bluuebunny/crossref_metadata_2025 huggingface dataset:
Measurement and Prediction of the Dynamic Characteristics of a Large Turbine Tilting-Pad Bearing Under High Circumferential Speeds
Authors: Kukla Sebastian , Hagemann Thomas , Schwarze Hubert
Published: November 2013

The identification results for the linear dynamic coefficients of a K-C model for a large tilting-pad bearing in load between pad configuration are presented for specific bearing loads between 1.0 and 2.0 MPa and circumferential speeds of 39 m/s and 78 m/s. The bearing with a double tilting support is lubricated by spray-bars and can be described by the following specifications: Five pads, 0.23 nominal preload, 60% offset, 56° pad arc angle, 500 mm inner diameter, 350 mm pad length and 1.28 per mill relative bearing clearance. The test rig and the test procedure are described in detail. For the determination of the dynamic coefficients, a harmonic force is induced by two unbalance-vibration generators being attached to the frame of the rig. The relative movement between bearing and shaft is detected by proximity probes between bearing housing and shaft. The bearing forces are identified by measurements of the entire film pressure distribution in both circumferential and axial direction. In the post processing of the data, the dynamic force components are determined by a Fourier-analysis. This procedure is well-established for fixed-pad bearings. However, the uncertainties of its capabilities for tilting-pad bearings are investigated and discussed in this study. The theoretical analyses with the code COMBROS are based on a calculation of linear perturbations for the predicted static properties. The measurement and the calculation procedures show very good agreement for fixed-pad bearings. For a tilting-pad bearing the results differ with increasing frequency ratio and rotational speed. The experimental results show very poor frequency dependence in load direction and a very high one in the orthogonal direction. Theoretically, the influence of the frequency ratio is comparable in both planes and pretty low due to the pivot offset and the high effective preload. While good agreement for the measured and predicted K-C model can be observed at the lower rotational and vibrational frequency the correspondence becomes worse with the increase of both. The identification procedure uses the fluid film force to determine the dynamic coefficients and assumes that this is equal to the load on the bearing in every time step. The results indicate that the experimental identification is uncertain due to the elasticity of the double tilting bearing support and the initiated dynamic effects of it. An improvement of the measurement that also identifies the limitation of the current procedures as well as simplifications in the theoretical analyses are discussed.
Numerical Investigation of Corner Separation on Compressor Cascade
Authors: Ling Jing , Du Xin , Wang Songtao , Wang Zhongqi
Published: November 2013

This paper studied the effects of suction surface corner separation on the aerodynamic performance and the effects of the blade parameters on suction surface corner separation in rectangular cascade. Corner separation alters the static pressure distribution on the suction surface, establish a C-Shape pressure distribution along spanwise, compared with open separation, closed separation intensifies the C-Shape pressure distribution, increases the streamwise adverse pressure gradient on the suction surface after the lowest pressure point. The diffusion capability in a closed separation was significantly lower than in an open separation on the separation zone, loss was larger than open separation. The changes of blade parameters have great effects on corner separation, not only affect the scale of separation zone, even they will change the separation form. This study show that with the increase of the blade thickness, the maximum thickness position moving afterwards and the increase of the deflection of mean camber line, the streamwise adverse pressure gradient on the suction surface after the lowest pressure point increase, the scale of separation zone increase, even the separation type changes from open separation to closed separation.
Consideration of Complex Support Structure Dynamics in Rotordynamic Assessments
Authors: Krüger Thomas D. , Liberatore Sauro , Knopf Eric , Clark Alastair
Published: November 2013

In rotordynamic analyses, support structures are commonly represented by lumped mass systems (single-degree-of-freedom, SDOF). This representation is easy to implement using standard rotordynamic tools. However, in reality the dynamic behaviour of the support structure (e.g. pedestals, casings, foundations) are in general much more complex. Only a multi-degree-of-freedom (MDOF) representation provides modelling close to reality.

For many applications the dynamic behaviour of the support structure significantly influences the rotordynamic characteristics of the shaft train and therefore needs to be included in the assessment. Due to this impact, a good quality of the dynamic model used for the support structure is imperative. Regarding the rotor itself, the modelling is well understood and the prediction quality is excellent, not least due to the jointless welded rotor design.

Numerous theoretical approaches exist for considering the complex dynamic behaviour of the support structure, all coming along with both drawbacks and opportunities. By discussing the characteristics of established approaches for modelling the support structure, the paper particularly presents an advanced theoretical approach based on a state-space representation using modal parameters.

A case study of a real shaft train is shown, including a comparison of achieved results using the SDOF and the presented MDOF approach. By validating with experimental results, the excellent prediction quality of the MDOF approach is confirmed. The implementation of this approach enabled to further improve the reliability and the efficiency, which means high accuracy combined with low computation time, in performing rotordynamic assessments.
The Flow Machines Course at the Technological Institute of Aeronautics for Mechanical-Aeronautical Engineering Undergraduate Course
Authors: Tomita Jesuino Takachi , Barbosa João Roberto , Bringhenti Cleverson
Published: November 2013

Undergraduate courses at Technological Institute of Aeronautics (ITA) are 5-years course, divided into Fundamental (2 years) and Professional (3 years). The Flow Machines, in the Mechanical-Aeronautical Engineering Course, is offered by the Turbomachines Department and is taught in the first semester of the fourth year (2nd professional year). In the course, the basic theory, unified for all machines, is presented in details for the students, emphasizing the physics of all processes involved in the fluid-machine energy transfer. Incompressible and compressible fluids are treated accordingly. The flow machines types are individually studied, focusing attention to their performance characteristics and range of applications. The preliminary design and off-design operation issues are discussed in details with the students, with emphasis on relevant aspects of each machine, like cavitation, stall and surge. The students are taught on how to choose the flow properties at the blade edges for the sake of preliminary design and off-design performance estimations. Loss models are introduced during the theory classes and popular models are presented. At this point, in-house computer codes and commercial software are presented to the students, who are asked to solve simple problems. The installation, operation and basic performance calculations are also presented for the students during the lab classes for several hydraulic machines installed at ITA laboratories. All course material is transferred for the students in pdf format before classes. In this work, the experience with the teaching process in flow machines at ITA, theory and laboratory, is described.
Potential Erroneous Interpretations of Pressure Signals as Beats During Established Rotating Stall
Authors: Pismenny Joseph , Levy Yeshayahou , Modelevsky Asaf
Published: November 2013

Investigation of the nature and properties of dynamic processes using different methods of processing of the measured signals may lead to erroneous interpretations and conclusions. One of the reasons for erroneous interpretations is applying only one analysis method. The use of two different methods allows reducing erroneous conclusions but does not eliminate them completely. Such erroneous conclusions concerning pressure oscillations during rotating stall in axial compressors are described when two conventional methods of information processing (auto-correlation functions and frequency characteristics) were used for analyzing processes with changing frequencies and amplitudes of oscillation. These methods have been used for the analysis of aerodynamic processes during little change in frequency (a process very close to the established). This led to an erroneous estimation of the characteristics of the investigated process, namely to the interpretation of a beating effect during established rotating stall. It is shown that the use of a third method — the method of spectrograms — may allow the correct interpretation of the process, showing the absence of beats and the existence of a small change of main frequency of the rotating stall during the process, interpreted as established process. At the same time, it is shown that in the initial transient stage of pressure oscillation prior to the establishment of the rotating stall, beating or processes close to it can be observed.
Direct Numerical Simulations of a Laminar Separation Bubbles on a Curved Plate: Part 2 — Flow Control Using Pulsed Vortex Generator Jets
Authors: Balzer Wolfgang , Fasel Hermann F.
Published: November 2013

Highly-accurate direct numerical simulations (DNS) are employed to investigate active flow control of laminar boundary layer separation by means of pulsed vortex generator jets (VGJs), i.e. by injecting fluid into the flow through a spanwise array of small holes. The uncontrolled flow configuration is represented by a laminar separation bubble developing on a curved-plate geometry modeling the convex suction-side curvature of the Pratt&Whitney “PackB” research blade. The simulation setup and uncontrolled flow results were presented in part I of the present paper. In this second part, particular focus is directed towards identifying the relevant physical mechanisms associated with VGJ control of low Reynolds number separation, as for example encountered in low-pressure turbine applications. The numerical results confirm findings of earlier flat-plate simulations, which showed that the control effectiveness of pulsed VGJs can be explained by the fact that linear hydrodynamic instability mechanisms are exploited. When pulsing with frequencies to which the (uncontrolled) separated shear layer is naturally unstable, instability modes are shown to develop into large-scale, spanwise-coherent structures. These structures provide the necessary entrainment of high-momentum fluid causing a much sooner reattachment of the separated flow compared to the uncontrolled flow and consequently leading to a significant reduction in performance losses.
Aero-Mechanical Optimization Design of a Transonic Fan Blade
Authors: Deng Xiangyang , Guo Fushui , Liu Yesheng , Han Pinlian
Published: November 2013

This paper presents the optimization design of a high bypass ratio civil fan blade with the consideration of aerodynamics, static and dynamic mechanics. The baseline fan blade was designed with a conventional approach without using automatic optimization techniques on both the aero side and the mechanical side. Therefore, the objective of this paper is to achieve a higher aero-mechanical performance under the multiple aerodynamic and mechanical constraints.

Before the optimization, the static stress and modal analysis are performed on the baseline fan blade with/without the introduction of the arc dovetail root and shank. The results are compared to investigate the necessity of including the arc root and shank in the aero-mechanical optimization.

With respect to the optimization process, the numerical design of experiment (DOE) by means of high fidelity CFD/FEA computations is firstly performed to construct the database for the initialization of Kriging surrogate mode. After that, the surrogate model is integrated with the optimization design process, and the non-dominated sorting genetic algorithm (NSGAII) is implemented to obtain the Pareto front, based on which the optimal design is selected. Utilizing this optimization process, both the aero-only and aero-mechanical optimizations are carried out.

The results show that the attenuation of the 3D shock wave strength between the middle and shroud span improves the overall aero performance of the fan blade in both the aero-only optimal design and the aero-mechanical optimal design. Compared with the aero-only optimal design, the aero-mechanical optimal design shows the efficiency penalty within all the operation range simulated, however, the mechanical performance is significantly enhanced by the mitigation of the static stress level on the entire arc dovetail root and shank as well as the increase of the resonance margin.
On Conservative CTOD Estimation Using Far Crack Deformation Field
Authors: Bednarz Piotr , Szwedowicz Jaroslaw
Published: November 2013

In general engineering practice, crack tip opening displacement (CTOD) is very convenient approach for prediction of the components fracture mechanics (FM) lifetime. FM lifetime calculations are defined very well in industry and the lifetime prediction methods based on the CTOD resolve linear and nonlinear material behavior for monotonic and cyclic responses. The experiments confirm that under plasticity conditions the crack tip blunts for small scale or large scale yielding while, crack flanks open against each other only under elastic conditions. However, the CTOD application requires a very fine mesh in order to predict a crack tip deformation in reliable manner. Therefore, much more engineering work have to be involved in fine FE modeling. The crack tip flank deformation is crucial parameter responsible for reliable prediction of the nonlinear energy release rate, which is obtained from Hutchinson-Rice-Rosengren solution and the Shih rule. In accordance with design guidelines, the nonlinear energy release rate obtained from the CTOD must be evaluated conservatively to meet demands of RAM (Reliability, Availability and Maintainability). By using far crack deformation field, the paper proposes an engineering approach, which predicts the CTOD in a conservative manner under elastic-plastic conditions. This novel method is validated numerically by applying the well-known J-integral approach.
One-Piece Flow Blade Tuning Process
Authors: Han Pinlian , Chen Tiefeng , Liu Yesheng
Published: November 2013

Blade tuning is one of the key tasks of the modern aircraft engine design. Generally, a couple of vibration modes of a blade need to be tuned by changing the distribution of its mass and/or stiffness to avoid the resonance at some specified rotating speed. The widely used traditional method for blade tuning is a loop-based process. It requires a number of design iterations between the aero engineer and the structure engineer. It usually takes a few weeks or up to a few months, depending on the experiences of the engineers, to reach an acceptable new design. The uncertainties caused by the non-convergence associated with the loop-based iteration process have led to serious issues for both project management and quality control. To avoid the problem, this paper presents a robust one-piece flow process based on the principle of the set-based concurrent engineering and applying the concept of Lean & DFSS.

Following the concept of Lean & DFSS process, the one-piece flow blade tuning starts with defining key parameters and objectives. These parameters consist of a set of design variables relevant to both structure and aero disciplines. The variation range of each parameter is predefined by the manufacture feasibility and the past design experiences.

Based on the principles of DOE, only a definite number of FEA models are developed and analyzed. From these analysis results, important parameters are selected and a family of response functions of those concerned objectives is formulated. The achievable ranges of the objectives are well determined accordingly.

Guided by these functions, the structure and aero engineers are then able to evaluate instantly any new design from the various combinations of the above-mentioned design parameters. A robust design can be obtained from the given sensitivity of the variance of those design parameters.

This methodology can be applied to blade and vane tuning for fans, compressors and turbines alike. In this paper an example of the application of this process is presented for a hollow fan blade tuning, which is accomplished within a week as planned. The methodology can be standardized and used in many other related engineering design processes as well.
Impact of Swirl on the Sensitivity of the Radial Mode Analysis in Turbomachinery
Authors: Laguna Juan D. , Bartelt Michael , Seume Joerg R.
Published: November 2013

Sound measurements in turbomachinery are a prerequisite for the study and consequent understanding of sound propagation mechanisms. For analyzing these measurements, the Radial Mode Analysis (RMA) is applied. This method decomposes the transmitted sound field in dominant acoustical modes at specific frequencies. Before an experimental campaign is carried out, measurement parameters are selected such that the uncertainty in the results from the application of the RMA is minimized.

In order to minimize uncertainties, a sensitivity analysis of the parameters which influence the overall error of the RMA is performed. This analysis focuses mainly on the output of a measurable quantity, namely on the propagating mode amplitudes. Using a numerical simulation, modal structures are generated based upon real turbine operating data with swirling flow and a characteristic operating temperature. The swirling flow is generated by adding an axial vortex to a constant flow-velocity profile. The results show that the sound field varies under consideration of swirling mean flow compared to uniform flow conditions. In the present case, higher-order modes dominate the propagating sound structure. The parameters studied for assessing the sensitivity are the signal-to-noise ratio of the measurement sensors, the number of triggered revolutions, the azimuthal spacing of the sensors, and a triggering delay. The sensitivity analysis gives a detailed insight into the measurement parameters influencing the output of the RMA, e.g. that small triggering delays cause appreciable measurement errors. This knowledge is used to define the requirements for high fidelity measurements.
Effects of Tip Clearance on the Stall Inception Process in an Axial Compressor Rotor
Authors: Yamada Kazutoyo , Kikuta Hiroaki , Furukawa Masato , Gunjishima Satoshi , Hara Yasunori
Published: November 2013

The paper presents experimental and numerical studies on the effects of tip clearance on the stall inception process in a low-speed axial compressor rotor with a large tip clearance. It has been revealed that in the small tip clearance case, shortly after the spike disturbance which results from the leading-edge separation near the rotor tip appears, the tornado-like vortex is generated by the separation, and soon the compressor falls into stall. In the large tip clearance case, the experiment showed that the performance characteristic differs from that in the small tip clearance case at near-stall conditions. This implies that the stall inception process differs with the tip clearance size. The flow phenomenon in the stall inception leading to such difference has been investigated in this study.

Pressure and velocity fields which were ensemble-averaged and phase-locked by the periodic multi-sampling technique were measured on the casing wall and downstream of the rotor, respectively. In addition, to capture the unsteady flow phenomena inside the rotor, “Instantaneous Casing Pressure Field Measurement” was carried out: instantaneous casing pressure fields in one rotor passage region were measured by 30 high response pressure transducers mounted on the casing wall. In order to investigate further details of near-stall flow field for the large tip clearance, DES (Detached Eddy Simulation) has been conducted using a computational mesh with 120 million points. The results are compared with those from previous studies for the small tip clearance.

As expected, the measurement results show notable differences in the near-stall flow field between the two tip clearance cases. The results from the casing pressure measurement show that high pressure fluctuation appears on the pressure side near the rotor leading-edge in the large tip clearance case. In the result of the velocity field measurement downstream of the rotor, high turbulence intensity is found near the casing in the large tip clearance case. The numerical results reveal that the vortex breakdown occurs in the tip leakage vortex and induces the oscillation of the tip leakage vortex with its unsteady nature. The flow phenomena confirmed in the experimental results are clearly explained by considering the breakdown of the tip leakage vortex. The vortex breakdown gives rise to not only large blockage but also the rotating disturbance through the interaction of the fluctuating tip leakage vortex with the pressure surface of the adjacent blade, and governs the stall inception process.
Touchdown Bearing Contact Forces in Magnetic Bearing Systems
Authors: Saket Fawaz Y. , Sahinkaya M. Necip , Keogh Patrick S.
Published: November 2013

Under contact-free levitation, rotors supported by active magnetic bearings have many advantages such as allowing near frictionless rotation and high rotational speeds. They also provide the designer the capability to achieve increased machine power density. However, magnetic bearings possess limited load capacity and operate under active control. Under certain operational conditions, the load capacity may be exceeded or a transient fault may occur. The rotor may then make contact with touchdown bearings and the ensuing rotor dynamics may result in transient or sustained contact dynamics. The magnetic bearings may have the capability to restore contact-free levitation, though this will require appropriate control strategies to be devised. An understanding of the contact dynamics is required, together with the relationship between these and applied magnetic bearing control forces. This paper describes the use of a contact force measurement system to establish the force relationship. The contact force components measured by the system are calibrated against forces applied by an active magnetic bearing. The data generated can be used to validate non-linear dynamic system models and aid the design of control action to minimize or eliminate contact forces.
Statistical Impact of Manufacturing Tolerances on Axial Gaps Between Vane Segments and the Rotor of Axial Flow Turbo-Compressors
Authors: Weihard Stefan , Hupfer Andreas , Kau Hans-Peter
Published: November 2013

Geometrical deviations arising from the manufacturing process significantly influence the axial gaps between rotating and nonrotating parts. To optimize axial gaps and thus reduce the overall length and weight of the compressor, it is essential to take the statistical nature of these deviations into account.

The focus of this paper is to identify complex three-dimensional effects caused by geometrical uncertainties. The results can be then used to define an appropriate level of detail for the simulation model in order to accurately predict the probability distribution of axial gaps.

In the simulation model the parts are assembled according to defined mechanical constraints and the paper presents mathematical models regarding this. A simplified 2D model of a sectional view and a detailed 3D model are implemented and investigated. A 2D tolerance analysis shows that a statistical gap tolerance can be defined based on geometrical parameters. It must be assumed that a certain percentage of all gap values will fall below the lower specification limit for this tolerance interval. A detailed 3D analysis, however, results in a change in the probability distribution of the gap values compared to the simplified 2D analysis. In addition to the standard deviation, the mean value of the gap is reduced significantly. Therefore, a simplified 2D approach may yield invalid results. What this effect is will depend on the configuration. One influencing factor is the number of vane segments in the compressor stage. A sensitivity analysis is presented which identifies and quantifies the impact of such important design parameters.
GPU Speed-Up for the Implicit Navier-Stokes Solver
Authors: Huang Junfeng , Lin Zhirong , Ma Can , Yuan Xin
Published: November 2013

In this study, an in-house higher-order, high resolution implicit CFD solver for Favre-averaged Navier-Stokes equation with two-equation turbulence model, which including fourth-order MUSCL TVD scheme for Right-Hand-Side and LU-SGS-GE scheme for Left-Hand-Side, is implemented to run on a single Graphic Process Unit (GPU) using the Compute Unified Device Architecture (CUDA).

The partly parallel model for implicit scheme is introduced and we have got a good acceleration using this partly parallel model. The case of VKI 59 is used to test the accelerations. Results obtained on a single GPU (NVDIA GeForce 560Ti, Memory: DDR5 1G 2004 Mhz) are compared with the results obtained on the single CPU (Intel i5 2.8–3.0 GHz, Memory: DDR3 8G 800 Mhz). The speed-up is about 25. The fourth-order MUSCL TVD used in the code is extended to the simulation of multi-species, and also speed-up by using GPU. The case of a single stage turbine with hot-spot inlet boundary is used to test the code with multi-species scheme using GPU. Furthermore, in the study, the shared memory is used to speed-up the code, which is introduced in details in the paper. In the end, the influence factors of the acceleration are discussed.
Aerodynamic Origin of Rotor-Rotor Interaction Noise From Unducted Propulsors
Authors: Danner Florian , Kendall-Torry Christofer , Kau Hans-Peter
Published: November 2013

The sound arising from blade row interaction in open rotor propulsion systems is known to significantly contribute to overall noise emissions. The present paper therefore addresses the origination of rotor-rotor interaction noise from a pair of unducted counter-rotating fans. The focus is on the aerodynamic mechanisms that involve sound generation, in order to provide the physical understanding required to find noise-reducing means. Detailed insight into the underlying phenomena is provided on the basis of numerical simulations applying the unsteady Reynolds-averaged Navier-Stokes equations. The interaction mechanisms are identified by extracting the time-dependent disturbances of the flow field in the respective rotor relative frame of reference. Conclusions on the sources of interaction noise and potential noise-reducing means are drawn by evaluating polar directivities, blade surface pressure distributions and propagation characteristics.
Predicting High-Lift LP Turbine Cascades Flows Using Transition-Sensitive Turbulence Closures
Authors: Pacciani Roberto , Marconcini Michele , Arnone Andrea , Bertini Francesco
Published: November 2013

This paper discusses the application of different transition-sensitive turbulence closures to the prediction of low-Reynolds number flows in high-lift cascades operating in low-pressure turbine (LPT) conditions. Different formulations of the well known γ – R̃eθt model are considered, and compared to a recently developed transition model based on the laminar kinetic energy (LKE) concept. All those approaches have been coupled to the Wilcox k – ω turbulence model. The performance of the transition-sensitive closures has been assessed by analyzing three different high lift cascades, recently tested experimentally in two European research projects (UTAT and TATMo). Such cascades (T106A, T106C, and T108) feature different loading distributions, different suction side diffusion factors, and they are characterized by suction side boundary layer separation when operated in steady inflow. Both steady and unsteady inflow conditions (induced by upstream passing wakes) have been studied. A particular attention has been devoted to the treatment of crucial boundary conditions like the freestream turbulence intensity and the turbulent length scale. A detailed comparison between measurements and computations, in terms of blade surface isentropic Mach number distributions and cascade lapse rates will be presented and discussed. Specific features of the computed wake-induced transition patterns will be discussed for selected Reynolds numbers. Some guidelines concerning the computations of high-lift cascades for LPT applications using RANS/URANS approaches and transition-sensitive closures will be finally reported.
Influence of Freestream Turbulence on the Aerodynamic Performance of Transonic Vanes
Authors: Edwards R. , Asghar A. , Allan W. D. E. , Woodason R. , LaViolette M.
Published: November 2013

The present authors have reported a noticeable reduction in the aerodynamic performance of turbine vanes which had been modified as a result of commonly applied repair processes. These tests were done at a low turbulence level to isolate the profile-only effect. In the present research, the effect of the same profile modification on the performance of the vanes was investigated at engine representative flow conditions by increasing the turbulence level and length scale. Since the tested vane profiles in the present research were synthesized using the profile of LPT vanes, the turbulence level was maintained at around 4% and the length scale was set at 2 cm. In the present investigation, calculations with computational fluid dynamics and measurements in a transonic cascade rig were carried out. The high turbulence level in the cascade rig was produced using a passive turbulence-generating grid and in CFD by specifying the desired level and length scale. Coordinates of the baseline profile were obtained from the LPT vanes of an in-service turboshaft engine using 3D optical scanning and digital modeling. The repaired vanes were synthesized using profiles representative of two specific repair types. In both methods, flow visualization was carried out using axial density gradient or schlieren and exit total pressure was obtained numerically or using a multihole probe. Further insight into the flow phenomenon was obtained by surface flow visualization in the cascade rig using a graphite and paraffin oil mixture and by computed surface pressure distributions on the vane. The shock pattern in the cascade for low and high turbulence flows was similar; however, the surface flow pattern exhibited a significant difference for the two conditions. The total pressure ratio and cascade loss also showed some differences.
Mathematical Development and Modelling of a Counter Balance Compensating Sleeve for the Suppression of Lateral Vibrations in High Speed Flexible Couplings
Authors: Kirk Antony , Knowles Grahame , Stewart Jill , Bingham Chris
Published: November 2013

High speed drive shafts are traditionally balanced using trim balance weights applied to the shaft ends. This paper considers the development and theoretical analysis of a novel and alternative strategy of balancing long flexible coupling shafts, whereby the trim balancing weights are applied by the means of a pair of ‘Balancing Sleeve’ arms that are integrally attached to each end of the coupling shaft. The trim balance weights are intended to apply a corrective centrifugal force to the coupling shaft in order to limit shaft end reaction forces. With increasing speed, the magnitude of the corrective force also increases due to the flexibility of the balance sleeve. This thereby counteracts the increased coupling shaft unbalance resulting from its own flexibility. Additionally, it is also found that the mechanism imparts a corrective bending moment to the coupling shaft ends, which has a tendency to limit deflection. The methodology is modelled as a rotating simply supported shaft with uniform eccentricity and allows application to the problem of drivetrain balancing of sub-15MW industrial gas turbines. Results show that reaction loads can theoretically be reduced from 10,000 N to approximately zero. The bending moment applied to the shaft is also shown to reduce shaft deflection theoretically to zero. In practical applications this will be unrealistic and achievable results show deflection theoretically reduced by half. Analysis of the balance sleeve feasibility is considered through use of a three-dimensional finite element model. Further to this paper, the aim is to develop a full dynamic model of both shaft and counterbalance sleeve, with verification coming from scaled, experimental test facilities.
Radial Inflow Turbine: Synchronous Vibration Failure
Authors: Duong Loc Q. , Thayalakhandan Nagamany , Hu Charlene X.
Published: November 2013

Radial inflow turbine is frequently used in small gas turbine application where ruggedness and simplicity are prime requisites. For compactness, the radial turbine is mounted back to back with a radial compressor resulting in an overhung rotor in both compressor and turbine wheel. In such configuration the structural integrity of the radial turbine is crucial since due to its large inertia its failure could result in an uncontained egress of the engine. This paper presents one class of radial inflow turbine wheel failure due to high cycle fatigue of synchronous nature caused by two different excitation sources, namely mechanics and aerodynamics. While the first source of excitation is associated with the dynamics of the rotor, the second one is related to the dynamic of fluid flow and the thermal characteristics of the combustor, expressed in terms of hot streak. These sources of excitation can act individually or in combination. Two distinct types of failures were illustrated — blade mode — and disc-blade coupling mode. The failure phenomenon is characterized by fatigue crack originated at location corresponding to maximum dynamic stress for each type of failures, followed by the released of one portion of the blade.

Analytical methods including finite element method, rotor dynamics analysis, and computational fluid dynamics are used to illustrate the root cause of such failure and also to its underlying solutions. Laser vibrometry and optical method were used to obtain the blade dynamic characteristics and to validate the solutions.
Numerical Investigation of Rotor-Stator Interaction in a High-Bypass Ratio Fan Stage
Authors: Yang Zhong , Wang Hengliang , Yan Jiachao , Deng Xiangyang
Published: November 2013

Both steady and time-accurate simulations of the flow through a high-bypass ratio fan stage consisting of transonic fan rotor blade (FAN), outlet guide vane (OGV) of bypass duct, and inlet guide vane (IGV) of low-pressure compressor (LPC) are performed at design operating point to address the fundamental questions regarding the impacts of unsteady rotor-stator interactions. The original ratios of fan blade number to OGV and IGV vane numbers are scaled to avoid the full wheel computation, thus reducing the amount of time-accurate computation effort. The comparison between time-averaged unsteady results and steady results shows minor differences in the performance of bypass and core ducts. The transient flow field investigation shows that the unsteady interaction between FAN and IGV is much stronger than that in the bypass duct. Considering aerodynamic response and excitation mechanism due to rotor-stator interaction, the unsteady pressure distributions are examined. The time traces and frequency contents of the unsteady pressure show complex rotor-stator interaction arising from the incoming wakes and potential effects of downstream blade rows.
Experimental Investigation of the Noise Generation and Propagation for Different Turning Mid Turbine Frame Setups in a Two-Stage Two-Spool Test Turbine
Authors: Faustmann C. , Lengani D. , Spataro R. , Marn A. , Göttlich E. , Heitmeir F.
Published: November 2013

The paper deals with the investigation of the noise generation in the two-stage two-spool test turbine located at the Institute for Thermal Turbomachinery and Machine Dynamics (ITTM) at Graz University of Technology. The rig went into operation within the EU-project DREAM, where the target was to investigate the aerodynamics of interturbine flow ducts. The facility is a continuously operating cold-flow open-circuit plant which is driven by pressurized air. The flow path contains a transonic turbine stage (HP) followed by a low pressure turbine stage consisting of a turning mid turbine frame and a counter-rotating LP-rotor.

Downstream of the low pressure turbine a measurement section is instrumented with acoustic sensors. The acquisition system consists of a fully circumferentially traversable microphone array located at the outer casing, while at the hub endwall a stationary flush mounted microphone is placed as a reference.

Additionally a new embedded concept for the turning mid turbine frame was tested. Here, two zero-lift splitters were located into the vane passage.

In order to evaluate the noise emission of the turbine the facility was instrumented with a new acoustic measurement setup which is presented in the paper. Therefore the emitted sound pressure level and the microphones signal spectra are compared for both configurations. The acoustic field was characterized by azimuthal modes by means of a microphone array traversed over 360 degrees.

In the multisplitter configuration, the propagating modes due to the HP turbine are found suppressed by 5 dB, while the increase in amplitude of the modes related to the LP turbine is negligible. The overall effect is a reduction of the acoustic emission for the turning mid turbine frame with embedded design.
Time-Resolved Near-Wake Measurements of a 2MW Wind Turbine
Authors: Mansour Michel , Atalayer Caglar , Chokani Ndaona , Abhari Reza
Published: November 2013

This paper presents time-resolved velocity measurements performed in the near wake of a multi-megawatt wind turbine, using a novel nacelle-mounted fast-response aerodynamic probe. The aerodynamic probe, which has been developed at ETH Zurich, consists of a hemispherical 5-hole probe equipped with fast-pressure sensors. The probe has a measurement uncertainty of ±0.1m/s and a measurement bandwidth of 65Hz. In addition to measurement of the three-dimensional wind velocity vector, the probe is instrumented for the real-time monitoring of meteorological conditions. The measured data are processed in real-time, stored on on-board and accessible via a GPRS modem. As the aerodynamic probe is installed adjacent to the wind turbine’s ultrasonic anemometer, the measurements of the two systems can be compared. The measured wind speeds are found to be in very good agreement and remains on an averaged within ±0.24m/s deviation to the ultrasonic anemometer. The measured yaw angle shows an average offset of −7.5°. This difference is observed since the ultrasonic anemometer does not accurately capture the turning of the flow across the wind turbine’s rotor. From the time-resolved measurements of the aerodynamic probe, the phase-lock averaged measurements show that over one blade passing period the turbulence intensity varies from 13 to 24%, with a maximum degree of anisotropy above 1.4. It is found that a hub passage vortex, which extends over more than 50% of the blade passage width, is present. Thus, from a turbine control perspective the actual placement of the ultrasonic anemometer, even when corrected, can lead to high yaw angle misalignment when the wind turbine is located in moderately or highly complex terrain.
A Systematic Approach to Estimate the Impact of the Aerodynamic Force Induced by Rotating Stall in a Vaneless Diffuser on the Rotordynamic Behavior of Centrifugal Compressors
Authors: Bianchini Alessandro , Biliotti Davide , Ferrara Giovanni , Ferrari Lorenzo , Belardini Elisabetta , Giachi Marco , Tapinassi Libero , Vannini Giuseppe
Published: November 2013

One of the main challenges of the present industrial research on centrifugal compressors is the need of extending the left margin of the operating range of the machines. As a result, interest is being paid in accurately evaluating the amplitude of the pressure fluctuations caused by rotating stall, which usually occurs prior to surge. The related aerodynamic force acting on the rotor can produce subsynchronous vibrations, which can prevent the machine to further operate, in case their amplitude is too high. These vibrations are often contained thanks to the stiffness of the oil journals.

Centrifugal compressors design is, however, going towards alternative journal solutions having lower stiffness levels (e.g. Active Magnetic Bearings or Squeeze Film Dampers), which hence will be more sensitive to this kind of excitation: consequently, a more accurate estimation of the expected forces in presence of dynamic external forces like those connected to an aerodynamically unstable condition is needed to predict the vibration level and the compressor operability in similar conditions.

Within this scenario, experimental tests were carried out on an industrial impeller operating at high peripheral Mach number. The dedicated test rig was equipped with several dynamic pressure probes that were inserted in the gas flow path; moreover, the rotor vibrations were constantly monitored with typical vibration probes located near the journal bearings.

The pressure field induced by the rotating stall in the vaneless diffuser was reconstructed by means of an ensemble average approach, defining the amplitude and frequency of the external force acting on the impeller. The calculated force value was then included in the rotordynamic model of the test rig: the predicted vibrations on the bearings were compared with the measurements, showing satisfactory agreement.

Finally, the prospects of the proposed approach are discussed by investigating the response of a real machine in high-pressure functioning when different choices of journal bearings are made.
Speed Dependency of Coupled Rotor-Blade Torsional Natural Frequencies
Authors: Ehehalt Ulrich , Becs Balazs , Zhou Xiaoping , Güllenstern Stefan
Published: November 2013

The natural frequencies of blades depend on the rotational speed of the rotor train as the stiffness changes with centrifugal loading. In the case of low pressure turbines with shrunk-on-disc design the coupled rotor-blade torsional natural frequencies can also show this property. For proper analysis of the speed dependency, a complete rotor-blade model which takes the elasticity of the blades into account is required.

In this paper the torsional natural frequencies calculated with a complete rotor-blade model are compared with those calculated with a model in which blade elasticity is not included. The analysis clearly demonstrates that calculations without blade elasticity lead to different natural frequencies. By modeling the complete rotor and taking blade elasticity into account, it is demonstrated that the torsional natural frequencies of a complete rotor-blade model can also become speed dependent. As a consequence, a distinction between the natural frequencies at nominal speed and natural frequency at critical speeds becomes necessary.

In the following, measured torsional natural frequencies at different rotating speeds of an individual low pressure rotor are presented. A comparison of the measured speed dependency of the torsional natural frequency with calculation results thereby taking the blade elasticity into account is conducted. The analysis shows that the measured speed dependency can be predicted with a high level of accuracy and can become important for modes which are dominated by the blades of the last stages.

As a consequence of this analysis, a clear distinction between natural frequency at nominal and at critical speed has to be made for certain rotor and blade designs. It is shown that the use of the Campbell diagram is highly beneficial for designing rotor trains with large blades with regard to their torsional vibration behavior.
Performance Improvements of Centrifugal Compressors Through Shroud Cavity Leakage Management
Authors: Basol A. M. , Abhari R. S.
Published: November 2013

This study numerically investigated the effects of the geometry modifications in the vicinity of the shroud cavity area of a high flow coefficient, multi-stage, inline centrifugal compressor on its efficiency. The modifications in the shroud cavity area cover the lean of the seal teeth geometries and their streamwise positioning. The baseline four teeth seal geometry has been modified which resulted in 15 % reduction in the leakage mass flow and increased the compressor’s efficiency by 0.17 % by even reducing the number of the teeth to three.

Modifications in the radial inlet duct geometry aimed to reduce the pressure difference across the shroud cavity by providing further static pressure recovery at the shroud cavity outlet. The modified inlet duct design resulted in a further 0.13 % rise in efficiency in spite of the minor 4 % additional drop in the leakage mass flow. The modified inlet duct performed better only in presence of the shroud cavity leakage flow. Excluding the leakage the modified inlet duct resulted in a lower efficiency value compared to the efficiency value obtained with the existing inlet duct. These findings point out a possible reduction in the mixing loss between the main flow and the shroud cavity leakage flow with the modified inlet duct design which reduced the Mach number level close to the shroud side wall due to the increased static pressure. As the final conclusion on the design of the radial compressors this work shows the importance of considering the leakages at the early stages of the compressor design even deciding on the meridional flow path.