bluuebunny/crossref_metadata_2025 huggingface dataset:
Design Considerations for High Pressure Turbo Expanders Supported by Active Magnetic Bearings
Authors: Innocenti Alice , Cangioli Francesco , Iurisci Giuseppe , Biliotti Davide
Published: November 2019

Nowadays industrial processes are going toward high power density solutions.

This can be read as a general increase in pressure (more than 200 barA) and fluid density indicating Turbo Expander (TEX) as a worthwhile solution to boost main process compressors and increasing process efficiency.

On the other hand, inlet high pressure and high density fluids could lead to strong pressure pulsations on the impeller with consequences on the rotordynamic behavior, rotor stability and bearing integrity.

For this purpose, a full annulus unsteady CFD analysis of the flow path through the inlet plenum, nozzles and impeller has been performed and analyzed in terms of resulting force FFT on the impeller.

The force signal is used as an input of the rotor model supported by Active magnetic bearings (AMB), including their control system and transfer function.

The aim of the study is the investigation of the influence of high density gas in TEX applications, coupling the results of unsteady CFD analysis with the dynamic response of the mechanical system.

The control system parameters of magnetic bearings are included into the analysis and a sensitivity to different inlet nozzles numbers and inlet pressure levels is performed verifying the feasibility of the rotor.
Effects of Wakes on Blade Endwall Heat Transfer in High Turbulence Intensity
Authors: Park Sehjin , Sohn Ho-Seong , Cho Hyung Hee , Moon Hee Koo , Han Yang Seok , Ueda Osamu
Published: November 2019

Different types of vortices, such as horseshoe vortex, passage vortex, corner vortex, cause high heat transfer distributions and complex heat transfer characteristics at the endwall of turbine blades. In addition, the endwall heat transfer is also affected when the main flow is highly turbulent and wakes are generated by the trailing edge of the vane. Detailed heat transfer measurements are necessary to protect the blades under harsh and complex flow conditions. Therefore, this study investigated the heat transfer characteristics on the blade endwall under flow conditions that simulate high turbulence intensity of the main flow and the generation of wakes by the trailing edge of the vane. The endwall heat transfer was measured using the naphthalene sublimation method. A turbulence generating grid was installed in a linear cascade to simulate the main flow with high turbulence intensity and a wake generator with a rod bundle was used to simulate the wakes generated by the trailing edge of the vane. In the case of high turbulence intensity without wakes, the main flow with high turbulence intensity (Turbulence intensity, T.I = 7.5%) had little impact on the effect of the horseshoe vortex and passage vortex on the heat transfer characteristics. However, increasing turbulence caused the endwall heat transfer to decrease near the pressure side of the blade and increase near the suction side of the blade. On the other hand, the wakes resulted in heat transfer characteristics similar to high turbulence intensity, but decreased heat transfer by horseshoe vortex and passage vortex. The endwall heat transfer distributions were similar regardless of the turbulence intensity (T.I = 1.2%, 7.5%) in the cases with wakes (Rod passing Strouhal number, S = 0.3). This means that the flow condition of S = 0.3 has a more significant influence on the endwall heat transfer than that of T.I = 7.5%.
Influence of an Additional Indexing Rotary Axis on Wire EDM Performance for the Automated Manufacture of Fir Tree Slots
Authors: Bergs T. , Tombul U. , Herrig T. , Klink A. , Welling D.
Published: November 2019

The demand for higher efficiency in aircraft propulsion engines leads to materials with increasing thermo-mechanical strengths and new designs inducing filigree geometries of blisks and disks. Because of new designs which induce tighter tolerances, the high mechanical process forces in conventional cutting processes like broaching cause inacceptable geometrical deviations and high tooling costs. Due to the electro-thermal material removal mechanism, electrical discharge machining (EDM) ensures a force free and thus precise machining.

The manufacture of fir tree slots in nickel-based alloys by wire EDM has been investigated in the last few years and the process was verified as an alternative technology for broaching. To get a better competitive position, the productivity can be prospectively increased by using an additional indexing rotary axis which ensures a precise and automated production of rotationally symmetric components and reduce production times e.g. for the manufacture of fir tree slots on a disk. Nevertheless, the application of these axes cause changed flushing conditions and can also affect the electrical contacting as well. Both influence the process performance and demand a technology development or adjustment of standard machining technologies. The influence of these changed machining conditions has not been investigated scientifically to date. In this paper, the surface integrity and process performance of fir tree slots machined by wire EDM on the machine table are compared with the manufacture by using an additional indexing rotary axis. The results of the investigations are supposed to create a basis for technology adaptions when using additional axes in wire EDM.
Critical Speeds Analysis for Dual Rotor Turbofan Engine Using Finite Element Method
Authors: Sun Kai , Wan Zhao , Song Huiying , Wang Shaohui
Published: November 2019

Intershaft bearing is widely adopted in dual rotor turbofan aircraft engines. Since this kind of dual rotor system has two different rotor speeds and the intershaft bearing leads to the coupling between HP rotor and LP rotor, the calculation of the critical speeds is much more complicated than that of the rotor systems without intershaft bearing. Compared to a single rotor system, the dual rotor system has more critical speeds which can be classified as critical speeds excited by HP rotor and that by LP rotor. In the paper, a finite element rotor model of a high-bypass turbofan jet engine with intershaft bearing is established for the study of critical speeds analysis. The general axisymmetric element is used to model the shafts and disks, and the blades are simplified to mass points. The main bearings including the intershaft bearing are set up with spring element. Assuming that the rotational speed ratio of the two rotors for the dual rotor system is a fixed number, the critical speeds are calculated using three methods based on the finite element rotor model. For the first method, the system critical speeds are obtained directly by Campbell diagram based on QR damped solution method. Then the synchronous unbalance response analyses are carried out and the rotor critical speeds are derived from the amplitude-frequency curves. For the last method, multiple group Campbell diagram analyses are conducted. With one rotor speed fixed at constant rpm N, we can change the speed of the other rotor to obtain one group of critical speeds. By varying speed N of the two rotors, a critical speeds data set can be obtained and plotted as a dual rotor critical speed map. The critical speeds can be easily extracted from the critical speed map according to the rotational speed curve of the engine. The study shows that the dual rotor system critical speeds calculated from above three methods are identical. For the first two methods, the rotational speed ratio of two rotors must be a known and fixed number, which is impossible in reality. The third proposal has no rotation speed relation restriction for rotors, and therefore is recommended for analyzing the critical speeds of aircraft engines with intershaft bearing.
Effects on Heat Transfer Coefficient and Adiabatic Effectiveness in Combined Backside and Film Cooling With Short-Hole Geometry
Authors: La Rosa Renzo , Pandit Jaideep , Ng Wing , Barker Brett
Published: November 2019

Heat transfer experiments were done on a flat plate to study the effect of internal counter-flow backside cooling on adiabatic film cooling effectiveness and heat transfer coefficient. In addition, the effects of density ratio (DR), blowing ratio (BR), diagonal length over diameter (L/D) ratio, and Reynolds number were studied using this new configuration. The results are compared to a conventional plenum fed case. Data were collected up to X/D = 23 where X = 0 at the holes, an S/D = 1.65 and L/D = 1 and 2. Testing was done at low L/D ratios since short holes are normally found in double wall cooling applications in turbine components. A DR of 2 was used in order to simulate engine-like conditions and this was compared to a DR of 0.92 since relevant research is done at similar low DR. The BR range of 0.5 to 1.5 was chosen to simulate turbine conditions as well. In addition, previous research shows that peak effectiveness is found within this range. Infrared (IR) thermography was used to capture temperature contours on the surface of interest and the images were calibrated using a thermocouple and data analyzed through MATLAB software. A heated secondary fluid was used as ‘coolant’ in the present study. A steady state heat transfer model was used to perform the data reduction procedure. Results show that backside cooling configuration has a higher adiabatic film cooling effectiveness when compared to plenum fed configurations at the same conditions. In addition, the trend for effectiveness with varying BR is reversed when compared with traditional plenum fed cases. Yarn flow visualization tests show that flow exiting the holes in the backside cooling configuration is significantly different when compared to flow exiting the plenum fed holes. We hypothesize that backside cooling configuration has flow exiting the holes in various directions, including laterally, and behaving similar to slot film cooling, explaining the differences in trends. Increasing DR at constant BR shows an increase in adiabatic effectiveness and HTC in both backside cooling and plenum fed configurations due to the decreased momentum of the coolant, making film attachment to the surface more probable. The effects of L/D ratio in this study were negligible since both ratios used were small. This shows that the coolant flow is still underdeveloped at both L/D ratios. The study also showed that increasing turbulence through increasing Reynolds number decreased adiabatic effectiveness.
Impact of Forecast Uncertainty on Wind Farm Profitability
Authors: Frate Guido Francesco , Ferrari Lorenzo , Desideri Umberto
Published: November 2019

The great amount of support schemes that initially fueled the fast, and often uncontrollable, Renewable Energy Sources (RESs) growth have been strongly reduced or revoked in many countries. Currently, the general trend is to try to equate the RESs to the traditional power plants. From the energy market point of view, this entails exposing the RESs more to the market competition and mechanics. This could be done, for example, requiring the stochastic RESs to submit a production schedule in advance and to be financially responsible for any deviation from this. This could push the Wind Farm (WF) operators to make accurate forecasts, fostering the electric system resiliency and an efficient use of balancing resources. From the forecasting point of view this is not a trivial problem, since the schedule submission is often due 10–12 hours before the actual delivery. Since forecast errors are unavoidable, the submitted schedule could turn out to be infeasible, forcing the WF to recur to correcting actions which are generally costly. Focusing on this, the analysis estimates the revenue reduction which would affect a WF operating in the energy market due to forecast errors. To do this in a realistic way, a case study is selected, and realistic forecast scenarios are generated using a copula approach. Important forecast error features like autocorrelation and dependency on forecasted power level and forecast lead-time are modeled. The revenue reduction due to balancing actions is calculated on an annual basis, using typical days, derived through the production data clustering. Losses ranging from 5% to 35% has been found, depending on the days and on the market prices. A sensitivity analysis to the costs of balancing actions is performed. In this way, the effect of different market architectures and, possibly, of different RESs penetration level is considered in the analysis. Finally, the effectiveness of the curtailment as a technique to reduce the impact of forecast errors in highly penalizing market environments is assessed.
A New Practical Approach to the Design of Industrial Axial Fans: Part I — Tube-Axial Fans With Very Low Hub-to-Tip Ratio
Authors: Masi Massimo , Lazzaretto Andrea
Published: November 2019

This paper presents a simple but complete design method to obtain arbitrary vortex design tube-axial fans starting from fixed size and rotational speed. The method couples the preliminary design method previously suggested by the authors ago with an original revised version of well-known blade design methods taken from the literature. The aim of this work is to verify the effectiveness of the method in obtaining high efficiency industrial fans. To this end, the method has been applied to a 315mm rotor-only tube-axial fan having the same size and rotational speed, and a slightly higher flow rate coefficient, as another prototype previously designed by the authors, which was demonstrated experimentally to noticeably increase the pressure coefficient of an actual 560mm industrial fan. In contrast, no constraints are imposed on the hub-to-tip ratio and pressure coefficient. The new design features a hub-to-tip ratio equal to 0.28 and radially stacked blades with aerodynamic load distribution corresponding to a roughly constant swirl at rotor exit. The ISO-5801 experimental tests showed a fan efficiency equal to 0.68, which is 6% higher than that of the previous prototype. The pressure coefficient is lower, but still 12% higher than that of the benchmark 560mm industrial fan.
A New Practical Approach to the Design of Industrial Axial Fans: Part II — Forward-Swept Blades With Low Hub-to-Tip Ratio
Authors: Masi Massimo , Lazzaretto Andrea
Published: November 2019

The authors previously suggested a simple method to design forward-swept axial-flow rotors with blades having low hub-to-tip and high aspect ratios. This design method was demonstrated experimentally to increase the aeraulic performance of a small tube-axial fan having unswept blades and 0.4 hub-to-tip ratio, while maintaining the efficiency in the entire operation range. However, the method has not yet been assessed by experimental tests of lower hub-to-tip ratio designs where the strong three-dimensionality of the actual blade passage flow could compromise its validity. This assessment is the object of the present paper, which is aimed at examining the practical effectiveness of the forward-swept blade design method for low hub-to-tip ratio tube-axial fans. To this end, past results of the authors’ work are supported here by the design of a new 315mm forward-swept industrial fan derived from the 0.28 hub-to-tip ratio design presented in Part I of this paper. The ISO-5801 aerodynamic performance tests at blade Reynolds number of approximately 60,000 show that the method permits the design of forward-swept industrial fans capable of pressure coefficients in excess of 0.02 at aeraulic efficiency well above 60%, in a wide range of flow rate coefficients and blade positioning angles. Moreover, the method allows obtaining a pressure coefficient equal to 0.021 at 70% maximum efficiency, with an improvement of both the stall margin and stable operation pressure curve of the unswept design, if applied in combination with the complete fan design method presented in Part I of this paper.
Analysis of U.S. Navy Rolls Royce 501-K34 Turbine Engine Removals 2008 to 2018
Authors: Russom Dennis , Patterson Jeffrey , Pineiro Ivan
Published: November 2019

The Rolls Royce 501-K34 gas turbine engine serves as the prime mover in the Ship Service Gas Turbine Generators (SSGTGs) of the U.S Navy’s USS ARLEIGH BURKE (DDG 51) Class Flight I and Flight II ships. At the time of this writing, there are 65 ships and 195 shipboard 501-K34 turbine engines which operate a total of about 400,000 hours per year. Engines periodically require removal from ships for depot repair. This paper discusses the guidelines that govern the removal process then discuss the 156 engine removals that occurred between January 2008 and November 2018.
Analysis of a Combined Cycle Exploiting Inlet Conditioning Technologies for Power Modulation
Authors: Sorce Alessandro , Giugno Andrea , Marino Daniela , Piola Stefano , Guedez Rafael
Published: November 2019

The high share of non-dispatchable renewable energy source generators in the electrical grid has increased the need for flexibility of Gas Turbine Combined Cycles (GTCC) already installed. To maximize not only the maximum power produced, via Power Augmentation Technologies (PATs), but also to reduce the Minimum Environmental Load (MEL), both OEMs and GTCC owners have adopted several technical solutions. This kind of flexibility has become, year-by-year, ever more crucial to guarantee GTCC economical sustainability. Amongst the solutions which can be adapted to guarantee GTCC flexibility, the Inlet Conditioning System is a particularly interesting technical solution, which can be installed without restrictions related to the different GT design.

In this paper, an evaluation of the compressor inlet temperature effect over the Combined Cycle performance is presented, with a focus on the bottoming Cycle impact. Different Inlet Conditioning Strategies are then compared considering the energy, and the environmental impact on GTCC behavior. The performance of a layout including a Thermal Energy Storage (TES) and a Heat Pump (HP) is then evaluated and compared to other technical solutions.
Thermo-Acoustic Analysis of a Realistic Liquid-Fueled GT Combustor
Authors: Graham Owen , Magina Nicholas , Wickersham A. J. , Han Fei , Sorato Sebastiano , Bethke Sven
Published: November 2019

Thermo-acoustic instabilities are an important consideration in the design of modern power generation gas turbine combustors. While the design process must consider many competing requirements, such as temperature profiles, emissions, robustness to auto-ignition and flameholding, thermoacoustics is one of the most challenging to predict, and therefore design for. This is particularly true in the case of liquid-fueled systems, where the phenomenon results from a complex system of coupled multi-physics phenomena: fuel atomization and transport, mixing, reactive kinetics and acoustics. Nevertheless, emissions-compliant liquid fuel capability is becoming increasingly important to GT operators, thus it is critical to be able to predict the thermoacoustic instabilities of these combustors.

In this work we present an approach to model the thermoacoustic feedback loop for a realistic liquid fuel nozzle in a single burner configuration. The approach is based on an analytical liquid-fuel diffusion flame model to provide the fluctuating heat release response to inflow perturbations. This is coupled with a 3D FEM description of the acoustic response of the single burner rig through a time-domain Green’s function model to predict the growth and saturation of pressure oscillations. The necessary flame model parameters are calibrated based on a range of test data obtained from the single burner rig with a tunable combustor length. The results are shown to compare well with test data across a range of operating conditions, and for two different nozzle geometries.
Improving Aerodynamic Performance of a Highly Loaded Compressor Based on Coanda Jet Flap
Authors: Du Juan , Li Yiwen , Li Zhihui , Li Jichao , Wang Zinan , Zhang Hongwu
Published: November 2019

A novel blowing flow control technique, called Coanda jet flap, to improve the performance of the highly loaded axial compressor is numerically and experimentally investigated in this study. A theoretical model is initially built to guide the preliminary design of the jet velocity. The design and effectiveness of the Coanda jet flap are then studied for two different blade designs: a highly-loaded double-arc blade cascade and a highly-loaded axial compressor stator. The optimal aerodynamic shape of the Coanda jet flap is achieved based on the genetic algorithm in conjunction with the artificial neural networks. A comparison of the flow details between the optimization result and the prototype is then conducted to understand the flow mechanisms responsible for the performance enhancement. The numerical and experimental results show that the Coanda jet effectively improves the aerodynamic performance of the highly loaded cascade. The total pressure loss is reduced by a maximum of 22.4% when the normalized mass flow rate of Coanda jet is equal to 1%. Meanwhile, the static pressure rise coefficient is increased by up to 14% with a 1.5% jet mass flow rate. The cantilevered stator vanes with Coanda jet flap is well-designed and embedded in a 1.5-stage highly loaded axial compressor stage. The numerical data shows that the total number of stator blades can be successfully reduced by 35% with the uncompromised overall performance when the Coanda jet flap is applied.
The Effect of Heat Transfer on Turbine Performance
Authors: Jardine Lachlan J. , Miller Robert J.
Published: November 2019

For over 50 years, high-pressure gas turbine blades have been cooled using air bled from the compressor. This cooling results in very high rates of heat transfer, both within the fluid and within the blade. The heat transfer often occurs across large differences in temperature and thus is highly irreversible. It is therefore surprising that little is understood about the effect of this heat transfer on turbine performance. This paper solves this problem by applying a new method known as mechanical work potential, or euergy, analysis. The key consequence of the analysis is that the value placed on all heat, relative to work, becomes set by the Joule (Brayton) cycle efficiency. This means that when heat is transferred locally within a flow, or when viscous reheat occurs, the value of this heat should be set by the Joule cycle efficiency. This paper demonstrates how the new method can be implemented both in the preliminary design systems and in the analysis of conjugate CFD solutions of complex engine representative components. The new method provides the cooling designer with a new way of raising turbine efficiency, a form of recuperation locally in the flow. This method offers the exciting potential to design cooling systems that, when added to a blade profile, actually reduces profile loss by up to 7.3%.
Steady Conjugate Heat Transfer Method for High Temperature Gradient in Turbomachinery Applications
Authors: Salem Rami , Marty Julien , Errera Marc
Published: November 2019

The high-pressure (HP) turbine and compressor blades are subjected to severe aero-thermal loads either caused by high inlet temperature resulting from the combustion chamber or by the shock wave at the blade surface. In this paper, a partitioned conjugate heat transfer (CHT) approach is used to assess 3D cases with high temperature gradient. On one hand, the high temperature resulting from the combustion chamber generates high azimuthal and radial non uniformities on the nozzle guide vane (NGV), known as hot streak. Since the engine efficiency is directly related to the turbine inlet temperature, manufacturers are seeking to improve thermal barrier coatings (TBC) to allow higher temperature. On the other hand, compressor rotor blades are also subjected to high thermal loads caused by the shock waves. Since the shock wave substantially contributes to the heat transfer, an accurate prediction of the wall temperature is thus required in order to properly estimate the rotor efficiency. However, the thermal load gradient caused by the hot streak, the use of thermal barrier coating or the shock wave have an impact on the overall computation stability. This paper assess the stability for a Dirichlet-Robin interface condition. Indeed an optimal relaxation parameter value is given for the Robin condition to provide a stable CHT computation. The impact on stability of the transient coupling time-step, the coupling frequency and the thermal conductivity is also assessed and the new transient coupling time-step defined allows to correctly stabilize the coupled simulation.
Impact of Different Volume Sizes on Dynamic Stability of a Gas Turbine Fuel Cell Hybrid System
Authors: Abrassi Alessio , Traverso Alberto , Tucker David , Liese Eric
Published: November 2019

A dynamic model is developed for a Micro Gas Turbine (MGT), characterized by an intrinsic free-spool configuration, coupled to large volumes. This is inspired by an experimental facility at the National Energy Technology Laboratory (NETL) called Hyper, which emulates a hybrid MGT and Fuel Cell system. The experiment and model can simulate stable and unstable operating conditions. The model is used to investigate the effects of different volumes on surge events, and to test possible strategies to safely avoid or recover from unstable compressor working conditions. The modelling approach started from the Greitzer lumped parameter approach, and it has been improved with integration of empirical methods and simulated components to better match the real Hyper plant layout and performance. Pressure, flow rate, and frequency plots are shown for the surge behavior comparing two different volume sizes, for cases where gas turbine shaft speed is uncontrolled (open loop) and controlled (closed loop). The ability to recover from a surge event is also demonstrated.
Physics-Based Part Orientation and Sentencing: A Solution to Manufacturing Variability
Authors: Lee Wen Yao , Dawes William N. , Coull John D.
Published: November 2019

Casting deviations introduce geometric variability that impacts the aerodynamic performance of turbomachinery. These effects are studied for a High Pressure Turbine (HPT) rotor blade from a modern aero-engine. 197 blades were measured using three-dimensional structured-light scanning (GOM scanning), and the performance of each blade is quantified using Reynolds-Averaged Navier-Stokes (RANS) simulations.

Casting variation is typically managed by applying geometric tolerances to determine the suitability of a component for service. The analysis demonstrates that this approach may not be optimal since it does not necessarily align with performance, in particular the capacity and efficiency. Alternatively, functional acceptance based on the predicted performance of each blade removes the uncertainty associated with geometric tolerancing and gives better performance control.

Building on these findings, the paper proposes a method to set the orientation of the fir-tree, which is machined after casting. By customizing the alignment of each blade, performance variability and scrap rates can be significantly reduced. The method uses predictions of performance to reorient the castings to compensate for the manufacturing-induced errors, without changing the design-intent blade geometry and with minimal changes to the manufacturing facility.
Time Domain Bloch Boundary Conditions for Efficient Simulation of Thermoacoustic Limit-Cycles in (Can-)Annular Combustors
Authors: Haeringer Matthias , Polifke Wolfgang
Published: November 2019

Thermo-acoustic eigenmodes of annular or can-annular combustion chambers, which typically feature a discrete rotational symmetry, may be computed in an efficient manner by utilizing the Bloch-wave theory. Unfortunately, the application of the Bloch-wave theory to combustion dynamics has hitherto been limited to the frequency domain.

In this study we present a time domain formulation of Bloch boundary conditions (BBC), which allows to employ them in time domain simulations, e.g. CFD simulations. The BBCs are expressed as acoustic scattering matrices and translated to complex-valued state-space systems. In a hybrid approach an unsteady, compressible CFD simulation of the burner-flame zone is coupled via characteristic-based state-space boundary-conditions to a reduced order model of the combustor acoustics that includes BBCs. The acoustic model with BBC accounts for cross-can acoustic coupling and the discrete rotational symmetry of the configuration, while the CFD simulation accounts for the nonlinear flow-flame-acoustic interactions. This approach makes it possible to model limit cycle oscillations of (can-)annular combustors at drastically reduced computational cost compared to CFD simulations of the full configuration, and without the limitations of weakly nonlinear approaches that utilize a flame describing function.

In the current study the suggested approach is applied to a generic multi-can combustor. Results agree well with a fully compressible CFD simulation of the complete configuration.
Reducing Compressor Shroud Leakage Flows by Raising the Stator Hub Line: Low Speed Tests
Authors: Schrapp Henner , Dodegge Arne , Gümmer Volker , Harvey Neil W. , Städing Jörn , Friedrichs Jens
Published: November 2019

The paper describes experimental investigations of an alternative shrouded stator concept in a 2.5 stage low speed compressor. The idea of this new concept is to raise the stator hub line by a small amount, thus decelerating the flow upstream of the shroud cavity due to the into wind step and raising the static pressure. Downstream of the cavity the out of wind step changes the streamline curvature thus lowering the static pressure locally. As a result, the static pressure difference across the shroud is lower and the shroud flow is reduced. Tests were done at three seal gap heights under stator 1, both with a “neutral” (in–line) hub and a six percent “bump shroud”, i.e. the hub is raised by six percent annulus height.

Performance measurements show the impact of the “bump shroud” geometry on the overall behavior of the compressor, i.e. efficiency and pressure ratio and the variation of these quantities with varying seal gap height. While the efficiency and pressure ratio of the compressor inevitably reduce with increasing seal gap height, the sensitivity of both is reduced by using “bump shrouds”. At small seal gap heights the “bump shroud” design behaves similarly to the neutral one, while at the design seal gap height it is superior. Thus, both the efficiency and the pressure ratio are less sensitive against seal gap height variations if the compressor is equipped with a raised hub line — leading to a more robust product. A similar behavior is seen at near stall conditions.

The analysis of five hole probe measurements reveals the reason for the improved efficiency. The stator 1 losses were significantly reduced by the introduction of the “bump shroud”. This is mostly due to the reduced amount of shroud flow and the subsequent reduction of hub cross–flow in the stator. A comparison of losses with and without the raised hub line show not only a reduction of the losses near the hub, but also adjacent to the suction side of the stator due to reduced migration of hub boundary layer fluid onto the vane.
Impact of Fluid Substitution on the Performance of an Axial Compressor Blade Cascade Working With Supercritical Carbon Dioxide
Authors: Muñoz Alejandro , Sánchez David , Savill Mark , Kipouros Timoleon , Tello-Castillo Carlos
Published: November 2019

Recent research on turbomachinery design and analysis for supercritical Carbon Dioxide (sCO2) power cycles has relied on Computational Fluid Dynamics. This has produced a large number of works whose approach is mostly case-specific, rather than of general application to sCO2 turbomachinery design.

As opposed to such approach, this work explores the aerodynamic performance of compressor blade cascades operating on air and supercritical CO2 with the main objective to evaluate the usual aerodynamic parameters of the cascade for variable boundary conditions and geometries, enabling ‘full’ or ‘partial’ similarity. The results present both the global performance of the cascades and certain features of the local flow (trailing edge and wake). The discussion also highlights the mechanical limitations of the analysis (forces exerted on the blades), which is the main restriction to applying similarity laws to extrapolate the experience gained through decades of work on air turbomachinery to the new working fluid.

This approach is a step towards the understanding and appropriate formulation of a multi-objective optimisation problem for the design of such turbomachinery components where sCO2 is used as the operating fluid. With this objective, the paper aims to identify and analyse what would be expected if a common description of such computational design problems similar to those where air is the working fluid were used.
Conceptual Flutter Analysis of Stepped Labyrinth Seals
Authors: Corral Roque , Vega Almudena , Greco Michele
Published: November 2019

A simple non-dimensional model to describe the flutter onset of two-fin straight labyrinth seals [1] is extended to stepped seals. The effect of the axial displacement of the seal is analyzed first in isolation. It is shown that this fundamental mode is always stable. In a second step, the combination of axial and torsion displacements is used to determine the damping of modes with arbitrary torsion centers. It is concluded that the classical Abbot’s criterion stating that seals supported in the low-pressure side of the seal are stable provided that natural frequency of the mode is greater than the acoustic frequency breaks down under certain conditions. An analytical expression for the non-dimensional work-per-cycle is derived and new non-dimensional parameters controlling the seal stability identified. It is finally concluded the stability of stepped seals can be assimilated to that of a straight through seal if the appropriate distance of the torsion center to the seal is chosen.
Non-Dimensional Parameters for Comparing Conventional and Counter-Rotating Turbomachines
Authors: Waldren J. J. , Clark C. J. , Grimshaw S. D. , Pullan G.
Published: November 2019

Counter-rotating turbomachines have the potential to be high efficiency, high power density devices. Comparisons between conventional and counter-rotating turbomachines in the literature make multiple and often contradicting conclusions about their relative performance. By adopting appropriate non-dimensional parameters, based on relative blade speed, the design space of conventional machines can be extended to include those with counter-rotation. This allows engineers familiar with conventional turbomachinery to transfer their experience to counter-rotating machines. By matching appropriate non-dimensional parameters the loss mechanisms directly affected by counter-rotation can be determined.

A series of computational studies are performed to investigate the relative performance of conventional and counter-rotating turbines with the same non-dimensional design parameters. Each study targets a specific loss source, highlighting which phenomena are directly due to counter-rotation and which are solely due to blade design. The studies range from two-dimensional blade sections to three-dimensional finite radius stages. It is shown that, at hub-to-tip ratios approaching unity, with matched non-dimensional design parameters, the stage efficiency and work output are identical for both types of machine. However, a counter-rotating turbine in the study is shown to have an efficiency advantage over a conventional machine of up to 0.35 percentage points for a hub-to-tip ratio of 0.65. This is due to differences in absolute velocity producing different spanwise blade designs.
A Step Towards an Intelligent Aerodynamic Design Process
Authors: Clark Christopher J.
Published: November 2019

A new type of section inverse design is presented that offers a 200 times speed up over conventional iterative methods. The method parametrises a blade section in terms of its aerodynamic duty and design style. The duty is defined by the inlet and outlet flow angles, and the Mach and Reynolds numbers at which the blade must operate. The aerodynamic style is represented by a normalised Mach number distribution, parametrised by four control variables. This is found to provide a wide range of desirable loading distributions.

For a given style and duty, the method obtains the section geometry and its performance using a radial basis function network. This network is trained, a priori, on a database of designs produced using a conventional inverse design method. The database covers a range of inlet and exit angles for a range of Mach numbers, at each duty aerofoil families are produced varying the four style parameters. During the database production the radial basis function (RBF) network is sequentially refitted and used to accelerate future designs convergence by providing an increasingly better initial guess.

Errors in prediction accuracy, of the fully trained system, are calculated by evaluating the obtained geometry and comparing its aerodynamic performance with the intent. The average exit flow angle error was calculated to be 0.074° from the intent. The root-mean-squared error between intent and achieved Mach fraction distributions averaged below 0.0275. The loss prediction featured average errors of 0.012% and hence provides a very accurate method of performance estimation.

It is highlighted that not all combinations of aerodynamic duty and design style are compatible. “Infeasible” regions within the design space where the aerodynamic intent is not possible exist. A method is applied to avoid these regions, where section errors are above average, ensuring accurate section generation.
Simulation, Test and Mitigation of 1/2X Forward Whirl Following Rotor Drop Onto Auxiliary Bearings
Authors: Kang Xiao , Palazzolo Alan
Published: November 2019

1/2X forward whirl repeatedly occurred after a test rotor spinning at 5,800 rpm was dropped onto ball bearing type auxiliary bearings AB, utilized as a backup for magnetic bearings. The measured contact forces that occurred between the rotor and the auxiliary bearing during the ½X subsynchronous vibration were about thirteen times larger than the static reaction force. The vibration frequency coincided with the rotor-support system natural frequency with the rotor at rest on the auxiliary bearing AB, an occurred at ½ of the rotor spin speed when dropped. The test rig provided measurements of rotor-bearing contact force, rotor orbit (vibrations), and rotational speed during rotor drop events. A simulation model was also developed and demonstrated that parametric excitation in the form of a Mathieu Hill model replicated the measured 1/2X forward whirl vibrations. The simulation model included a nonlinear, elastic-thermal coupled, ball bearing type auxiliary bearing model. The transient model successfully predicted the 1/2X vibration when the rotor was passing 5800RPM as well, and the simulation results quantitatively agreed well with the test results in the frequency domain. Several approaches for mitigating the 1/2X forward whirl were presented such as adding an elastomer O-ring or waviness spring in the AB support system. Measurements confirmed that adding AB dampers effectively mitigated the ½ subsynchronous forward whirl and significantly reduced the contact forces.
Retrofittable Solutions to Keep Existing Gas Turbine Power Plants Viable and Profitable in an Increasingly Dynamic Power Generation Market: Validation of Low Pressure Drop FlameSheet™ Combustor
Authors: Hernandez Fred , Rizkalla Hany
Published: November 2019

As renewable energy sources continue their global energy market penetration, new natural gas fired power plant installations have decreased significantly. The reduction in new installed capacity has increased pressure on operators to profitably maintain and expand their existing fleet capability. Retrofitting existing gas turbines to increase baseload power output, expand fuel flexibility and provide a wider operating load range are key natural gas fired power plant market demands. The FlameSheet™ combustor system addresses these considerations with a novel “dual-zone burn system” design that reduces emissions, increases fuel flexibility and reduces pressure losses to improve thermal cycle efficiency. The present work presents the results of FlameSheet™ installations into GE 7F.03 heavy duty gas turbines at two commercial sites. The first installation combined FlameSheet™ with PSM’s Gas Turbine Optimization Package (GTOP) to provide higher output through a combination of lower combustor pressure drop, higher mass flows and an increase in firing temperature, while maintaining sub-9ppm NOx emissions across the expanded operating range. Results are also presented for a second site on a unit that operates with up to 5% hydrogen blend into the baseline natural gas, where a reduction in NOx to sub-4 ppm levels at a typical 7F.03 baseload point has been safely and reliably achieved. Both results continue to demonstrate that fuel flexibility and expanded operational windows are possible to “future proof” existing gas turbine installations at a fraction of the cost of a new unit installation.
Evaluating the Effects of Transient Purge Flow on Stator-Rotor Seal Performance
Authors: Berdanier Reid A. , DeShong Eric T. , Thole Karen A.
Published: November 2019

As modern engine designs target higher efficiencies through increased turbine inlet temperatures, critical turbine components are at increased risk of damage from conditions exceeding material melting temperatures. In particular, improperly designed underplatform hardware components are susceptible to damage when hot main gas path flow is ingested into the stator-rotor cavity. While all turbines inherently experience transients during operation, a majority of turbine tests have been executed using steady operating conditions, and routine transient events are not well understood. To address this need, the present study utilizes a continuous-duration, one-stage test turbine operating with true-scale engine hardware and seal geometries at engine-representative flow conditions. The nature of the continuous-duration facility uniquely supports direct assessment of transient events through its ability to transition between steady-state operating conditions. Specifically, the effects of a transient purge flow were investigated in this study to identify general trends for transient events in a full-scale engine. Results from multiple measurement techniques in the wheelspace region show an interdependence of transient purge flow with a thermal lag of the underplatform hardware. Through experiments conducted at different coolant-to-main gas path temperature ratios, the use of pressure measurements as an indicator of fully-purged behavior was introduced, and a thermally-driven influence on rim seal performance was quantified.
Efficient Passage-Spectral Method for Unsteady Flows Under Stall Conditions
Authors: Romera David , Corral Roque
Published: November 2019

This paper presents an efficient method of approximating unsteady flows using a blockwise discrete spatial Fourier series for the modeling of three-dimensional non-axisymmetric flows without making any hypothesis about its temporal periodicity. The method aims at capturing the long wavelength flow patterns which are present in many unsteady problems of industrial interest, such as compressor stability, with a drastic reduction in computational resources. The method is intended to be used to compute flows exhibiting large-scale instabilities and where the fundamental frequency of the problem is not known beforehand. The approach discretizes the domain using a finite number of blocks or passages, where the flow variables at the supposedly periodic boundaries are continuously updated using the spatial Fourier coefficients of a uniformly spaced set of reduced-passage domains. The NASA rotor 67 under stall conditions has been used as verification validation case to demonstrate the effectiveness and viability of the proposed modeling strategy. The comparison between the solutions obtained with the discrete Fourier series and the full-annulus solution shows that accurate solutions can be obtained with a low number of harmonics. The new method has been applied to investigate the rotating stall inception of the NASA rotor 67 for clean and distorted inlet flow near stall operating conditions. The method is shown to accurately reproduce the full-annulus solution with a few spatial harmonics, capturing the characteristic features of the complex flow induced by the tip leakage vortex breakdown. The computational cost in this application has been reduced by a factor of between three and seven, although this number heavily depends on the ratio between the number of retained harmonics and the number of blades.
Validation of CFD Analyses Results With Instrumented Ground and Flight Tests Measurements of an Aerobatic Turboprop Aircraft Inlet
Authors: Pitel Welnitz Daniela , Vlach Tomas , Lombardi Luca , Faltot Pierre-Jean , Vertenoeuil Philippe , D’Ercole Michele
Published: November 2019

With the onset of affordable high-performance computing and cloud computing technologies, numerical methods like computational fluid dynamics and the finite element method are spreading to all the branches of industry. The ability to perform such analyses allows engineers to speed up development and refine designs even in early stages of development without the need for expensive experimental setups. However, it is desirable to build up confidence in these results by experimental validation. GE Aviation Czech has been extensively using a variety of CFD tools to predict flow behavior in the field of internal aerodynamics — especially for design of compressor and turbine stages, combustion chambers, inlet and exhaust systems. Development of a new aerobatic engine prototype and its installation is currently under way. A series of back-to-back analyses conducted to verify performance of the aerobatic inlet and compare it with a reference commuter engine installation. The results were presented in article “GT2018-76398 Analysis and Testing of an Aerobatic Turboprop Aircraft Inlet”. A variety of critical parameters such as ram factor, pressure losses, inlet heating, and engine inlet pressure and temperature distortions were investigated. These parameters affect engine performance, operability, and compressor stability. Afterward, a unique instrumentation, data acquisition chain, and test program was developed in cooperation with the airframer. This made it possible to record all of the required parameters during ground and flight tests, including basic aerobatic maneuvers. The recorded data was then used for validation of performed CFD analyses with boundary conditions matching the conditions of ground and flight tests. Confidence in CFD results backed up by well-matched test data allows to conduct future design changes without the need to perform additional expensive experimental trials.