bluuebunny/crossref_metadata_2025 huggingface dataset:
Numerical Investigation of the Interaction Between Upstream Purge Flow and Mainstream in a Highly-Loaded Turbine
Authors: Jia Wei , Liu Huoxing
Published: September 2014

In modern high pressure turbine, a certain amount of cooling air is bled off from compressor and then directly injected into the inter-stage gap between the stationary and rotating components. This paper presents a computational study of the interaction between mainstream flow and purge flow, with the objectives of evaluating the impacts of purge flow on turbine aerodynamic design parameters, tracing the loss sources involved with the injection of purge flow and describing the secondary flows near the hub region. The purge flow through the rim seal has been varied between 0–2% of the main flow and the axial position of rim seal has been also changed. Steady-state simulation using a 3D RANS solver is presented with particular emphasis on the mechanisms of loss production.

It is found that purge flow has a primary effect on the spanwise distribution of turbine aerodynamic design parameters, especially near the hub region. The losses brought about by the injection of purge flow can be divided into four parts: reaction redistribution between vane and blade in one stage, a shear layer between purge flow and mainstream flow due to different circumferential momentum, hub passage vortex interaction and decrease of output work near the end wall. However these four loss sources are not independent of each other. Shear induced vortex (SIV) and slot leakage vortex (SLV) appear near the hub region after purge flow is introduced. The shear induced vortex is formed due to the shear interaction between mainstream flow and purge flow which develop into hub passage vortex. The slot leakage vortex is formed due to the relative motion of the cavity disks and its strength is relatively weak compared with the shear induce vortex. The results gained from this paper may give some useful guidelines for turbine aerodynamic design and end wall profile optimization.
Sensitivity Analysis of Autoignition Simulation at Gas Turbine Operating Conditions
Authors: Prause Juliane , Noll Berthold , Aigner Manfred , Syed Khawar
Published: September 2014

The demand to reduce CO2 emissions favors the use of alternative hydrogen-rich fuels, which can stem from pre-combustion carbon capture or power-to-gas technologies. These fuels are characterized by a higher reactivity and reduced ignition delay time compared to natural gas. Therefore, current combustor designs need to be adapted to the new requirements. Numerical modeling greatly assists the further development of such systems. The present study aims to determine how far a sophisticated combustion CFD method is able to predict autoignition at real engine conditions.

Scale-resolving computations of autoignition were performed at elevated pressure (15 bar) and intermediate temperatures (> 1000 K). The conditions are similar to those occurring in premixing ducts of reheat combustors. A nitrogen-diluted hydrogen jet is injected perpendicularly into a stream of hot vitiated air. The scale-adaptive simulation method (SAS) as proposed by Menter and co-workers has been applied. The chemistry is captured by direct inclusion of detailed kinetics. Subgrid fluctuations of temperature and species are considered by an assumed probability density function (PDF) approach. The results are compared with appropriate experimental reference data. The focus of the present work is set on the identification of the major sources of uncertainty in the simulation of autoignition. Despite the very challenging operating conditions, satisfactory agreements could be obtained within experimental uncertainties.
Numerical Investigation of Inlet Distortion on the Stall Inception of a Radial Compressor
Authors: Vagnoli S. , Verstraete T.
Published: September 2014

In this paper unsteady CFD computations are used to study the stability and the rotating stall inception of a transonic radial compressor, considering two different configurations influencing the inlet flow. The prediction of the surge and stall limit for a centrifugal compressor is a key parameter for operating such machines, and hence requires attention already in the early design phase. Moreover, the operating margin can be dramatically influenced by non-uniform inlet conditions due to for example bended pipes mounted close to the inlet, as found more often today due to space limitation and optimization. In this case, it is not trivial to understand how the performance of the machine will be affected.

The influence on the stability limit of an elbow installed in front of the compressor inlet is analyzed in this paper by means of unsteady RANS simulations: results for two different configurations, with elbow and with straight inlet placed in front of the compressor, are compared. All simulations are done modeling the whole annulus of the radial machine, using high performance computing, in order to accurately take into account the non-periodic phenomena leading to stall inception. Moreover, mechanisms leading to flow separation for unstable solutions are studied, pointing out how they are influenced by the unsteady interactions of the tip leakage vortices with the main flow and by the distorted flow from the inlet elbow.
Unsteady Effects of a Generic Non-Axisymmetric Rotor Endwall Contour on a 1½ Stage Turbine Test Rig at Off Design Conditions
Authors: Dunn Dwain , Snedden Glen , von Backström Theodor
Published: September 2014

With the current drive to improve fuel efficiency and reduce emissions, in gas turbine engines various methods have been investigated. Previously it has been shown that a generic rotor endwall contour could improve the efficiency of a 1½ stage test turbine at design conditions. The current investigation looked at the increased and decreased loading conditions to determine if the contour introduces detrimental effects at off design conditions.

A previous unsteady analysis of the design condition found that the contoured rotor does have an effect on the flow field, reducing the magnitude of the hub endwall secondary flow region as well as reducing fluctuations in the velocity.

Experimental results showed that the increased load case presented with an increase in hub endwall secondary flow structure when compared to the design case. This increase was to be expected due to the increased turning of the flow due to the increased loading operating condition. The contoured rotor had a weaker hub endwall secondary flow system, with the high momentum flow distributed more in the span wise direction. The variation in the velocity was also found to be smaller for the contoured rotor.

The decreased loading case showed similar improvements, but the extent of the change was less due to the lower turning of the flow (due to a faster rotor).

The numerical results show that the hub endwall secondary flow vortex of the contoured rotor was not as tightly wrapped as that of the annular rotor. The rotor outlet flow was thus more uniform due to the more dispersed vortex system. As seen with the experimental results, the extent of the change due to the contoured rotor changes with loading. The differences present in the decreased loading case being relatively insignificant.

It was concluded that the generic contour does not introduce any unsteady effects at off design conditions that were not observed in the design case.
Hydrogen Enriched Combustion Testing of SIEMENS Industrial SGT-400 at Atmospheric Conditions
Authors: Lam Kam-Kei , Geipel Philipp , Larfeldt Jenny
Published: September 2014

In order to further extend the turbine fuel flex capability, a test under atmospheric conditions of a full-scale SGT-400 burner was performed to study the combustion behavior when operating on hydrogen enriched natural gas.

A high speed camera was installed in the rig to investigate the flame dynamics on different operation conditions. NOx emissions were measured for all presented conditions. The combustion system was instrumented with thermocouples on all the key locations to allow flame position monitoring and to avoid flame attachment on the hardware. Further measurements included static pressure probes to monitor combustor pressure drop.

The test was conducted in a systematic matrix format to include the most important combustion parameters in order to identify their individual effects on the combustion behaviors. The quantity of hydrogen in natural gas, fuel split, air preheat temperature, air reference velocity and flame temperature were the combustion related variables studied in the presented test campaign. The volumetric hydrogen quantity could be increased to 30% maintaining stable operation for all measured conditions. Higher hydrogen contents up to 80 vol-% were reached without flash back tendency. A glowing spark igniter prevented testing at even higher hydrogen contents. Hydrogen enriched gas showed higher NOx emissions and improved blowout limit. Hydrogen blending in the fuel also reduced the combustor pressure drop, lowered the prechamber temperature and raised the pilot tip temperature.
Developing a Reduced-Order Model of Non-Synchronous Vibration in Turbomachinery Using Proper Orthogonal Decomposition Methods
Authors: Clark Stephen T. , Besem Fanny M. , Kielb Robert E. , Thomas Jeffrey P.
Published: September 2014

The paper develops a reduced-order model of non-synchronous vibration (NSV) using proper orthogonal decomposition (POD) methods. The approach was successfully developed and implemented, requiring between two and six POD modes to accurately predict CFD solutions that are experiencing non-synchronous vibration. This POD method was first developed and demonstrated for a transversely-moving, two-dimensional cylinder in cross-flow. Later, the method was used for the prediction of CFD solutions for a two-dimensional compressor blade.

This research is the first to offer a proper orthogonal decomposition approach to the reduced-order modeling of non-synchronous vibration in turbomachinery. Modeling non-synchronous vibration is especially challenging because NSV is caused by complicated, unsteady flow dynamics; this initial study helps researchers understand the causes of NSV, and aids in the future development of predictive tools for aeromechanical design engineers.
Numerical Simulation of Lean Premixed Stagnation Flames
Authors: Lee H. C. , Mohamad A. A. , Jiang L. Y. , Bergthorson J. M. , Salusbury S. D.
Published: September 2014

Lean premixed combustion counterflow model coupled with detailed chemical kinetics mechanisms are used to study the effects of methane dilution on syngas mixture numerically. The work critically evaluates and examines the effects of methane additions at various methane/syngas ratios and various equivalence ratios (φ = 0.6, 0.65, and 0.7) using the CANTERA package. The results of this study increase the feasibility of incorporating syngas mixture diluted with methane based fuel for lean premixed gas turbine engines. This study indicates that the flame velocity of a syngas mixture (H2:CO-75:25) can be reduced to the flame speed of a methane mixture at an equivalence ratio of 0.6 and standard conditions (1 atm and 298 K) by mixing it with 50% of methane. This could reduce the flashback propensity that syngas mixtures will incur due to the significantly high flame speed. Moreover, this study substantiates that the current detailed mechanism is capable of predicting the one-dimensional reacting flows and it matches remarkably well with experimental data. The approach employed herein can be used to optimize and validate detailed chemical kinetics mechanism without the need to correct the flame speed to zero strain or stretch free from the measured datasets.
Rotordynamic Performance of a Negative-Swirl Brake for a Tooth-on-Stator Labyrinth Seal
Authors: Childs Dara W. , Mclean James E. , Zhang Min , Arthur Stephen P.
Published: September 2014

In the late 1970’s, Benckert and Wachter (Technical University Stuttgart) tested labyrinth seals using air as the test media and measured direct and cross-coupled stiffness coefficients. They reported the following results: (1) Fluid pre-swirl in the direction of shaft rotation creates destabilizing cross-coupled stiffness coefficients, and (2) Effective swirl brakes at the inlet to the seal can markedly reduce the cross-coupled stiffness coefficients, in many cases reducing them to zero. In recent years, “negative-swirl” swirl brakes have been employed that attempt to reverse the circumferential direction of inlet flow, changing the sign of the cross-coupled stiffness coefficients and creating stabilizing stiffness forces. This study presents test results for a 16-tooth labyrinth seal with positive inlet preswirl (in the direction of shaft rotation) for the following inlet conditions: (1) No swirl brakes, (2) Straight, conventional swirl brakes, and (3) Negative-swirl swirl brakes. The negative-swirl swirl-brake designs were developed based on CFD predictions. Tests were conducted at 10.2, 15.35, and 20.2 krpm with 70 bars of inlet pressure for pressure ratios of 0.3, 0.4, 0.5.

Test results include leakage and rotordynamic coefficients. In terms of leakage, the negative-swirl brake configuration leaked the least, followed by the conventional brake, followed by the no-brake design. Normalized to the negative-swirl brake configuration, the conventional-brake and no-brake configurations mass flow rate were greater, respectively, by factors of 1.04 and 1.09.

The direct stiffness coefficients are negative but small, consistent with past experience. The conventional swirl brake drops the destabilizing cross-coupled stiffness coefficients k by a factor of about 0.8 as compared to the no-brake results. The negative-swirl brake produces a change in sign of k with an appreciable magnitude; hence, the stability of forwardly-precessing modes would be enhanced.

In descending order, the direct damping coefficients C are: no-swirl, negative-swirl, conventional-swirl. Normalized in terms of the no-swirl case, C for the negative and conventional brake designs are, respectively, 0.7 and 0.6 smaller.

The effective damping Ceff combines the effect of k and C. Ceff is large and positive for the negative-swirl configuration and near zero for the no-brake and conventional-brake designs.

The present results for a negative-brake design are very encouraging for both eye-packing seals (where conventional swirl brakes have been previously employed) and division-wall and balance-piston seals where negative shunt injection has been employed.
Some Aspects on Inlet Blockage Affecting the Performance of a Heavy Duty Gas Turbine’s Exhaust Diffuser
Authors: Schaefer Philipp , Gieß Peter-Anton , Finzel Conrad , Hofmann Willy H.
Published: September 2014

Exhaust diffusers of heavy duty gas turbines significantly improve the performance of gas turbines. These diffusers typically differ from the simple and well investigated annular or conical diffusers because of the incorporated struts. The investigated model of such a typical exhaust diffuser consists of an annular duct with two rows of integrated struts and a Carnot diffuser downstream of the annular part. In this paper the influence of inlet blockage on the performance of gas turbine diffusers is numerically investigated. 3D-RANS calculations are performed applying the k-ω-SST-model. The numerical inlet conditions for the baseline are given by detailed experimental data acquired in a test rig. The results show that the impact of the inlet blockage on the diffuser flow can be split into two aspects, which both influence diffuser pressure recovery. The first aspect is the well-known reduction of effective area of the core flow leading to reduced pressure recovery feasibility. Secondly, it is observed that the inlet blockage has a significant influence on the structure of secondary flow in the vicinity of the diffuser struts affecting the diffuser pressure recovery. Using a simple correlation of the size of the secondary flow and the core flow the influence on pressure recovery is estimated. In sum, the aerodynamic performance of the annular diffuser and the first row of strut is of high importance and mainly depends on the inlet boundary conditions. Accordingly, these geometries need to be designed to their inlet conditions.
New Approach of Whole Engine Structural System Design
Authors: Andoh Masaharu , Honda Tatsuhito , Takamatsu Kikuo
Published: September 2014

Fuel-efficient jet engines are developed by many companies. In the conventional design approach of a jet engine, many parameters are investigated independently. Consequently, a developed jet engine by this approach meets conventional design criteria but there is still room for improvement of engine performance. In this research, the new design approach of a jet engine that integrates these design objectives is developed and performance of a jet engine is evaluated. The goal is to develop the design approach of a fuel-efficient and robust jet engine.

In current design, a jet engine is developed with a focus on not only performance improvement but also robustness. Therefore, Taguchi method is adopted in order to assess robustness. By this method, a jet engine is optimized based on the analysis that minimizes deterioration and variation of SFC. As a result, more efficient engine design is realized with the new design approach that directly deals with SFC.
Shower Head and Trailing Edge Cooling Influence on Transonic Vane Aero Performance
Authors: Saha Ranjan , Fridh Jens , Fransson Torsten H. , Mamaev Boris I. , Annerfeldt Mats , Utriainen Esa
Published: September 2014

An experimental investigation on a cooled nozzle guide vane has been conducted in an annular sector to quantify aerodynamic influences of shower head and trailing edge cooling. The investigated vane is a typical high pressure gas turbine vane, geometrically similar to a real engine component, operated at a reference exit Mach number of 0.89. The investigations have been performed for various coolant-to-mainstream mass-flux ratios. New loss equations are derived and implemented regarding coolant aerodynamic losses. Results lead to a conclusion that both trailing edge cooling and shower head film cooling increase the aerodynamic loss compared to an uncooled case. In addition, the trailing edge cooling has higher aerodynamic loss compared to the shower head cooling. Secondary losses decrease with inserting shower head film cooling compared to the uncooled case. The trailing edge cooling appears to have less impact on the secondary loss compared to the shower head cooling. Area-averaged exit flow angles around midspan increase for the trailing edge cooling.
Accelerated 3D Aerodynamic Optimization of Gas Turbine Blades
Authors: Amtsfeld Philipp , Lockan Michael , Bestle Dieter , Meyer Marcus
Published: September 2014

State-of-the-art aerodynamic blade design processes mainly consist of two phases: optimal design of 2D blade sections and then stacking them optimally along a three-dimensional stacking line. Such a quasi-3D approach, however, misses the potential of finding optimal blade designs especially in the presence of strong 3D flow effects. Therefore, in this paper a blade optimization process is demonstrated which uses an integral 3D blade model and 3D CFD analysis to account for three-dimensional flow features. Special emphasis is put on shortening design iterations and reducing design costs in order to obtain a rapid automatic optimization process for fully 3D aerodynamic turbine blade design which can be applied in an early design phase already.

The three-dimensional parametric blade model is determined by up to 80 design variables. At first, the most important design parameters are chosen based on a non-linear sensitivity analysis. The objective of the subsequent optimization process is to maximize isentropic efficiency while fulfilling a minimal set of constraints. The CFD model contains both important geometric features like tip gaps and fillets, and cooling and leakage flows to sufficiently represent real flow conditions.

Two acceleration strategies are used to cut down the turn-around time from weeks to days. Firstly, the aerodynamic multi-stage design evaluation is significantly accelerated with a GPU-based RANS solver running on a multi-GPU workstation. Secondly, a response surface method is used to reduce the number of expensive function evaluations during the optimization process. The feasibility is demonstrated by an application to a blade which is a part of a research rig similar to the high pressure turbine of a small civil jet engine. The proposed approach enables an automatic aerodynamic design of this 3D blade on a single workstation within few days.
Experimental Study on Pressure Losses in Circular Orifices for the Application in Internal Cooling Systems
Authors: Binder Christian , Kinell Mats , Utriainen Esa , Eriksson Daniel , Bahador Mehdi , Kneer Johannes , Bauer Hans-Jörg
Published: September 2014

The cooling air flow in a gas turbine is governed by the flow through its internal passages and controlled by restrictors such as circular orifices. If the cooling air flow is incorrectly controlled, the durability and mechanical integrity of the whole turbine may be affected. Consequently, a good understanding of the orifices in the internal passages is important. This study presents experimental results for a range of pressure ratios and length-to-diameter ratios common in gas turbines including even very small pressure ratios. Additionally, the chamfer depth at the inlet was also varied. The results of the chamfer depth variation confirmed its beneficial influence on decreasing pressure losses. Moreover, important effects were noted when varying more than one parameter at a time. Besides earlier mentioned hysteresis at the threshold of choking, new phenomena were observed, e.g. a rise of the discharge coefficient for certain pressure and length-to-diameter ratios. A correlation for the discharge coefficient was attained based on the new experimental data with a generally lower error than previous studies.
Considerations About the Necessary Mesh Density of Bearings in Detailed Finite Element Models
Authors: Kober Markus , Kühhorn Arnold , Stelldinger Enrico , Keskin Akin
Published: September 2014

In order to predict properly the behavior of very complex mechanical models, much focus needs to be done on the level of model details as well as the techniques used to predict component movements based on mechanical and thermal loads. In particular bearings and joints play a significant role in big model assemblies such as aero-engines where most of the structures are modeled by solid elements rather than using traditional shell or beam elements. This approach is very complex and expensive in terms of computational effort however can lead to increased simulation accuracy when done properly.

This publication will answer questions related to the modeling techniques required for detailed bearings (not only rotor bearings) and joints of an aero-engine application where these components are not idealized or simplified, i.e. the ball in a ball joint is meshed by solid elements. The investigation will discuss the question about the number of elements needed to mesh a ball joint in such a way that a correct rotation is possible. If the mesh is too coarse, a rotation would be impossible due to the faceting of the rotating parts. It will be shown that a proper rotation depends on a correct relation between the radius of the inner ball, the gap size of the bearing and the number of nodes in the circumferential direction. For this relation an analytical formula has been derived and successfully tested at some examples. Furthermore, some finite element codes offer the possibility of smoothed contact surfaces. These options improve the situation but they might cause issues too. For this reason these smoothed contact options are also considered under the aspects mentioned above.
Suppression of Secondary Flows in an Axial Flow Turbine Rotor With a Novel Blade Design Concept
Authors: Yamada Kazutoyo , Furukawa Masato , Shibata Takanori , Nakakido Satoshi , Oka Nobuhito
Published: September 2014

This paper presents a novel blade design concept about control of the secondary flow in axial turbines to improve the internal efficiency. A rotor blade for the high and intermediate pressure stage of a steam turbine was designed by one-dimensional inverse method incorporated with throughflow calculation.

In the throughflow calculation, the axisymmetric Navier-Stokes equations are solved, assuming that the meridional flow is axisymmetric and viscous. To take into account the blade loading, a blade force is introduced as a body force to the governing equations. The blade force contains the inviscid blade effect only, namely, the pressure difference across a blade. The blade force acts in the direction perpendicular to a three-dimensional blade camber surface, which is constructed from camber lines stacked in blade height direction, so that the flow convects along the surface. The camber line is calculated by one-dimensional inverse method, based on a given blade loading distribution and the meridional velocity distribution from the throughflow calculation. The present throughflow calculation method was validated for a direct problem, using blade geometry of existing turbine rotor. The blade force with the camber surface in the present method was able to express the secondary flow inside the turbine by modifying the camber surface appropriately. As a result, the calculation result showed good agreement with the experimental result.

The turbine rotor blade was redesigned with the present method, modifying the chord-wise blade loading distribution so as to suppress the development of the secondary flow. The way of modifying the chord-wise blade loading distribution is based on a new idea, which can be actualized by the present design method. It was confirmed that the secondary flow was successfully suppressed as intended in the designed rotor.
The Impact of Modified Corrugated Bump Structures on the Rotor Dynamic Performance of Gas Foil Bearings
Authors: Hoffmann Robert , Pronobis Tomasz , Liebich Robert
Published: September 2014

Gas foil bearings (GFBs) have been successfully introduced in the field of high speed turbo machineries. Low drag friction, high speed operation and the omission of an oil system are some advantages of bump type foil bearings. However, experimental and numerical investigations have shown sub synchronous vibrations, which affect the rotor dynamic behavior. Several methods and devices have been introduced to decrease these vibrations (e.g. viscoelastic foil bearings, shims and side feed pressurization). This current paper examines the effect of different bump foil configurations on the rotor dynamic performance. Different bump stiffness distributions in axial and circumferential direction are considered for a set of loadings (5, 15, 30 and 60 N) and rotor speeds (6,000–70,000 rpm). To evaluate the onset speed of sub synchronous vibration a linear stability analysis is applied. It uses the linearized bearing parameters stiffness and damping. The results show, that a variation of stiffness distributions may enlarge the stability range. Simulations indicate that a non-uniform circumferential stiffness distribution is very effective to avoid sub synchronous vibrations, due to smaller cross coupling effects.
Topology Optimization in Structural Design of a LP Turbine Guide Vane: Potential of Additive Manufacturing for Weight Reduction
Authors: Seppälä Joona , Hupfer Andreas
Published: September 2014

A low pressure turbine guide vane of an aircraft engine is structurally redesigned for additive manufacturing (AM). AM is known to provide more design freedom than conventional manufacturing methods, which encourages the implementation of numerical optimization methods in the design process in order to reduce weight by eliminating unneeded material. One such method is called topology optimization (TO), which finds the optimal material distribution inside a fixed design space. Using commercial software, TO is conducted to find the optimal geometry. The guide vane is subject to gas loads. During optimization, constraints for bending deformation and Eigen frequencies are applied. The design space consists of the airfoil interior and the shrouds, leaving aerodynamic surfaces untouched. Several TO approaches are examined and the result is preliminarily evaluated in a stationary coupled temperature-displacement FEA with take-off loading conditions. The results indicate a potential weight reduction of 19% but with a rise in temperature gradients. An enlarged shroud geometry would enable even greater weight reduction.
Numerical Investigation of Impeller Tip Tailoring and its Impact on Aerodynamic Design of a Centrifugal Compressor’s Impeller
Authors: Mahajan Ashvin , Baert Lieven , Leborgne Michaël , Lonfils Timothée , Parwatha I. Gede , Timoshadchenko Denis
Published: September 2014

The current research focuses on the aerodynamic design of a centrifugal compressor and the effect of tip tailoring on the aerodynamic impeller efficiency. To this extent a high-fidelity multi-point design optimization process has been developed and exploited on a high pressure ratio transonic impeller. By manipulating the shape of the impeller blades and endwalls and by including advanced geometrical features such as winglets on the impeller blades, the behavior of the impeller flow has been investigated. Here, the results of three-dimensional RANS simulations with the Spalart-Allmaras turbulence model on a structured multi-block mesh is used for the evaluation of the flow characteristics.

In the context of radial machines, the results of the aerodynamic design optimization show an important improvement of the impeller isentropic efficiency compared to the reference impeller, with a significant contribution from the presence of the impeller tip winglets. Furthermore, the integration of the impeller winglet has encouraged this study to provide a detailed analysis on the impeller flow structures in order to have a better understanding of the effects of tip tailoring on impeller performance.
An Efficient Workflow for Accurate Flutter Stability Analyses and Application to a State of the Art Compressor Rotor
Authors: Micallef Derek , Witteck Dirk , Wiedermann Alexander , Mailach Ronald
Published: September 2014

During the aeromechanical design process of turbomachinery blading, one of the main goals is to improve the blade loading which may lead to a higher risk of flutter. To avoid flutter induced blade failure during operation, the final blade design has to fulfill certain aero mechanical requirements. These refer to the permitted static and dynamic stress levels as well as the aeroelastic stability constraint of flutter for the whole operating range. In this contribution, an efficient workflow for three-dimensional viscous flutter stability analyses will be presented using the three-dimensional viscous flow solver TBLOCK and the open-source software package CalculiX for FE modal analyses. For this purpose, the workflow is applied to the first compressor rotor of a state of the art gas turbine. The flutter analysis is performed for several operating points to predict an accurate flutter envelope for the whole operating range of the investigated compressor stage. To reduce the numerical effort, only the first two mode shapes are considered with respect to different shaft speeds. In addition, phase-shifted boundary conditions are applied to all flutter calculations using the traveling wave mode domain taking all possible inter-blade phase angles into account. The results of the flutter analysis show no indications for flutter within the projected operating range of the rotor and for the considered mode shapes. In conclusion, the described workflow is able to determine the critical flutter stability boundaries of the investigated compressor rotor with reasonable numerical effort.
Influential Parameters on Structural Damping Values of Turbine Blades
Authors: Hentschel Olaf P. , Panning-von Scheidt L. , Wallaschek J. , Denk M. , Masserey P.-A.
Published: September 2014

In turbomachinery blading the avoidance of High Cycle Fatigue (HCF) failures is of great importance. The prediction and reduction of vibration amplitudes is of primary objective in context of HCF risk reduction. For this reason the quantification of the mechanical damping is of essential relevance.

During the last decades the research has been focused on the usage of nonlinear calculation tools to predict vibration amplitudes of blades. These calculations require the specification of contact parameters as well as of material damping values. Especially for weakly damped systems like turbine blades, it is necessary to specify adequate structural damping values which determine the accuracy of the calculated transfer function. However, many researchers use uncertain structural damping values to calculate transfer functions.

In this paper, an experimental setup for specimen specific damping determination in a vacuum chamber is presented. Three different clamping mechanisms, as well as the mechanism for specimen excitation, are introduced. A suspended-like specimen clamping, where the specimens are clamped in their nodes of vibration, is described first. To analyze potential influence of the clamping procedure, a comparison is given to specimens clamped on one side (cantilever beams), where the magnitude of the clamping force is chosen in a way that the friction loss is minimized. To allow an application of static stresses the specimens are clamped on both sides in the third approach. The excitation of the analyzed specimens is performed with the help of a voice-coil actuator. Damping values are determined by analyzing the decay curve, which is measured with a laser doppler vibrometer. Further experimental results show the influence of ambient pressure, frequency, amplitude, geometry, mode shape, static stress and clamping mechanism on the specimen specific damping value.
A New Simulation and Control Model for a Variable Speed Variable Pitch Direct Drive Large Offshore Wind Turbine Generator With Integrated Energy Storage
Authors: Kazemtabrizi B. , Hogg S.
Published: September 2014

A comprehensive control and simulation model has been presented in this paper for a large multi-megawatt Variable Speed Variable Pitch Wind Turbine Generator (WTG) with Energy Storage System (ESS) integration. The generator is of a Permanent Magnet Synchronous Generator (PMSG) type which is then decoupled from the grid using a Fully-Rated back-to-back Converter (FRC) link. A Rotor Flux Oriented Control (RFOC) strategy has been adopted in order to extract the maximum output electromagnetic torque from the generator according to a Maximum Power Point Tracking (MPPT) profile for any wind speed lower than rated wind speed. The MPPT method presented in this paper relies on the turbine’s optimal output torque curve. The ESS makes provisions for enhanced performance especially during faults on the system. Dynamic performance of a super-capacitor makes it ideal as the integrated ESS in the WTG for satisfying such performance constraints. Results indicate that the WTG integrated with a super-capacitor ESS provides the required enhanced performance in terms of DC link voltage stability and limiting output power fluctuations during sudden variations in the wind speed as well as enhanced Fault Ride-Through (FRT) capability.
Two-Dimensional Modeling of Shaft Precessional Motions Induced by Blade/Casing Unilateral Contact in Aircraft Engines
Authors: Salvat Nicolas , Batailly Alain , Legrand Mathias
Published: September 2014

In the present work, the focus is made on the occurrence of precessional motions of the shaft — whirling motions — in bladed-disk assemblies, initiated by direct blade/casing contacts in one stage of an aircraft engine. These contact events are favored by increasingly reduced blade-tip clearances and are expected to occur during standard operating conditions. However, it has been shown that potentially harmful interactions may arise and threaten the engine structural integrity.

A 2D in-plane model of an aircraft engine fan stage is built with a set of curved beams for the casing and an assembly straight beams for the bladed-disk. The flexibility of the shaft is reflected by two linear springs attached to the center node of the disk. Contact is initiated through a prescribed casing distortion and the two structures are then left free to interact. Equations of motion are solved via explicit time-marching and contact forces are computed with Lagrange multipliers method allowing to fully satisfy non-penetration conditions. Friction is accounted for through a Coulomb law and permanent sliding is assumed.

Three types of regimes are identified, namely: (1) damped, (2) sustained, (3) divergent, and both forward and backward shaft precessional motions are witnessed. It is shown that the vibratory response of the bladed-disk mainly lies on the first nodal diameter of the first family of modes regardless of the rotational velocity or the type of regime detected. The risk of failure arising from these contact events is highlighted, in particular in the presence of forward whirl, and the need for accurate predictive tools in early design phases of the engine is emphasized.
Creating Reduced Kinetics Models That Satisfy the Entropy Inequality
Authors: Jones Nathan H. , Cizmas Paul G. A. , Slattery John C.
Published: September 2014

In simulating chemically reacting flows, the differential entropy inequality (the local form of the second law of thermodynamics) must be satisfied in addition to the differential mass, momentum and energy balances. Previously we have shown that entropy violations occur when using a global/reduced mechanism. Herein we show that entropy violations also occur when using a detailed/skeletal/reduced mechanism. Using a recent theorem [1], we illustrate how to modify a reduced chemical kinetics model to automatically satisfy the differential entropy inequality. The numerical solution of a methane laminar flame was improved when using reduced chemical kinetics modified in this way. In addition, an ad hoc temperature limiter is no longer necessary.
Effect of Variable Properties and Radiation on Convective Heat Transfer Measurements at Engine Conditions
Authors: Greiner Nathan J. , Polanka Marc D. , Rutledge James L. , Shewhart Andrew T.
Published: September 2014

Experiments measuring film cooling performance are often performed near room temperature over small ranges of driving temperature. For such experiments, fluid properties are nearly constant within the boundary layer and radiative heat transfer is negligible. Consequently, the heat flux to the wall is a linear function of driving temperature. Therefore, the convective heat transfer coefficient and adiabatic wall temperature can be extracted from heat flux measurements at two or more driving temperatures.

For large driving temperatures, like those seen in gas turbine engines, significant property variations exist within the boundary layer. In addition, radiative heat transfer becomes sufficiently large such that it can no longer be neglected. As a result, heat flux becomes a non-linear function of driving temperature. Thus, for these high temperature cases, ambient temperature methods utilizing a linear heat flux assumption cannot be employed to characterize the convective heat transfer.

The present study experimentally examines the non-linearity of heat flux for large driving temperatures flowing over a flat plate. The results are first used to validate the temperature ratio method presented in a previous study to account for variable properties within a boundary layer. This validation highlighted the need to account for the radiative component of the overall heat transfer. A method is subsequently proposed to account for the effects of both variable properties and radiation simultaneously. Finally, the method is validated with the experimental data.

While this methodology was developed in a flat plate rig, it is applicable to any relevant configuration in a hot environment. The method is general and independent of the overall radiative component magnitude and direction. Overall, the technique provides a means of quantifying the impact of both variable properties and the radiative flux on the conductive heat transfer to or from a surface in a single experiment.
Minimization of Heat Load due to Secondary Reactions in Fuel Rich Environments
Authors: Shewhart Andrew T. , Polanka Marc D. , Robertson Jacob J. , Greiner Nathan J. , Rutledge James L.
Published: September 2014

The demand for increased thrust, higher engine efficiency, and reduced fuel consumption has increased the turbine inlet temperature and pressure in modern gas turbine engines. The outcome of these higher temperatures and pressures is the potential for unconsumed radical species to enter the turbine. Because modern cooling schemes for turbine blades involve injecting cool, oxygen rich air adjacent to the surface, the potential for reaction with radicals in the mainstream flow and augmented heat transfer to the blade arises. This result is contrary to the purpose of film cooling. In this environment there is a competing desire to consume any free radicals prior to the flow entering the rotor stage while still maintaining surface temperatures below the metal melting temperature.

This study evaluated various configurations of multiple cylindrical rows of cooling holes in terms of both heat release and effective downstream cooling. Results were evaluated based on a new Wall Absorption parameter which combined the additional heat available from these secondary reactions with the length of the resulting flame to determine which schemes protected the wall more efficiently. Two particular schemes showed promise. The two row upstream configuration reduced the overall augmentation of heat by creating a short, concentrated reaction area. Conversely, the roll forward configuration minimized the local heat flux enhancement by spreading the reaction area over the surface being cooled.
Study on the Regression Method of Compressor Map Based on Partial Least Squares Regression Modeling
Authors: Ying Yulong , Sun Bo , Peng Shuhong , Wu Keliang , He Lei , Xu Qiang
Published: September 2014

One of the most troublesome problems in the development of a component-based engine model is the compressor modeling because of the strong dependence of its performance on rotational speed and the treatment of compressor characteristic curves plays a significant role in modeling and simulation of gas turbine for the analysis of its off-design performance and thus it is crucial to describe compressor map exactly.

Usually part of rotational speed characteristic curves of compressor including on-design operating point are known during actual modeling and simulation, and reasonable interpolation and extrapolation have to be done in order to make good use of flow characteristics and efficiency characteristics under more constant rotational speed lines during off-design performance simulation.

Due to that the accuracy of traditional approaches for component characteristic treatment is not satisfactory, and the performance of interpolation and extrapolation of artificial neural network is poor, a linear multiple regression method, i.e., partial least-squares regression method was proposed in this paper.

Partial least-squares regression modeling method was used to reproduce the compressor maps. Different polynomial functions of various powers were used to obtain respectively expressions of compressor maps. Fitting accuracy and performance of interpolation and extrapolation of this method were analyzed, and the simulating experimental results show that partial least-squares regression method can ensure good fitting accuracy and good performance of interpolation and extrapolation for compressor thermodynamic modeling with both maximum RMS errors less than 0.5%.

It can be expected that the application of partial least-squares regression modeling has a certain reference value to improve the solution accuracy for thermodynamic model of industrial gas turbine.
Analysis of Circumferential Fluctuation in Compressor Cascades
Authors: Wan Ke , Tang Mingzhi , Jin Donghai , Gui Xingmin
Published: September 2014

This paper presents the relation between circumferential fluctuation and the geometric and flow parameters. The governing equations are derived by circumferentially averaging the three-dimensional (3D) Navier-Stokes equations. Different types of compressor cascades are simulated and the circumferential fluctuation terms are extracted according to the definition of circumferential average. Three different blade profiles are chosen, including CDA, C4 and NACA65 profile, respectively. The peak value of circumferential fluctuation terms often occurs at the leading or the trailing edge and increases as the radius grows. Meanwhile, the circumferential fluctuation terms exist at the inlet of the blade which can be accurately calculated. 0°, 15° and 30° camber angles are chosen to study the influence of camber angle. When the camber angle is smaller, the flow is more uniform and therefore, the value of circumferential fluctuation is lower. Different incidence angles are compared to discuss the relationship between circumferential fluctuation and incidence angle. For specific term of circumferential fluctuations, the distribution curves are different.
Method for Selecting Master Degrees of Freedom for Rotating Substructure
Authors: Zuo Yanfei , Wang Jianjun , Ma Weimeng , Zhai Xue , Yao Xinyu
Published: September 2014

A method of selecting master degrees of freedom (DOFs) for rotating substructure is presented in this paper to obtain reduced 3D rotor models. Fixed modes of the substructure below thrice the operating frequency are analyzed. According to each mode shape, the DOFs at where main kinetic energy locates are selected as master DOFs to decrease missing of dynamic coupling. Additional DOFs may be selected based on traditional substructure method. In the stationary reference frame, frequency-dependent gyroscopic effects can be included as damping matrices changing with spin speed. Besides, by selecting appropriate substructure, localized damping and key parts of the rotor for analysis can be kept the same as the original model. A reduced model of a high pressure rotor amply demonstrated the capability of the method in reducing the model size and increasing the computational efficiency with less than two percent error.
Fatigue Crack Growth of Multiple Load Path Structure Under Combined Fatigue Loading: Part I — Numerical Simulation
Authors: Hu Dianyin , Wei Jiaming , Liu Huawei , Si Wulin , Wang Rongqiao
Published: September 2014

A new numerical method for the analysis of fatigue crack growth in the multiple load path structure under low-high cycle combined fatigue (L-HCCF) loading is proposed in this paper. Firstly, vibrational stress is obtained through dynamic response analyses considering joint dry friction between the turbine blade and the turbine disc based on equivalent linearization and micro-slip model. Then, the crack growth in the fir-tree attachment is simulated under vibrational stress superimposed with low cycle fatigue loading through fracture mechanics (FM) analysis. Vibrational loading in the fir-tree attachment is redistributed with the crack propagation because of this multiple load path structure. Thus, dynamic response analysis is re-performed on the turbine attachment. At last, the crack growth life of multiple load path structure under L-HCCF loading is predicted based on the linear cumulative damage model. The predicted life agrees well with the experimental data of actual turbine component, which verifies that the new numerical method for the analysis of fatigue crack growth in the multiple load path structure under L-HCCF loading is reasonable and feasible.