bluuebunny/crossref_metadata_2025 huggingface dataset:
Validation of Large-Eddy Simulation for the Prediction of Compressible Flow in an Axial Compressor Stage
Authors: Gourdain Nicolas
Published: November 2013

A better understanding of unsteady flows (including turbulence) is a necessary step towards a breakthrough in the design of modern gas turbine components. LES emerges as a very promising method to improve both knowledge of complex physics and reliability of flow solver predictions. However, there is still a lack of evidences in the literature that LES is applicable for rotor/stator configurations at industrial Reynolds numbers. In that context, the objective of the present work is to investigate the capability of LES to predict the turbulent flow in a stage of an axial compressor. The studied configuration operates at conditions relevant to industrial applications (M = 0.5, Re = 5 × 105). The whole 3D flow is considered, including the rotor tip clearance, as well as the natural periodicity of the compressor (i.e. the exact ratio between the number of rotor blades and stator vanes). First a validation of LES is obtained by comparing results on different mesh densities, in order to evaluate the grid requirements necessary to perform a wall-resolved LES (the finest grid is made of o(109) points to represent 3 rotor and 4 stator passages). Typical mesh scales are compared to the Kolmogorov length scale to estimate the regions where the subgrid scale model acts. Results obtained with both URANS and LES are then compared at nominal operating conditions. The analysis focuses on two dataset: time-averaged quantities (boundary layer profiles, turbulent kinetic energy, etc). and unsteady flow data. An evaluation of the flow physics shows that URANS predicts much more losses than LES in this configuration (discrepancy on efficiency is about 6%). The analysis indicates that LES predicts a laminar to turbulent transition at 50% of the rotor chord, leading to thinner boundary layers in both rotor and stator than with URANS. Actually LES also predicts the development of high-energy frequencies in the tip region that are uncorrelated to the blade passing frequency, related to the pulsation of the tip leakage flow.
Reduced Order Modeling Based on Complex Nonlinear Modal Analysis and its Application to Bladed Disks With Shroud Contact
Authors: Krack Malte , Panning-von Scheidt Lars , Wallaschek Jörg , Siewert Christian , Hartung Andreas
Published: November 2013

The design of bladed disks with contact interfaces typically requires analyses of the resonant forced response and flutter-induced limit cycle oscillations. The steady-state vibration behavior can efficiently be calculated using the Multi-Harmonic Balance method. The dimension of the arising algebraic systems of equations is essentially proportional to the number of harmonics and the number of degrees of freedom (DOFs) retained in the model. Extensive parametric studies necessary e.g. for robust design optimization are often not possible in practice due to the resulting computational effort.

In this paper, a two-step nonlinear reduced order modeling approach is proposed. First, the autonomous nonlinear system is analyzed using a Complex Nonlinear Modal Analysis technique based on the work of Laxalde and Thouverez [1]. The methodology in [1] was refined by an exact condensation approach as well as analytical calculation of gradients in order to efficiently study localized nonlinearities in large-scale systems. Moreover, a continuation method was employed in order to predict nonlinear modal interactions. Modal properties such as eigenfrequency and modal damping are directly calculated with respect to the kinetic energy in the system. In a second step, a reduced order model is built based on the Single Nonlinear Resonant Mode theory. It is shown that linear damping and harmonic forcing can be superimposed. Moreover, similarity properties can be exploited to vary normal preload or gap values in contact interfaces. Thus, a large parameter space can be covered without the need for re-computation of nonlinear modal properties. The computational effort for evaluating the reduced order model is almost negligible since it contains a single DOF only, independent of the original system.

The methodology is applied to both a simplified and a large-scale model of a bladed disk with shroud contact interfaces. In contrast to [1], the contact constraints account for variable normal load and lift-off in addition to dry friction. Forced response functions, backbone curves for varying normal preload and excitation level as well as flutter-induced limit cycle oscillations are analysed and compared to conventional methods. The limits of the proposed methodology are indicated and discussed.
Probabilistic Structure-Mechanical Assessment of Rotor Discs Considering Geometry Variations
Authors: Reuter Ilko , Weiss Thomas , Voigt Matthias , Vogeler Konrad , Schlums Hartmut , Becker Karl-Helmut , Fischersworring-Bunk Andreas
Published: November 2013

For rotating critical parts, like compressor or turbine discs of aero engines, it is essential to perform reliable life predictions. Probabilistic methods are ideal to investigate these life predictions with regard to their sensitivities and robustness. Beside other system properties, the variation of geometrical disc parameters has a strong influence on the physical system behavior.

Within this paper, the system behavior of a turbine disc is assessed with regard to its sensitivities, to enable the continuous optimization of the disc and to verify a new post-processing method. To suit this purpose, a process chain was developed from CAD-model to lifing analysis and embedded into a Monte Carlo Simulation (MCS). As input parameters geometrical parameters and their optimization ranges are used. The statistical evaluation of an MCS with regard to sensitivities is based on comparison of topologically similar locations. Similarity is achieved by the standard operation of “mesh morphing”, in which the original mesh of the initial geometry is deformed onto the new geometry. This method is limited to small variations. Another method is remeshing of the new geometry. This makes direct comparison at identical FE-nodes impossible and post-processing is done based on significant local features. The post-processing method described in this paper is a combination of aforementioned approaches. Based on this new method, the result variables von-Mises stress and life cycles are investigated for their sensitivities to geometrical parameters within the disc bore.
Rotordynamic Crack Diagnosis: Distinguishing Crack Depth and Location
Authors: Varney Philip , Green Itzhak
Published: November 2013

The goal of this work is to establish a condition monitoring regimen capable of diagnosing the depth and location of a transverse fatigue crack in a rotordynamic system. The success of an on-line crack diagnosis regimen hinges on the accuracy of the crack model used. The model should account for the depth of the crack and the localization of the crack along the shaft. Negating the influence of crack location on system response ignores a crucial component of real cracks. Two gaping crack models are presented; the first simulates a finite-width manufactured notch, while the second models an open fatigue crack. An overhung rotordynamic system is modeled, imitating an available rotordynamic test rig. Four degree-of-freedom equations of motion for both crack models are presented and discussed, along with corresponding transfer matrix techniques. Free and forced response analyses are performed, with emphasis placed on results applicable to condition monitoring. It is demonstrated that two identifiers are necessary to diagnose the crack parameters: the 2X resonance frequency and the magnitude of the 2X component of the rotor angular response at resonance. First, a contour plot of the 2X resonant shaft speed versus crack depth and location is generated. The magnitude of the 2X component of the rotor’s angular response along the desired contour is obtained, narrowing the possible pairs of crack location/depth to either one or two possibilities. Practical aspects of the diagnosis procedure are then discussed.
Impact of Measurement Uncertainty in the Characteristic Maps of a Turbocharger on Engine Performance
Authors: Mai Holger , Vogt Mathias , Baar Roland , Kinski Andreas
Published: November 2013

The main goal of current engine development is to increase power density and efficiency and to minimize engine emissions. The idea is to obtain the desired power output with a highly charged combustion engine in combination with exhaust gas turbocharging and a very small engine displacement, so-called downsizing.

The selection of a turbocharger is based on the maps of the turbine and compressor, which are usually measured on a test bench. They also provide important boundary conditions on the engine process simulation of a supercharged engine with this turbocharger. In general, a very accurate measurement of the characteristic maps is desired to ensure the best possible matching.

However, random and systematic errors have an impact on the measurement results. In order to assess the quality of the measured and calculated values, it is necessary to determine the uncertainties of the measurement variables as accurately as possible, particularly the error propagation in calculating the efficiencies.

The uncertainties are based on a systematic uncertainty component of the sensor and the confidence interval. In this way the measurement uncertainty is estimated by linear and geometric combination of the calculated random and systematic uncertainties. After that, the respective uncertainty contributions and the identification of the relevant parameters that influence the resulting measurement uncertainty are evaluated. Knowing the measurement uncertainties of the characteristic maps of a turbocharger, the influence on engine operation will be determined with a one-dimensional engine process simulation model. Consequently, the determined measurement uncertainty will be applied as a deviation on the efficiencies and will be investigated in a GT POWER simulation. The impact of the measurement uncertainty on the engine performance is shown on the basis of load steps.
Experimental and Computational Analysis of a Multistage Axial Compressor Including Stall Prediction by Steady and Transient CFD Methods
Authors: Cornelius Christian , Biesinger Thomas , Galpin Paul , Braune André
Published: November 2013

Siemens Energy has commissioned an extensive multi-year experimental and numerical (CFD) project to improve its ability to design for and predict compressor stall. The experimental test rig is a half scale six stage axial compressor. The goal of this work is to provide insight into how best to predict the compressor performance map and in particular the stall point by applying state-of-the-art multiple blade row CFD simulation tools. A preliminary CFD analysis quantified numerical, model and systematic error on the first stage of the compressor. Subsequent steady (Mixing Plane) and transient (Time Transformation) CFD simulations of the entire six stage compressor are compared to each other and to experimental data. Both the steady and transient simulations are shown to be computationally efficient and in very good agreement with the experimental data across the full performance map, up to stall inception on multiple speedlines. Physical explanations of the key flow features observed in the experiment, as well as of the differences between the predictions and experimental data, are given.
On the Coupling of Direct Design and Optimization Techniques for Mitigating the Turbomachinery Performance Test Risk
Authors: Tajiri Keiji , Zhao Jinhui , Hohlweg William C. , Zhang Haijie
Published: November 2013

Automatic optimization techniques have been used in recent years to facilitate more rapid analyses of different design options with multiple performance objectives. Typically, this process has been used during new product development. In this paper, a design system is presented, which enables the multipoint, multi-objective optimization of the centrifugal compressor stage aerodynamic components. Moreover, it is applied to a design modification of a multistage compressor, during the manufacturing cycle, for risk mitigation. The system is based on the application of the Isight code for coupling of one dimensional direct design and analysis with multi-objective genetic algorithms, design of experiment, and response surface method. The design system was applied to a redesign of the diffuser, crossover, and return channel of two stages in a multistage compressor. The geometry parameterization is performed by a one dimensional analysis method where the diffuser width, crossover inlet and exit width and associated inner and outer radii, are used to describe the meridional flow path while holding the return vane geometry unchanged. Centrifugal compressor performance parameters, such as polytropic head and efficiency at the client rated point, head rise to surge, and choke flow capacity are evaluated during the optimization process. The example confirmed the validity of the system to perform the optimization of turbomachine components in a time efficient manner to meet production schedule. The system also allowed for a sensitivity analysis of the impact of geometry parameters on the aerodynamic performance, contributing to the development of guidelines for manufacturers to design new products and mitigate the performance risk on test floor.
A Theoretical Model for Flow Instability Inception in Transonic Fan/Compressors
Authors: Sun Xiaofeng , Liu Xiaohua , Sun Dakun
Published: November 2013

This paper applies a theoretical model, which has been developed recently, to calculate the flow instability inception of axial transonic fan/compressors system. After the mean flow field is computed by steady CFD simulation, a body force approach, which is a function of flow field data, is taken to represent the effects of discrete blades on the flow field and duplicate the physical sources of flow turning and loss. Further by applying appropriate boundary conditions and spectral collocation method, a group of homogeneous equations will yield from which the stability equation can be derived. The singular value decomposition method is adopted over a series of fine grids in frequency domain to solve the resultant eigenvalue problem, and the onset point of flow instability can be judged by the imaginary part of the resultant eigenvalue. The present investigation is to validate the feasibility of calculating the stall onset point for single stage transonic compressor. It is shown that this model is capable of predicting instability inception point of transonic flow with reasonable accuracy, and it is sustainable in terms of computational cost for industrial application. It is shown that this model can provide an unambiguous judgment on stall inception without numerous requirements of empirical relations of loss and deviation angle. It provides a possibility to check over-predicted stall margin during the design phase of new high speed fan/compressors. In addition, the effect of flow compressibility on the stall onset point calculation for transonic rotor is studied.
Open Rotor Designs for Low Noise and High Efficiency
Authors: Khalid S. Arif , Wojno John P. , Breeze-Stringfellow Andy , Lurie David P. , Wood Trevor H. , Ramakrishnan Kishore , Paliath Umesh
Published: November 2013

Building upon the successes of the UDF® program in the 1980’s, open rotor designs for high flight speed efficiency and low community noise have been developed at GE in collaboration with NASA and the FAA. Targeting a narrow body aircraft with 0.78 cruise Mach number, the cost-share program leveraged computational fluid dynamics (CFD), computational aero-acoustics (CAA), and rig scale testing to generate designs that achieved significant noise reductions well beyond what was attained in the 1980’s while substantially retaining cruise performance. This paper presents overall propeller net efficiency and acoustic assessments of GE’s modern open rotor designs based on measured rig data and the progression of the technology from the 1980’s through the present. Also discussed are the effects of aft rotor clipping, inter-rotor spacing, and disk loading. This paper shows how the two-phase design and scale model wind tunnel test program allowed for test results of the first design phase to feed back into the second design phase, resulting in 2–3% improvement in overall propeller net efficiency than the best efficiency design of the 1980’s while nominally achieving 15–17 EPNdB noise margin to Chapter 4 (when projected to full scale for a prescribed aircraft trajectory and installation). Accounting for trades and near term advancements, such a propulsion system is projected to meet the goal of 26% fuel burn reduction relative to CFM56-7B powered narrow body aircraft.
Investigation of Fan Rotor Interaction With Pressure Disturbance Produced by Downstream Pylon
Authors: Enoki Tomonori , Kodama Hidekazu , Kusuda Shinya
Published: November 2013

This paper presents an investigation of fan rotor interaction with potential pressure disturbances produced by a downstream pylon. Three-dimensional unsteady viscous analyses are performed for two fan rotor-stator-pylon configurations with different axial gaps between the stator and the pylon, and compared with the experimental results. To clarify the impact of the rotor-pylon interaction on the potential pressure flow field, a numerical analysis for the configuration in which a fan rotor is removed is also performed and compared with the numerical results with fan rotor. Actuator disk analyses are also performed to interpret the flow structures observed in the experiments and the numerical results. It is found that a fan rotor-stator interaction also exists in the fan flow field, and this may impact on the upstream propagating potential flow that dominates the unsteady forces acting on the rotor blades. A coupled analysis between fan rotor and stator is essential to accurately predict the unsteady blade force.
A Modified Theta Projection Creep Model for a Nickel-Based Super-Alloy
Authors: Day W. David , Gordon Ali P.
Published: November 2013

Accurate prediction of creep deformation is critical to assuring the mechanical integrity of heavy-duty, industrial gas turbine (IGT) hardware. The classical description of the creep deformation curve consists of a brief primary, followed by a longer secondary, and then a brief tertiary creep phase. An examination of creep tests at four temperatures for a proprietary, nickel-based, equiaxed, super-alloy revealed many occasions where there is no clear transition from secondary to tertiary creep. This paper presents a new creep model for a Nickel-based super-alloy, with some similarities to the Theta Projection (TP) creep model by Evans and all [1].

The alternative creep equation presented here was developed using meaningful parameters, or θ’s, such as: the primary creep strain, time at primary creep strain, minimum (or secondary) creep rate, and time that tertiary creep begins. By plotting the first and second derivative of creep, the authors were able to develop a creep equation that accurately matches tests. This creep equation is identical to the primary creep portion of the theta projection model, but has a modified second term. An additional term is included to simulate tertiary creep. An overall scaling factor is used to satisfy physical constraints and ensure solution stability. The new model allows a constant creep rate phase to be maintained, captures tertiary creep, and satisfies physical constraints.

The coefficients of the creep equations were developed using results from 27 creep tests performed at 4 temperatures. An automated routine was developed to directly fit the θ coefficients for each phase, resulting in a close overall fit for the material. The resultant constitutive creep model can be applied to components which are subjected to a wide range of temperatures and stresses. Useful information is provided to designers in the form of time to secondary and tertiary creep for a given stress and temperature. More accurate creep predictions allow PSM to improve the structural integrity of its turbine blades and vanes.
Theoretical Study on the Origin of Radial Cracks in Labyrinth Seal Fins due to Rubbing
Authors: Pychynski Tim , Dullenkopf Klaus , Bauer Hans-Jörg
Published: November 2013

This paper presents a theoretical approach to analyse the macroscopic thermo-mechanical behaviour of labyrinth seal fins (knife edges) during and after a rubbing event. The study helps to understand the mechanism behind radial cracking in labyrinth fins and identifies its driving parameters. To begin with, a generic analytical model is developed in order to determine the elastic thermo-mechanical stresses in a thin rotating disk representing the seal fin for a given radial temperature distribution and contact pressure. Secondly, the analytical model is used to explain the high tensile stresses which may result from a rubbing process in labyrinth seals and which are a prerequisite for radial cracks to occur. The driving parameters and simple criteria to compare different materials with respect to cracking are discussed in detail. Crack initiation criteria and fracture mechanics are not considered in this paper.

Finally, the analytical model is validated for various seal fin geometries and rub conditions using a three-dimensional Finite-Element model. The interaction of the seal fin with the stator is not taken into account in this numerical simulation. Instead, the rubbing process is modeled by assuming a time-constant surface friction heat flux and contact pressure on the seal fin tip. Both friction heat flux and contact pressure were varied in a sensitivity study in order to account for different rubbing conditions.
Towards Massively Parallel Large Eddy Simulation of Turbine Stages
Authors: Wang Gaofeng , Papadogiannis Dimitrios , Duchaine Florent , Gourdain Nicolas , Gicquel Laurent Y. M.
Published: November 2013

The context of integrated numerical simulations of gas turbine engines by use of high-fidelity Computational Fluid Dynamic (CFD) tools recently emerged as a promising path to improve engines design and understanding. Relying on massively parallel super-computing such propositions still have to prove feasibility to efficiently take advantage of the ever increasing computing power made available worldwide. Although Large Eddy Simulation (LES) has recently proven its superiority in the context of the combustion chamber of gas turbine, methodologies need to be developed and start addressing the problem of the turbomachinery stages, if integrated simulations based on LES are to be foreseen. In the proposed work an in-house code and strategy, called TurboAVBP, is developed for turbomachinery LES thanks to the coupling of multi-copies of the unstructured compressible reacting LES solver AVBP, designed to run efficiently on high performance massively parallel architectures. Aside from the specificity of such wall bounded flows, rotor/stator LES type simulations require specific attention and the interface should not interfere with the numeric scheme to preserve proper representation of the unsteady physics crossing this interface. A tentative LES compliant solution based on moving overset grids method is proposed and evaluated in this work for high-fidelity simulation of the rotor/stator interactions. Simple test cases of increasing difficulty with reference numerical are detailed and prove the solution in handling acoustics, vortices and turbulence. The approach is then applied to the QinetiQ MT1 high-pressure transonic turbine for comparison with experimental data. Two configurations are computed: the first one is composed of 1 scaled stator section and 2 rotors while the second computation considers the geometrically accurate periodic quarter of the machine, i.e. 8 stators and 15 rotors to test scalability issues of such applications. Although under-resolved, the LES pressure profiles on the stator and rotor blades appear to be in good agreement with experimental data and are quite competitive compared to the traditional (Unsteady) Reynolds-Averaged Navier-Stokes (RANS or URANS) modeling approach. Unsteady features inherently present in these LES underline the complexity of the flow in a turbine stage and clearly demand additional diagnostics to be properly validated.
Nozzle Guide Vane Assembly for Air-Cooled Retrofit Turbine Case Kit
Authors: Dabiru Venkata Rambabu , Gorte Devidasa Pai , Colantoni Simone
Published: November 2013

Variable Guide Vanes are common to many of the Gas Turbines. Nozzle Guide Vane (NGV) is a variable nozzle present in the hot gas path of a twin-shaft gas turbine. In addition to guiding the hot gases onto the buckets, the NGV also controls the energy split between the high-pressure turbine and power turbine. The NGV is rotated to the desired position for different machine operability requirements through an actuation system. A twin shaft gas turbine equipped with NGV allows higher operational flexibility and higher efficiency at partial load/speed.

MS3002F is a heavy duty gas turbine developed by GE in 1960’s with water cooling system for the turbine casing and the NGV. Over the period of time, technology has evolved and water cooling system in Gas Turbines has become obsolete, as it leads to uneven cooling of the casing and scaling that cause performance losses, unplanned outages and down-time. To cater the existing fleet with improved reliability and availability, a new product introduction program was launched to design air-cooled system to replace the water-cooled system, within the boundaries of retrofit ability. The air cooling is achieved by extracted secondary flow air; the reduction on performance due to this is compensated by increasing the firing temperature. Owing to the removal of water cooling and increased firing temperature, the turbine casing and hot gas path components including NGV have to be redesigned.

The present paper discusses the redesign of the NGV assembly for new air-cooled configuration. This redesign includes geometry modification, assembly interface definition (contact geometry and coating), and material selection. The key challenge is to redesign the NGV with no cooling (neither water nor extracted secondary flow) while maintaining the steady state clearances of the NGV assembly as those of water-cooled kit. These redesigned components are validated for structural integrity under loads affecting fatigue, creep and oxidation. The verification of the functional integrity of NGV actuation system (that was originally designed for water-cooled system) for all operating conditions relevant to the new air-cooled turbine case kit is also discussed.
Validation of FE Simulated Structural Dynamic Behaviour of Heavy Duty Gas Turbines: A Case Study
Authors: Ugel Diego , Clark Alastair , Knopf Eric
Published: November 2013

The increasing market demand for operational flexibility has highlighted the importance of gas turbines optimized for fast transient load variations. Dynamics is a key element in achieving these requirements.

The advanced FE modeling techniques used by ALSTOM for dynamic simulations of the GT (including rotor, bearings and structural parts) have been taken further by the detailed validation of the models used.

The validation exercise involved cold testing with impact hammer and shaker of individual parts and sub-assemblies at the ALSTOM Power Plant in Switzerland.

The influence of operational conditions has been investigated by warm testing, operating the engine with imbalance weights. Important eigenmodes / displaced shapes have been identified during the validation phases and compared with predicted mode shapes. For defined modes, the understanding is increased with particular reference to their excitability.

On the analytical side, key parameters, which affect the dynamic behavior, have been identified by using a detailed systematic approach. Factors affecting individual parts, sub-assemblies and final GT assembly are examined, physically understood and adjusted accordingly. Thus the accuracy of the prediction has been improved in both structural dynamic and rotordynamic assessments.

Furthermore, the guidelines defined for these parameters have been applied to the dynamic calculation for design modifications, successfully introduced on medium sized heavy duty gas turbines for service retrofit. Finally the validation exercise has enabled ALSTOM to improve the understanding of vibration data from engines in the field.
Stator Wake Research in Axial-Radial Combined Compressor With Inlet Distortion
Authors: Li Du , Lao Dazhong , Zhao Ben , Yang Ce
Published: November 2013

A combined compressor consisting of an axial rotor, a stator and a centrifugal rotor was used in a vehicle engine turbocharger because of its better performance compared with a single stage compressor under space restrictions. There have been many studies on the inlet distortion effect on multistage axial compressors; however, few studies have been performed for axial-radial combined compressors. To investigate stator wake characteristics, an axial-radial compressor was unsteadily simulated with three-dimensional Reynolds averaged Navier–Stokes equations by “domain-scaling” the rotor/stator interface method under uniform, circumferential distorted and combined distorted total pressure inlet conditions. In addition to the axial-radial configuration, the unsteady method of analysis was applied to a stand-alone axial stage by the same unsteady method. A comparison shows that at the vane stator outlet, a radial distortion is formed and different total pressure loss occurs at different spans. The circumferential interfaces between the distorted and non-distorted regions generate different flow characteristics due to different pitchwise pressure gradient directions and the absence of a downstream rotor. The stator wake causes more flow loss in the distorted case but less flow angle fluctuation compared to the uniform inlet condition. The downstream potential effect and inlet distortion creates strong pressure pulsation at the stator blade surface. A stator wake oscillation characteristics study shows disturbing effects on the stator wake from the upstream axial rotor, and the downstream radial rotor acts differently both spanwise and pitchwise.
On Inflow Distortion Decay in the Blade Cascades
Authors: Vassiliev Vladimir , Kappis Wolfgang , Granovsky Andrey , Shchaulov Vladimir
Published: November 2013

Flow distortions in front of turbomachines can lead to additional mechanical and thermal loading of the blades, and may influence the operational stability of the engine and the lifetime of its parts. In current design practice the impact of inflow distortions is accounted by safety margins, which are often conservative. Therefore the knowledge of real limits helps to increase operational flexibility and recommended service intervals.

The distortions inside turbomachines have a wide spectrum of sizes and frequencies depending on their origin. Some of them were extensively studied in the recent years, including wakes interaction with blades, hot streaks propagation in turbines, and some other phenomenon. However, there are areas of practical importance, which are not yet covered by available studies. One of this distortion types is investigated in the present work.

These are distortions of a size, comparable with the inlet guide vane pitch of a compressor or the nozzle guide vane pitch of a turbine, both steady in the absolute frame of reference. Such distortions in front of a compressor can be caused, for example, by fouling of the intake, or by some structures purposely installed inside the intake. The distortions in front of the expansion turbine can be caused by structures.

Main purpose of the study is to provide an understanding of the decay mechanism of distortions considered. For this purpose the numerical and theoretical analysis of inflow distortion propagation inside the set of stationary and rotating blades cascades is performed. These cascades represent the mid-section of a typical multistage compressor and turbine.

The analysis showed that the decay mechanism is different for temperature distortions and total pressure distortions, and that the latter one is stronger affected by the blading configuration.
Prediction of Flow in an Exhaust Gas Turbine Diffuser With a Scale-Adaptive Simulation Model
Authors: Volkmer Silke , Schatz Markus , Casey Michael , Montgomery Matthew
Published: November 2013

The prediction of the flow in a gas turbine exhaust diffuser of a combined cycle power plant is particularly difficult as maximum performance is obtained with highly loaded diffusers, which operate close to boundary layer separation. CFD (computational fluid dynamics) simulations then need to cope with complex phenomena such as smooth wall separation, recirculation, reattachment, blockage and free shear layer mixing. Recent studies based on the RANS (Reynolds-Averaged-Navier-Stokes) approach demonstrate the challenge for two-equation turbulence models to predict separation and mixing of the flow correctly in such highly loaded diffusers and identify that more accurate methods are needed. Hence, the application of a hybrid Scale-Adaptive Simulation (SAS) is investigated and the CFD results are compared with experimental results from an in-house test rig.

In the present study the flow in a model exhaust diffuser (for heavy-duty gas turbine diffuser applications typical Reynolds number 1.5×106 and inlet Mach number 0.6) is examined with unsteady RANS (URANS) simulations with the SST (Shear Stress Transport) model as well as a hybrid Scale-Adaptive Simulation (SAS) model. The SAS model switches from URANS to a mode similar to a Large Eddy Simulation (LES) in unsteady flow regions to resolve various scales of detached eddies. The current study shows that with the SST model similar results are obtained with RANS and URANS simulations, whereas the more complex SAS model leads to a much better resolution of the unsteady fluctuations. However, the time-averaged results of the SAS calculations overpredict the blockage of the separation and hub wake. This results in an underprediction of the pressure recovery and the mixing of the flow compared to the simpler two-equation models and also compared to experimental results.
Aerodynamic Loss Measurements in a Compressible Flow Vane Cascade Showing the Influence of Reynolds Number Lapse
Authors: Mihelish M. P. , Ames F. E.
Published: November 2013

Turbofan engines are designed to operate over a range of conditions depending on the mission of the aircraft. Many unmanned aerial vehicles (UAVs) have been designed to use turbofans and these aircraft can operate at service ceilings up to 65,000 ft. At these altitudes the pressure drops to about 6% of ambient conditions and Reynolds numbers throughout the turbine drop precipitously. Understanding and designing for the influence of these large changes in engine Reynolds numbers over the aircraft’s operating range is key to the success of the aircraft and its mission. The present paper documents the influence of chord exit Reynolds number (90,000 to 720,000), inlet turbulence (0.8% and 9%), and exit Mach number (0.7, 0.8, and 0.9) on the exit losses of a first vane design consistent with turbofan engines designed to power high altitude UAVs.

These aerodynamic loss measurements were acquired in a new closed loop compressible flow facility. This closed loop wind tunnel was designed to test turbine cascades over a wide range of Reynolds numbers by controlling the pressure in the tunnel. The current measurements were acquired using a five-hole cone probe controlled with a sealed two-axis traversing system. This first stage vane test section was configured in a three full passage four-vane linear cascade arrangement with upper and lower bleed flows. The low turbulence condition was developed using a flow conditioning section consisting of two perforated plates and two fine-mesh screens and a 4.7 to 1 area ratio nozzle. The high turbulence condition was developed using a simulated combustor. The present results show a significant increase in exit losses with decreasing Reynolds number. This increase in losses is largely due to the thickening of the wake.
Correlation of Parameters to Instability and Chaos of a Horizontal Flexible Rotor Ball Bearing System
Authors: Gupta T. C. , Gupta K.
Published: November 2013

The higher order effects from ball bearing nonlinearities cause complex vibration characteristics in rotor ball bearing systems. The sources of nonlinearities are internal radial clearance, Hertzian contact forces between balls and races and varying compliance effect. The same authors in their earlier work have identified the sets of parameters corresponding to instability and chaos for a horizontal flexible rotor supported on deep groove ball bearing. To the best of author’s knowledge, there is not much work reported in the literature on the dynamic analysis for instability and chaos, which is based on energy functions and bearing loads. Extending the preceding research work in the present paper by using a typical set of parameters and specifications of rotor ball bearing system, a correlation of parameters to instability and chaos is attempted using different energy functions associated with the dynamical system.

A generalized Timoshenko beam finite element formulation is used to model the flexible rotor shaft. To achieve the convergence of solution with smaller number of elements, shape functions are derived from the exact solutions of governing differential equations of Timoshenko beam element. The sources of excitation are rotating unbalance and parametric excitation due to varying compliance of ball bearing during motion. For the bearing used in the present paper, the ratio of these excitation frequencies comes out to be an irrational number. Therefore, the dynamic response would be quasi-periodic with time period equal to infinity. To extend the use of non-autonomous shooting method to derive quasi-periodic solution, the fixed point algorithm (FPA) proposed in the literature is used to deduce the time period for non-autonomous shooting algorithm. The shooting method otherwise is used only to derive periodic solutions. Thus the non-autonomous shooting method coupled with fixed point algorithm (FPA) is used to compute the quasi-periodic solution, which also gives the monodromy matrix. The eigenvalues of the monodromy matrix, called Floqoet multipliers, give information about instability. The chaotic nature of the dynamic response is established by the maximum value of Lyapunov exponent.

Once the instability and chaos is confirmed based on computed values of Floquet multipliers and Lyapunov exponents, the nature of the work done (positive or negative) by different conservative and non-conservative forces and moments during motion are analyzed and the fundamental causes, which make the system response unstable and / or chaotic, are established.
Flexible Seal Strip Design for Advanced Labyrinth Seals in Turbines
Authors: Herrmann N. , Dullenkopf K. , Bauer H.-J.
Published: November 2013

The paper discusses the potential benefit of flexible seal strips in labyrinth seals for turbines. By reducing the radial stiffness compared to a standard straight and stiff knife, seal clearance could be reduced without significantly reducing the seal durability and long-term performance. As contact between the seal strips and the rotor can occur especially during transient operating phases, a more flexible design of the seal strips can prevent damage and wear, keeping the discharge rates constantly low. However, the pressure difference across the fin will cause a deflection of the seal strip due to the increased flexibility and thus creating an additional possible risk for an unwanted contact. Pressure balanced designs and supports on the low pressure side are used on the investigated seal designs to eliminate that risk.

To give evidence of possible performance gain a standard labyrinth seal configuration is compared to two configurations with segmented and curved seal strips. In a first step, the discharge coefficient and the leakage rates for the nominal seal design are calculated using two-dimensional CFD. In order to investigate the impact of a worn seal tip on the leakage flow, the geometry change due to a rubbing event is simulated with FEA tools. Therefore, a specific high-speed wear model is implemented and calibrated by experimental data, enabling the correct cooling effects and plastic deformation. The discharge coefficient and the leakage mass flow rates of the worn geometry are then again modeled with CFD for the various seal configurations and compared to the unworn state.

The study shows that a wise combination of the advantages of flexible curved seal strips can be used to reduce the leakage rates significantly, improving the life time of seal elements at the same time.
A New Partial Admission Method for Turbocharger Turbine Control at Off-Design
Authors: Challand Sebastian , Dirschauer Eckart , Ilievski Mirko , Casey Michael , Schatz Markus
Published: November 2013

A new method for turbocharger control in automotive applications is presented. It is called MEDUSA (Multiple Exhaust DUct with Source Adjustment, European patent application number: 21326 - EP) and is a partial admission system consisting of several separate flow channels that connect the exhaust duct of the engine and individual nozzle segments of the turbine. By opening or closing the individual flow channels using external valves, the turbine flow can be adjusted, hence allowing the whole turbocharger to be controlled. Due to the use of external valves, the system is considerably more robust than other variable geometry systems based on variable inlet guide vanes and thus becomes suitable for application to spark-ignition motors at high temperature.

The paper presents a theoretical assessment of this innovative control system, based on one dimensional considerations and CFD simulations. The CFD-calculations of the MEDUSA-system are compared to those of a turbocharger turbine controlled with a variable inlet nozzle. The results indicate that the performance and operating range of the new system is comparable, or even better, than the currently used variable nozzle systems, especially at low load conditions. This indicates that further experimental work is justified as it could become considerably more effective than the typical waste gate systems used in spark ignition engines and provides a new solution for the turbocharger control in these applications. So far, only radial turbines have been considered for application of this method but it could also be used for mixed-flow or axial turbines.
A Numerical Study of Squeeze Film Dampers
Authors: Boppa Praneetha , Morrison Gerald L. , Sekaran Aarthi
Published: November 2013

Squeeze film dampers (SFDs) are used in the high speed turbomachinery industry as a means to reduce vibration amplitude, provide damping, and improve dynamic stability of the rotor bearing system. Past numerical studies analyzing SFDs, have been computationally expensive and time consuming. The present study investigates the feasibility of applying a steady state solver to obtain computational efficiency while ensuring that the parameters of interest are captured. This is done via the application of the Moving Reference Frame (MRF) solver in ANSYS® Fluent. A steady state solver in an absolute frame of reference was used to produce whirling motion of the rotor. The results are validated by comparison to the experimental data of Delgado [1]. The numerical model shows good agreement with these results.
Extended Turbomachinery Aeromechanical Model
Authors: Romanov Artyom , Di Mare Luca
Published: November 2013

An extended version of the previously presented linearized one-dimensional turbomachinery performance model is described. The current version of the model is capable of performing forced response and flutter simulations on several stationary and/or rotating bladerows. The amplitude of the perturbation is assumed small thus impact of the perturbations of several sources may be superimposed. The distortion propagation analysis may be performed in the early stages of the design process, or whenever a quick solution is desirable, having only minimal information about the studied geometry.

The approach has a block structure, where each block represents a bladed passage or the empty space between. The blocks contain linearized gas relations that relate the gas state to known changes of enthalpy, entropy and momentum. The blade blocks are represented using the extended semi-actuator disk theory, where the flow inside the passage assumed to be one-dimensional [9]. The model considers frequency scattering for the rotating bladerows and is also using a complete package of linearized loss- and deviation correlations, providing more realistic results. The approach extends the previously presented methods by Amiet [2], [3] and Kaji&Okazaki [4], [5], being capable to handle harmonic distortions of various wavelength-to-chord ratios. Minimal assumptions are made about the studied geometry and nature of the gas, allowing to perform unsteady flow analysis not only on the idealized cases, but on more complex, realistic geometry.

Appropriate non-reflecting boundary conditions are applied at the boundaries of each block, using the hyperbolic characteristic theory, thus facilitating multi-blaredow domains setup and allowing running more complex cases, involving both forced response and flutter.

A number of idealized cases presented by Amiet and Kaji & Okazaki are reproduced to validate the model against the reference data, where a good comparison is achieved. The approach is also tested on the HP compressor of modern design for forced response simulations. Several multi-bladerow cases are also studied.
Integration of Fluid Thermodynamic and Transport Properties in Conceptual Turbomachinery Design
Authors: Thorbergsson Egill , Grönstedt Tomas , Robinson Christopher
Published: November 2013

The interest in designing gas turbine cycles that use unconventional working fluids is increasing. To be able to design the turbomachinery for these cycles the design programs need to be modified. This paper describes how a streamline curvature compressor design program, a one-dimensional compressor design program and a cycle performance program are linked to a thermodynamic properties program. A case study is described where the streamline curvature program is used to design a three stage compressor that has air as a working fluid. The design is completed both using a semi-perfect gas assumption and the new modification using a state of the art real gas data thermodynamic properties program. If not properly implemented real gas data retrieval may completely dominate calculation times. An algorithm targeting high numerical efficiency intended for through-flow multi-dimensional optimization is outlined. Various implementation recommendations associated with the integration of the real gas data program into the one-dimensional compressor design program and the cycle program are described.