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Controls, Diagnostics and Instrumentation

2002;():1-7. doi:10.1115/GT2002-30019.

A diagnostic tool able to detect faults that may occur in a gas turbine power plant at an early stage of their emergence is of a great importance for power production. In the present paper, a diagnostic tool, based on Feed Forward Neural Networks (FFNN), has been proposed for gas turbine power plants with a condition monitoring approach. The main aim of the proposed diagnostic tool is to reliably detect not only every considered single fault, but also two or more faults that may occur contemporarily. Two different FFNNs compose the proposed diagnostic tool. The first network, that is not-fully connected, operates a fault pre-processing in order to evaluate the influence of the single fault variable on the single fault condition. The second FFNN detects the fault conditions by means of an iterative process. Such a diagnostic tool has been applied to a mathematical model of a single shaft gas turbine for power generation, resulting able to detect the 100% of single faults and the 80% of combined faults.

Commentary by Dr. Valentin Fuster
2002;():9-17. doi:10.1115/GT2002-30020.

A method for estimating performance parameters in jet engines with limited instrumentation has been developed. The technique is applied on a nonlinear steady state performance code by making simultaneous use of a number of off-design operating points. A hybridized optimization tool using a genetic algorithm to obtain an initial estimate of the performance parameters, and a gradient method to refine this estimate, has been implemented. The method is tested on a set of simulated data that would be available during performance testing of a PW100 engine. The simulated data is generated assuming realistic noise levels. The technique has been successfully applied to the estimation of ten performance parameters using six simulated measurement signals. The determination of identifiability (the property governing whether the performance parameters of the model can be uniquely determined from the measured data) and the selection of parameters in the performance model has been based on the analysis of the system Hessian, i.e. the multidimensional second derivative of the goal function. It is shown, theoretically as well as in practice, how the process of selecting model parameters can be approached in a systematic manner when nonlinear multi-point problems are studied.

Commentary by Dr. Valentin Fuster
2002;():19-27. doi:10.1115/GT2002-30021.

This paper describes a new approach to the development of a fault diagnostics and prognostic capability for an advanced cycle gas turbine. It is based on techniques using sensor based and model based information. Sensor based information is the actual information obtained from the real engine and the model based information comes from the data obtained from engine performance model simulation with a permutation of implanted faults taking into account sensor noise and bias. The approach adopted here is to minimize an objective function which represents the difference between the actual and simulated data and the minimized objective function allows us identify the nature of fault. After the initial success with simple cycle engines, it was decided to extend this technique to advanced cycle engines. The technique is being tested on an in-house model of an intercooled recuperated engine with variable geometry similar to the ICR-WR21cycle. A detailed analysis of the technique applied to simple cycle and advanced cycle will be presented.

Commentary by Dr. Valentin Fuster
2002;():29-36. doi:10.1115/GT2002-30022.

The profitability of industries that use gas turbines is very much dependent on the satisfactory performance of their engines. Deterioration in performance of engines, results in loss of power and thermal efficiency which together contributes to higher fuel costs, lower revenue and reduced profitability. The purpose of a performance monitoring system is to identify any faults developing at engine component level and the extent of the deterioration. (e.g. the extent to which compressor fouling has occurred). The ability of such a system to predict the impact on power available is also important. This paper discusses the application of a performance monitoring system in detecting an implanted fault in an actual gas turbine.

Commentary by Dr. Valentin Fuster
2002;():37-44. doi:10.1115/GT2002-30023.

An integrated machine condition monitoring system was developed, installed, tested, commissioned and successfully operated on a floating production, storage and offloading vessel (FPSO) in the North Sea. This system combines an existing vibration and process monitoring system with focused performance monitoring capability that has been implemented with the cooperation of the oil company end-user, a thermodynamics specialist consultant and a monitoring system supplier. Implementation of this integrated monitoring system strategy with advanced performance monitoring is partly based on the end-user’s requirements to optimize their operation and maintenance functions to improve competitiveness. The system has already been in use for one year and has demonstrated the ability to detect faults at an early stage of development, such as the compressor degradation and gas turbine fouling described in this paper. The same system has also been implemented in other oil & gas and power applications around the world with similar, positive results.

Topics: FPSO
Commentary by Dr. Valentin Fuster
2002;():45-51. doi:10.1115/GT2002-30024.

A new method for the data validation of gas turbines is introduced that allows to assess engine degradation. The degree of degradation is derived from the analytical method of Gas Path Analysis (GPA). The method, built on GPA, allows to isolate faulty measurements. The method repeatedly estimates degradation from a set of measurement values. By systematically excluding measurement values from the calculation, differing estimations are evoked which indicate faulty measurement sets. The procedure also allows to distinguish between state changes and measurement errors in an easy fashion.

Topics: Gas turbines
Commentary by Dr. Valentin Fuster
2002;():53-63. doi:10.1115/GT2002-30025.

Gas turbine performance analysis programs convert raw measurements (thrust, fuel flow, gas path temperatures and pressures) into engine and component figures of merit (e.g., specific fuel consumption, component efficiencies). The derived parameters allow a diagnostician to recognize a poorly performing engine and help to identify the root cause for the problem. These goals can be foiled by measurement problems or by analysis assumptions embedded in the programs. A gifted analyst learns to recognize measurement or assumption problems via characteristic “fingerprints” in the program output. The latest data analysis tools tackle the same issues using a weighted-least-squares algorithm which estimates sensor errors at the same time component health is being determined. The weighted-least-squares algorithm modifies the “fingerprint” of a measurement problem and also that of a problem-free solution. To develop intuition for these new programs, the analyst must understand how weighted-least-squares works and how normal and problem situations are reflected in the output. This paper describes the weighted-least-squares algorithm, as implemented in GE’s TEMPER program, with the goal of stimulating a new intuition for interpreting gas path analysis results.

Commentary by Dr. Valentin Fuster
2002;():65-73. doi:10.1115/GT2002-30026.

A method of identifying the gradual deterioration in the components of jet engines is presented. It is based on the use of an engine model which has the capability to adapt component condition parameters, so that measured quantities are matched. The main feature of the method is that it gives the possibility to identify performance deviations in a number of parameters larger than the number of measured quantities. This is achieved by optimizing a cost function which incorporates not only measurement matching terms but also terms expressing various constraints resulting from the physical knowledge of the deterioration process. Time series of data representing deterioration scenarios are used to demonstrate the method’s capabilities. The test case considered is a twin spool partially mixed turbofan, representative of present day large civil aero engines. Implementation aspects, related to both the measurement set and the identification algorithms are discussed. An interpretation of the output of the method in function of different parameters entering the diagnostic problem is presented.

Commentary by Dr. Valentin Fuster
2002;():75-82. doi:10.1115/GT2002-30027.

Today many methods are available for gas turbine flow path analysis. Some of them are very simple but yet very useful, since they give an indication of the compressor capacity with almost no calculation effort. The state of the art today is the heat and mass balance models (HMB), which are more sophisticated. This paper presents a general overview of these methods, including the most recent trend, Artificial Neural Networks (ANN). In the future, the ANN-based flow path analysis system will probably, to some extent, replace the HMB-based systems, or become a complementary tool for monitoring and performance analysis of power production units. This paper will give a comprehensive explanation of how to build a flow path analysis system in an equation-solving package (e.g. spreadsheet program), by using relationships presented here. This may give a system that is well within the capabilities of most commercially available systems, used and developed by consultant companies (third party companies).

Commentary by Dr. Valentin Fuster
2002;():83-91. doi:10.1115/GT2002-30028.

Thermal and flow diagnostics of power units makes use of diagnostic relations i.e. relations between fault signatures (sets of symptoms) and geometry degradation of its components. Determining symptoms may base on thorough thermal measurements of the cycle. However, numerous apparatuses in the cycle are not or cannot be properly equipped for necessary measurements. Examples of such apparatuses in a steam turbine are external glands and nozzle box sealings. The paper studies the applicability of a selected type of Artificial Neural Network, ANN, as a diagnostic relation for locating faulty apparatuses in HP and IP turbine casings, including their sealing systems. The obtained results can be assessed as good for single faults, and satisfactory for multiple faults of the cycle components. The examined type of ANN can be used e.g. in a modular hierarchical diagnostic system proposed by Gluch & Krzyzanowski, 1998, 1999.

Commentary by Dr. Valentin Fuster
2002;():93-100. doi:10.1115/GT2002-30029.

In one possible model of distributed power generation a large number of users will operate individual, gas turbine powered, cogeneration systems. These systems will be small, relatively inexpensive, and installed in locations without ready access to gas turbine maintenance experts. Consequently an automated method to monitor the engine and diagnose its health is required. To remain compatible with the low cost of the power system the diagnostics must also be relatively inexpensive to install and operate. Accordingly a minimum number of extra sensors should be used and the analysis performed by a common personal computer system. The current work automates the diagnosis of component faults by comparing the engine’s operating trends to the trends for known faults. This allows the relative percentage chance of each fault occurring to be determined. The likelihood of each fault is then compared, to determine which component is degrading. The technique can be adapted to compare the engines historic operating trend or a single operating point. In this initial work a computer model was used as a test bed and 5 faults were introduced individually. The technique successfully diagnosed the faulty component using either the operating trend or a single operating point.

Commentary by Dr. Valentin Fuster
2002;():101-108. doi:10.1115/GT2002-30030.

The diagnostic ability of Probabilistic Neural Networks (PNN) for detecting sensor faults on gas turbines is examined. The structure and the features of a PNN, for sensor fault detection, are presented. It is shown that with the proposed formulation, a powerful tool for sensor fault identification is produced. A particular feature of the PNN produced is the ability to detect sensor faults even in the presence of engine component malfunction, as well as on deteriorated engines. In such situations, the size of bias that can be identified increases. The way to establish the limits of sensor bias that can be detected is presented along with results from application to test cases with realistic noise magnitudes. The diagnostic procedure proposed here is also supported by an engine performance model. The data used for setting up and testing the PNN are generated by such a model.

Commentary by Dr. Valentin Fuster
2002;():109-118. doi:10.1115/GT2002-30031.

A method for diagnosing component faults of jet engines is presented. It uses non-linear gas path analysis techniques to determine the values of health parameters, with the help of a suitably formulated engine simulation model. The incentive of the method is to achieve the determination of the values of component health indices when a limited number of measured quantities is available, which do not allow the determination of all the fault indices simultaneously. A combinatorial approach is introduced, in order to circumvent the problem of the insufficient information for determining a full set of indices. After obtaining a set of possible solutions, a selection procedure is applied to isolate the ones that can give the actual fault identity. Quantification of the fault comes at a final step, when the faulty component has been identified. Different scenarios of faults on a twin spool partially mixed turbofan engine are considered in order to demonstrate the effectiveness of the method. The limitations of the method are also discussed.

Commentary by Dr. Valentin Fuster
2002;():119-126. doi:10.1115/GT2002-30032.

The paper presents an analysis of the effect of changing the fuel on the performance of industrial gas turbines and examines the impact of such a change on methods used for engine condition assessment and fault diagnostics. A similar analysis is presented for the effects of water injection in the combustion chamber (which is usually done for reducing NOx emissions). First, the way of incorporating the effect of fuel changes and water injection into a computer model of gas turbine performance is described. The approach employed is based on the change of (a) working fluid properties, (b) turbomachinery components performance. The model is then used to derive parameters indicative of the “health” of a gas turbine and thus diagnose the presence of deterioration or faults. The impact of ignoring the presence of an altered fuel or injected water is shown to be of a magnitude that would render a diagnostic technique that does not incorporate these effects ineffective. On the other hand, employing the appropriate physical modeling makes the diagnostic methods robust and insensitive to such changes, being thus able to provide useful diagnostic information continuously during the use of a gas turbine.

Commentary by Dr. Valentin Fuster
2002;():127-134. doi:10.1115/GT2002-30033.

In the paper, Expert Systems (ESs) developed to support gas turbine engine maintenance and diagnostics are presented. The ESs are applied to turbofans and Auxiliary Power Units and are developed both in procedural (Visual Basic) and declarative (Turbo Prolog) languages. The paper reports some examples of ES utilization, so highlighting high interactivity and user-friendly interface. Moreover, for each ES, the main working features as well as strong and weak points are put into evidence.

Commentary by Dr. Valentin Fuster
2002;():135-143. doi:10.1115/GT2002-30034.

Upgrading military engine test cells with advanced diagnostic and troubleshooting capabilities will play a critical role in increasing aircraft availability and test cell effectiveness while simultaneously reducing engine operating and maintenance costs. Sophisticated performance and mechanical anomaly detection and fault classification algorithms utilizing thermodynamic, statistical, and empirical engine models are now being implemented as part of a United States Air Force Advanced Test Cell Upgrade Initiative. Under this program, a comprehensive set of real-time and post-test diagnostic software modules, including sensor validation algorithms, performance fault classification techniques and vibration feature analysis are being developed. An automated troubleshooting guide is also being implemented to streamline the troubleshooting process for both inexperienced and experienced technicians. This artificial intelligence based tool enhances the conventional troubleshooting tree architecture by incorporating probability of occurrence statistics to optimize the troubleshooting path. This paper describes the development and implementation of the F404 engine test cell upgrade at the Jacksonville Naval Air Station.

Topics: Engines
Commentary by Dr. Valentin Fuster
2002;():145-152. doi:10.1115/GT2002-30035.

In this paper a feedforward neural network is used to model the fuel flow to shaft speed relationship of a Spey gas turbine engine. The performance of the estimated model is validated against a range of small and large signal engine tests. It is shown that the performance of the estimated models is superior to that of the estimated linear models.

Commentary by Dr. Valentin Fuster
2002;():153-161. doi:10.1115/GT2002-30036.

This paper presents extensive use of dynamic simulation of Compressed Air Energy Storage Gas Turbine (CAES G/T) for control system design, control logic development and software validation which significantly reduced development time and cost of the control system and contributed to successful demonstration of a 2,000 kW pilot plant with technical risk mitigation. The CAES G/T is one of the electrical load leveling power plants. High-pressure air compressed by motor-driven compressors in nighttime and stored in underground reservoir is provided to the CAES G/T for power generation in day time. Based on extensive simulation, we finalized control system configuration and control component specification. Developed control logic was tested by comprehensive simulation tests covering almost all expected operations such as start, acceleration, load application and rejection, recuperator active/non-active mode transfer etc. Finally, we conducted hardware in the loop simulation test to assure Electronic Control Unit (ECU) function and performance. An actual ECU and a real time simulator with the CAES G/T model, sensor models and actuator models were used in this test. Tests at a 2,000 kW pilot plant started in January 2001. The full load rejection test showed 9.9% overspeed of the rotor, which is 1% less than the simulation predicted value and within the regulation limit. Only minor control parameter and logic adjustments were required during the tests. Regular operation of the plant started in June 2001.

Commentary by Dr. Valentin Fuster
2002;():163-168. doi:10.1115/GT2002-30038.

The paper addresses the problem of dynamic modelling of gas turbines for condition monitoring purposes. Identification of dynamic models is performed using a novel Markov chain technique. This includes identifiability analysis and model estimation. When identifying the model, experimental data should be sufficiently informative for identification. So far, identifiability analysis is weak formed and workable solutions are still to be developed. A possible technique is proposed based on non-parametric models in the form of controllable Markov chains. The second step in systems identification is the model estimation. At this stage, Markov chains are introduced to provide more functionality and versatility for dynamic modelling of gas turbines. The Markov chain model combines the deterministic and stochastic components of the engine dynamics within a single model, thus providing more exact and adequate description of the real system behaviour and leading to far more accurate health monitoring.

Commentary by Dr. Valentin Fuster
2002;():169-182. doi:10.1115/GT2002-30039.

A novel high-fidelity real-time simulation code based on a lumped, non-linear representation of gas turbine components is presented. The aim of the work is to develop a general-purpose simulation code useful for setting up and testing control equipments. The mathematical model and the numerical procedure are specially developed in order to efficiently solve the set of algebraic and ordinary differential equations that describe the dynamic behavior of the gas turbine engine. The paper presents the model and the adopted solver procedure. The code, developed in Matlab-Simulink using an object-oriented approach, is flexible and can be easily adapted to any kind of plant configuration. For high-fidelity purposes, the mathematical model takes into account the actual composition of the working gases and the variation of the specific heats with the temperature, including a stage-by-stage model of the air-cooled expansion. Simulation tests of the transients after load rejection have been carried out for a single-shaft heavy-duty gas turbine and a double-shaft industrial engine. Time plots of the main variables that describe the gas turbine dynamic behavior are shown and the results regarding the computational time per time step are discussed.

Commentary by Dr. Valentin Fuster
2002;():183-187. doi:10.1115/GT2002-30040.

This paper is focused on the control-related problems of a staged combustion system for a large civil high-bypass ratio engine. Methods to obtain optimal fuel distribution and an analysis of the fuel distribution control system are presented in the paper. Additionally, an overview of a closed-loop control scheme based on AFR estimation is described.

Commentary by Dr. Valentin Fuster
2002;():189-195. doi:10.1115/GT2002-30041.

This article describes the design and development of a model-based control system for a large commercial aero gas turbine engine. The control system, referred to as the Integrated Margin Management (IMM) control, exploits a real-time engine model (RTEM) to estimate control loop feedback signals, enabling the implementation of nontraditional control modes. These nontraditional control modes include algorithms for controlling, optimizing, and/or trading off margins to key operational limits such as thrust, compressor stability, combustor stability, turbine life, redline limits, and emissions. An overview of the results produced with the IMM controller design illustrates the feasibility of this approach for commercial aero gas turbine applications.

Commentary by Dr. Valentin Fuster
2002;():197-207. doi:10.1115/GT2002-30042.

Active combustion control has been accomplished in many laboratory and real-world combustion systems by fuel modulation as the control input. The modulation is commonly achieved using reciprocating flow control devices. These demonstrations have been successful because the instabilities have been at relatively low frequencies (∼200 Hz) or the scale of demonstration has been small enough to require very small levels of modulation. A number of real-world instabilities in gas turbine engines involve higher frequencies (200–500 Hz) and attenuation requires the modulation of large fractions of the engine fuel flow rate (hundreds of pounds per hour). A spinning drum valve was built to modulate fuel for these applications. Tests showed that this device provided more than 30% flow modulation up to 800 Hz for liquid fuel flows of greater than 400 lbm/hr. This paper describes the performance of the valve in flow bench tests, open-loop forcing and closed-loop instability control tests. The closed loop tests were done on a single-nozzle combustor rig which exhibited a limit-cycling instability at a frequency of ∼280 Hz with an amplitude of ∼7 psi. It also encounters an instability at 575 Hz under a different set up of the rig, though active control on that instability has not been investigated so far. The test results show that the spinning valve could be effectively used for active instability control, though the control algorithms need to be developed which will deal with or account for actuator phase drift/error.

Commentary by Dr. Valentin Fuster
2002;():209-217. doi:10.1115/GT2002-30043.

Due to the transient operation of short duration facilities (0.2–1.0s running time), fidelity in temperature reproduction requires both minimum steady-state errors and a frequency response above 20Hz. Even with the smallest thermocouple wire diameter (∼12μm), badly designed probes may suffer from unsteady heat conduction between wires and supports. The resulting error is often much larger than steady errors such as the effect of recovery factor. In this paper, the origins of steady and unsteady measurement errors are described and evaluated. An analytical modeling of the transient convection/conduction problem is presented. A number of probe designs are described and evaluated at different Reynolds number. The dynamic response is tested in a hot jet apparatus that generates temperature steps, with jet velocities up to 150m/s. The influence of the length/diameter ratio, the type of support and the presence of a shield on the dynamic response are addressed. The unsteady behavior of a thermocouple probe can be replicated with a combination of first order systems, which defines the transfer function of the probe. The relevant parameters are found by using an optimization routine that fits the numerical system response to the experimental response. This numerical model can be reversed to perform a frequency compensation of the measured data.

Commentary by Dr. Valentin Fuster
2002;():219-226. doi:10.1115/GT2002-30044.

In this paper an optimized probe based on in house experience and literature studies for use in full scale gas turbines for power generation is presented. The main advantages of its design are the insensitivity of the probe head to unsteady wakes of blades, its high strength and suitability for high temperatures. Furthermore it is shown that the calibration function is unambiguous. The second part of the paper deals with correction methods for comparison to numerical methods such as difference in position, mass flow, bending of the probe and blockage effects. The methods are discussed by means of examples.

Commentary by Dr. Valentin Fuster
2002;():227-234. doi:10.1115/GT2002-30045.

This paper describes preparatory work towards three dimensional flowfield measurements downstream of the rotor in an industrial, multistage, axial compressor, using a pneumatic pressure probe. The probe is of the steady state four hole cobra probe type. The design manufacture and calibration of the probe is described. CFD calculations have been undertaken in order to assess the feasability of using such a probe in the high speed compressor environment where space is limited. This includes effects of mounting the probe in close proximity to the downstream stator blades and whether it is necessary to adjust the calibration data to compensate for these effects.

Topics: Compressors , Probes
Commentary by Dr. Valentin Fuster
2002;():235-242. doi:10.1115/GT2002-30046.

Current unsteady pressure sensors have a limiting upper temperature range and with few exceptions cannot survive at the temperatures experienced in gas turbine aero-engines. This paper describes a design and development study of an air-cooled commercially available unsteady pressure transducer capable of operation at temperatures exceeding 900 °C. The research objective for this work is the following: To design a cooling adapter, using air as the cooling media, capable of protecting a standard unsteady pressure transducer, whose maximum operating temperature is around 250 °C. in a gas turbine engine environment where temperatures typically reach 800–l500 °C. In addition the provision of thermal protection must not adversely effect the measurement of unsteady pressure and the cooling adapter and transducer assembly must be small enough to access critical parts of the engine. Current transducer can operate at temperatures exceeding 250 °C; the purpose of this paper is to demonstrate the additional protection offered by air-cooling. The paper describes the validation experiments conducted for this design, the level of thermal protection achieved and the frequency response of the transducer/cooling jacket assembly.

Commentary by Dr. Valentin Fuster
2002;():243-254. doi:10.1115/GT2002-30047.

Gas Turbine, GT, control methodology applied to power generation is being evaluated. Corrected parameter control methodology has been adopted for this purpose. This method uses the corrected physical ambient conditions such as pressure, temperature and humidity in controlling the GT operations. Humidity correction becomes increasingly important in this control scheme. The following are the reasons for accurate and robust humidity measurement: (1) Humidity measurement is important to the operation control of the dry low NOX , DLN, combustor system. (2) GT inlet performance enhancing devices, such as evaporative coolers and inlet foggers, depend upon the accurate humidity measurement to determine the amount of water needed for inlet temperature depression. (3) Humidity measurement is used to determine the amount of water to be injected in the combustor for NOX abatement when running on liquid fuel as an alternative to natural gas fuel. In order to obtain accurate and reliable humidity readings, several commercially available humidity sensors were extensively tested and evaluated in a controlled laboratory environment. The sensors were tested for their measurement accuracy, saturation conditions, power interruption and surge, sudden temperature changes and medium air speed. Test ambient temperature ranges from −30 °C to 50 °C. This covers the operating ambient conditions range for the Gas Turbine. The test criterion is that the error in the response of the sensor shall not exceed ±1 °C from the test reference for all the tests conducted on the sensors. The combustion requirements for Dry Low NOX operations and mode transfer dictate this criterion. Also, as a DLN requirement, error in specific humidity shall not exceed 0.904 g/g of air. This test criterion also satisfies the water injection requirements for NOX abatement and inlet performance enhancing devices. The results show that for ±1 °C error in the sensor measurement, the resulting error in NOX calculation is less than 0.2 ppm. The test results show that all sensors except the current one in use have met the test criterion. The current sensor, General Eastern DT-2, has a large measurement error in the order of ±5 °C. Programs have been launched to field test and evaluate these sensors in order to replace the current one.

Topics: Sensors , Gas turbines
Commentary by Dr. Valentin Fuster
2002;():255-260. doi:10.1115/GT2002-30048.

The knowledge of component stresses and operating conditions (pressure, temperature, vibration) within the rotating parts of aircraft engines and other turbomachines is essential to evaluate performance and load conditions. The rotating data acquisition is accomplished using telemetry systems that have to fulfill extremely high mechanical and thermal requirements. Despite these restrictions, an increasing number of measurement positions have to be recorded simultaneously with a very high bandwidth to resolve oscillations up to 50 kHz or even more. In the following, the requirements on a modern telemetry system will be presented in detail. In addition to the analogue systems used today MTU Aero Engines and Manner Sensortelemetrie have developed a new digital system that enables the simultaneous measurement of 48 dynamic channels with a bandwidth of 50 kHz and improved signal quality. The data is transmitted from the rotating to the stationary side using a high frequency carrier while the electric power is supplied inductively.

Commentary by Dr. Valentin Fuster
2002;():261-269. doi:10.1115/GT2002-30049.

At present, one of the most problematic topics associated with the application of optical pyrometry to in-service use on gas turbine aeroengines is the fouling of the system optics. Even though only one optical surface is, in general, exposed to the turbine environment the resulting deposition of particulates, from the combustion process or atmospheric ingestion, can greatly inhibit the benefits of utilising such an instrument for direct temperature measurement. The particulates that deposit on the pyrometer lens act as an interference filter by absorbing a portion of the thermal radiation from the target, for example the turbine blades. As a control function input, this will then bias the indicated temperature low and thus permit higher turbine temperature operation resulting in blade temperatures in excess of their intended limits. In practice, a purge air system is therefore incorporated into turbine pyrometer designs in order to minimise the number of particulates that can potentially deposit and build-up on the exposed system optics. This paper examines and compares the flow fields in two purge air designs that have already gained acceptance in-service, namely the military RB199 and civil GE90 pyrometer purging systems. This study of the airflow within actual purge designs will allow gas turbine engineers develop an understanding of the fluid flow structures and how they presently impede the achievement of sufficient lens cleanliness.

Topics: Pyrometers
Commentary by Dr. Valentin Fuster
2002;():271-280. doi:10.1115/GT2002-30050.

The reliability of the gas path components (compressor, burners and turbines) of a gas turbine (GT) is usually high when compared with other GT systems such as fuel and control. However, their availability could be relatively low as high downtimes are normally associated with these components when subjected to forced outages. One way of improving availability is by improved maintenance practices that involve applying such approaches as condition based monitoring (CBM). Unfortunately, this cannot be achieved without the existence of a proper instrumentation set that can adequately and repeatedly track down the levels of deterioration in these components, thereby allowing for optimally scheduled maintenance. Different engine handles (operating point or parameter that is held constant with respect to other parameters) would require different instrumentation sets for proper gas path fault diagnosis. Sometimes, the instrumentation sets used makes the required diagnostic analysis impossible. Furthermore, allowing redundancy in instrumentation, unless specified with knowledge of the diagnostic technique to be used, is not only unnecessary but also cost ineffective. The central theme of this paper is to present a means of attaining an optimum instrumentation set using a non-linear gas path analysis (NLGPA) programme. Firstly, some of the common gas path faults are considered, some theoretical backing is given to the principles involved in this work, the implications of unoptimised instrumentation set as viewed from the users’ perspective is examined and finally, results presented for the NLGPA approach when applied to a two-shaft and a three-shaft industrial gas turbine. Also, we show how the engine handle can affect the choice of the instrumentation set.

Commentary by Dr. Valentin Fuster
2002;():281-288. doi:10.1115/GT2002-30051.

The feasibility of an innovative minimally intrusive sensor for monitoring the hot gas stream at the turbine inlet in high performance aircraft gas turbine engines was demonstrated. The sensor uses passive fiber-optical probes and a remote readout device to collect and analyze the spatially resolved spectral signature of the hot gas in the combustor/turbine flowpaths. Advanced information processing techniques are used to extract the average temperature, temperature pattern factor, and chemical composition on a sub-second time scale. Temperatures and flame composition were measured in a variety of combustion systems including a high pressure, high temperature combustion cell. Algorithms for real-time temperature measurements were developed and demonstrated. This approach should provide a real-time temperature profile, temperature pattern factor, and chemical species sensing capability for multi-point monitoring of high temperature and high pressure flow at the combustor exit with application as an engine development diagnostic tool, and ultimately, as a real-time active control component for high performance gas turbines.

Commentary by Dr. Valentin Fuster
2002;():289-299. doi:10.1115/GT2002-30052.

The objective of this study was to obtain instantaneous planar laser induced fluorescence (PLIF) images of OH in a laboratory-scale, gas-turbine combustor (LSGTC) with a pre-mixed, swirl-stabilized, natural gas flame. Instantaneous PLIF images of OH were obtained at each of four operating conditions (high swirl and medium swirl at fuel equivalence ratios of 0.80 and 0.65). Comparison of the instantaneous images illustrates the stochastic nature of the flame structure. Pixel by pixel statistical analysis of each collection of images allowed both mean and standard deviation images to be generated, and analysis at selected locations has allowed probability density functions to be obtained in various regions of the flame structure. PLIF images of OH, along with visual photographs and video recordings, showed a wide variation in flame structure for the different operating conditions. The variations in flame shapes are primarily a result of the effect of the swirl intensity and fuel equivalence ratio. Changes in the airflow rate over an order of magnitude do not seem to affect the visual flame structure in this experiment. Operation at φ = 0.80 produced the most stable flames with both injectors. The flame with the high swirl injector was more coalesced and closer to the injector than with the medium swirl injector. At φ = 0.65, the flame was quite unstable for both swirl injectors. With the medium swirl injector, the flame would oscillate between two different flame structures, one that was more or less attached to the vortex funnel, and one that was lifted well above the vortex funnel. The MS case at φ = 0.65 was at the very edge of the lean flammability limit, and would on occasion extinguish.

Commentary by Dr. Valentin Fuster
2002;():301-311. doi:10.1115/GT2002-30053.

The objective of this study was to obtain simultaneous axial/radial and axial/tangential velocity measurements in a laboratory-scale, gas-turbine combustor (LSGTC) with a pre-mixed, swirl-stabilized, natural gas flame. Velocity measurements were obtained at each of four operating conditions (high swirl and medium swirl at fuel equivalence ratios of 0.80 and 0.65). Example results of mean and standard deviation axial, radial, and tangential velocities are included in this paper for the high swirl (HS), φ = 0.80 case (most stable flame) and the medium swirl (MS), φ = 0.65 case (least stable case). Additionally, example probably density distributions (PDF) for the axial velocity at the 80 mm axial location are presented for the same two cases.

Commentary by Dr. Valentin Fuster
2002;():313-321. doi:10.1115/GT2002-30054.

This objective of this study was to obtain instantaneous gas temperature measurements using a coherent anti-Stokes Raman spectrometer (CARS) in a laboratory-scale, gas-turbine combustor (LSGTC) with a pre-mixed, swirl-stabilized, natural gas flame. These measurements complement PLIF measurements of OH radical, and LDA measurements of velocity which are presented in companion papers [1,2]. Gas temperature measurements were obtained at each of four operating conditions (high swirl and medium swirl at fuel equivalence ratios of 0.80 and 0.65). Results of mean and standard deviation measurements are included in this paper for all four test cases. Additionally, example probably density functions (PDF) for the gas temperature at the 40 mm, 60 mm, and 80 mm axial locations are presented for the for the high swirl (HS) φ = 0.80 case (most stable flame) and the medium swirl (MS) φ = 0.65 case (least stable flame).

Commentary by Dr. Valentin Fuster
2002;():323-331. doi:10.1115/GT2002-30055.

The determination of degradation of first row gas turbine blades during e.g. hot gas path inspections is very important: Misjudgement can cause severe damage in the gas turbine or lead to an unwanted reduction of the operational life of the blades. A novel method, called Tintell (a Dutch acronym for in-situ degradation determination during hot gas path inspections), to improve this determination is developed. The method is designed to be applicable without demounting the casing of the turbine. Only small openings are required for installing measuring equipment. The procedure for applying Tintell consists of three main phases: • selection of blades and critical area on blades, in order to reduce the amount of time required for measurements; • measurements of degradation; • determination of remnant life and prognosis of future degradation. Selection can be realised by temperature measurements (e.g. pyrometry) on the blades during operation, by acceptance tests and/or by strain measurements on all blades. A new optical strain measuring technique is under development for this, applying boroscope holes near the first row of blades and using the cooling holes on the blades as optical markers. The determination of remnant life is realised by comparing the results of the measurements with calculations produced by applying a sophisticated three-dimensional finite element model of the blades (based on ANSYS). With this model a prognosis of the degradation during a next operational period can be calculated. This quantitative information will help the operator of the gas turbine with the decision, whether or not to replace or refurbish blades.

Topics: Blades
Commentary by Dr. Valentin Fuster

Cycle Innovations

2002;():333-340. doi:10.1115/GT2002-30109.

Solid oxide fuel cell (SOFC) and gas turbine hybrid power generation systems have gained more and more attention with regard to the development of the high performance distributed energy systems. The SOFC can be combined with a gas turbine because the SOFC operating temperature of about 1000°C matches the turbine inlet temperature. In this study, we proposed the multi-stage type SOFC/GT combined system and compared the system performance of it with that of other combined systems using the thermal efficiency and exergy evaluation. It is noted that the thermal efficiency of the 3-stage type SOFC/GT combined system can reach more than 70% (HHV) at low pressure ratio.

Commentary by Dr. Valentin Fuster
2002;():341-349. doi:10.1115/GT2002-30110.

A dynamic system model is presented for a 1 MW Molten Carbonate Fuel Cell/Gas Turbine (FC/GT) hybrid power generation system. Results are shown for both a fuel step reduction at the anode inlet, and a voltage step reduction to the fuel cell. Results showed significant thermal consequences at the cathode inlet for both perturbations. To mitigate these thermal consequences, methods of airflow management were examined and indicated that reducing the total gas turbine air flow worked better than an air bleed arrangement. As one option for air flow management, a control for the turbine speed was studied. These results are also presented showing that the thermal conditions are managed far better with such a control method.

Commentary by Dr. Valentin Fuster
2002;():351-360. doi:10.1115/GT2002-30111.

Design-point and part-load characteristics of a gas turbine-solid oxide fuel cell hybrid micro generation system, of which total power output is 30 kW, are investigated for its prospective use in the small distributed energy systems. A cycle analysis of the hybrid system has been performed to obtain general strategies of highly efficient operation and control. The method of analysis has been compared with previous results, of which power output values are set in the range from 287 to 519 kW. Then, the part-load performance of the 30 kW system has been evaluated. Two typical operation modes, i.e., constant and variable rotation speed gas turbine operation are considered. It is found that the variable speed mode is more advantageous to avoid performance degradation under part-load conditions. Operating under this mode, despite of 10% adiabatic efficiency drop in the gas turbine components, the generation efficiency can be maintained over 60% (LHV) in the power output range from 50 to 100%.

Commentary by Dr. Valentin Fuster
2002;():361-370. doi:10.1115/GT2002-30112.

The present paper reports a detailed technological assessment of two concepts of integrated micro gas turbine and high temperature (SOFC) fuel cell systems. The first concept is the coupling of micro gas turbines and fuel cells with heat exchangers, maximising availability of each component by the option for easy stand-alone operation. The second concept considers a direct coupling of both components and a pressurised operation of the fuel cell, yielding additional efficiency augmentation. Based on state-of-the-art technology of micro gas turbines and solid oxide fuel cells, the paper analyses effects of advanced cycle parameters based on future material improvements on the performance of 300–400 kW combined micro gas turbine and fuel cell power plants. Results show a major potential for future increase of net efficiencies of such power plants utilising advanced materials yet to be developed. For small sized plants under consideration, potential net efficiencies around 70% were determined. This implies possible power-to-heat-ratios around 9.1 being a basis for efficient utilisation of this technology in decentralised CHP applications.

Commentary by Dr. Valentin Fuster
2002;():371-378. doi:10.1115/GT2002-30113.

Molten Carbonate Fuel Cell/Gas Turbine (MCFC/GT) hybrid power systems represent a modern, efficient and clean alternative to the currently used marine propulsion systems. The objective of this paper is to present the results found from the application of MCFC/GT hybrid power systems to marine propulsion, and in particular to present the results of the off-design performance of a COGAFC system (Combined Gas Turbine and Fuel Cell System). The results presented are subjected to the current uncertainties on MCFC power systems derived from its early stage of development. It is, then, the interest of the authors to summarise the results of the research work done, providing to the lectors the understanding and a general view of which are the concerns, the benefits, and which should be the next steps on the implementation of these systems. The study is summarised into two papers: “Molten Carbonate Fuel Cell Gas Turbine Combined Cycle for Marine Propulsion. Part A: Design Point Operation” (Basurto et al., 2002), that describes the selection of the design point, and “Molten Carbonate Fuel Cell Gas Turbine Combined Cycle for Marine Propulsion. Part B: Part Load Operation”, that describes the off-design performance of the system. The study is based on previous work published by the authors on the integration of MCFCs with gas turbines (Basurto et al., 2001).

Commentary by Dr. Valentin Fuster
2002;():379-386. doi:10.1115/GT2002-30114.

Molten Carbonate Fuel Cell/Gas Turbine (MCFC/GT) hybrid power systems could represent a modern, efficient and clean alternative to the currently used marine propulsion systems. The objective of this paper is to present the results of the study of a MCFC/GT hybrid power systems used on marine propulsion. The results are quite promising, but they are subjected to the current uncertainties derived from MCFCs early stage of development. Therefore, the interest of the authors is to summarise the research work done and the results, providing the understanding and a general view of the main concerns, benefits, identifying the next steps on the development of these systems. The study is summarised into two papers: “Molten Carbonate Fuel Cell Gas Turbine Combined Cycle for Marine Propulsion. Part A: Design Point Operation”, that describes the selection of the design point, and “Molten Carbonate Fuel Cell Gas Turbine Combined Cycle for Marine Propulsion. Part B: Part Load Operation” (Basurto et al., 2002), that describes the off-design performance of the system, and it compares the system against conventional diesel and gas turbine systems. The study is based on previous work published by the authors on the integration of MCFCs and gas turbines (Basurto et al., 2001).

Commentary by Dr. Valentin Fuster
2002;():387-395. doi:10.1115/GT2002-30115.

This paper addresses the off-design analysis of a hybrid system (HS) based on the coupling of an existing Ansaldo Fuel Cells (formerly Ansaldo Ricerche) molten carbonate fuel cell (MCFC) stack (100 kW) and a micro gas turbine. The MCFC stack model at fixed design conditions has previously been presented by the Authors [1]. The present work refers to an off-design stack model, taking into account the influence of the reactor layout, current density, air and fuel utilisation factor, CO2 recycle loop, cell operating temperature, etc. Finally, the design and off-design model of the whole hybrid system is presented. Efficiency at part load condition is presented and discussed, taking into account all the constraints for the stack and the micro gas turbine, with particular emphasis on CO2 recycle control.

Commentary by Dr. Valentin Fuster
2002;():397-404. doi:10.1115/GT2002-30116.

The relatively innovative gas turbine based power cycles R–ATR and R–REF (Recuperative – Auto Thermal Reforming GT cycle and Recuperative–Reforming GT cycle) here proposed are mainly aimed to allow the upstream CO2 removal by the way of natural gas fuel reforming. The power unit is a Gas Turbine (GT), fuelled with reformed and CO2 cleaned syngas produced by adding some basic sections to the simple GT cycle: • Auto Thermal Reforming (ATR) for the R-ATR solution, where the natural gas is reformed into CO, H2 , CO2 , H2 O and CH4 ; this endothermic process is completely sustained by the heat released from the reactions between the primary fuel (CH4 ), exhausts and steam. • Water Gas Shift Reactor (WGSR), where the reformed fuel is, as far as possible, shifted into CO2 and H2 by the addition of water. • Water Condensation, in order to remove a great part of the fuel gas humidity content (this water is totally reintegrated into the WGSR). • CO2 removal unit for the CO2 capture from the reformed fuel. Among these main components, several heat recovery units are inserted, together with GT Cycle recuperator, compressor intercooler and steam injection in combustion chamber. The CO2 removal potential is close to 90% with chemical scrubbing using an accurate choice of amine solution blend: the heat demand is completely provided by the power cycle itself. The possibility of applying steam blade cooling by partially using the water released from the dehumidifier downstream the WGSR has been investigated: in these conditions, the R-ATR has shown an efficiency range of 44–46%. High specific work levels have also been observed (around 450–550 kJ/kg). These efficiency values are satisfactory, especially if compared with ATR combined cycles with CO2 removal, more complex due to the steam power section. If regarded as an improvement to the simple GT cycle, R-ATR shows an interesting potential if directly applied to a current GT model; however partial redesign with respect to the commercially available version is required. Finally, the effects of the reformed fuel gas composition and conditions on the amine CO2 absorption system have been investigated, showing the beneficial effects of increasing pressure (i.e. pressure ratio) on the thermal load per kg of removed CO2 .

Commentary by Dr. Valentin Fuster
2002;():405-412. doi:10.1115/GT2002-30117.

In the context of the reduction of the carbon dioxide (CO2 ) emissions as prescribed by the Kyoto protocol, this paper describes a thermodynamic performance analysis of new gas turbine combined cycles with no emissions of CO2 and nitrogen oxides. Three new similar cycles belonging to the same typology are proposed. These cycles use water/steam as working fluid, which is compressed in liquid and vapour phase, and the internal combustion process, which takes place between syngas and pure oxygen. The top Brayton cycle and the bottom Rankine cycle are integrated together. The syngas is produced by steam-natural gas reforming with internal chemical heat recovery. The CO2 produced in the combustion is captured simply by water condensation from the exhaust gas and liquefied to be stored. A simulation analysis has been performed to evaluate the net efficiency and the net specific work of the cycles. Varying the most important operative variables and using the least square regression and 2k factorial design techniques, a very large sensitivity analysis has permitted to highlight the performance behaviour of the cycles. Including the energy penalty due to the liquefaction of CO2 and to the oxygen production and adopting standard operative conditions, the LHV-based net efficiency and the net specific work may exceed 50% and 1000 kJ/kg, respectively.

Commentary by Dr. Valentin Fuster
2002;():413-420. doi:10.1115/GT2002-30118.

The gas turbine system GRAZ CYCLE has been thoroughly studied in terms of thermodynamics and turbomachinery layout. What is to be presented here is a prototype design for an industrial size plant, suited for NG-fuel and coal and heavy fuel oil gasification products, capable to retain the CO2 from combustion and at the same time able to achieve maximum thermal efficiency. The authors hope for an international cooperation to make such a plant available within a few years.

Commentary by Dr. Valentin Fuster
2002;():421-428. doi:10.1115/GT2002-30119.

The Cheng Cycle gas turbine has enjoyed its 25th anniversary since its conception. More than 100 sites around the world including the United States, Japan, Australia, Italy, Germany, and the Netherlands have used the Cheng Cycle. A chronology will be presented in this paper which will highlight the steps taken to develop the fully automated, load following power and cogeneration system. The Cheng cycle operates with a steam to air ratio trajectory that has its highest “peak efficiency” at the onset of a turbine’s operation. The peak efficiency point was coined as the Cheng point by Dr. Urbach [ref.1] of the US Navy’s David Taylor Research Center. Many thermodynamic and professional textbooks refer to the original Dual Fluid Cycle as the Cheng Cycle. Besides the high efficiency feature, the Cheng Cycle is mechanically simple and flexible in operation. It can put power on line faster than a combined cycle, and it has extremely clean emissions at low cost. The future performance of the Advanced Cheng Cycle will also be projected.

Topics: Gas turbines , Cycles , Steam
Commentary by Dr. Valentin Fuster
2002;():429-437. doi:10.1115/GT2002-30120.

In this paper the thermoeconomic analysis of gas turbine plants with fuel decarbonisation and carbon dioxide sequestration is presented. The study focuses on the amine (MEA) decarbonisation plant lay-out and design, also providing economic data about the total capital investment costs of the plant. The system is fuelled with methane that is chemically treated through a partial oxidation and a water-gas shift reactor. CO2 is captured from the resulting gas mixture, using an absorbing solution of water and MEA that is continuously re-circulated through an absorption tower and a regeneration tower: the decarbonised fuel gas is afterwards burned in the gas turbine. The heat required by CO2 sequestration is mainly recovered from the gas turbine exhausts and partially from the fuel treatment section. The reduction in efficiency and the increase in energy production costs due to fuel amine decarbonisation is evaluated and discussed for different gas turbine sizes and technologies (microturbine, small size regenerated, aeroderivative, heavy duty). The necessary level of carbon tax for a conventional plant without a fuel decarbonisation section is calculated and a comparison with the Carbon Exergy Tax procedure is carried out, showing the good agreement of the results.

Commentary by Dr. Valentin Fuster
2002;():439-445. doi:10.1115/GT2002-30123.

Cheng Power Systems, Inc. following the successful Cheng Cycle development based on the Allison 501KH, is developing a 50%+ efficiency medium-power-range Advanced Cheng Cycle. The recent development work involved the selection of a candidate gas turbine which possesses the following attributes: (1) a single shaft, (2) advanced compressor and turbine aerodynamic design, and (3) F-Class firing temperature. The targeted results were a 30MW+ output and 50%+ efficient intermediate-load Advanced Cheng Cycle with the simple-cycle performance characteristics of quick startup and shutdown and a $/kW cost comparable to simple-cycle machines, but at a combined-cycle efficiency. A candidate engine was selected which has the following performance characteristics: a compressor pressure ratio of 18:1, a turbine inlet temperature of 1250°C, and a net efficiency of 35%. Cheng Power has a unique software program which predicts the energy balance at various ambient conditions. The program takes into account the massive cooling air flows incorporated into advanced gas turbines and the heat recovery boiler performance characteristics when developing our performance analyses. The uniqueness of the Cheng Cycle is that it selects a trajectory of steam-to-air ratios to maintain high efficiency for the entire operating range of the power system, while offering competitive efficiency, simple hardware, the possibility of retrofit, fast response to load change, and feasible cogeneration plant operations. The quick startup of the Advanced Cheng Cycle is suitable for variable load operation in the mid range, i.e. 8 to 10 hours per day. This paper provides the energy balance of the proposed plant and its variation with ambient temperature. The paper also addresses the emission characteristics of this engine, which provides low NOx and CO emissions at levels that satisfy most U.S. EPA requirements without SCR or any other add-on devices.

Topics: Cycles
Commentary by Dr. Valentin Fuster
2002;():447-455. doi:10.1115/GT2002-30124.

Steam injection in gas turbines has been used for many years to increase the power output as well as the efficiency of the system and, more recently, to reduce the formation of NOx during the combustion. The major drawback in steam-injected gas turbine technology is the need of large amounts of fresh water that is eventually lost into the atmosphere along with the exhaust gases. Nowadays, fresh water is not readily available in many places due to either local water shortages or environmental legislation that protects water sources from depletion and pollution. In order to deal with water constraints, water recovery systems (WRS) to recuperate the injected steam from the exhaust gases and return it to the steam injection system can be implemented. In this project, computer models for two different WRS configurations have been developed and tested. The computer models allow finding the optimum size, power requirements and capital costs of the heat exchangers involved in a particular WRS configuration. The models can also simulate the performance of WRS during a given period of time, calculating the energy consumed by fans and pumps in the process. This paper explains the details of the computer models and illustrates, as an example, the results obtained when both WRS configurations are applied to the GE LM2500 gas turbine. These results support the technical and economic feasibility of steam recovery for medium-size steam-injected gas turbines.

Commentary by Dr. Valentin Fuster
2002;():457-464. doi:10.1115/GT2002-30125.

The evaporative gas turbine (EvGT) cycle is an advanced cycle with a high electrical efficiency and specific power output. The heat in the exhaust gas is used to evaporate water directly into the compressed combustion air. This increases the volumetric flow rate through the expander without increasing the compressor work. In part flow evaporative cycles, only a fraction of the compressed air passes through the humidification system; the rest of the air bypasses the humidifier. In this study, exergy analyses of different part flow cases for two evaporative cycles have been performed, in order to evaluate part flow humidification. The evaporative cycles are based on the industrial gas turbine GTX100 and the aeroderivative Trent. This paper is divided into two parts. The first part contains an introduction to the EvGT cycle and the methods used in the study. The second part contains the results, discussion, and conclusions.

Commentary by Dr. Valentin Fuster
2002;():465-473. doi:10.1115/GT2002-30126.

This paper is the second part of a two-part paper. The first part contains an introduction to the evaporative gas turbine (EvGT) cycle and the methods used in the study. The second part contains the results, discussion, and conclusions. In this study, exergy analysis of EvGT cycles with part flow humidification based on the industrial GTX100 and the aeroderivative Trent has been performed. In part flow EvGT cycles, only a fraction of the compressed air is passed through the humidification system. The paper presents and analyzes the exergetic efficiencies of the components of both gas turbine cycles. The highest cycle exergetic efficiencies were found for the full flow case for the GTX100 cycles and for the 20% part flow case for the Trent cycles. The largest exergy destruction occurs in the combustor, and the exergetic efficiency of this component has a large influence on the overall cycle performance. The exergy destruction of the heat recovery system is low.

Commentary by Dr. Valentin Fuster
2002;():475-484. doi:10.1115/GT2002-30127.

The Evaporative Gas Turbine Pilot Plant has been in operation at Lund Institute of Technology in Sweden since 1997. In this cycle low-grade heat in the flue gases is utilized for water evaporation into the compressed air in the humidification tower. This result in, amongst others, power augmentation, efficiency increase and lower emissions. This article presents the experimental and theoretical results of the humidification tower, in which simultaneous heat and mass transfer occurs. A theoretical model has been established for the simultaneous heat and mass transfer occurring in the humidification tower and it has been validated with experiments. The humidification tower in the pilot plant can be operated at several operating conditions. An after-cooler makes it possible to chill the compressor discharge air before entering the humidification tower. The saturation temperature of the incoming compressed air can thereby be varied from 62 to 105 °C at the operating pressure of 8 bar(a). It has been shown that the air and water can be calculated throughout the column in a satisfactory way. The height of the column can be estimated with an error of 10% compared with measurements. The results from the model are most sensitive of the properties of the diffusion coefficient, viscosity and thermal conductivity due to the complexity of the polar gas mixture of water and air.

Commentary by Dr. Valentin Fuster
2002;():485-492. doi:10.1115/GT2002-30128.

In this paper, we have proposed a new type of gas turbine cycle based on innovative combination of a newly designed HAT cycle with an externally fired boiler (EFHAT). In this manner, “dirty” fuels such as coal, biomass and etc. can be more efficiently used than before, and the water can be recovered. It is different from the conventional HAT cycle. In particular, the temperature of clean humid air discharging to atmosphere is not limited by the dew point, and the latent heat of steam can be utilized to generate hot water entering the humidifier. Therefore, the humidification ability is significantly enhanced, which is beneficial to improvement on system efficiency. As a result, with a gas turbine of 1073 K TIT, the net efficiency of the new system may reach as high as 43.0%. Furthermore, we have identified that air saturation can make a good temperature match in system, which results in effective utilization of both quality and quantity of energy at low and/or middle temperature range. With the principle of cascade utilization of energy and the methodology of system integration, we have opened up a new orientation for the clean coal power generation system.

Commentary by Dr. Valentin Fuster
2002;():493-499. doi:10.1115/GT2002-30129.

In this paper an alternative to the so-called “oxy-fuel” combustion for CO2 capture is evaluated. “Chemical looping combustion” (CLC), is closely related to oxy-fuel combustion as the chemically bound oxygen reacts in a stoichiometric ratio with the fuel. In the CLC process the overall combustion reaction takes place in two reaction steps in two separate reactors. In the reduction reactor, the fuel is oxidised by the oxygen carrier, i.e. the metal oxide MeO. The metal oxide is reduced to a metal oxide with a lower oxidation number, Me, in the reaction with the fuel. In this manner, pure oxygen is supplied to the reaction with the fuel without using a traditional air separation plant, like cryogenic distillation of air. The paper presents a thermodynamic cycle analysis, where CLC is applied in a Humid Air Turbine concept. Main parameters are identified, and these are varied to examine the influence on cycle efficiency. Results on cycle efficiency are presented and compared to other CO2 capture options. Further, an evaluation of the oxygen carrier, metals/oxides, is presented. An exergy analysis is carried out in order to understand where losses occur, and to explain the difference between CLC and conventional combustion. The oxidation reactor air inlet temperature and the oxidation reactor exhaust temperature have a significant impact on the overall efficiency. This can be attributed to the controlling effect of these parameters on the required airflow rate. An optimum efficiency of 55.9% has been found for a given set of input parameters. Crucial issues of oxygen carrier durability, chemical performance and mechanical properties have been idealized, and further research on the feasibility of CLC is needed. Whether or not the assumption 100% gas conversion holds, is a crucial issue and remains to be determined experimentally. Successful long-term operation of chemical looping systems of this particular type has not yet been demonstrated. The simulation points out a very promising potential of CLC as a power/heat generating method with inherent capture of CO2 . Exergy analysis show reduced irreversibilities for CLC compared to conventional combustion. Simulations of this type will prove useful in designing CLC systems in the future when promising oxygen carriers have been investigated in more detail.

Commentary by Dr. Valentin Fuster
2002;():501-507. doi:10.1115/GT2002-30131.

The economic and design optimisation of gas turbine cycles using biomass fuel gives a clear picture of which kind of cycle is more suitable for a given application, regarding capital and fuel costs. Due to its low specific energy, raw fuel transportation plays a large role in the final cost of electricity. Other factors of influence are the availability of the biomass in each season and the power plant location. An economic and exergy destruction optimisation of a BIGGT, biomass integrated gasification/gas turbine, and EFGT, externally fired gas turbine, cycles is carried out in this paper. Then the sensitivity of the systems to fuel costs, as well as to investment life, is assessed in order to evaluate the influence of these parameters in the final optimised cost of electricity. The Genetic Algorithms, GA, technique, a well known robust technique, is used for the optimisation in this work. The GA tool, GENIAL 1.1® has been written in Fortran77 computer language and was developed by Widell, 1997. The software for the cycle design performance was developed under Fortran90. The EFGT cycle seems to be very promising in terms of cost of electricity and efficiency. The calculations also show that the BIGGT cycle still needs further improvements in the gasification process regarding costs and performance. The cycle used as reference is the well known gas natural/gas turbine cycle, NGGT.

Commentary by Dr. Valentin Fuster
2002;():509-516. doi:10.1115/GT2002-30132.

Off-design steady performance and operating characteristics of single and two shaft gas turbines for electric power generation have been investigated comparatively. A set of balance equations has been derived based on validated component models. A simultaneous calculation scheme has been employed, which is flexible to various engine configurations. Part-load performance analyses of two commercial gas turbines have been carried out to compare operating characteristics between single and two shaft engines. The predicted performance characteristics of both engines coincide soundly with the manufacturer’s data and also correspond with the inherent characteristics of each configuration. The adoption of the VIGV modulation has been addressed in order to examine the possibility of leveling up the heat recovery capacity by maintaining a high turbine exhaust temperature (TET) when those gas turbines are used for combined cycle plants. Maintaining TET at its design value as far as the VIGV modulation allows has been simulated and it has been determined that the TET control is possible at up to 40% and 50% load in the single and two shaft engine, respectively. Combined cycle performances have also been investigated for two engine configurations in different operating modes. While the VIGV modulation produces a favorable influence over the combined cycle performance of the single shaft configuration, the two shaft engine does not appear to be effectively improved by the VIGV modulation since the degradation of gas turbine performance counteracts the advantage of the higher performance of the bottoming (steam turbine) cycle.

Commentary by Dr. Valentin Fuster
2002;():517-525. doi:10.1115/GT2002-30133.

An alternative configuration for a regenerative gas turbine engine cycle is presented that yields higher cycle efficiencies than either simple or conventional regenerative cycles operating under the same conditions. The essence of the scheme is to preheat compressor discharge air with high temperature combustion gases before the latter are fully expanded across the turbine. The efficiency is improved because air enters the combustor at a higher temperature, and hence heat addition in the combustor occurs at a higher average temperature. The heat exchanger operating conditions are more demanding than for a conventional regeneration configuration, but well within the capability of modern heat exchangers. Models of cycle performance exhibit several percentage points of improvement relative to either simple cycles or conventional regeneration schemes. The peak efficiencies of the alternative regeneration configuration occur at optimum pressure ratios that are significantly lower than those required for the simple cycle. For example, at a turbine inlet temperature of 1300°C (2370°F), the alternative regeneration scheme results in cycle efficiencies of 50% for overall pressure ratios of 22, whereas simple cycles operating at the same temperature would yield efficiencies of only 43.8% at optimum pressure ratios of 50, which are not feasible with current compressor designs. Model calculations for a wide range of parameters are presented, as are comparisons with simple and conventional regeneration cycles.

Topics: Gas turbines
Commentary by Dr. Valentin Fuster
2002;():527-535. doi:10.1115/GT2002-30135.

One of the remedies to reduce the major emissions production of nitric oxide (NOx ), carbon monoxide (CO) and unburned hydrocarbon (UHC) from conventional gas turbine engine combustors at both high and low operating conditions without losing its performance and stability is to use variable geometry combustors. This type of combustor configuration provides the possibility of dynamically controlling the airflow distribution of the combustor based on its operating conditions and therefore controlling the combustion in certain lean burn conditions. Two control schemes are described and analyzed in this paper: both are based on airflow control with variable geometry, the second including fuel staging. A model two-spool turbofan engine is chosen in this study to test the effectiveness of the variable geometry combustor and control schemes. The steady and dynamic performance of the turbofan engine is simulated and analyzed using an engine transient performance analysis code implemented with the variable geometry combustor. Empirical correlations for NOx , CO and UHC are used for the estimation of emissions. Some conclusions are obtained from this study: • With variable geometry combustors significant reduction of NOx emissions at high operating conditions and CO and UHC at low operating condition is possible; • Combustion efficiency and stability can be improved at low operating conditions, which is symbolized by the higher flame temperature in the variable geometry combustor; • The introduced correlation between non-dimensional fuel flow rate and air flow ratio to the primary zone is effective and simple in the control of flame temperature; • Circumferential fuel staging can reduce the range of air splitter movement in most of the operating conditions from idle to maximum power and have the great potential to reduce the inlet distortion to the combustor and improve the combustion efficiency; • During transient processes, the maximum moving rate of the hydraulic driven system may delay the air splitter movement but this effect on engine combustor performance is not significant.

Commentary by Dr. Valentin Fuster
2002;():537-545. doi:10.1115/GT2002-30136.

This paper presents a methodology to optimize the part load behavior of complex power plant cycles. As free optimization parameters the traditional continuous control parameter and the activation/deactivation of defined plant components are considered resulting in a mixed integer non-linear programming problems (MINLP). The procedure starts with a continuous process on either side of the non-linearity in part load, while refining the steps as it approaches the discontinuity. It is shown that good convergence around the non-linearity can be found with the present scheme. For part load operation a number of continuous and binary free optimization parameters are available creating a challenging optimization problem. The developed procedure is applied to a conventional steam cycle power plant, which is parallel repowered with a modern gas turbine. The resulting power plant layout is a hybrid coal and gas fired combined cycle. As objective function the maximized overall thermal efficiency and the minimized fuel costs are two examples chosen. Investigating the minimized fuel costs as the objective function the optimized operation strategy is found to be an unique function of the fuel price ratio between coal and gas for the chosen layout. Finally we show, that the operation strategy can be notably improved by considering the deactivation of cycle components for minimizing the fuel costs and for maximizing the cycle efficiency. For example the cycle efficiency can be improved up to 2% by deactivating the high pressure feed water preheating. The fuel costs are reduced by 20% for a particular load point by deactivating the gas turbine.

Topics: Stress , Coal , Optimization , Cycles
Commentary by Dr. Valentin Fuster
2002;():547-556. doi:10.1115/GT2002-30137.

The total cost to purchase and operate a gas turbine engine is indeed quite substantial. Out of this, the biggest portion is the operating cost. In order to keep it as low as possible, a prime concern is to keep temperatures at the entry of the turbine at low levels. Therefore the effect of engine degradations should be understood and analysed, as well as the impact of the major hot section failure modes. This paper examines an alternative cruising method of constant speed for the Hercules C-130H, powered by the Alison T56-A-15 turboprop engine. This study is being done on a comparative basis to the constant power cruising method, which is applied until now. It is concerning the life savings deduced out of creep. Moreover, the effect of several engine degradations on engine life, for both constant speed and constant power cruising methods has been investigated.

Topics: Engines
Commentary by Dr. Valentin Fuster
2002;():557-562. doi:10.1115/GT2002-30138.

A major cause of noise at airports are the run-up tests conducted after engine maintenance. The acoustic enclosures currently used to alleviate the problem are expensive and not very effective and can only be used during favorable wind conditions. Delays and complaints are commonplace. The reason why engines on stationary aircraft are at least twice as noisy as engines in flight is investigated and the findings used to propose more efficient and cost effective methods of noise reduction. This approach has also revealed a possible cause of premature and unexpected failure of combustors and turbines.

Commentary by Dr. Valentin Fuster
2002;():563-570. doi:10.1115/GT2002-30139.

An instrument for promoting CO2 emission reductions, taking the Kyoto Protocol goal into account, could be the assignment to energy conversion plants of a monetary charge linked to their specific emission intensity, usually called Carbon Tax. There are two main problems closely connected with this approach: the estimation of the charge (that must be related to the “external” cost associated with CO2 emission) and the choice of the strategy to determine the amount of the imposed charge. In this paper an analytical procedure proposed by the authors and called Carbon Exergy Tax (CET) for the evaluation of CO2 emission externalities is presented. It is based on the thermoeconomic analysis of energy systems, which allows Second Law losses to be quantified in monetary terms: the resulting cost represents the taxation that is to be applied to the energy system under examination, calculated without any arbitrary assumption. Since the complete procedure of the CET evaluation is too complex to become a feasible instrument of energy policy, hereby, after applying the procedure to some conventional and advanced power plants, gas-, oil- and coal-fuelled, a new generalised approach, based on the results of the complete CET procedure, is proposed. The generalised CET evaluation requires much less information about the energy system and thus a simple and effective energy policy rule to manage global warming is obtained and available.

Commentary by Dr. Valentin Fuster
2002;():571-580. doi:10.1115/GT2002-30140.

An open/closed gas-turbine simple Brayton cycle or Brayton-Rankine gas- and steam-turbine combined-cycle power-producing system is proposed, with the gas turbine recirculating a large portion of partly expanded high-temperature gas into an inverse mixing ejector. The inverse mixing ejector uses injected-gas velocity that is necessarily greater than jet-gas velocity to increase the hot-gas pressure up to the compressor-discharge level. This is a necessary condition for achieving very high cycle thermal efficiency. Maximum combined-cycle thermal efficiency can be expected to reach up to about 80%, up to an appropriate temperature-level Carnot-cycle efficiency. The inverse mixing ejector can operate in either subsonic or supersonic (necessary for higher cycle thermal efficiencies) regions of gas velocity. The gas turbine cycle can operate in either simple-cycle, single-intercooled-cycle or multi-intercooled-cycle mode.

Topics: Ejectors , Cycles
Commentary by Dr. Valentin Fuster
2002;():581-590. doi:10.1115/GT2002-30141.

The fossil fuel reserves are limited. In addition, usable energy supply has a considerable impact on the environment, even if some effects, which are usually alleged, are far from being fully established. Natural gas is often found in remote locations far from developed industrial nations. Where possible, the gas is transported by pipeline to the end user. However, where oceans separate the gas source and the user, or there are other difficulties, the only viable way to transport the gas is to convert it into liquid natural gas (LNG) and to convey it using insulated LNG tankers. This paper outlines the results of an examination of a complex system, employing solar energy, for the production of electrical energy and the vaporization and superheating of LNG. It is to be remarked that, differently from the usual combined systems, both the thermal source and the thermal sink are exergy sources.

Commentary by Dr. Valentin Fuster
2002;():591-599. doi:10.1115/GT2002-30144.

An explorative and preliminary study has been done by a small crew of students of the Ecole des Applications Militaires de l’Energie Atomique (Cherbourg FRANCE) under the directives and councils of TECHNICATOME and the CEA (Commissariat à l’Energie Atomique). Main results: • The HTR reactor types are interesting for equipping military great ship like an air craft carrier, the general design as result of a preliminary study shows that mass and size of the propulsion system can be more favourable than with PWR reactors.

Commentary by Dr. Valentin Fuster
2002;():601-606. doi:10.1115/GT2002-30145.

The Pebble Bed Modular Reactor (PBMR) power station concept is currently being developed in South Africa by ESKOM and its partners. This high temperature gas cooled reactor is based on the three-shaft recuperative inter-cooled closed loop Brayton cycle. Detailed cycle analysis was performed with the Flownet thermo-hydraulic network simulation software developed at the Engineering Faculty of Potchefstroom University in South Africa. Using Flownet simulations is especially useful for component design and integration. It furthermore enables the study of complex load following and load rejection scenarios and the design of suitable controller algorithms. One of the most severe load control scenarios is that of full load rejection due to the loss of the grid power. This paper discusses the two control concepts that showed the most promising results for full load rejection.

Commentary by Dr. Valentin Fuster
2002;():607-614. doi:10.1115/GT2002-30146.

The Starting up and Shutting down of a closed cycle gas turbine power plant needs special attention due to the inter-dependable nature of the components. Achieving self-sustainability in a fast and efficient way within the mechanical constraints is the challenge in the start-up of a closed cycle. The Nuclear reactor as the heat source will add more complexity to the system. The paper looks into the various options available for the start up and shutdown of a closed cycle Helium turbine using a gas cooled reactor as the heat source. A comparative analysis of these options is carried out by simulating various operating scenarios using a Transient Simulation Computer Programme especially prepared for an HTGR Project called PBMR (Pebble Bed Modular Reactor), which is being carried out in South Africa. The simulation was focused on the power conversion side of the plant, which includes all the Turbocompressors, Turbogenerator, Heat exchangers, Valves etc. Based on the analysis and its findings, an outline of a start up and shutdown procedure for a 3-shaft Closed Cycle Turbine Power Plant using hot gas injection is proposed in the paper.

Commentary by Dr. Valentin Fuster
2002;():615-622. doi:10.1115/GT2002-30147.

Expected doubling of marine trade within the next two decades, threats of global warming amplified by the increased consumption of fossil fuels, globalization of world economy resulting in growing need for rapid ocean transport of time sensitive freight, and recent rise in the fossil fuel prices prompted the Society of Naval Architects and Marine Engineers (SNAME) to initiate a study to examine power plant options for the next generation of high-speed merchant ships. Emerging nuclear power technologies, which might be applicable to such ships, including long core life light water reactors, heavy liquid metal cooled reactors, and gas cooled reactors are discussed. Results of a study comparing economic benefits of nuclear and conventional gas turbine merchant ship propulsion systems are reported. Finally, cost and performance characteristics that would make nuclear power a viable alternative for high-speed merchant ships are identified.

Topics: Nuclear power , Ships
Commentary by Dr. Valentin Fuster
2002;():623-630. doi:10.1115/GT2002-30148.

This paper describes the conceptual design and cost estimation of a 600MW(t) HTGR-GT power plant, which has been completed in the framework of the HTGR-GT feasibility study project in the duration of FY 1996 to FY 2000. The project is assigned to JAERI by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) (former Science and Technology Agency) in Japan. The inlet and outlet gas temperatures in the reactor are 460°C and 850°C, respectively. Helium gas pressure is 6MPa. The gas turbine system type is an intercooled recuperative direct cycle. Designs of reactor and gas turbine are presented. The main feature of the plant is a relatively large 600 MW(t) HTGR, horizontal single shaft helium turbine and divided power conversion vessel, that is, a turbomachine vessel and heat exchanger one. Their main specifications and drawings are presented. As a result of cost estimation, an economically attractive construction cost and a power generation cost have been obtained.

Commentary by Dr. Valentin Fuster
2002;():631-638. doi:10.1115/GT2002-30149.

Many GE frame gas turbines have a unique 90-degree tailpipe exhaust system that contains struts, diffusers, and turning vanes. As confirmed in a recent report by GE and other authors [1], it is known in the industry that this tailpipe design has large pressure losses. In this recent report a pressure loss as high as 60 inches of water (0.15 kgs/sqcm) was cited. Due to the flow separations they create, the report indicates that the struts can cause very high-pressure losses in the turbine. The report also states that these pressure losses can vary with different turbine load conditions. Cheng Fluid Systems and Cheng Power Systems have conducted a study aimed at substantially reducing these pressure losses. Flow control technology introduced to the refinery industry, i.e., the Cheng Rotation Vane (CRV) and the Large Angle Diffuser (LAD) can be used to mitigate the flow separation and turbulence that occurs in turns, bends, and large sudden expansions. Specifically the CRV addresses the flow separations in pipe turns, and the LAD addresses the flow problems that occur with large sudden expansion areas. The paper will introduce the past experience of the CRV and LAD, and will then use computer simulations to show the flow characteristics around a new design. First, the study meticulously goes through the entire GE exhaust system, starting with the redesign of the airfoil shape surrounding the struts. This new design has a larger angle of attack and minimizes the flow separations over a much wider operating range. Second, the pros and cons of the concentric turning vanes are studied and it is shown that they are more flow restrictive, rather than flow enhancing. Third, it is shown that the highly turbulent rectangular box type exhaust ducting design, substantially contributes to high noise levels and pressure losses. In this paper a completed design will be shown that incorporates a new airfoil shape for the struts, and by using CRV flow technology in combination with the LAD flow technology, the pressure recovery can be enhanced. If the pressure losses could be reduced by 40 inches of water (0.10 kgs/sqcm), the turbine efficiency could be increased by 5%, and the power output could be increased by 6%.

Commentary by Dr. Valentin Fuster
2002;():639-646. doi:10.1115/GT2002-30150.

A scaling method for characteristics of gas turbine components using experimental data or partially given data from engine manufacturers was newly proposed. In case of currently used traditional scaling methods, the predicted performance around the on-design point may be well agreed with the real engine performance, but the simulated performance at off-design points far away from the on-design point may not be well agreed with the real engine performance generally. It would be caused that component scaling factors, which were obtained at on-design point, is also used at all other operating points and component maps are derived from different known engine components. Therefore to minimize the analyzed performance error in the this study, firstly component maps are constructed by identifying performances given by engine manufacturers at some operating conditions, then the simulated performance using the identified maps is compared with performances using currently used scaling methods. In comparison, the analyzed performance using the currently used traditional scaling method was well agreed with the real engine performance at the on-design point but had maximum 12% error at off design points within the flight envelope of a calculation example turboprop engine. However the performance result using the newly proposed scaling method had maximum 6% reasonable error even at all flight envelope.

Topics: Gas turbines
Commentary by Dr. Valentin Fuster
2002;():647-654. doi:10.1115/GT2002-30151.

In order to improve the performance of the combined cycle, much effort has been spent over the past decade on increasing gas turbine performance. As a contrast to this, the present work focuses on possibilities for combined cycle performance enhancement through present and expected future steam cycle and boiler technology. The use of various heat recovery steam generators, (single and dual pressure) with or without supplementary firing are studied, in combination with steam turbine admission temperatures of up till 973 K. Supplementary firing is applied either in the entire gas turbine exhaust duct or in part of it, in a so-called split-stream boiler (SSB). Furthermore, the flashing of pressurized water from an overdimensioned economiser in the SSB, to produce steam for gas turbine vane cooling is studied. Many of the supplementary fired cycles studied are found to have a thermal efficiency superior of the unfired cycles, based on the same gas turbines. Hence, available steam technology and expected future development mean that most of the cycles studied are realistic concepts that merit further attention in the quest for more efficient power production.

Topics: Cycles , Steam
Commentary by Dr. Valentin Fuster
2002;():655-662. doi:10.1115/GT2002-30152.

The Universal Electric Machine (UEM) separates the speed of the magnetic field on the rotor from the mechanical speed of the rotor. This fundamental ability has numerous consequences that are very appealing in the power industry. The UEM is capable of generating AC power at a frequency that is independent of the mechanical drive speed on the rotor. Thus the UEM generator is a single product that can efficiently convert power into either 50 Hz or 60 Hz regardless of prime mover speed. The UEM maintains a steady-state frequency through a load transient, however severe, from full load to no load or vice versa. As a motor, the UEM uses this same ability to control speed independently of line frequency. The UEM simulates the torque-rpm characteristics of a large mechanical prime mover as the torque increases with mechanical speed. The UEM is capable of automatic synchronization of the electrical output/input to the power source, in frequency, phase angle and voltage. Finally by using modern power electronics, the configuration of the machine is brushless. These properties are ideally suited for both startup and power generation of large gas turbines. The UEM is under development and testing. A patent has been granted in the United States and international patent applications are being submitted.

Commentary by Dr. Valentin Fuster
2002;():663-673. doi:10.1115/GT2002-30410.

Many gases, including carbon dioxide and argon, have been considered as alternatives to air as working fluids in a number of design studies for closed and semi-closed gas turbine engines. In many of these studies, it has been assumed that if the gas constant R and specific heat ratio (isentropic exponent) γ are included in the speed and flow parameters, the compressor map or turbine characteristic is applicable to other working fluids. However, similarity arguments show that the isentropic exponent itself is a criterion of similarity and that the turbomachinery characteristics, even when appropriately non-dimensionalized, will in principle vary as the γ of the working fluid varies. This paper examines the effect of γ on turbomachinery characteristics, mainly in terms of compressors. The performance of a centrifugal compressor stage was measured using air (γ = 1.4), CO2 (γ = 1.29), and argon (γ = 1.67). For the same values of the non-dimensional speed and mass flow, the pressure ratio, the efficiency, and the choking mass flow were found to be significantly different for the three test gases. The experimental results have been found to be consistent with a CFD analysis of the impeller. Finally, it is shown that the changes in performance can be predicted reasonably well with simple arguments based mainly on one-dimensional isentropic flow. These arguments form the basis for correction procedures that can be used to project compressor characteristics measured for one value of γ to those for a gas with a different value.

Commentary by Dr. Valentin Fuster
2002;():675-682. doi:10.1115/GT2002-30411.

A concept for natural-gas fired power plants with CO2 capture has been investigated using exergy analysis. The present approach involves decarbonization of the natural gas by authothermal reforming prior to combustion, producing a hydrogen-rich fuel. An important aspect of this type of process is the integration between the combined cycle and the reforming process. The net electric power production was 47.7% of the Lower Heating Value (LHV) or 45.8% of the chemical exergy of the supplied natural-gas. In addition, the chemical exergy of the captured CO2 and the compression of this CO2 to 80 bar represented 2.1% and 2.7%, respectively, of the natural-gas chemical exergy. For a corresponding conventional combined cycle without CO2 capture, the net electric power production was 58.4% of the LHV or 56.1% of the fuel chemical exergy. A detailed breakdown of irreversibility is presented. In the decarbonized natural-gas power plant, the effect of varying supplementary firing (SF) for reformer-feed preheating was investigated. This showed that SF increased the total irreversibility and decreased the net output of the plant. Next, the effects of increased gas-turbine inlet temperature and of gas-turbine pressure ratio were studied. For the conventional plant, higher pressure led to increased efficiency for some cases. In the decarbonized natural-gas process, however, higher pressure ratio led to higher irreversibility and reduced thermal-plant efficiency.

Commentary by Dr. Valentin Fuster
2002;():683-690. doi:10.1115/GT2002-30412.

The use of hydrogen as an aviation fuel can be beneficial for the reduction of CO2 emissions, if renewable energy sources are used for hydrogen production. Pure hydrogen fuel produces no CO2 in flight. NOx emissions can be significantly lower for hydrogen fuelled combustors than for current kerosene fuelled combustors. Other advantages derive from the high energy content, which reduces the necessary fuel mass, and from the availability of a valuable heat sink, useful to improve cycle performance. The present paper (based on the EU Cryoplane Project) focuses on the use of hydrogen in aero gas turbine engines. It studies the differences in performance produced by of its cryogenic properties in unconventional cycles. Three novel concepts are applied to a turbofan aero engine; for each cycle the improvement in performance at take-off and cruise is presented. An estimation of the weight and size of the engine is then made.

Commentary by Dr. Valentin Fuster
2002;():691-701. doi:10.1115/GT2002-30413.

In this paper a method to evaluate the expansion of condensing steam in presence of incondensable gases is presented. The expansion of such a mixture involves thermodynamic processes very different from those related to the expansion of pure steam (as it happens in conventional steam power plants) or to the expansion of a gaseous mixture without steam condensation (as it happens in gas turbine power plants, combined and mixed cycles). In this paper these thermodynamic processes are investigated and an evaluating method is developed. In particular using this method it is possible to evaluate the excess turbine back pressure, that is the pressure increase with respect to the steam saturation pressure, and the specific expansion work of the mixture. The proposed method is then applied in order to evaluate the performance of an hydrogen/oxygen (fuel/oxidiser) cycle, where the presence of incondensable gases into condensing steam is unavoidable owing to the processes that give H2 from fossil fuels and O2 from air. The results of the developed investigation confirm that the presence of incondensable gases can not be neglected for a realistic evaluation of H2 /O2 cycle performance.

Topics: Gases , Cycles
Commentary by Dr. Valentin Fuster
2002;():703-713. doi:10.1115/GT2002-30414.

An integrated method for power plant analysis, including rotating component matching and CFD simulation of the combustion process, is applied to the study of gas turbines supplied with hydrogenated fuels originating from the natural gas reforming. The method proposed by the authors allows estimation of the power plant performance and emission in the gas turbine operating range. A comparison is then carried out between the plant behaviour with conventional fuelling and with decarbonised fuel supply. Attention is also paid to the study of the combustion regimes with either natural gas or fuels with increasing hydrogen contents, in order to achieve a realistic insight of both the temperature distributions and the growth of nitric oxides throughout the combustion chamber.

Commentary by Dr. Valentin Fuster
2002;():715-719. doi:10.1115/GT2002-30447.

The profits that can be gained by use of inlet air cooling on gas turbines has been recognised for quite some time now and the systems installed throughout the world have shown the users in the gas turbine field that cooling indeed can be used to boost power at times when the ambient temperature reaches or exceeds the ISO rating temperature of the gas turbine. Drawback however being that the initial investment asked of the gas turbine user is rather large thus only justifying a cooling system in regions where the outdoor temperatures exceed the ISO rating time and again due to the climate in that region. Lately gas turbine users in colder climates have become interested in power augmentation during their short summer, however there is no justification for an investment like necessary when installing one of the presently available systems on the market. As the question reached us from more and more of our clients it stimulated us to go out and search for a low-investment solution to this problem. This resulted in the world’s first low pressure gas turbine inlet cooling system.

Commentary by Dr. Valentin Fuster
2002;():721-727. doi:10.1115/GT2002-30448.

Analytical model is proposed to account for carbon emission behaviour during replacement of power source from fossil to renewable energy in which sustainability of energy supply is stressed. Analyses show that energy payback time (EPT) should be much shorter than the doubling time of manufacturing cycle to secure adequate available energy during as well as after the replacement. Nuclear, small hydropower and photovoltaic cell are taken as representative candidates and investigated as an option to replace fossil power until mid-century. Nuclear and small hydropower can be a promising candidate but photovoltaic cell needs further development efforts to reduce EPT to avoid energy expense after the replacement.

Commentary by Dr. Valentin Fuster
2002;():729-736. doi:10.1115/GT2002-30509.

Over the last few years a number of papers have discussed the progress on studies and thoughts on small-scale nuclear power. Nuclear power conversion systems aiming for the t of the non-utility markets, such as the stand-alone heat generation, Combined Heat & Power production, stand-alone electricity conversion and ship propulsion. The design of these installations must fully comply with the philosophies as are common in these markets, where the expression “the engine is a means to an end” applies. So design to cost, design to be operated by non professional energy producers, to be managed by a pool-management system, maintained, repaired and overhauled by replacement, etc. The paper will discuss such a design. So far all papers mentioned have discussed the gas turbine directly coupled to the heat source. However the helium turbine is considered quite a challenge for the gas turbine industry, so alternatives had to be found. At the moment the possibilities of gas turbines with an indirect heat source (to burn refuse, wood, refinery waste, etc.) are getting much more attention. The paper therefore will discuss how an inherently safe, well proven, nuclear heat source can be coupled by an Intermediate Heat Exchanger to a recuperative, existing but adapted gas turbine.

Topics: Nuclear power
Commentary by Dr. Valentin Fuster
2002;():737-743. doi:10.1115/GT2002-30510.

The paper discusses uranium as a new fuel for gas turbines used as energy conversion installations for the markets of: stand-alone heat production, combined heat and power generation, stand-alone electricity production and as prime mover on board ships. This development is a logical step in a historical trend in energy conversion. The paper discusses the availability of the fuel, uranium and the construction of the fuel which makes this combination of gas turbine and uranium suitable for the non-utility markets.

Commentary by Dr. Valentin Fuster
2002;():745-753. doi:10.1115/GT2002-30511.

Recent media articles about nuclear power renaissance are encouraging, but this controversial topic is far from being embraced by major industrial powers. The fact is, that within the next two to three decades or so most of the first generation US nuclear power plants, currently producing about 20 percent of the nation’s electrical power, will be near the end of their design lives. In addition to providing needed power, a major argument put forward for the introduction of next generation smaller and safer nuclear plants relates to the growing concern about greenhouse gas emission and global warming. However, overcoming public and institutional resistance to nuclear power remains a formidable endeavor, and in reality the introduction of new plants in sufficient numbers to significantly impact the market will not be realized for several decades. Clearly vision is needed to define the requirements for new nuclear plants that will meet the needs of consumers by say the middle of the 21st century. Market forces will mandate changes in the energy supply sector, and to be in concert with environmental concerns new nuclear plants must have operational flexibility. In addition to economical electrical power, energy needs in the future could include hydrogen production in slgnificant quantity (for fuel cells in the transportation and power sectors) and fresh water by desalination for urban, industrial and agricultural users. The High Temperature Reactor (HTR) has the capability to meet these projected needs. With an established technology base, and successful plant operation in Germany, the helium cooled pebble bed reactor (PBR) must be regarded as a leading second generation nuclear plant. Operational versatility by virtue of its high temperature capability is assured, and high availability can be realized with its on-line refueling approach. While the multipurpose HTR may be several decades away from playing a significant rote in the commercial market place, this paper emphasizes the need for technical planning today to establish a nuclear heat source adaptable to both a high efficiency helium timed cycle gas turbine and large scale hydrogen production facilities, thus extending the role of nuclear power beyond just the supply of electrical power.

Commentary by Dr. Valentin Fuster
2002;():755-761. doi:10.1115/GT2002-30512.

Nuclear power currently only serves the market segment of large scale base load electricity generation. Other energy markets, like cogeneration and heat production or market segments like the smaller scale (but still industrial) electricity production are entirely served with fossil fuels (and hydropower). When these fuels at acceptable prices are being depleted and if actively marketed, and if an inherently safe small-scale nuclear plant could be developed and marketed, a huge market could emerge for this new form of nuclear power. Pebble Bed High Temperature Reactor technology is most suitable for designing small inherently safe nuclear reactors. The oldest small designs were meant for application as district heating plants, making full use of the self-controlling features of nuclear reactors. The ACACIA concept (AdvanCed Atomic Cogenerator for Industrial Applications) is a design for industrial cogeneration, producing 13.6 MW of electricity and 17 tons of industrial quality steam per hour, with a total efficiency of 63%. In case the electricity production would be maximized at the expense of the steam quality, an electrical output of 16.5 MW could be achieved, and the plant efficiency would rise to 86% (electric efficiency 41%). The heat source is a pebble bed reactor with 40 MW of thermal power. The energy conversion system is a direct recuperated helium cycle with a radial compressor and an axial helium turbine. A number of operational and safety related transients have been calculated with two different simulation codes. The safety related transient analyses show the reactor power and the fuel temperature bebaviour after a full loss of coolant accident, and illustrate the inhrently safe nature of the plant. The operational transient simulations show the suitability of the system for an industrial user. Furthermore, the transport of radioactive fission products within the primary circuit has been analyzed. A cost study shows high kWh-costs compared to large scale generating plants, but the treatment of scaling factors for this particular case needs continued attention. However, for those areas in the world without fossil fuel supply networks and with only small-scale demand, ACACIA will still be an economic option. To improve matching with non-utility market needs, the current ACACIA design will be adapted from a direct cycle system to an indirect cycle system, where primary cycle will be strictly separated from the remainder of the plant. A conceptual comparison with the direct cycle system will be discussed.

Commentary by Dr. Valentin Fuster
2002;():763-770. doi:10.1115/GT2002-30513.

This paper presents an investigation of the degradation effects that gas and steam turbine cycles components have on combined cycle (CCGT) power plant performance. Gas turbine component degradation effects were assessed with TurboMatch, the Cranfield Gas Turbine simulation code. A new code was developed to assess bottoming cycle performance deterioration. The two codes were then joined to simulate the combined cycle performance deterioration as a whole unit. Areas examined were gas turbine compressor and turbine degradation, HRSG degradation, steam turbine degradation, condenser degradation, and increased gas turbine back-pressure due to HRSG degradation. The procedure, assumptions made, and the results obtained are presented and discussed. The parameters that appear to have the greatest influence on degradation are the effects on the gas generator.

Commentary by Dr. Valentin Fuster
2002;():771-778. doi:10.1115/GT2002-30514.

It has always been thought by the gas turbine industry that steam injection will shorten the effective life of certain gas turbine parts. Recently it was shown that a number of steam injected Cheng Cycle Rolls-Royce Allison 501KH gas turbines, accumulated more than 2.5 million logged hours of operation and with a prolonged parts life. The “hot parts” of a Rolls-Royce Allison 501KH gas turbine engine that are of concern, are the first stage nozzle, the first stage blade, and the second stage nozzle. These parts are all air cooled through the first stages internal passages. (The second stage blades and on down are not internally cooled.) The concern raised in many gas turbine institutions is that the metal temperatures of these hot parts, due to the heat conductivity properties of injected steam, will make them deteriorate faster. An experiment was completed using a steam injected Cheng Cycle, on an Allison 501KH gas turbine engine. In the experiment, a substantial number of thermocouples were attached to the surfaces of the turbines hot parts. This engine had a steam injection rate of up to 18% airflow. The experimental results showed that if steam could be properly mixed with the cooling air before the air enters into the cooling passages of the hot parts, the metal temperatures did not increase. During the operation of the engines, it was recorded that the hot parts lifetime increased from 25,000 hours before the hot parts section had to be overhauled, to 42,000 hours (on average) before they needed to be overhauled. This paper will report the measurement installation in detail. The results before and after steam injection in the hot parts sections of the Rolls-Royce Allison 501KH engine will also be discussed.

Commentary by Dr. Valentin Fuster
2002;():779-787. doi:10.1115/GT2002-30515.

A whole gas-turbine engine model has been developed incorporating all of the key turbomachinery aerothermal relationships. The aim of the model has been to predict trends in gas-turbine performance with a high degree of confidence that they reflect real engine design limitations. Simple cycles, recuperated, inter-cooled, and inter-cooled recuperated cycles can be assessed across a wide of range of operating parameters. The model is spreadsheet-based with additional macro programming. The major part of it is concerned with establishing representative overall turbine characteristics. A non-integer number of stages is determined as a function of technology level inputs. Individual stage geometry features are derived allowing the calculation of the coolant requirements and efficiencies. The results of various studies are presented for a number of cycle types. The resulting trends are believed to be sensible because of the realistic turbine features. Confidence in the method is established by the modelling of a number of existing industrial gas turbines.

Commentary by Dr. Valentin Fuster
2002;():789-798. doi:10.1115/GT2002-30516.

A performance simulation model of a turboprop engine, the PT6A-62, which is the power plant of KT-1, was developed to predict the steady-state behaviors using the SIMULINK® model. The SIMULINK model consists of subsystems to represent engine components such as intake, compressor, combustor, compressor turbine, power turbine and exhaust nozzle. For validation, performance parameters calculated using the SIMULINK model were compared with the results using GASTURB model. The steady-state performance analysis using the developed SIMULINK model was performed. Performance parameters, such as the mass flow rate, the compressor pressure ratio, the fuel flow rate, the specific fuel consumption ratio and the turbine inlet temperature, were conducted to evaluate validity of the SIMULINK model at various cases. The first case was the uninstalled condition at various altitudes from sea level to 9144m (30000ft) with fixed M.N. = 0. And the second case was the installed condition at various altitudes from sea level to 7620m (25000ft) with fixed M.N. = 0. The third case was the installed condition at altitudes of 1524m (5000ft) and 3048m (1000ft) and at the M.N. = 0.1, 0.2 and 0.3 in ECS operation ECS. In this investigation, it was confirmed that the results using the SIMULINK model were well agreed with the results using the GASTURB model within maximum 6.5%.

Topics: Engines , Simulation
Commentary by Dr. Valentin Fuster
2002;():799-807. doi:10.1115/GT2002-30517.

Although increasing the turbine inlet temperature has traditionally proved the surest way to increase cycle efficiency, recent work suggests that the performance of future gas turbines may be limited by increased cooling flows and losses. Another limiting scenario concerns the effect on cycle performance of real gas properties at high temperatures. Cycle calculations of uncooled gas turbines show that when gas properties are modelled accurately, the variation of cycle efficiency with turbine inlet temperature at constant pressure ratio exhibits a maximum at temperatures well below the stoichiometric limit. Furthermore, the temperature at the maximum decreases with increasing compressor and turbine polytropic efficiency. This behaviour is examined in the context of a two-component model of the working fluid. The dominant influences come from the change of composition of the combustion products with varying air/fuel ratio (particularly the contribution from the water vapour) together with the temperature variation of the specific heat capacity of air. There are implications for future industrial development programmes, particularly in the context of advanced mixed gas-steam cycles.

Topics: Gas turbines
Commentary by Dr. Valentin Fuster
2002;():809-817. doi:10.1115/GT2002-30519.

With the objective of performing reliable innovative gas-turbine cycle calculations, a new procedure aimed at evaluating blade cooling performance is presented. This complete analytical (convective and film) blade cooling modelling provides the coolant mass flow and pressure loss estimation, and is a useful tool in the field of innovative gas turbine cycle analysis, mainly when alternative fluids are considered. In this case, in fact, the conventional semi-empirical data based on the use of air as traditional coolant and working media are no longer suitable. So the analytical approach represents a way of properly investigating alternative cooling methods and fluids. In the presented analysis the effects of internal blade geometry on cooling performance are summarised by the Z parameter, which also highly affects the coolant flow pressure losses. Since existing technology represents a natural starting point for the assessment of Z, the model is able to automatically estimate a proper value relying only on available semi-empirical data which were established for air-cooled gas turbine blades. When alternative fluids are considered, the same estimated value of Z is still maintained for the calculation, with the result of investigating the performance of existing blade technology for novel operational conditions. This represents an example of how the analytical approach, supported by conventional air cooled blade semi-empirical data, appears as an innovative tool in the analysis of novel gas turbine cycles. In fact, the simulation results for the cooled blade were easily employed on the whole system level (Gas Turbine).

Commentary by Dr. Valentin Fuster
2002;():819-826. doi:10.1115/GT2002-30550.

The relatively innovative gas turbine based power cycles R-ATR and R-REF (Recuperative – Auto Thermal Reforming GT cycle and Recuperative – Reforming GT cycle) here proposed, are mainly aimed to allow the upstream CO2 removal by the natural gas fuel reforming. The 2nd part of the paper is dedicated to the R-REF cycle: the power unit is a Gas Turbine (GT), fuelled with reformed and CO2 cleaned gas, obtained by the addition of several sections to the simple GT cycle, mainly: • Reformer section (REF), where the reforming reactions of methane fuel with steam are accomplished: the necessary heat is supplied partially by the exhausts cooling and, partially, with a post–combustion. • Water Gas Shift Reactor (WGSR), where the reformed fuel is, shifted into CO2 and H2 with the addition of water. • CO2 removal unit for the CO2 capture from the reformed and shifted fuel. No water condensing section is adopted for the R-REF configuration. Between the main components, several heat recovery units are applied, together with GT Cycle recuperator, compressor intercooler and steam injection into the combustion chamber. The CO2 removal potential is close to 90% with chemical absorption by an accurate choice of amine solution blend: the heat demand for amine regeneration is completely self-sustained by the power cycle. The possibility of applying steam blade cooling (the steam is externally added) has been investigated: in these conditions, the RREF has shown efficiency levels close to 43–44%. High values of specific work have been observed as well (around 450–500 kJ/kg). The efficiency is slightly lower than that found for the R–ATR solution, and 2–3% lower than CRGTs with CO2 removal and steam bottoming cycle, not internally recuperated. If compared with these, the R-REF offers higher simplicity due to absence of the steam cycle, and can be regarded as an improvement to the simple GT. In this way, at least 5–6 points efficiency can be gained, together with high levels of CO2 removal. The effects of the reformed fuel gas composition, temperature and pressure on the amine absorption system for the CO2 removal have been investigated, showing the beneficial effects of increasing pressure (i.e. pressure ratio) on the specific heat demand.

Commentary by Dr. Valentin Fuster
2002;():827-834. doi:10.1115/GT2002-30551.

CO2 emissions reduction has become an important topic, especially after Kyoto protocol. There are several ways to reduce the overall amount of CO2 discharged into the atmosphere, for example using alternative fluids such as steam or CO2 . It is therefore interesting to analyze the consequences of their usage on overall performances of gas turbine and blade cooling systems. The presence of steam can be associated with combined or STIG cycle, whereas pure carbon dioxide or air-carbon dioxide mixtures are present in innovative cycles, where the exhaust gas is recirculated partially or even totally. In this paper we will analyze a commercial gas turbine, comparing different fluids used as working and cooling fluids. The different nature of the fluids involved determines different external heat transfer coefficients (external blade surface), different internal heat transfer coefficients (cooling cavities) and affects film cooling effectiveness, resulting in a change of the blade temperature distribution. Results show that the presence of steam and CO2 could determine a non negligible effect on blade temperature. This means that cooling systems need a deep investigation. A redesign of the cooling system could be required. In particular, results show that steam is well suited for internal cooling, whereas CO2 is better used in film cooling systems.

Topics: Cooling , Fluids , Blades
Commentary by Dr. Valentin Fuster
2002;():835-843. doi:10.1115/GT2002-30611.

A universal mathematical model (UMM) has been developed and applied to the LAJ (for Labinov, Armstrong, and Judkins) cycle, a new combined-cycle, fossil-fuel power system. The UMM includes static and dynamic models of the system. The static model allows for thermodynamic and thermochemical analyses of the basic system components (reformer, turbine, membrane separator, fuel cell, air compressor, heat exchanger, and other components) and the entire system. Equilibrium compositions of reforming products are defined by minimizing Gibbs free energy of the mixtures using the Lagrangian multiplier method. The dependence of the main system parameters on pressure (P), temperature (T), and water-to-methane molar ratios (N) at the steam reformer have been evaluated. For selected reforming parameters, viz., P = 4.0 MPa and T = 1200 K, the degree of methane conversion is near 95% with N = 5. However, in view of mass and size limitations on equipment, a lower value of N = 3 is preferred, in which case the degree of methane conversion is 88%. The dependence of the system static model parameters on N has been investigated, and economic characteristics of the model have been evaluated for an output power of 250 kW. It is shown that when, N = 3, the fuel cost contribution to overall electricity costs is 1 cent/kWh.

Commentary by Dr. Valentin Fuster
2002;():845-852. doi:10.1115/GT2002-30612.

Oak Ridge National Laboratory (ORNL) has developed a novel system for combined-cycle power generation, called the LAJ cycle. This system could serve as a basis for the development of a new generation of high-efficiency combined cycles. In one of several possible configurations of the new combined-cycle fossil fuel power system, natural gas enters the system at 4.0 MPa and about 300 K, is heated and reformed, and is transferred to a turbine at 4.0 MPa and 1200 K. The gas expands in the turbine to 0.6 MPa and 800 K, and then flows successively to heat exchangers and a condenser-separator, after which it is separated into two gas streams, one containing principally CO with some CH4 and water vapor and the other containing pure H2 . The CO and H2 flow to separate fuel cells and undergo electrochemical oxidation with the concomitant production of electricity. Separate streams of water and carbon dioxide (CO2 ) are produced, making this cycle compatible with carbon mitigation strategies based on sequestration. Model calculations indicate combined-cycle efficiencies greater than 70% based on the lower heating value of natural gas. The high efficiencies realized result from a combination of the high-pressure natural gas reformate expansion and the highly efficient CO and H2 fuel cells. Most of the power derives from the fuel cells in the system.

Commentary by Dr. Valentin Fuster
2002;():853-859. doi:10.1115/GT2002-30647.

The following study was undertaken on the assumption that hydrocarbon-based fuels may not be acceptable in the very long term , because of environmental concerns. A possible future fuel is hydrogen, and this study explores a novel proposition for a civil airliner using hydrogen fuel. The technical challenges of this preliminary investigation were: a) the integration of an electric power plant (Fuel Cell) into a Blended Wing Body (BWB) aircraft, and b) to investigate the possibility of reducing the aircraft’s profile drag by boundary layer re-energization. For the re-energization of the boundary layer and for propulsion during cruise, the study considered High-Speed/High Specific Power (HS/HSP) motors, situated at the trailing edge (TE) of the center body, driving fans. Re-energizing the boundary layer of the center body, would reduce the profile drag of the aircraft and hence, the total fuel burn. The take-off requirements of the aircraft were met, by high by-pass ratio (BPR) turbofan lift engines, operating on hydrogen, for a V/STOL (Pachidis, 2000b).

Commentary by Dr. Valentin Fuster
2002;():861-867. doi:10.1115/GT2002-30649.

In order to model the performance of a gas-turbine engine in the sub-idle region, particularly for starting and windmilling, it is necessary to use compressor characteristics which describe their operation and low speeds. As most compressor tests are conducted only in the range of operating speeds normally encountered, the resultant characteristics must be extrapolated in order to define the low speed compressor maps. In this paper, a variety of techniques for the extrapolation of axial-flow compressor characteristics are presented and evaluated. These include those presented by De-You & Zhong-Fan [4], Agrawal & Yunis [1] and Converse & Giffen [3]. The ease and reliability of the extrapolation methods are compared. Problems associated with the prediction of losses within the turbomachinery are highlighted, particularly in respect of compressor operation at high flows and low rotational speeds. This is due to the creation of highly off-design values of incidence which are not encountered during above-idle running, but which may be found during windmilling.

Topics: Compressors , Modeling
Commentary by Dr. Valentin Fuster
2002;():869-876. doi:10.1115/GT2002-30650.

Fuel cells have resulted to be very attractive power generation systems, promising highly efficient electricity generation with very low environmental impact. Although still in embryonic infancy high-temperature fuel cells, the very high energetic efficiency can be further increased by their integration in hybrid cycles. While a wide variety of potential bottoming technologies for exploiting the high temperature exhaust gases waste heat is available, a lot of research effort is needed to determine the optimal integration of well established technologies with these very novel conversion devices. This work, using results from previous works, in which a MCFC/Gas Turbine hybrid plant was defined and the working parameters optimised both with a GT simple cycle and with steam injection and post combustion, starts from choosing an existing gas turbine technology with suitable working parameters to perform a system evaluation and integration. Since the gas turbine nominal maximum temperature cannot be reached by the only means of heat exchange from the MCFC plant section, post combustion has been considered and the turbine has been simulated both in nominal and in off design conditions in order to find an optimal solution. The work takes into consideration not only the main plants modifications of the existing components to allow for system integration but also the most relevant issues connected with altered working conditions.

Commentary by Dr. Valentin Fuster
2002;():877-885. doi:10.1115/GT2002-30651.

The current drive for low specific fuel consumption (SFC) has resulted in increasing the bypass ratio (BPR) to improve the propulsive efficiency. Conventional high BPR engines face several practical limits when augmenting BPR i.e. an increase in the number of low-pressure turbine stages and in weight of the engine. This paper is an account of an investigation of the tip-turbine driven propulsion fan (TTDPF) as one potential solution. In particular, the TTDPF is considered as the main propulsion source for the Blended Wing Body Aircraft (BWB) developed by the College of Aeronautics at Cranfield University. The concept combines a gas generator(s) in the fuselage in combination with three or four over-wing mounted propulsion fans driven by tip-turbines. A double pass single stage configuration for the tip-turbine is assessed. This comprises two partial admission segments sharing the same circular annulus. The paper considers some special features of this novel engine concept. For example, the complex ducting (volutes) between the two passes and the aerodynamic arrangement of a two-pass turbine with a single rotor are investigated. Mechanical integrity issues and nacelle sizing are also considered. Finally, estimates are made of the noise generated by the fan and the exhaust configuration and a comparison is made with a traditional high by-pass ratio turbofan. The study concludes that the potential benefits in terms of SFC are somewhat overshadowed by the less favourable outcomes in terms of weight, nacelle size, noise and tip-turbine efficiency.

Topics: Propulsion , Turbines
Commentary by Dr. Valentin Fuster
2002;():887-893. doi:10.1115/GT2002-30652.

Solid waste, and bio-residuals in general, are usually disposed of or alternatively converted into energy by means of medium to big scale power plants. For isolated communities, usually in protected natural areas, this turns into high energy and waste management costs because of their intrinsic distance from landfills and power plants. Considering also the electric dependency from the grid, small towns are commonly showing low sustainability. This paper focuses on both problems by evaluating the economic feasibility and the global warming contribution of an innovative micro scale waste to energy system based on a microturbine fuelled by waste pyrolysis gas. The plant reaches high efficiency, considering the scale, because of its high regenerative rate and is tailored to the waste disposal needs of Giano Dell’Umbria a small town in central Italy. The economic analysis was carried out, with the Net Present Value method, to determine the expected capital cost of the plant considering that the innovative technology utilized does not allow a reliable cost evaluation. The global warming contribution was calculated considering CO2 and CH4 avoided emission from landfilling and the better CO2 emission rate of such a technology with respect to the status quo. Results obtained show an acceptable cost positioning for the plant that makes it an interesting solution for distributed waste to energy systems. Executive projecting and construction of the proposed technology was funded and a pilot plant will be built and tested in 2002, in a laboratory facility of the University of Perugia.

Commentary by Dr. Valentin Fuster
2002;():895-903. doi:10.1115/GT2002-30653.

High efficiency Hybrid Systems (HS) based on the coupling of Solid Oxide Fuel Cells (SOFCs) and Gas Turbines (GT) are analysed in this paper through the use of two different approaches: simplified and detailed SOFC models. The simplified model, already presented by the Authors1 , is very useful for HS design and off-design analysis. The detailed model, developed by the Authors2 and verified through the use of available experimental data, allows the complete description of the SOFC reactor’s internal behaviour to be obtained. The detailed model can also be utilised for HS modelling. Both models are presented and discussed in this paper, and they are compared to each other. The results obtained for the stand-alone SOFC reactor, and the HS design point configuration are presented and carefully discussed, also taking into account the non linear SOFC response.

Commentary by Dr. Valentin Fuster
2002;():905-911. doi:10.1115/GT2002-30665.

A Universal Mathematical Model (UMM) has been developed and applied to a combined-cycle, fossil-fuel power system. The UMM includes static and dynamic models of the system. The static model allows for thermodynamic and thermochemical analyses of the basic system components (reformer, turbine, membrane separator, fuel cell, air compressor, heat exchanger, and other components) and the entire system. The dynamic model provides for mode-to-mode (a partial load to a full or nominal load) time determination for the individual system components and for the entire system. System transient modes were studied, and it was determined that the reforming reactor transition time should be no less than 200 sec, which results in a system mode-to-mode transition time of three to four minutes.

Commentary by Dr. Valentin Fuster

Marine

2002;():913-921. doi:10.1115/GT2002-30260.

Many Navies around the world have either committed to or are considering utilizing an integrated electric propulsion and ship service power system for their next generation of surface combatants. An integrated system provides for greater operational flexibility, efficiency, and survivability as described in Reference [1]. Two examples of this concept are the Type 45 Destroyer program for the Royal Navy and the DD (X) program for the US Navy. The machinery plant for the Type 45 will include both gas turbine and diesel generators sets, and although not determined yet, the DD (X) plant will undoubtedly include gas turbine prime movers. The US Navy has been evaluating a gas turbine generator based Integrated Power System (IPS) architecture at the Land Based Engineering Site (LBES) at the Naval Surface Warfare Center, Ship Systems Engineering Station (NSWCCD-SSES) in Philadelphia, Pa since 1999. This paper will describe the IPS configuration, test program, gas turbine generators, gas turbine generator operational experience, and recommendations for future systems.

Commentary by Dr. Valentin Fuster
2002;():923-931. doi:10.1115/GT2002-30261.

The Naval Surface Warfare Center, Carderock Division (NSWCCD) Gas Turbine Emerging Technologies Code 9334 was tasked by NSWCCD Shipboard Energy Office Code 859 to research and evaluate fouling resistant compressor coatings for Rolls Royce Allison 501-K Series gas turbines. The objective of these tests was to investigate the feasibility of reducing the rate of compressor fouling degradation and associated rate of specific fuel consumption (SFC) increase through the application of anti-fouling coatings. Code 9334 conducted a market investigation and selected coatings that best fit the test objective. The coatings selected were Sermalon for compressor stages 1 and 2 and Sermaflow S4000 for the remaining 12 compressor stages. Both coatings are manufactured by Sermatech International, are intended to substantially decrease blade surface roughness, have inert top layers, and contain an anti-corrosive aluminum-ceramic base coat. Sermalon contains a Polytetrafluoroethylene (PTFE) topcoat, a substance similar to Teflon, for added fouling resistance. Tests were conducted at the Philadelphia Land Based Engineering Site (LBES). Testing was first performed on the existing LBES 501-K17 gas turbine, which had a non-coated compressor. The compressor was then replaced by a coated compressor and the test was repeated. The test plan consisted of injecting a known amount of salt solution into the gas turbine inlet while gathering compressor performance degradation and fuel economy data for 0, 500, 1000, and 1250 KW generator load levels. This method facilitated a direct comparison of compressor degradation trends for the coated and non-coated compressors operating with the same turbine section, thereby reducing the number of variables involved. The collected data for turbine inlet, temperature, compressor efficiency, and fuel consumption were plotted as a percentage of the baseline conditions for each compressor. The results of each plot show a decrease in the rates of compressor degradation and SFC increase for the coated compressor compared to the non-coated compressor. Overall test results show that it is feasible to utilize anti-fouling compressor coatings to reduce the rate of specific fuel consumption increase associated with compressor performance degradation.

Commentary by Dr. Valentin Fuster
2002;():933-942. doi:10.1115/GT2002-30262.

As part of the Gas Turbine Condition Based Maintenance (CBM) Program, Naval Surface Warfare Center, Carderock Division Code 9334 conducted compressor fouling testing on the General Electric LM2500 and Rolls Royce/Allison 501-K Series gas turbines. The objective of these tests was to determine the feasibility of quantifying compressor performance degradation using existing and/or added engine sensors. The end goal of these tests will be to implement an algorithm in the Navy Fleet that will determine the optimum time to detergent crank wash each gas turbine based upon compressor health, fuel economy and other factors which must be determined. Fouling tests were conducted at the Land Based Engineering Site (LBES). For each gas turbine, the test plan that was utilized consisted of injecting a salt solution into the gas turbine inlet, gathering compressor performance and fuel economy data, analyzing the data to verify sensor trends, and assessing the usefulness of each parameter in determining compressor and overall gas turbine health. Based upon data collected during these fouling tests, it seems feasible to accomplish the end goal. Impact Technologies, who analyzed the data sets for both of these fouling tests, has developed a prognostic modeling approach for each of these gas turbines using a combination of the data and probabilistic analysis.

Commentary by Dr. Valentin Fuster
2002;():943-959. doi:10.1115/GT2002-30263.

This paper discusses the performance of various high pressure turbine (HPT) blade coatings applied to refurbished LM2500 components that were operated in a frigate class marine application. In the early 1990s, an LM2500 propulsion gas turbine engine was removed from the Australian Navy ship HMAS DARWIN for overhaul. In an effort to reduce the cost of overhaul, it was proposed to refurbish the HPT blades instead of the standard practice of replacing the blades with new parts. Included in the proposal to replace the blades with refurbished parts, various coatings were applied in order to evaluate the cost effectiveness of refurbishing blades during overhaul vice replacing with new components. The HPT “Rainbow” Rotor is a paired blade configuration and was built up in 1991 using refurbished blades coated with the standard BC 21 (overlay CoCrAlY) and an industrial standard platinum aluminide (diffusion) coating. In addition, BC 23 coating was applied to several new production blades and installed in the Rainbow Rotor as a control to evaluate the refurbished parts performance. In May 2000, the Rainbow Rotor set of LM2500 blades was removed from service after having accumulated in excess of 11,500 operating hours. This paper details the coating compositions tested and the resultant metallurgical analysis of these blades/coatings.

Topics: Coatings , Rotors , Blades , Seas
Commentary by Dr. Valentin Fuster
2002;():961-967. doi:10.1115/GT2002-30264.

The Navy Landing Craft Air Cushion (LCAC) Service Life Extension Program (SLEP) upgrades the current TF40B gas turbine engine and analog control system on the LCAC to an Enhanced TF40B (ETF40B) gas turbine with a Full Authority Digital Engine Control (FADEC) system. This upgrade and enhancement will provide additional engine horsepower, increased engine reliability, and modern digital engine control equipment to the LCAC. The success of the ETF40B engine development program has been an ongoing effort between the Navy, the LCAC craft builder Textron Marine & Land Systems (TM&LS), and the engine manufacturer Honeywell Engine and Systems. This paper will document and outline the differences between the TF40B and ETF40B and the efforts of the ETF40B 150 hour endurance qualification test.

Topics: Gas turbines
Commentary by Dr. Valentin Fuster
2002;():969-975. doi:10.1115/GT2002-30265.

The U.S. Navy has experienced problems with liquid fuel nozzles used on the Rolls Royce (formerly Allison) 501K series marine gas turbine engines. The 501K engines used by the U.S. Navy power Ship Service Gas Turbine Generators (SSGTGs) on a number of destroyer and cruiser class ships. Over roughly the last 25 years, 3 different nozzle designs have been employed, the latest and current nozzle being a piloted air blast design. The primary failure modes of these designs were internal fuel passage coking and external carbon deposits. The current piloted air blast design has a hard time replacement requirement of 1500 hours. This life is considered unacceptable. To improve fuel nozzle life, the Navy and Turbine Fuel Technologies (formerly Delavan) teamed in a fast track program to develop a new fuel nozzle with a target life of 5000 hours and 500 starts. As a result, an air assist/air blast nozzle was developed and delivered in approximately 6 months. In addition to the nozzle itself, a system was developed to provide assist air to the fuel nozzles to help atomize the fuel for better ignition. Nozzle sets and air assist systems have been delivered and tested at the NSWC Philadelphia LBES (Land Based Engineering Site). In addition, nozzle sets have been installed aboard operating ships for in-service evaluations. During the Phase one evaluation (July 2000 to June 2001) aboard USS Porter (DDG 78) a set of nozzles accumulated over 3500 hours of trouble free operation, indicating the target of 5000 hours is achievable. As of this writing these nozzles have in excess of 5700 hours. The improvements in nozzle life provided by the new fuel nozzle design will result in cost savings through out the life cycle of the GTGS. In fact, the evaluation nozzles are already improving engine operation and reliability even before the nozzles’ official fleet introduction. This paper describes the fuel nozzle and air assist system development program and results of OEM, LBES and fleet testing.

Topics: Fuels , Gas turbines , Nozzles , Navy
Commentary by Dr. Valentin Fuster
2002;():977-984. doi:10.1115/GT2002-30266.

The WR-21 gas turbine engine will be employed by the Royal Navy and potentially by the United States and French Navies in their future Integrated Full Electric Powered Surface Combatants. The Intercooled Recuperated (ICR) advanced cycle means that in a Warship power system a single WR-21 engine sits on the throne of the realm that traditionally would have been occupied by two gas turbine engines, one for ‘cruise’ and one for ‘boost’; not forgetting that it is also doing the job of at least two diesel generators in our traditional example. This performance will provide Warship operators with an unprecedented opportunity to configure the Warship propulsion plant to return exceptional Platform Life Cycle Cost reductions in peacetime while retaining warfighting operational capability in time of conflict. The Royal Navy is the first user of the WR-21 ICR gas turbine engine in its Type 45 Air Defense destroyer, an artists impression of which is shown in Figure 1. The vessel is a 7500 tonne monohull, fitted with an integrated electric propulsion plant comprising two WR-21 Gas Turbine Alternators (GTAs), the prime mover side of which is capable of delivering 25 MW (ISO) and the Alternator side of which is rated at 21.6 MWe (0.9 pf lagging), 4.16KV. These GTAs in combination with a pair of diesel generators rated at around 2 MWe (0.9 pf lagging) will provide electrical power to two 20 MWe (0.9 pf lagging) 4.16 KV electric propulsion motors and to the ship’s non propulsion consumer electrical distribution system. Any combination of generator set can provide any consumer with electrical power. In their crudest form any generator set that forms part of the Type 45 power system may be simply regarded as Mega Watts towards the installed power total. The division of priority and delivery of power to meet the Command’s requirements will require skilful and subtle engineering of the control systems that will be used to operate the power system and precise definition of the operating philosophy and principles for the platform. In a Warship that has only four sources of electrical power the principles of survivability and prime mover independence are fundamental. The limitations of operating electrical generation machinery are established. This paper examines how the WR-21 will be capable of providing power to the Command of the Type 45 as an integral part of the Warship power system in all states of operational readiness for war.

Commentary by Dr. Valentin Fuster
2002;():985-989. doi:10.1115/GT2002-30267.

An advanced three stage filtration/separation air intake system (Compact II) is introduced in this paper. The system was developed to meet the current and expected future market demands for gas turbine combustion air treatment in a marine environment. Developing and testing of the Compact II are subjects of this paper.

Commentary by Dr. Valentin Fuster
2002;():991-998. doi:10.1115/GT2002-30268.

The Royal Navy is pursuing the ‘All Electric’ ship under its Marine Engineering Development Strategy. This strategy envisages the use of long life, fuel efficient, advanced cycle marine gas turbine alternator sets in an Integrated Full Electric Propulsion (IFEP) system which includes the wide scale electrification of auxiliary systems. The IFEP system favoured by the Royal Navy requires just four prime movers for a typical European sided Frigate or Destroyer; two high power units (2 × 20–25MW), one medium power (1 × 4–8MW), one low power unit (1 × 1–2MW), and employs single engine operation at sea. The high power unit, the WR21, has already completed development. In December 2000 the Royal Navy placed a contract on Turbomeca Limited of France to develop a 1.8 MW advanced cycle gas turbine alternator. The paper provides details of the development of this 1.8MW gas turbine alternator know as the Advanced Cycle Low-power Gas Turbine Alternator (ACL GTA). It describes the basic engine design including recuperator, outlines the predicted performance, and gives details of the directly coupled high speed alternator and associated power electronic to provide the 800V DC output. The overall programme is discussed, along with the outstanding issues. Reference is made to the GTA’s ability to compete in a highly competitive market dominated by diesel driven alternators.

Commentary by Dr. Valentin Fuster
2002;():999-1011. doi:10.1115/GT2002-30269.

Marine gas turbines have been used for many decades in a diverse range of commercial and naval marine vessels, almost exclusively for main propulsion duties in a number of different configurations. As well as providing an outline of the scope of operation, this paper aims to discuss the key Life Extension Program’s and Cost Reduction Strategies developed by the UK Ministry of Defence in support of the two international collaborative Memoranda of Understanding (MoU) for the marine Olympus, Tyne and Spey gas turbines. Where available, discussion is supported with evidence from emerging equipment maintenance policies, equipment modifications and data collected from components and engines returned from the fleet for repair or overhaul. In addition, and in terms of the economy of scale advantages that the arrangements offer, an assessment of accumulated savings and projected financial return is provided with an insight into the operational benefits and improved capability that the program’s realise.

Topics: Gas turbines
Commentary by Dr. Valentin Fuster
2002;():1013-1023. doi:10.1115/GT2002-30270.

The Naval University of Gdynia (NUG) over the years have developed quite an extensive Condition Monitoring and Fault Diagnosis system and has applied this for some time now on Naval Gas Turbines of the Polish Navy. An outline of the system and its use is given in the paper. The Royal Netherlands Naval College, together with Delft University of Technology is active in the field of advanced modelling of marine systems. A broad outline of work done in that area forms the second part of the paper. Recently it has been decided to start a collaboration between the two research groups. The aims of this collaboration will be clarified.

Commentary by Dr. Valentin Fuster
2002;():1025-1033. doi:10.1115/GT2002-30271.

As part of the Naval gas turbine CBM effort, diagnostic and prognostic algorithms that utilize state-of-the-art probabilistic modeling and analysis technologies are being developed and implemented onboard Navy ships. The algorithms under development and testing will enhance gas turbine preventative maintenance in such areas as compressor on-line/crank wash and fuel nozzle replacement. In one application, the prognostic module assesses and predicts compressor performance degradation due to salt ingestion. From this information, the optimum time for on-line water washing or crank washing can be determined from a cost/benefit standpoint. A second application diagnoses the severity of fuel nozzle fouling in real-time during startup. This paper discusses the diagnostic and prognostic modeling approaches to these maintenance issues and their implementation for an Allison 501-K34 gas turbine engine onboard a DDG 51 class guided missile destroyer.

Commentary by Dr. Valentin Fuster
2002;():1035-1039. doi:10.1115/GT2002-30272.

All United States Navy LM2500 gas turbine engines use a hydromechanical Main Fuel Control to meter the amount of fuel and air introduced into the engine based upon power demand and ambient conditions. The Main Fuel Control is one of the highest failure items on the LM2500 engine and is frequently the cause of costly high-speed engine stalls resulting from mis-rigged throttle or stator vane positioning linkages. Efforts are underway to upgrade the LM2500 with Woodward Governor Company’s state-of-the-art Digital Fuel Control system, subsequent to successful integration testing. The system consists of an off-engine fuel metering valve/actuator, a hydraulic pump/servo valve, and Variable Stator Vane hydraulic actuators equipped with electronic positioning feedback devices. The system will be controlled via one of two engine controllers; the Woodward MicroNet for CG-47 class ships and the Lockheed Martin Universal Engine Controller (UEC) “Plus” for DDG-51 class ships. This paper covers the development, land-based testing, shipboard testing, and fleet implementation of the Digital Fuel Control system in addition to associated engine controllers on the LM2500 engine.

Commentary by Dr. Valentin Fuster
2002;():1041-1050. doi:10.1115/GT2002-30419.

This paper will discuss the conversion of the US Navy LHD assault ship from conventional steam propulsion to gas turbine mechanical drive with all electric auxiliaries. The LHD 8 will provide gas turbine mechanical drive and electric propulsion via Controllable Pitch Propellers (CPP) in a Combined Operation Diesel Electric or Gas Turbine (CODLOG) configuration. The primary propulsion is from gas turbine engines with an auxiliary electric drive to provide economical low speed operations while the ship is operating in the littoral zone. The paper will discuss how the gas turbine drive in concert with an electric “loiter” motor drive will be used to provide the most efficient drive combination for each operating scenario. The paper will also discuss the process, trade-offs, and constraints placed on the designers of incorporating a new propulsion plant into an existing ship design. Interfaces with other ship systems presented significant challenges. The text presents the constraints and issues involved in the design process by addressing all major design impacts and significant design concerns. Since the Navy first introduced the General Electric LM2500, the standard gas turbine installation has changed very little and all major surface combatants since the early 1970s have utilized a very similar design. The paper will discuss how the LM2500+ installation on LHD 8, the first of its kind in a military application, will capitalize on the existing design while at the same time changing to meet new requirements, standards and regulations. The paper will also discuss the changes brought about by adopting commercial practices and standards and capitalizing on commercial experiences particularly in areas such as engine qualification.

Topics: Gas turbines , Navy , Ships , Steam
Commentary by Dr. Valentin Fuster
2002;():1051-1058. doi:10.1115/GT2002-30673.

The first section of this paper will discuss the aimed mission and need for the latest Royal Netherlands Navy’s (RNLN) ship class, the Luchtverdedigings en Commando Fregat (LCF) and its derived requirements for the propulsion plant. The choice of a CODOG (Combined Diesel or Gas) propulsion installation as opposed to COGOG (Combined Gas or Gas) or CODLAG (Combined Diesel Electric and Gas), will be explained. The difficult choice between the Rolls Royce Spey SM1C and the General Electric LM2500 gas turbines for the boost role will be explained showing how history largely shaped the choice. The second section of the paper will deal with the challenges of the Spey gas turbine installation, the development of a digital fuel control system, the problems encountered and the challenges yet to be resolved. Discussions on the provisioning and support arrangements will also be made. The last part of the paper will make a short reconnaissance in the near future of gas turbine ship propulsion and the broad thoughts that are shaping the RNLNs thinking.

Commentary by Dr. Valentin Fuster
2002;():1059-1064. doi:10.1115/GT2002-30675.

Naval Surface Warfare Center, Carderock Division - Ship Systems Engineering Station (NSWCCD-SSES) successfully completed testing of a new Full Authority Digital Control (FADC) system for gas turbine control. This system will be back-fit onto Model 139 Ship Service Gas Turbine Generator Sets (SSGTGs) on the U.S. Navy’s Ticonderoga (CG-47) class cruisers. The FADC will be a direct replacement of the original Model 139 Local Operating Panel (LOCOP) and will control the Allison 501-K17 gas turbine. The new control system provides for standardized installation across a wide variety of existing configurations. The development program leveraged off of the design work done for the AG9140 FADC currently being installed on DDG 51 Class ships. The result was a state-of-the-art system ready for shipboard installation in a short period of time, providing commonality of look and feel across platforms. This paper describes the CG-47 FADC and details the development and testing conducted on a Model 139 SSGTG at the NSWCCD-SSES DDG 51 Gas Turbine Land Based Engineering Test Site (LBES). The test program included all modes of SSGTG operation, including starts, shutdowns, and generator operations under varying load conditions.

Commentary by Dr. Valentin Fuster
2002;():1065-1072. doi:10.1115/GT2002-30676.

The benefits of utilizing Condition Based Maintenance (CBM) over a time based preventative approach to reduce life cycle costs has been well documented over the years. The U.S. Navy has been working through several separate programs, such as Smartship, Integrated Condition Assessment System (ICAS), Accelerated Capabilities Initiative (ACI), to develop this technology for use in the fleet. Recently, an effort was initiated to coordinate these programs in an application on marine gas turbines. This paper describes the phased process of CBM implementation on marine gas turbines and the resulting benefits of reduced life cycle costs and condition based engineering plant alignment.

Commentary by Dr. Valentin Fuster

Oil and Gas Applications

2002;():1073-1082. doi:10.1115/GT2002-30275.

The paper presents a method for gas turbine performance prediction which uses compressor and turbine performance maps obtained by using generalized stage performance curves matched by means of the “stage–stacking” procedure. In particular, the overall multistage compressor performance is predicted using generalized relationships between stage efficiency, pressure coefficient and flow coefficient, while the multistage turbine performance is predicted by modeling each turbine stage by a series of two nozzles, a fixed one (stator) and a moving one (rotor). The characteristic of the proposed method is that the unknown parameters defining the generalized stage performance curves are determined by combining a Cycle Program with the compressor and turbine performance maps obtained using the “stage–stacking” procedure, and by searching for the values of the unknown parameters which better reproduce, by means of the Cycle Program, the overall performance and thermodynamic data measured on a gas turbine.

Commentary by Dr. Valentin Fuster
2002;():1083-1089. doi:10.1115/GT2002-30276.

Recently, diagnostic approaches based on Artificial Intelligence have become very attractive. In particular Neural Networks (NNs) seem to have suitable characteristics for gas turbine diagnostics. This paper deals with the activities carried out for: • selecting the most appropriate NN structure for gas turbine diagnostics; • developing a NN for the detection, isolation and assessment of single and combined causes of performance degradation in a two shaft industrial gas turbine; • testing both the NN performance in recognizing causes of performance degradation and robustness in presence of scarce and/or wrong input data. The data used in all these phases in order to train and test the NN have been generated using a non-linear Cycle Program. So, the Cycle Program becomes a data generator, which may be integrated with data derived from field experience, while the diagnostic function is performed by the NN.

Commentary by Dr. Valentin Fuster
2002;():1091-1101. doi:10.1115/GT2002-30277.

The flow regimes and pressure drop of air-oil two-phase flow in a half-inch diameter pipe over a wide range of test conditions have been investigated. The flow regimes were identified with the aid of a 1000 frames per second high-speed camera. The current flow regime data show significant differences in the transitional boundaries from the flow regime maps of Mandhane et al. (1974), Taitel and Dukler (1974) and Spedding and Nguyen (1980). The pressure drop measurements were compared to the predictions from four existing pressure drop models: Homogeneous, Martinelli (1948), Chisolm (1973) and Olujic (1985). The Chisolm and Martinelli models were found to be the most accurate, with an average error of about 35 percent. A capacitance sensor for instantaneous void fraction measurement was developed. Results indicate the data from the sensor could be used to identify the different flow regimes.

Commentary by Dr. Valentin Fuster
2002;():1103-1109. doi:10.1115/GT2002-30278.

Centrifugal compressors used in the pipeline market generate very strong noise, which is typically dominated by the blade passing frequency and its higher harmonics. The high level noise is not only very disturbing to the people living nearby the installation site but also causes expensive structural failures in the downstream piping. A novel design of Helmholtz array has been developed to address this type of noise problem. Computational studies show that the installation of the Helmholtz array acoustic liner on the compressor diffuser walls is very effective in reducing noise level of the compressor, especially the dominant blade passing frequency noise. The acoustic liner design has been built and tested at an installation site by the customer. The data clearly shows that the use of acoustic liners is indeed very effective in the reduction of both the noise and the vibration levels of the machine.

Topics: Pipes , Vibration
Commentary by Dr. Valentin Fuster
2002;():1111-1121. doi:10.1115/GT2002-30279.

Major challenges are related to compressor and driver integration during run down. In order to understand these challenges, the pipeline compressor facility at Troll Kollsnes gas treatment plant, Norway, has been subjected to detailed trip testing and dynamic simulation analysis. The plant includes five pipeline compressors and is utilised as a pilot for analysing the transient response of a 40 MW compressor driven by a variable speed electric motor. The compressor control and protection system include an anti-surge and a hot gas bypass system. Vibration records have shown that under power outage the compressors were exposed to violent vibrations. Further investigation revealed that during a short power outage, the compressor enters the surge- and rotating stall area under certain operating scenarios. The rotating stall response resulted in reduced operating range and flexibility for the pipeline compressors. Specific precautions had to be taken to prevent the compressor from running into the low flow operating area of the performance envelope. Dynamic simulations cover important aspects related to the transient scenario analyses performed to reveal the root cause of the compressor problems. The simulation system enables sophisticated plant models to be configured from high quality standard model algorithm building blocks. Verification of the model blocks have been performed against plant records in order to validate the transient predictions. The paper reports experience from testing and verification of compressor and driver integration with reference to transient behaviour during run down. This includes the validation of the dynamic models, which apply both to the design and commissioning phase where actual plant trip tests should be used to verify the design and stability margins.

Topics: Compressors
Commentary by Dr. Valentin Fuster
2002;():1123-1135. doi:10.1115/GT2002-30280.

Industrial gas turbine manufacturers began offering engines configured with dry low emissions (DLE) control in 1992. In the past ten years the performance and emissions reductions have been well demonstrated by DLE equipment. To date DLE gas turbines have relied on lean premixed combustion technology to achieve emissions reductions of 8 to 10 fold from “conventional” diffusion flame engines. The significant new content incorporated for DLE combustion systems has required industrial gas turbine manufacturers and users to work with greater synergy to overcome significant challenges. As evidence of this ultimately successful integration, DLE gas turbines are now as common in service as conventional diffusion flame engines. With thousands of DLE units sold one would expect that DLE gas turbines are now a mature product. In many aspects, this is true. However, emissions regulations and other market drivers have continued to change, forcing DLE equipment to continually evolve. A Solar history of DLE gas turbine developments, capabilities, and experiences are provided to give operators background and knowledge to reduce field issues and maximize availability of their DLE gas turbines. Design limitations and problems encountered in the field are discussed along with the steps that were taken to resolve them. Recommendations on DLE engine operation to avoid unscheduled downtime are presented. Design improvements to reduce emissions further and improve system flexibility are summarized.

Commentary by Dr. Valentin Fuster
2002;():1137-1142. doi:10.1115/GT2002-30281.

In general, two approaches have been followed so far in gas turbine maintenance procedures to determine correct inspection intervals: “no interdependency” or “interdependency” between number of starts and number of running hours. The first approach is based on the assumption that starts and running hours induce different deterioration processes not correlated to each other. Accordingly, the number of starts defines the life limit for cyclic duty operation where low-cycle fatigue phenomena dominate, while the number of running hours define the continuous operation life limit for which erosion, corrosion and creep are controlling factors. The “interdependency” approach instead assumes that failure is produced by combination of low-cycle fatigue and continuous degradation mechanisms: in this scheme the frequency of starts becomes a fundamental parameter in order to determine the optimal maintenance interval. A statistical and reliability engineering methodology to validate the first or rather the second line of action is described in the paper. The population on which the study was conducted is made up by GE Oil & Gas PGT10 gas turbines that are in operation worldwide with fleet operation totaling 1,5 million hours. Most of the cases examined consist of mechanical drive applications for natural gas production, storage and transportation, with significant combination of both intermittent and continuous operation. Hot gas path components have been chosen for examination in consideration of their sensitivity to effects of both cyclic fatigue stress and wear mechanisms. The analysis concentrates on transition piece and combustion liner, both having scored a number of failures statistically significant for the purposes of this study. This analysis is considered the key to optimize inspection intervals and therefore achieve extended machine life. The methodology, based on Weibull data analysis, has been applied to a restricted sample of machines that operate in “standard” conditions, corresponding to gas fuel utilization, mechanical drive service with homogeneous load factor and very low number of trips. The study shows that interdependency between starts and running hours does exist and, given the number of starts, the corresponding running hours can be evaluated, and the inspection intervals appropriately predicted. Further developments of this study will be aimed at evaluating maintenance factors for “non standard” conditions such as dual fuel combustion systems, generator drive and operation with higher number of trips etc.

Commentary by Dr. Valentin Fuster
2002;():1143-1152. doi:10.1115/GT2002-30282.

Gas compression systems consist of a number of turbo machinery components: A Gas Turbine consisting of a gas producer turbine, and a power turbine; one or several gas compressors or gas compressor sections; and possibly a gearbox. The end-user usually defines very specific operating conditions. However, the acceptance testing is often performed on the individual components rather than the overall system. Intuitively, one would rather expect criteria that are based on the overall system operation. This paper suggests a different approach of defining the acceptance criteria, such that it is based on the performance of the overall system and thus ties the acceptance criteria to the operational goals of the installation. The review also includes comments on some mechanical aspects of factory and field-testing of turbomachinery. Because the above approach involves the entire system, the verification of the acceptance criteria needs to be addressed. It is therefore described how the overall system performance can be verified, even if the operating conditions during the test are different from the specified acceptance conditions. The advantage of this approach is that it avoids over-specifying equipment due to adding tolerances upon tolerances, while at the same time specifying what really counts for the operational success of the project.

Commentary by Dr. Valentin Fuster
2002;():1153-1160. doi:10.1115/GT2002-30283.

Alliance Pipeline Limited (APL) pipeline is a 1850 miles transmission line delivering natural gas from British Columbia (Canada) to Chicago (Illinois USA). The Pipeline crosses remote areas and is subject to adverse winter weather conditions that may temporarily limit accessibility to sites where boosting stations are located. By considering APL requirements in terms of turbo compressor maintenance and operation level, a system providing real time and historical monitoring capabilities was designed to support pipeline operation with the newest available analysis techniques. Thanks to current communication technologies the system allows for a stable link between equipment and OEM (Original Equipment Manufacturer - GE Oil&Gas - O&G) that is operating the system since pipeline commissioning in November 2000. This paper describes the system architecture and processes to deliver advanced condition monitoring and proactive maintenance planning for the APL turbo compressing fleet.

Commentary by Dr. Valentin Fuster
2002;():1161-1171. doi:10.1115/GT2002-30538.

The present paper deals with several problems arising in the stabilisation of surge in compression systems. From a theoretical standpoint the system requires a high gain type controller to be stabilised. On the other hand, the real system is affected by persistent measurement noise whose negative effect on stability is strongly amplified by the high gain. Therefore, a suitable filtering of the system output is necessarily required. The bandwidth of the filter is subject to opposite constraints: if it is too large it affects the attenuation property; if it is too narrow, the large phase-lag introduced at low frequencies may compromise the system stabilisation, which requires a fast actuator reaction. This paper analyses all these aspects concurring in the stabilisation problem and the consequent trade-off in the control design. The results of experimental and numerical studies are provided with reference to the active control of surge in a multistage centrifugal compressor.

Topics: Compressors , Surges
Commentary by Dr. Valentin Fuster
2002;():1173-1183. doi:10.1115/GT2002-30539.

This paper presents the application of feed forward neural networks to the performance control of a gas transmission compressor. It is estimated that a global saving in compressor fuel gas of 1% could prevent the production of 6 million tonnes of CO2 per year, [1]. Results of compressor model testing suggest that compressor speed can be estimated to within ± 2.5%. The neural network property of function approximation is used to predict compressor speed for given process constraints and instrument input sets. The effects of training set size, instrument noise, reduced input sets and extrapolation from the training domain, are quantified. Various neural network architectures and training schema were examined. The embedding of a neural network into an expert system is also discussed.

Topics: Compressors
Commentary by Dr. Valentin Fuster
2002;():1185-1193. doi:10.1115/GT2002-30592.

Industrial Gas Turbines allow operation with a wide variety of gaseous and liquid fuels. To determine the suitability for operation with a gas fuel system, various physical parameters of the proposed fuel need to be determined: Heating value, dew point, Joule-Thompson coefficient, Wobbe Index and others. This paper describes an approach to provide a consistent treatment for determining the above physical properties. Special focus is given to the problem of determining the dew point of the potential fuel gas at various pressure levels. A dew point calculation using appropriate equations of state is described, and results are presented. In particular the treatment of heavier hydrocarbons, and water is addressed and recommendations about the necessary data input are made. Since any fuel gas system causes pressure drops in the fuel gas, the temperature reduction due to the Joule-Thompson effect has to be considered and quantified. Suggestions about how to approach fuel suitability questions during the project development and construction phase, as well as in operation are made.

Commentary by Dr. Valentin Fuster
2002;():1195-1202. doi:10.1115/GT2002-30593.

In 1996, Cogeneration-Kraftwerke Management Steiermark (CMST), OMV Cogeneration, together with local partners, built a 25Mwel gas turbine plant with a hot water boiler for thermal energy to be used by a car manufacturer and the municipality Graz, Austria. The plant is driven by a FT8-30 (JT8D-219) Pratt & Whitney (P&W) jet engine, accumulating 8200 operating hours per annum. This paper outlines the technical experience and related problems with the existing equipment in the light of variable operating conditions and the investments for efficiency augmentation of the gas turbine trains. A joint-venture between Cogeneration Kraftwerke Management Oberösterreich GmbH (CMOÖ) and OMV Cogeneration GmbH as well as Energie AG. CMOÖ has operated the Combined Heat Power CHP Plant (50 MW el) in the paper mill SCA GRAPHIC LAAKIRCHEN based on contracting since 1994. Because of a extension of the paper mill the energy supply had to be increased. So the delivery of two steam boilers with each 30 t steam per hour and water treatment took place in August 2001. The plant-extension will operate as an independent unit and will guarantee the full availability of the energy supply. Commercial operation will start in January 2002.

Commentary by Dr. Valentin Fuster

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