Tronic system, and describes test performance and field experience.

Commentary by Dr. Valentin Fuster
1982;():V005T14A008. doi:10.1115/82-GT-319.
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Traditionally, vibration monitoring and protection equipment has been totally separate from the diagnostic and data acquisition equipment as used for rotating machinery information systems. Application oriented utilization of multiple microprocessors in a distributed processing system can virtually eliminate this artificial barrier. The design philosophy, block diagram, and operating results obtained from actual field-installed units will be presented. In addition, its use with a central Host Processor computer based total plant rotating machinery information system will be discussed.

Commentary by Dr. Valentin Fuster

Education

1982;():V005T15A001. doi:10.1115/82-GT-177.
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This report presents a broad overview of the scope of research presently being supported by NASA in aircraft propulsion. Aircraft systems work is performed to identify the requirements for the propulsion system that enhance the mission capabilities of the aircraft. It is an important source of innovation and creativity that drives the direction of propulsion research. In a companion effort, component research of a generic nature is performed to provide a better basis for design and provides an evolutionary process for technological growth that increases the capabilities of all types of aircraft. They are both important.

Commentary by Dr. Valentin Fuster
1982;():V005T15A002. doi:10.1115/82-GT-192.
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Since 1968, an advanced educational program in the fluid dynamics of turbomachinery has been offered by the ASME Turbomachinery Institute at Iowa State University. Initiated by concerned individuals to help meet the need for high-level, continuing education in this field of specialization, the course appears to be accomplishing its original intent. The success of the program can be attributed to a number of factors including a good faculty, an eager and qualified group of participants, and scholarly surroundings. As might be expected, timely and thoughtful planning, good luck and competent support are also essential. Of the many lessons learned about this kind of educational effort, several seem important enough to report in this paper.

Commentary by Dr. Valentin Fuster
1982;():V005T15A003. doi:10.1115/82-GT-203.
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A timely review of selected examples of research in aeropropulsion being sponsored by the U. S. Army covering the full spectrum of relevant topics is given. The discussion in this paper is focussed on aerodynamic components, mechanical elements and materials with emphasis on those fundamental issues which strongly influence engine reliability and life-cycle cost, particularly reduced fuel consumption. Specific research investigations on compressors, combustors, turbines, mechanical components, materials and cycle analysis are described and new desired research initiatives are elaborated. The reader can acquire a rather complete overview of Army activities related to small gas turbine engines from this paper.

Topics: Army
Commentary by Dr. Valentin Fuster
1982;():V005T15A004. doi:10.1115/82-GT-204.
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Under certain circumstances, hot gases containing particulates may be expanded through a turbine. The erosion damage due to the particulates interacts with the high temperature oxidation processes. The interaction may be positive: the oxide layer may be more erosion-resistant than the substrate. The interaction may be negative: the erosion can remove the protective oxide resulting in accelerated metal loss. If the gases contain corrosive materials such as alkali sulfates as well as the particulates, further interactions are possible. These processes are of importance in gas turbine expanders for pressurized fluidized bed combustors burning coal, and several research projects are in progress to study them.

Commentary by Dr. Valentin Fuster
1982;():V005T15A005. doi:10.1115/82-GT-315.
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At the Technical University of Berlin, the usual Fluid Mechanics Course for engineering students has been offered since 1977 in two different ways simultaneously, between which the students may choose. One is the traditional scheme of lectures with additional example classes; students can obtain a typewritten summary of the course without the examples. As an alternative the students are provided with a complete text of the course arranged for self-study together with worked examples. This text has now been published as a book (H. Schade, E. Kunz: Strömungslehre. Berlin, New York: de Gruyter 1980). In the alternative course the students meet in groups of about five with a professor or a teaching assistant once a week to discuss the section of the text which they had been given as a reading assignment in the previous week. All students, whether they have attended the traditional or the alternative course, must pass the same written and oral examinations at the end of the one-year course. The advantages and disadvantages of both methods will be discussed and an evaluation will be given.

Commentary by Dr. Valentin Fuster

Process Industries

1982;():V005T16A002. doi:10.1115/82-GT-38.
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This paper presents the results of Gibbs & Hill, Inc.’s studies, engineering, and implementation of a cogeneration plant consisting of gas turbines and heat recovery steam generators. The plant is designed for fluctuating power supply and stable steam supply requirements to a multistage flash (MSF) type Desalination Plant. This paper emphasizes the advantages of an Integrated Power/Desalination Plant System Optimization as against separate optimization of power and desalination plants.

Commentary by Dr. Valentin Fuster
1982;():V005T16A003. doi:10.1115/82-GT-83.
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Gas turbines furnished with heat recovery equipment generally have maximum cycle efficiency when the gas turbine is operated at its ambient capability. At reduced gas turbine output the cycle performance can fall off rapidly as gas turbine exhaust temperature drops, which reduces the heat recovery equipment performance. This paper reviews the economic gains which can be realized through use of several control modes which are currently available to optimize the cycle efficiency at part load operation. These include variable inlet guide vane (VIGV) control for single-shaft units, and combined VIGV and variable high pressure set (compressor) speed control for two-shaft units. In addition to the normal control optimization mode to maintain the maximum exhaust temperature, a new control mode is discussed which allows airflow to be modulated in response to a process signal while at constant part load. This control feature is desirable for gas turbines which supply preheated combustion air to fired process heaters.

Commentary by Dr. Valentin Fuster
1982;():V005T16A004. doi:10.1115/82-GT-95.
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Combined gas-steam cycles have been analyzed from the thermodynamic point of view. Suitable thermodynamics indices — explained in Appendix A — have been utilized. The parameters that most influence efficiency have been singled out and their ranges of variability have been specified. Calculations have been carried out — see Appendix B — taking into account the state of the art for gas turbines and the usual values for the quantities of steam cycles. The results are given. The maximal gas turbine temperature has been varied between 800°C and 1400°C. The gas turbine pressure ratio has been analyzed in the range of 2–24. Afterburning has also been taken into consideration. Maximal efficiency curves and the corresponding specific work curves (referred to the compressed air) related to the parameters of the analysis are given and discussed.

Topics: Cycles , Steam
Commentary by Dr. Valentin Fuster
1982;():V005T16A005. doi:10.1115/82-GT-111.
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The part load behavior of cogeneration with open cycle gas turbines and standard power control by temperature variation is poor if total fuel utilization is considered. Therefore, alternative control methods and their effect on total fuel utilization are analyzed. A simple method with a good partload behavior is proposed. The heat balances of waste heat utilization from constant speed, single shaft gas turbines are shown and discussed. Additional heat production with auxiliary burners in waste heat boilers is analyzed and a more effective solution with supplementary boilers is proposed. The combined power and heat production characteristics with compressor backflow and supplementary boilers point to the outstanding cogeneration behavior of simple open cycle gas turbines. The very good power to heat ratio and the high exhaust gas temperature of the turbine enables an efficient combination with bottoming steam cycles and compression and absorption heat pumps. This indicates the potential of fuel utilization for heating purposes.

Topics: Heat , Gas turbines
Commentary by Dr. Valentin Fuster
1982;():V005T16A006. doi:10.1115/82-GT-140.
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In a typical LPG recovery plant, heat is needed for an amine reboiler, as well as additional horsepower to compress residue gas to pipeline pressure. With the design of a power recovery package, waste heat from turbine exhaust can be utilized to supplement the necessary horsepower and to fulfill the amine reboiler duty requirement. This reduces the number of turbines needed to achieve the compression and hence the operating cost. This paper discusses the design of such a system using propane as the power fluid, both from the application point of view as a pure heat medium system, and a power recovery system.

Commentary by Dr. Valentin Fuster
1982;():V005T16A007. doi:10.1115/82-GT-206.
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Technical and economic aspects of utilization of steam and gas turbine plans for district heating are presented and discussed. Combined use of these plants for heat and electrical generation is considered. Advantages of flexible and combined operation, wide load variation and increase of the district heating supply temperature are presented and discussed.

Commentary by Dr. Valentin Fuster
1982;():V005T16A008. doi:10.1115/82-GT-277.
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In high pressure centrifugal pumps the magnitude of the critical speed is strongly affected by the presence of liquid in the close clearance spaces of sealing rings and throttle bushings. The stiffening and damping effect resulting from the non-axisymmetric pressure drop across such gaps is often large compared with the elastic stiffness of the shaft. In many cases it raises the critical speed far above the “dry” value, and in some cases it eliminates the critical speed altogether. As a consequence, design specifications based on shaft critical speeds, calculated disregarding the effects of the fluid, become quite meaningless. In this paper simplified equations are derived for the calculation of the effect of the fluid gap. The results have been verified by measurements on a test pump. Sample calculations for a typical multistage boiler feed pump are included as illustration.

Commentary by Dr. Valentin Fuster
1982;():V005T16A009. doi:10.1115/82-GT-283.
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This paper discusses the results of two years continuous operation of automatic self-cleaning air filtration systems designed to provide gas turbine protection in a desert environment subject to high ambient concentrations of sand, dust and salt. The condition of the gas turbines and associated pulse-jet air filtration systems, after two years continuous operation, are described. Filter operating costs are also analyzed.

Commentary by Dr. Valentin Fuster
1982;():V005T16A010. doi:10.1115/82-GT-301.
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Two years’ operation of a new high efficiency gas turbine in the process industry are reviewed. Operational problems are detailed, and the inspection procedures and intervals developed are discussed in relation to the needs of the users and equipment vendor.

Commentary by Dr. Valentin Fuster
1982;():V005T16A012. doi:10.1115/82-GT-309.
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The paper covers the basic thermodynamics of the combined cycle concept and illustrates that energy conservation is possible by coupling the Joule and Rankine cycles. It discusses the optimisation of steam conditions and outlines the concept of the unfired combined cycle. “Carnot efficiency” and “pinch points” are shown to be important as is the concept of “specific work” as it relates to the gas turbine in arriving at the best overall cycle efficiencies. The importance of the efficiency characteristic of the steam turbine is emphasised and it is shown that this characteristic will determine the overall cycle efficiency. It is suggested that steam turbine manufacturers should design and develop steam turbines to match the advanced gas turbines available to-day and so enchance the overall efficiency which can presently be obtained from to-day’s combined cycle. Market forces will tend to bring this about as evidenced by the growing interest being shown in this concept both in the UK and Europe.

Commentary by Dr. Valentin Fuster

Technology Resources

1982;():V005T17A001. doi:10.1115/82-GT-8.
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Special purpose steam turbine driver efficiency improvements offer significant energy saving incentives, particularly in today’s environment of soaring energy costs. Efficiency gains of up to ten percentage points are possible over past designs with up-to-date technology and can provide substantial operating cost savings for current energy costs. This paper reviews (1) governing factors for turbine efficiency improvement, (2) currently implemented design modifications, and (3) recommended approach to developing the “next generation” steam turbine technology.

Commentary by Dr. Valentin Fuster

General

1982;():V005T18A002. doi:10.1115/82-GT-307.
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With introduction of the Spruance Class Destroyers, gas turbines have been used to power both destroyers and frigates. However, to achieve maximum crusing range, high efficiency propulsion systems are required. The scope of this paper is limited to a basic description of propulsion plant configurations, using gas turbines as prime movers. These systems serve as candidates for U.S. Navy ship propulsion. Comparisons are limited to fuel loading, because of its impact on ship design. Although annual fuel consumption is a more accurate measure of fuel efficiency, it requires a specific ship design. This is beyond the scope of this paper. Of the systems considered, the maximum reduction in required fuel loading (for a specific cruising range) was 20%. To achieve optimum efficiency, cross-connect ability (for a two propeller shaft system) is essential. Potential cross-connect methods include mechanical, electrical and steam features. The first two are described in this paper.

Commentary by Dr. Valentin Fuster

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