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RESEARCH PAPERS

Commentary by Dr. Valentin Fuster
J. Eng. Power. 1980;102(1):5-12. doi:10.1115/1.3230232.

The general effect of Reynolds Number on axial flow compressors operating over a sufficiently wide range is described and illustrated by experimental data for four multistage axial compressors. The wide operating range of military aircraft engines leads in the back stages of high pressure ratio compression systems to three distinctly different regimes of operation, characterized by the boundary layer conditions of the cascade flow: • laminar separation, • turbulent attached flow with hydraulically smooth blade surface, • turbulent attached flow with hydraulically rough blade surface. Two “critical” Reynolds Numbers are defined, the “lower critical Reynolds Number” below which laminar separation occurs with a definite steepening of the efficiency/Reynolds Number relation and an “upper critical Reynolds Number” above which the blade surface behaves hydraulically rough, resulting in an efficiency independant of Reynolds Number. The permissible blade surface roughness for hydraulically smooth boundary layer conditions in modern high pressure ratio compression systems is derived from experimental data achieved with blades produced by grinding, electrochemical machining and forging. A correlation between the effect of technical roughness and sand type roughness is given. The potential loss of efficiency in the back end of compression systems due to excessive blade roughness is derived from experimental results. The repeatedly experienced different sensitivity of front and back stages towards laminar separation in the low Reynolds Number regime is explained by boundary layer calculations as a Mach Number effect on blade pressure distribution, i.e. transonic versus subsonic flow.

Commentary by Dr. Valentin Fuster
J. Eng. Power. 1980;102(1):14-18. doi:10.1115/1.3230214.

A GT-225 regenerative gas turbine engine was used to obtain engine performance data with varied rates of simulated internal leakage. Regenerator hot surface rim and crossarm seal leaks as well as regenerator cold surface seal leaks were simulated. Instrumentation was included in external ducting to measure the leakage rates. Data were taken at three gasifier turbine speeds with turbine inlet temperature held constant by means of the variable power turbine nozzle and also with varied turbine inlet temperatures resulting from operation with the nozzle fixed at the design area. Component performance data were calculated from the engine measurements to determine the loss mechanisms associated with the leakage. The GT-225 was most sensitive to hot surface rim seal leakage, followed closely by the hot surface crossarm seal leakage, and then the cold surface leakage. Performance was degraded much more when turbine inlet temperature was fixed (by about a factor of three for specific fuel consumption) than for the fixed geometry mode. The effects of leakage became less severe as gasifier turbine speed was increased. The engine performance deterioration due simply to the flow bypass is compounded by induced losses in regenerator and compressor performance leading to very large changes in engine performance, e.g., specific fuel consumption increased up to 80 percent and power decreased as much as 40 percent with 8 percent additional leakage.

Commentary by Dr. Valentin Fuster
J. Eng. Power. 1980;102(1):19-27. doi:10.1115/1.3230222.

This paper describes an experimental measurement of the effects of uneven blade spacing on the acoustic and aero-thermodynamic characteristics of high-speed, high-pressure-ratio fan rotors at two selected spacing configurations. A test rig, consisting of inlet guide vanes and transonic rotor blades, was employed to explore the redistribution of harmonic sound energy into a series of multiple tones of lower sound pressure level. The measured data indicated that a ten percent modulated rotor exhibited a six to eight decibel decrease in the sound pressure level as compared with the original first blade passage frequency harmonic. Disadvantages in aerodynamic performance resulting from spacing modulation were not so unfavorable for the ten percent modulated blades. However, with five percent modulated blades, serious deterioration in aerodynamic performance was observed particularly near the blade tip section, which produced an unfavourable acoustic signature. A calculation method, assuming a pulse event for each blade sound pressure, provided agreeable results with the measured data.

Commentary by Dr. Valentin Fuster
J. Eng. Power. 1980;102(1):28-34. doi:10.1115/1.3230228.

A method for computation of the flow field around an arbitrary airfoil cascade on an axially symmetric blade-to-blade surface was developed which takes into account the development and separation of the blade surface boundary layers and mixing in the wake. The method predicts the overall fluid turning and total pressure loss in the context of an inviscid-viscous interaction scheme. The inviscid flow solution is obtained from a compressible flow matrix method. The viscous flow is obtained from a differential boundary layer method which calculates laminar, transitional and turbulent boundary layers. Provisions for the calculation of laminar and turbulent separation regions were added to the viscous scheme. The combined inviscid-viscous interaction scheme described yields results which are quantitatively consistent with experimental data. This suggests that the physical basis for the interactive system is correct and justifies continued exploration and use of the method.

Commentary by Dr. Valentin Fuster
J. Eng. Power. 1980;102(1):35-41. doi:10.1115/1.3230229.

The reheat gas turbine cycle combined with the steam turbine Rankine cycle holds new promise of appreciably increasing power plant thermal efficiency. Apparently the cycle has been overlooked and thus neglected through the years. Research and development is being directed towards other gas turbine areas because of the world energy crunch; and in order to focus needed technical attention to the reheat cycle, this paper is presented, using logic and practical background of heat recovery boilers, steam turbines, gas turbines and the process industry. A critical analysis is presented establishing parameters of efficiency, cycle pressure ratio, firing temperature and output. Using the data developed, an analysis of an actual gas generator, the second generation LM5000, is applied with unique approaches to show that an overall 50 percent efficiency power plant can be developed using today’s known techniques and established base-load firing temperatures.

Commentary by Dr. Valentin Fuster
J. Eng. Power. 1980;102(1):42-49. doi:10.1115/1.3230230.

Part I presented an analysis of the simple and reheat gas turbine cycles and related these cycles to the combined gas turbine Rankine cycle. Part II uses the data developed in Part I and applies the second generation LM5000 to a combined cycle using a steam cycle with 1250 psig 900 FTT (8.62MPa and 482°C) steam conditions; then the reheat gas turbine is combined with a reheat steam turbine with steam conditions of 2400 psig and 1000/1000 FTT (16.55 MPa and 538/538° C). A unique arrangement of the superheater is discussed whereby part of the steam heat load is shifted to the reheat gas turbine to obtain a minimum heat recovery boiler stack temperature and a maximum cycle efficiency. This proposed power plant is projected to have a net cycle efficiency of 50 percent LHV when burning distillate fuel.

Commentary by Dr. Valentin Fuster
J. Eng. Power. 1980;102(1):50-60. doi:10.1115/1.3230231.

Pure tone noise, blade row vibrations, and aerodynamic losses are phenomena which are influenced by stator and IGV blade wake production, decay, and interaction in an axial-flow compressor. The objective of this investigation is to develop a better understanding of the nature of stator and IGV blade wakes that are influenced by the presence of centrifugal forces due to flow curvature. A single sensor hot wire probe was employed to determine the three mean velocity components of stator and IGV wakes of a single stage compressor. These wake profiles indicated a varying decay rate of the tangential and axial wake velocity components and a wake profile similarity. An analysis, which predicts this trend, has been developed. The radial velocities are found to be appreciable in both IGV and the stator wakes. This wake data as well as the data from other sources are correlated in this paper. Appreciable static pressure gradient across the wake is found to exist near the trailing edge of both stator and IGV.

Commentary by Dr. Valentin Fuster
J. Eng. Power. 1980;102(1):61-67. doi:10.1115/1.3230235.

It can be argued that the vectored thrust engine is one of the best propulsion systems for V/STOL combat aircraft. The next step is to consider the problems to be overcome in integrating such a powerplant into the airframe. A critical installation issue, and one which is not unique to vectored thrust systems, is that of hot exhaust gas recirculation. In this paper some of Rolls-Royce’s hot gas recirculation experience is described, covering, in particular, the creation of a specialized recirculation test facility and some sample test results. The application of the results to a vectored thrust installation is discussed, together with some of the implications for the aircraft designer. Finally, some comments are made regarding the requirements of other V/STOL propulsion systems to avoid excessive hot gas recirculation.

Commentary by Dr. Valentin Fuster
J. Eng. Power. 1980;102(1):68-74. doi:10.1115/1.3230236.

The development of supercritical cascades follows experience with supercritical single profiles. In cascade arrangement strong inter-blade influences exist in the transonic region, so that a desired pressure distribution, starting from the contour shape, is not easy to realize. For these cases, a design method has been developed in which the boundary conditions can be prescribed in a simple and clear manner. Thickness and deflection are not restricted, since the full potential equation is treated. Variation of the axial velocity density ratio is provided for. The solution by relaxation leads to short computing times. Experimental comparisons for turbine and compressor cascades show the applicability of the method.

Commentary by Dr. Valentin Fuster
J. Eng. Power. 1980;102(1):75-80. doi:10.1115/1.3230237.

This paper presents the results of experimental investigations on the three-dimensional flow and performance characteristics of a free vortex axial flow fan rotor, with a freely rotating and braked inlet guide vane row. The influences of axial distance between the inlet guide vane row and the rotor inlet, inlet guide vane setting angle and shape, partial omission of guide vanes at the hub and tip regions on the return flows have been studied and optimum axial distance and setting angle that will improve the useful operating range of the fan were determined. Use of freely rotating inlet guide vanes at high flow volumes and braked inlet guide vanes at low flow coefficients resulted in a reduction of return flows and an increase of the stable operating range of the axial fan rotor by more than 35 percent and this combination has yielded higher efficiencies as well in the extended region of stable operation.

Commentary by Dr. Valentin Fuster
J. Eng. Power. 1980;102(1):81-87. doi:10.1115/1.3230238.

Film cooling has been studied on the rotor blade of a large scale (low speed) model of a high pressure turbine first stage. Film coolant was discharged from single holes on the pressure and suction surfaces of the airfoil. For each blowing site the coolant to free stream mass flux ratio and density ratio were varied from 0.5 to 1.5 and from 1.0 to 4.0 respectively. Both surface flow visualization and local film cooling adiabatic effectiveness data were obtained. The observation was made that although it can have a strong radial component, the trajectory of the film coolant was very insensitive to coolant flow conditions. The existence of the radial component of the film coolant trajectory was found to have a strong impact on the nature of the effectiveness distribution. The data have been compared with data taken by other investigators on flat surfaces and in plane cascades. Agreement between the flat plate data and the suction surface data was reasonably good. However, the pressure surface results showed a much faster decay of the effectiveness than did the flat plate data due to effects thought to be related to both curvature and radial flow.

Commentary by Dr. Valentin Fuster
J. Eng. Power. 1980;102(1):88-95. doi:10.1115/1.3230239.

Laser-Doppler velocimetry was used to investigate the secondary flow in the endwall region of a large-scale turbine inlet-guide-vane passage. The mean and turbulent velocities were measured for three different test conditions. The different test conditions consisted of variations in the blade aspect ratio and the inlet boundary-layer thickness or all three cases, a passage vortex was identified and its development documented. The turbulent stresses within the vortex were found to be quite low in comparison with the turbulence in the inlet boundary layer.

Commentary by Dr. Valentin Fuster
J. Eng. Power. 1980;102(1):96-103. doi:10.1115/1.3230240.

The strong pressure gradients that take place in high performance transonic axial flow compressors induce important viscous interaction effects. In order to estimate these effects and to calculate the flow field in the inviscid core, the perfect gas approach is necessary. A three-dimensional stationary characteristic method is used for such an approach, and numerical results are given for the flow field in: • the blade channel of a supersonic cascade with converging side walls, • the blade channel of an annular supersonic cascade, • the inlet region of a linear supersonic blade cascade with converging side walls.

Commentary by Dr. Valentin Fuster
J. Eng. Power. 1980;102(1):104-112. doi:10.1115/1.3230203.

This paper describes a joint General Electric/ARAMCO program for improving rotating gas turbine parts lives in desert environments with high concentrations of alkali salts and sulfur-containing fuels. A description of the environment is given along with a definition of the hot corrosion problem. A series of buckets coated with experimental or developmental coatings produced by a variety of processes such as pack cementation, physical vapor deposition, plasma spray and sheet claddings are described and results from actual field trials are discussed. Pt-Cr-Al pack cementation coatings were observed to perform well. Next generation overlay coatings of the MCr-AlY-type applied by plasma spraying electron beam and sheet cladding processes also performed well.

Commentary by Dr. Valentin Fuster
J. Eng. Power. 1980;102(1):113-119. doi:10.1115/1.3230204.

Efficiency of a gas turbine is reduced by vane tip losses in the compressor section; also, vane tip/rotor rubs can produce catastrophic failure of a gas turbine engine. This paper describes a test rig and experimental data obtained evaluating abrasive coatings for clearance control between the vanes and the rotor in a gas turbine compressor. Plasma/flame sprayed oxides, carbides and bonded abrasives were tested under conditions duplicating those encountered in the compressor section. The effectiveness of the coatings in grinding away the vane tips to provide minimum clearance without damage to the vane or rotor was determined. Coatings with rough, sharp abrasive grains were most effective. The laboratory tests were confirmed in engine development tests and by application to a compressor rotor now in operation.

Commentary by Dr. Valentin Fuster
J. Eng. Power. 1980;102(1):120-123. doi:10.1115/1.3230205.

This paper deals with the design and application of a 29,000 bhp (21,625 KW) gas turbine-compressor unit to perform the duties of high pressure ratio/low volume (storage) and low pressure ratio/high volume (transmission). To achieve this wide range of requirements, a single gas turbine was matched with two tandem driven centrifugal compressors. The paper describes the considerations and the techniques used to select the gas turbine, compressor aerodynamic performance and match the gas turbine and compressors.

Commentary by Dr. Valentin Fuster
J. Eng. Power. 1980;102(1):124-127. doi:10.1115/1.3230206.
Abstract
Commentary by Dr. Valentin Fuster
Commentary by Dr. Valentin Fuster
J. Eng. Power. 1980;102(1):132-135. doi:10.1115/1.3230209.

The objective of this paper is to discuss key considerations associated with selection of Gas Turbine Driven LNG (Liquefied Natural Gas) Turbo Compressors. The selection of any compressor and the ultimate performance is greatly affected by the gas turbine driver chosen. The selection of gas turbine/compressor packages is even more critical when the compressor is to operate on refrigeration service such as in an LNG plant. The selection, performance and operation will be discussed for complete gas turbine compressor packages.

Commentary by Dr. Valentin Fuster
J. Eng. Power. 1980;102(1):136-140. doi:10.1115/1.3230210.

The causative phenomena of subsynchronous vibration, or re-excitation of the critical speed of a rotor, are discussed. Two rotor systems designed to control this type of vibration are presented. One system employs tuned, oil film damper bearings while the other features a stiffened rotor geometry. Stability thresholds of each design are compared based on high-pressure closed loop testing.

Commentary by Dr. Valentin Fuster
J. Eng. Power. 1980;102(1):141-147. doi:10.1115/1.3230212.

A design method has been developed to produce nozzleless turbine casings which provide a centrifugal turbine wheel with a uniform inlet state. The analysis includes the effect of wall friction and has been found to accurately predict the mass flow versus pressure ratio characteristics of nozzleless casings. The uniform inlet state provided by this design approach provides turbine wheel/casing configurations with near optimum efficiency and a very low aerodynamic blade vibration excitation level. The model has been extended to produce variable area casings to simulate a simplified variable casing geometry. Testing has verified the accuracy of the approach both in the design point and variable geometry cases. Also depicted are new insights into turbine wheel design constraints discovered when using a variable geometry turbine casing.

Commentary by Dr. Valentin Fuster
J. Eng. Power. 1980;102(1):148-152. doi:10.1115/1.3230213.

In order to provide the basis for alloy selection in future turbines using pulverized coal, an investigation is undertaken to obtain a basic understanding of the mechanisms of erosion at high temperatures. The test equipment has been designed to simulate the aerodynamic and thermodynamic conditions in the turbine. This facility has the capability of providing between ambient and a 1093°C (2000°F) environment temperature for erosion testing of various materials. The effects of high temperature on the erosion rate was determined and the test results from 304 stainless steel alloy are presented.

Commentary by Dr. Valentin Fuster
J. Eng. Power. 1980;102(1):153-159. doi:10.1115/1.3230215.

Gas cycles are being studied for solar energy power plants on account of the attractive prospects they offer for an efficient heat source utilization. By using a particular arrangement applicable to open or closed recuperative gas cycles, consisting of a heat generator partly bypassing the low pressure side of the recuperator, further improvements can be effected in gas turbine systems. They result in favorable conditions for power and high temperature heat cogeneration, for combined gas and steam cycles, and for flexible plant operation. Specific aspects of solar energy are investigated. They mainly concern variations in operating conditions, energy storage, energy conversion efficiency and combined cycles. Applications are made to open and closed cycle power plants. As the combination of a solar receiver with a fossil-fired auxiliary heat source is considered, fossil-fired power plants with an auxiliary solar heating are examined.

Commentary by Dr. Valentin Fuster
J. Eng. Power. 1980;102(1):160-161. doi:10.1115/1.3230216.

The Solar powered gas turbine plant has a storage system which is also capable of supplying instant heat at short interruptions of sunshine by clouds. The storage tank is not under pressure and is built of inexpensive material.

Commentary by Dr. Valentin Fuster
J. Eng. Power. 1980;102(1):162-168. doi:10.1115/1.3230217.

The axial flow compression system of a modern gas turbine engine normally delivers a large quantity of airflow at relatively high velocity. The sudden stoppage (and reversal) of this flow when an engine surges can result in structural loads in excess of steady state levels. These loads can be quite complex due to inherent asymmetry in the surge event. The increasing requirements for lighter weight engine structures, coupled with the higher pressure ratio cycles required for minimizing fuel consumption, make the accurate prediction of these loads an important part of the engine design process. This paper is aimed toward explaining the fluid mechanics of the surge phenomenon and its impact on engine structures. It offers relatively simple models for estimating surge-induced loads on various engine components. The basis for these models is an empirical correlation of surge-induced inlet overpressure based on engine pressure ratio and bypass ratio. An approximate estimate of the post-surge axial pressure distribution can be derived from this correlation by assuming that surge initiation occurs in the rear of the compression system.

Commentary by Dr. Valentin Fuster
J. Eng. Power. 1980;102(1):169-177. doi:10.1115/1.3230218.

This paper contains the results of the measurements of fluctuating pressures on the mid-span profile surfaces of both rotor and stator blades for several points of operation. By the aid of rotating probes behind the rotor, the shapes of the rotor wakes were measured, too. All the measurements have been performed twice, at first with guide vanes in front of the rotor, and afterwards without the latter. The results of the measurements are evaluated with respect to the parameters involved, like Strouhal-number, reduced frequency, and circumferential Mach number. The flow analysis is done for various wake-fields behind the rotor correspondent to different operation points. Emphasis is given to the establishment of correlations for the blade-row interaction and to the comparison of the measured pressure distributions with computed results according to well-known theoretical approaches.

Commentary by Dr. Valentin Fuster
J. Eng. Power. 1980;102(1):178-186. doi:10.1115/1.3230219.

A study of the effects of rotor-blade geometry, steady loading, and distortion wavelength on the distortion flow field upstream and downstream of an unstalled isolated rotor shows that the attenuation of the distortion as it passes through the rotor is a function of the blade stagger angle and the distortion-reduced frequency or the ratio of rotor blade spacing to the distortion wavelength s/l . Maximum attenuation occurs when s/l = 0.5 and increases with increasing stagger angle. Little influence of rotor incidence angle or steady loading was observed.

Commentary by Dr. Valentin Fuster
J. Eng. Power. 1980;102(1):187-192. doi:10.1115/1.3230220.

A conceptual design was made of an 80,000-shp fossil-fired closed-cycle helium turbine system for naval propulsion applications. Various oil-fired heater and power conversion system configurations were investigated, and a reference system was selected based on a trade-off between the system performance and the component size and weight characteristics which would yield the maximum pay load capability for the ship type considered. The conceptual design of major components was made based on the parametric design results identified from a comprehensive computer program incorporating extensive past design experiences. The conceptual design drawing for the propulsion engine is presented, and a preliminary propulsion system layout applicable to a conceptual high-speed destroyer is shown. The analysis indicates that no severe integration problem is anticipated for this type propulsion engine for lightweight ship propulsion applications.

Commentary by Dr. Valentin Fuster
J. Eng. Power. 1980;102(1):193-201. doi:10.1115/1.3230221.

A method for analyzing the viscous flow through turbomachine rotors is presented. The field analysis is based on the solution of the full Navier-Stokes equations over the rotor blade-to-blade stream channels. An Alternating-Direction-Implicit method is employed to carry out the necessary numerical integration of the elliptic governing equations. The flow analysis may be applied to various types of turbomachine rotors. Preliminarily, only the case of laminar flows are considered in this paper. The flow characteristics within the rotors of a radial inflow turbine and a radial bladed compressor are investigated over a wide range of operating conditions. Excellent results are obtained when compared with existing experimental data. The method of this analysis is quite general and can deal with wide range of applications. Possible modification of the present study to deal with turbulent flow cases are also identified.

Commentary by Dr. Valentin Fuster
J. Eng. Power. 1980;102(1):202-208. doi:10.1115/1.3230224.

A dynamic model of an atmospheric pressure fluidized-bed steam generation system is presented which allows digital simulation and analytical controller design. The nonlinear, time-invariant, deterministic, continuous-time model is derived in state-space form from conservation relations, empirical correlations and system design data. The model has been verified for steady-state and transient performance with measured data from experimental test runs. Transient responses of several process variables, following independent step disturbances in coal feed rate and air flow, are illustrated.

Commentary by Dr. Valentin Fuster
J. Eng. Power. 1980;102(1):209-214. doi:10.1115/1.3230225.

Flame stabilization by bluff-bodies has been investigated to highlight the role of recirculation zone on the phenomenon. It has been observed that close correlations exist between heat exchange from recirculation zone and flame stability as controlled by recirculation strength.

Topics: Flames , Stability , Heat
Commentary by Dr. Valentin Fuster
J. Eng. Power. 1980;102(1):215-222. doi:10.1115/1.3230226.

The choice of working fluid has a significant impact on the size and design characteristics of turbines for closed cycle OTEC (Ocean Thermal Energy Conversion) power systems. This paper examines turbine sizes and speeds for various candidate working fluids. The turbine performance and design limits are strongly influenced by blade stress criteria which have been ignored by previous investigators. Illustrative design parameters are given for a turbine using ammonia and scaling parameters are listed to compare the power outputs of turbines using other fluids. The design of a turbine for open-cycle OTEC power systems is largely dictated by the very high specific volume of the exhaust steam at a pressure of about 0.14 psia. In order to minimize the cost of turbines and generators through economy of scale, it is desirable to maximize the power output of a single turbine, and this leads to very large diameters and blade lengths. This paper explores the considerations which influence the choice of turbine size, blade length, speed, power output and efficiency.

Commentary by Dr. Valentin Fuster

DISCUSSIONS

TECHNICAL BRIEFS

J. Eng. Power. 1980;102(1):223-224. doi:10.1115/1.3230227.

Four types of sensors were used to make both dynamic and time-averaged flow measurements in a cold turbine rig to determine the magnitude of errors in time- averaged total-pressure measurement at a station 5 1/2 blade cords downstream from the rotor. The errors turned out to be negligible. The sensors and their intended use are discussed.

Commentary by Dr. Valentin Fuster

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