Books by Alexios Alexiou
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Papers by Alexios Alexiou
Energies, 2024
A review of existing research on signatures of gas turbine faults is presented. Faults
that influ... more A review of existing research on signatures of gas turbine faults is presented. Faults
that influence the aerothermodynamic performance of compressors and turbines, such as fouling,
tip clearance increase, erosion, variable geometry system malfunction, and object impact damage,
are covered. The signatures of such faults, which are necessary for establishing efficient gas path
diagnostic methods, are studied. They are expressed through mass flow capacity and efficiency
deviations. The key characteristics of the ratio of such deviations are investigated in terms of
knowledge existing in published research. Research based on experimental studies, field data, and
results of detailed fluid dynamic computations that exist today is found to provide such information.
It is shown that although such signatures may be believed to have a unique correspondence to the
type of component fault, this is only true when a particular engine and fault type are considered. The
choice of diagnostic methods by developers should, thus, be guided by such considerations instead
of using values taken from the literature without considering the features of the problem at hand.
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Applied Sciences, 2024
Data from the steady-state operation of gas turbine engines are used in gas path diagnostic proce... more Data from the steady-state operation of gas turbine engines are used in gas path diagnostic procedures. A method to identify steady-state operation is thus required. This paper initially explains and demonstrates the factors that cause a deviation in engine health when transient data are used for diagnosis and shows that there is a threshold in the slope of time traces, below which the variation in engine health parameters is acceptable. A methodology for deriving a criterion for steady-state operation based on actual flight data is then presented. The slope of the exhaust gas temperature variation with time and the size of its time-series window, from which this slope is determined, are the required parameters that must be specified when applying this criterion. It is found that the values of these parameters must be selected so that a sufficient number of steady-state points are available without compromising the accuracy of the diagnostic procedure.
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Aerospace, 2024
In a time when low emission solutions and technologies are of utmost importance regarding the sus... more In a time when low emission solutions and technologies are of utmost importance regarding the sustainability of the aviation sector, this publication introduces a reduced-order physics-based model for combustion chambers of aeroengines, which is capable of reliably producing accurate pollutant emission and combustion efficiency estimations. The burner is subdivided into three volumes, with each represented by a single perfectly stirred reactor, thereby resulting in a simplified three-element serial chemical reactor network configuration, reducing complexity, and promoting the generality and ease of use of the model, without requiring the proprietary engine information needed
by other such models. A tuning method is proposed to circumvent the limitations of its simplified configuration and the lack of detailed geometric data for combustors in literature. In contrast to most similar frameworks, this also provides the model with the ability to simultaneously predict the combustion efficiency and all pollutant emissions of interest (NOx, CO and unburnt hydrocarbons) more effectively by means of implementing a detailed chemical kinetics model. Validation against three correlation methods and actual aeroengine configurations demonstrates accurate performance and emission trend predictions. Integrated within two distinct combustion chamber low-emission preliminary design processes, the proposed model evaluates each new design, thereby displaying the ability to be employed in terms of optimizing a combustor’s overall performance given its sensitivity to geometric changes. Overall, the proposed model proves its worth as a reliable and valuable tool for use towards a greener future in aviation.
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Aerospace, 2023
An approach for preliminary aero-engine design, incorporating a mean-line code for the design of ... more An approach for preliminary aero-engine design, incorporating a mean-line code for the design of axial-flow, multi-stage compressors, is presented. The compressor mean-line code is developed and integrated within a framework for the preliminary design and assessment of aero-engine concepts. It is then combined with modules for compressor map generation, multipoint engine design, steady-state and transient engine off-design performance and aircraft mission analysis. Implementation examples are presented, demonstrating the determination of the optimal
combination of compressor and engine design parameters for achieving minimum fuel burn over a specific aircraft mission, while obeying constraints that guarantee operability over the entire flight envelope. Constraints related to compressor stability during transient maneuvers between idle and static take-off conditions and engine temperature limits at maximum take-off are respected by the final design. The results demonstrate the potential for design trade-offs between engine performance at the aircraft mission level and compressor aerodynamic stability.
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Aerospace, 2023
Disks in gas turbines are optimized for minimum weight, while satisfying both geometry and stress... more Disks in gas turbines are optimized for minimum weight, while satisfying both geometry and stress constraints, in order to minimize the engine production, operation, and maintenance costs.
In the present paper, a tool is described for the preliminary mechanical design of gas turbine disks. A novel formulation is presented, where the disk weight minimization is achieved by maximizing the stresses developed in the disk. The latter are expressed in the form of appropriately defined design and burst margins. The computational capabilities of the tool developed are demonstrated through comparisons to calculations with a higher fidelity tool. The importance of accurately calculating
thermal stresses is demonstrated and the ability of the tool for such calculations is discussed. The potential and efficiency of the tool are illustrated through a proposed re-design of the disks of a well-documented ten-stage compressor. Finally, the integration of the tool into an overall engine design framework is discussed.
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Journal of Engineering for Gas Turbines and Power
Propulsion diagnostic method evaluation strategy (ProDiMES) offers an aircraft engine diagnostic ... more Propulsion diagnostic method evaluation strategy (ProDiMES) offers an aircraft engine diagnostic benchmark problem where the performance of candidate diagnostic methods is evaluated while a fair comparison can be established. In the present paper, the performance evaluation of a number of gas turbine diagnostic methods using the ProDiMES software is presented. All diagnostic methods presented here were developed at the Laboratory of Thermal Turbomachinery of the National Technical University of Athens (LTT/NTUA). Component, sensor, and actuator fault scenarios that occur in a fleet of deteriorated twin-spool turbofan engines are considered. The performance of each diagnostic method is presented through the evaluation metrics introduced in the ProDiMES software. Remarks about each methods performance as well as the detectability and classification rates of each fault scenario are made.
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Energy
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Journal of Engineering for Gas Turbines and Power, 2016
A simulation environment allowing steady state and transient modeling is used for assessing sever... more A simulation environment allowing steady state and transient modeling is used for assessing several gas turbine based cycles proposed for solar hybridization. First, representative open cycle gas turbine configurations, namely, (a) single shaft (SS), (b) recuperated single-shaft, (c) twin shaft (TS), and (d) two-spool three-shaft, intercooled, recuperated, are evaluated. The importance of design point selection in terms of solar share value is highlighted. Solar steam injection gas turbine cycle (STIG) alternatives, namely, solar steam only and solar/fuel gas steam, are then assessed. Finally, the concept of a dual fluid receiver (DFR) for exploiting the rejected solar power by producing steam during sunny hours with high irradiation is demonstrated. The effects of hybridization on performance and operability are established and evaluated. Solarization effect on performance is estimated in terms of annual produced power and fossil fuel savings. The results indicate that the spool ar...
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Volume 3: Coal, Biomass and Alternative Fuels; Cycle Innovations; Electric Power; Industrial and Cogeneration; Organic Rankine Cycle Power Systems, 2016
Coupling of high fidelity component calculations with overall engine performance simulations (zoo... more Coupling of high fidelity component calculations with overall engine performance simulations (zooming) can provide more accurate physics and geometry based estimates of component performance. Such a simulation strategy offers the ability to study complex phenomena and their effects on engine performance and enables component design changes to be studied at engine system level. Additionally, component interaction effects can be better captured. Overall, this approach can reduce the need for testing and the engine development time and cost. Different coupling methods and tools have been proposed and developed over the years ranging from integrating the results of the high fidelity code through conventional performance component maps to fully-integrated three-dimensional CFD models. The present paper deals with the direct integration of an in-house two-dimensional (through flow) streamline curvature code (SOCRATES) in a commercial engine performance simulation environment (PROOSIS) with the aim to establish the necessary coupling methodology that will allow future advanced studies to be performed (e.g. engine condition diagnosis, design optimization, mission analysis, distorted flow). A notional two-shaft turbofan model typical for light business jets and trainer aircraft is initially created using components with conventional map-defined performance. Next, a derivative model is produced where the fan component is replaced with one that integrates the high fidelity code. For both cases, an operating line is simulated at sea-level static take-off conditions and their performances are compared. Finally, the versatility of the approach is further demonstrated through a parametric study of various fan design parameters for a better thermodynamic matching with the driving turbine at design point operation.
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Journal of Engineering for Gas Turbines and Power, 2016
The paper presents a thorough analysis of the historical data and results acquired over a period ... more The paper presents a thorough analysis of the historical data and results acquired over a period of two years through an on-line real-time monitoring system installed at a combined heat and power (CHP) plant. For gas turbine health and performance assessment, a gas path analysis tool based on the adaptive modeling method is integrated into the system. An engine adapted model built through a semi-automated method is part of a procedure which includes a steam/water cycle simulation module and an economic module used for power plant performance and economic assessment. The adaptive modeling diagnostic method allowed for accurate health assessment during base and part load operation identifying and quantifying compressor recoverable deterioration and the root cause of an engine performance shift. Next, the performance and economic assessment procedure was applied for quantifying the economic benefit accrued by implementing daily on-line washing and for evaluating the financial gains if ...
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Volume 1: Aircraft Engine; Fans and Blowers; Marine, 2016
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This paper reports on heat transfer measurements made on a rotating test rig representing the int... more This paper reports on heat transfer measurements made on a rotating test rig representing the internal disc-cone cavity of a gas turbine H.P. compressor stack. Tests were carried out for a range of flow rates and rotational speeds at similar non -dimensional conditions to an actual engine. The rig also had a central drive shaft, which could be made to either rotate in the same direction as the discs, contra-rotate relative to the discs, or remain static. Measurements of heat transfer were obtained from a conduction solution method using measured surface temperatures as boundary conditions. The heat transfer measurements from the inner surface of the cone reveal two regimes of heat transfer: one governed by rotation the other by the action of the throughflow. In the rotationally dominated regime, the heat transfer from the inner surface of the cone is higher for a co-rotating shaft than for either a static or contra-rotating shaft. In the throughflow dominated regime the heat transfe...
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Journal of Engineering for Gas Turbines and Power, 2012
This paper presents a method of modeling contra-rotating turbomachinery components for engine per... more This paper presents a method of modeling contra-rotating turbomachinery components for engine performance simulations. The first step is to generate the performance characteristics of such components. In this study, suitably modified one-dimensional mean line codes are used. The characteristics are then converted to three-dimensional tables (maps). Compared to conventional turbomachinery component maps, the speed ratio between the two shafts is included as an additional map parameter and the torque ratio as an additional table. Dedicated component models are then developed that use these maps to simulate design and off-design operation at the component and engine levels. Using this approach, a performance model of a geared turbofan with a contra-rotating core (CRC) is created. This configuration was investigated in the context of the European program “NEW Aero-Engine Core Concepts” (NEWAC). The core consists of a seven-stage compressor and a two-stage turbine without interstage stat...
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International Journal of Heat and Fluid Flow, 2000
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Experimental Heat Transfer, 2000
ABSTRACT Link: http://www.tandfonline.com/doi/abs/10.1080/08916150050175471#tabModule This articl... more ABSTRACT Link: http://www.tandfonline.com/doi/abs/10.1080/08916150050175471#tabModule This article reports on heat transfer measurements made on a rotating test rig representing the internal disc-cone cavity of a gas turbine high-pressure (H.P.) compressor stack. Tests were carried out for a range of flow rates and rotational speeds at engine representative nondimensional conditions. The rig also had a central drive shaft, which could rotate in the same direction as the discs, contrarotate relative to the discs, or remain static. Measurements of heat transfer were obtained from a conduction solution method using measured surface temperatures as boundary conditions. Results from the outer surface of the cone are in reasonable agreement with theoretical predictions for the heat transfer from a free cone in turbulent flow. The heat transfer measurements from the inner surface of the cone reveal two regimes of heat transfer: one governed by rotation, the other by action of the throughflow. In the rotationally dominated regime, the heat transfer from the inner surface of the cone is higher for a co-rotating shaft than for either a static or contra-rotating shaft. In the throughflow-dominated regime the heat transfer shows little consistent dependence on the direction of shaft rotation. Tests carried out at different values of surface-to-fluid temperature difference add support to the hypothesis that in the rotationally dominated regime the heat transfer occurs through a process of free convection, where the buoyancy force is induced by rotation. The heat transfer from the disc is significantly lower than that from the inner surface of the cone and more or less insensitive to the sense of shaft rotation. The disc average Nusselt numbers show similar behavior to those from the inner surface of the cone and suggest that the disc heat transfer too is governed either by rotationally induced buoyancy or by the axial throughflow.
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MATEC Web of Conferences
A method is presented for optimizing burnt fuel and flight time of civil aircraft missions, under... more A method is presented for optimizing burnt fuel and flight time of civil aircraft missions, under the scope of a full-scale free route airspace implementation as well as usage of actual environmental data, focusing on ambient temperature and pressure. A computational analysis model, using online available aircraft data is considered, for flight envelopes defined by the user, is used as the base tool. The model is validated over real flight data, before it is used as the main tool for optimization of flight missions. Flight paths are analyzed by considering both their vertical and horizontal aspect. Optimal paths are derived, for each chosen mission, through an optimization process that takes advantage of the flexibilities that become available by implementation of Free Route Airspace. Vertical flight path is shown to play very important role in achieving optimal flights, while horizontal paths also offer optimality possibilities, with a strong dependence on weather conditions.
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International Journal of Turbomachinery Propulsion and Power, 2021
A mean-line compressor performance calculation method is presented that covers the entire operati... more A mean-line compressor performance calculation method is presented that covers the entire operating range, including the choked region of the map. It can be directly integrated into overall engine performance models, as it is developed in the same simulation environment. The code materializing the model can inherit the same interfaces, fluid models, and solvers, as the engine cycle model, allowing consistent, transparent, and robust simulations. In order to deal with convergence problems when the compressor operates close to or within the choked operation region, an approach to model choking conditions at blade row and overall compressor level is proposed. The choked portion of the compressor characteristics map is thus numerically established, allowing full knowledge and handling of inter-stage flow conditions. Such choking modelling capabilities are illustrated, for the first time in the open literature, for the case of multi-stage compressors. Integration capabilities of the 1D code within an overall engine model are demonstrated through steady state and transient simulations of a contemporary turbofan layout. Advantages offered by this approach are discussed, while comparison of using alternative approaches for representing compres-sor performance in overall engine models is discussed.
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Books by Alexios Alexiou
Papers by Alexios Alexiou
that influence the aerothermodynamic performance of compressors and turbines, such as fouling,
tip clearance increase, erosion, variable geometry system malfunction, and object impact damage,
are covered. The signatures of such faults, which are necessary for establishing efficient gas path
diagnostic methods, are studied. They are expressed through mass flow capacity and efficiency
deviations. The key characteristics of the ratio of such deviations are investigated in terms of
knowledge existing in published research. Research based on experimental studies, field data, and
results of detailed fluid dynamic computations that exist today is found to provide such information.
It is shown that although such signatures may be believed to have a unique correspondence to the
type of component fault, this is only true when a particular engine and fault type are considered. The
choice of diagnostic methods by developers should, thus, be guided by such considerations instead
of using values taken from the literature without considering the features of the problem at hand.
by other such models. A tuning method is proposed to circumvent the limitations of its simplified configuration and the lack of detailed geometric data for combustors in literature. In contrast to most similar frameworks, this also provides the model with the ability to simultaneously predict the combustion efficiency and all pollutant emissions of interest (NOx, CO and unburnt hydrocarbons) more effectively by means of implementing a detailed chemical kinetics model. Validation against three correlation methods and actual aeroengine configurations demonstrates accurate performance and emission trend predictions. Integrated within two distinct combustion chamber low-emission preliminary design processes, the proposed model evaluates each new design, thereby displaying the ability to be employed in terms of optimizing a combustor’s overall performance given its sensitivity to geometric changes. Overall, the proposed model proves its worth as a reliable and valuable tool for use towards a greener future in aviation.
combination of compressor and engine design parameters for achieving minimum fuel burn over a specific aircraft mission, while obeying constraints that guarantee operability over the entire flight envelope. Constraints related to compressor stability during transient maneuvers between idle and static take-off conditions and engine temperature limits at maximum take-off are respected by the final design. The results demonstrate the potential for design trade-offs between engine performance at the aircraft mission level and compressor aerodynamic stability.
In the present paper, a tool is described for the preliminary mechanical design of gas turbine disks. A novel formulation is presented, where the disk weight minimization is achieved by maximizing the stresses developed in the disk. The latter are expressed in the form of appropriately defined design and burst margins. The computational capabilities of the tool developed are demonstrated through comparisons to calculations with a higher fidelity tool. The importance of accurately calculating
thermal stresses is demonstrated and the ability of the tool for such calculations is discussed. The potential and efficiency of the tool are illustrated through a proposed re-design of the disks of a well-documented ten-stage compressor. Finally, the integration of the tool into an overall engine design framework is discussed.
that influence the aerothermodynamic performance of compressors and turbines, such as fouling,
tip clearance increase, erosion, variable geometry system malfunction, and object impact damage,
are covered. The signatures of such faults, which are necessary for establishing efficient gas path
diagnostic methods, are studied. They are expressed through mass flow capacity and efficiency
deviations. The key characteristics of the ratio of such deviations are investigated in terms of
knowledge existing in published research. Research based on experimental studies, field data, and
results of detailed fluid dynamic computations that exist today is found to provide such information.
It is shown that although such signatures may be believed to have a unique correspondence to the
type of component fault, this is only true when a particular engine and fault type are considered. The
choice of diagnostic methods by developers should, thus, be guided by such considerations instead
of using values taken from the literature without considering the features of the problem at hand.
by other such models. A tuning method is proposed to circumvent the limitations of its simplified configuration and the lack of detailed geometric data for combustors in literature. In contrast to most similar frameworks, this also provides the model with the ability to simultaneously predict the combustion efficiency and all pollutant emissions of interest (NOx, CO and unburnt hydrocarbons) more effectively by means of implementing a detailed chemical kinetics model. Validation against three correlation methods and actual aeroengine configurations demonstrates accurate performance and emission trend predictions. Integrated within two distinct combustion chamber low-emission preliminary design processes, the proposed model evaluates each new design, thereby displaying the ability to be employed in terms of optimizing a combustor’s overall performance given its sensitivity to geometric changes. Overall, the proposed model proves its worth as a reliable and valuable tool for use towards a greener future in aviation.
combination of compressor and engine design parameters for achieving minimum fuel burn over a specific aircraft mission, while obeying constraints that guarantee operability over the entire flight envelope. Constraints related to compressor stability during transient maneuvers between idle and static take-off conditions and engine temperature limits at maximum take-off are respected by the final design. The results demonstrate the potential for design trade-offs between engine performance at the aircraft mission level and compressor aerodynamic stability.
In the present paper, a tool is described for the preliminary mechanical design of gas turbine disks. A novel formulation is presented, where the disk weight minimization is achieved by maximizing the stresses developed in the disk. The latter are expressed in the form of appropriately defined design and burst margins. The computational capabilities of the tool developed are demonstrated through comparisons to calculations with a higher fidelity tool. The importance of accurately calculating
thermal stresses is demonstrated and the ability of the tool for such calculations is discussed. The potential and efficiency of the tool are illustrated through a proposed re-design of the disks of a well-documented ten-stage compressor. Finally, the integration of the tool into an overall engine design framework is discussed.