bluuebunny/crossref_metadata_2025 huggingface dataset:
Microturbine Rotational Speed Selection
Authors: Rodgers C.
Published: November 2013

This paper delves upon the Aero-Thermodynamic performance and Mechanical design aspects of microturbines comprising a single shaft radial compressor driven by a single stage radial inflow turbine with a combustor and recuperator sized to directly drive a permanent magnet type high speed generator with an output power in the 5–10KW bracket and commensurate rotational speeds in the 100–200 krpm range.

It is initially shown that stipulation of a cycle design point output power, turbine inlet or exit temperatures, and compressor pressure ratio delivering optimum thermal efficiency inherently confines rotational speed selection, and that independent rotational speed choice away from those identified optimum speed regimes may result in cycle thermal efficiency compromises. Confining the cycle analysis within temperature limits of cost competitive superalloys and foil materials reveals that the achievement of optimum thermal efficiency is more dependent on temperature at the turbine exit rather than at inlet.

Albeit the choice of rotational speed is of particular importance in the compressor and turbine design it moreover is dominant in the mechanical design of the rotating assembly in terms of high speed bearing life and shaft dynamic stability.

As a consequence rotating assembly and bearing design options suitable for direct drive permanent magnet generators are reviewed and recommendations offered as to the prime candidate assemblies for future microturbines in the 5.0 to 10.0 kW power output range.
Electro Compression a Challenging Alternative: How and Why to Choose a Gas Turbine or an Electric Motor to Drive a Centrifugal Compressor
Authors: Alas Pascal , Noulette Eric
Published: November 2013

In the past decade, electric motors have become a challenging alternative to gas turbines to drive centrifugal compressors [1] [7], mainly because of the progress of Variable Frequency Drives main components allowing to drive high power motors. Low Electricity prices (mainly in France) and environmental concerns have contributed to spread that technology across Europe. As flexibility needs and environmental constraints will grow in the gas transmission business, the option to drive the centrifugal compressors with an electric motor will be taken into consideration more and more often either for technical, regulatory or economic reasons in future compressor station projects [2].

GRTgaz, the main gas transmission company in France, decided in the early 2000’s to enlarge and renew its compressor fleet (42 units to be installed between 2004 and 2014 in a 5 to 14 MW power range). GRTgaz is taking that option into account for each of these projects considering the following criteria, related to 3 main considerations:

Operational needs:

• Availability, reliability

• Rangeability needs

• Flexibility needs

• Type of compressor station (head, peak, network core)

• Access to the electrical grid

• Available space for the site

• …

Environmental constraints:

• Environmental Footprint

• Air emissions

• Noise

• …

The full possession cost for a 30-year period:

• Investment cost

• Energy cost

• Maintenance cost

• CO2 quota prices

This paper is proposing to develop how these criteria are assessed, what hypotheses are made to help the decision and to share GRTgaz feedback on the projects that have already been completed. Some examples will be presented.
Comments on a Newly-Identified Destabilizing Rotordynamic Mechanism Arising in Vertical Hydraulic Turbines and the Back Shrouds of Centrifugal Impellers
Authors: Childs Dara W. , Muhammed Ameen
Published: November 2013

In three 2010 papers, Tsujimoto, Ma, Song, and Horiguchi [1–3] discussed and explained a novel destabilizing mechanism arising in both hydraulic turbines and the back surface of vertical pump impellers. The destabilizing mechanism can be explained via a reaction force-moment model that includes both the customary radial displacement vector of an impeller plus the pitch and yaw degrees of freedom. This coupling between radial displacements and tilt plus the coupling of the shaft support structure can create negative damping. In 1993, Verhoeven, et al. [4] identified negative damping arising from U-shaped wearing-ring seals as causing a super-synchronous instability in a horizontal coke-crusher pump. However, several case studies have been presented of super-synchronously unstable pumps for which (until now) no explanation could be provided.

Tsujimoto-Song started with a 2DOF model for a disk suspended vertically via a cantilevered shaft. They used an f = ma model for the lateral displacements of the disk and used flexibility coefficients to account for reaction forces and moments from the back shroud of the impeller. The present work starts with a 4DOF model that includes the disk’s displacements and pitch and yaw degrees of freedom. Guyan reduction is used to create two reduced 2DOF models, Model A that retains the displacements and discards the rotations and Model B that retains the rotations and discards the displacements. Model A produces a requirement for instability that is inconsistent with Tsujimoto-Song’s experience and predictions. However, it is useful in predicting the reaction moments produced by a nominally planar precession of the impeller.

The instability requirement of Model B is consistent with Tsujimoto’s experience and predictions. A comparison of the predicted reaction moments of Model A and Tsujimoto’s reaction-moment data supports the instability predictions of Model B (and Tsujimoto-Song) that the instability arises due to coupling between the displacement and rotation degrees of freedom in the 4 × 4 damping matrix.
Consequential Damage of Blade Failures in Single Shaft Gas Turbines
Authors: Wettstein Hans E.
Published: November 2013

In history of single shaft gas turbines many blade failures have happened. In most cases the consequential damage has been limited to the blading in the downstream gas path. This is unavoidable because of the impact of debris. But in some cases the related unbalance of the rotor has caused additionally extended damage during the run-down of the engine. In the worst publicly known case the result was a complete destruction of the engine with disintegration of the rotor. The general rotordynamic literature indicates features and criteria to prevent such a high sensitivity to unbalance. Some manufacturers use internal design criteria for this purpose. A sometimes mentioned criterion in this respect is: “To keep the rotation speed below the frequency of the first bending mode of the rotor”.

The aim of this paper is to track generally the history of gas turbine blade failures and to explain the design features of rotors and bearing support structures, which lead to high or low risk of secondary damages after blade failures. This is not new but a description from a gas turbine design point of view is missing so far.

Both theory and experience indicate that an exact prediction of the response amplitudes to a large unbalance from a blade failure is very difficult. On the other hand the most important criteria for a robust behavior in this respect are simple to describe. And they could easily be specified and verified by manufacturers, customers or insurers.
Assessment of High Pressure Cooled and Uncooled Turbine Blade Wakes via RANS and URANS at Engine Scale Conditions
Authors: Kopriva James E. , Laskowski Gregory M. , Sheikhi M. Reza H.
Published: November 2013

Numerical simulations (RANS and URANS) are quantitatively and qualitatively compared to both trailing-edge cooled and uncooled turbine blade experimental data. The numerical studies are carried out at engine scale conditions. Focus is given to the turbulence model’s ability to predict the following flow quantities: turbine blade loading, boundary layer, base pressure, and wake profiles. RANS and URANS grid converged solutions are assessed. URANS provided accurate quantitative assessment of trailing-edge shedding frequency and base pressures, while qualitatively highlighting energy separation in the wake.
Flutter Prediction of a Transonic Fan With Travelling Wave Using Fully Coupled Fluid/Structure Interaction
Authors: Im Hong-Sik , Zha Ge-Cheng
Published: November 2013

This paper uses a fully coupled fluid/structure interaction (FSI) to investigate the flutter mechanism of a modern transonic fan rotor with a forward travelling wave. To induce an initial travelling wave for the blade structure, an initial BC that can facilitate each blade to vibrate with a time lag by a given nodal diameter (ND) is implemented. Unsteady Reynolds-averaged Navier-Stokes (URANS) equations are solved with a system of structure modal equations in a fully coupled manner. The 5th order WENO scheme with a low diffusion E-CUSP Riemann solver is used for the inviscid fluxes and a 2nd order central differencing is used for the viscous terms. A half annulus sector is used for the flutter simulations with a time shifted phase lag boundary condition at the circumferential boundaries. The present FSI simulations show that the shock instability causes the flutter. When the detached normal shock moves further upstream in a direction normal to the blade chord, the interaction of the detached normal shock with tip leakage vortex creates more serious blockage to the blade passage that can introduce an aerodynamic instability to the blade structure due to the incoming flow disturbance, resulting in flutter. The flutter of the transonic fan observed in this study occurs at the 1st mode before the stall. The predicted flutter boundary agrees well with the experiment.
Influence of a Finite Width Micro-Tab on the Spanwise Lift Distribution
Authors: Holst D. , Bach A. B. , Nayeri C. N. , Paschereit C. O.
Published: November 2013

The results of surface pressure measurements are presented in this paper to gain further insight into the lift changing influence of finite width micro-tabs, especially in adjacent airfoil sections. Micro-tabs are a promising concept for load control on wind turbines. Local pressure distributions were measured in several rows of pressure taps in the vicinity of the finite width micro-tab attached to a FX 63-137 profile at low Reynolds numbers. The investigation focuses on length dependency, chordwise position, and interaction between two micro-tabs. Additionally, stereo Particle-Image-Velocimetry measurements were conducted to study the structure, sense of rotation, and influence of tab-induced tip vortices, as well as the impact of a finite width micro-tab on the model’s near wake. Experiments reveal relative changes of more than 30 % in the pressure coefficient distribution upstream of several micro-tab configurations. Furthermore, increments of 20 % are recorded in neighbouring sections not directly controlled by micro-tabs. Even higher changes are obtained in the region between two tabs. These improvements are attained due to local and global changes in the effective camber.
Modeling of Abradable Coating Removal in Aircraft Engines Through Delay Differential Equations
Authors: Salvat Nicolas , Batailly Alain , Legrand Mathias
Published: November 2013

In modern turbomachinery, abradable materials are implemented on casings to reduce operating tip clearances and mitigate direct unilateral contact occurrences between rotating and stationary components. However, both experimental and numerical investigations revealed that blade/abradable interactions may lead to blade failures.

In order to comprehend the underlying mechanism, an accurate modeling of the abradable removal process is required. Time-marching strategies where the abradable removal is modeled through plasticity are available but another angle of attack is proposed in this work. It is assumed that the removal of abradable liners shares similarities with machine tool chatter encountered in manufacturing. Chatter is a self-excited vibration caused by the interaction between the machine and the workpiece through the cutting forces, and the corresponding dynamics are efficiently captured by delay differential equations. They differ from ordinary differential equations in that previous states of the system are involved. This mathematical framework is here employed for the exploration of the blade stability during abradable removal.

The proposed tool advantageously features a reduced computational cost and consistency with existing time-marching solution methods. Potentially dangerous interaction regimes are accurately predicted and instability lobes match both flexural and torsional modal responses. Essentially, the regenerative nature of chatter in machining processes can also be attributed to abradable coating removal in turbomachinery.
A Novel Weldless Foils-Case Attachment for Gas Foil Bearings
Authors: Di Liscia Marcelo H. , Matute Jose A. , Diaz Gabriela S. , Diaz Sergio E.
Published: November 2013

Gas Foil Bearings (GFB) have a wide field of applications, from air cycle machines to microturbomachinery. A GFB basically consists of a foil (top foil) that lies over a corrugated foil (bump foil) that acts as a compliant spring. The rotation of the shaft introduces the fluid into the bearing and the pressure generated into the gap between the shaft and the top foil supports the rotor. The foils are attached to the case in a fixed point; usually they are welded to the bearing case. In addition to being rather cumbersome to build, this welded union is a potential point of failure. This work presents a new proposal of foils and case union. Foils are not welded to the case; they are placed or slid into a groove which avoids the strain concentration. This also permits the replacement of any foil in case of damage and to change the bump foil to modify the load capacity or the rotordynamic coefficients if required. The proposed assembly is experimentally tested by measuring its static stiffness in different radial directions of the fixed point. Results for static stiffness are presented and discussed.
Identification of Multi-Concurrent Fault in a Steam Turbine Rotor System Using Model-Based Method
Authors: Chen Jingming , Jiang Dongxiang , Liu Chao
Published: November 2013

Fault identification and diagnosis of steam turbine generator unit is very important for safely and economically operation of a power plant. Currently, on-line monitoring is already widely utilized for alarming and recording in steam turbine. However, large amount of on-line monitoring data is not fully utilized to realize on-line fault diagnosis and identification, especially multi-concurrent fault. In the present study, model-based method was used for on-line vibrational fault identification and diagnosis based on rotordynamics. A 660MW supercritical steam turbine rotor system was modeled using FEM. Single faults, such as mass unbalance, local shaft bow and transverse crack, and multi-concurrent faults were simulated. Fault-induced changes of equivalent loads were analyzed to figure out fault type, location and severity. For model-based method, fault diagnosis accuracy is influenced by the model accuracy and signal noise. Model sensitivity was studied in this research by comparing the influence of different model error and SNR (Signal to Noise Ratio). It was used to evaluate the degree of confidence of the diagnosis result. The smaller was the model sensitivity, the higher was the degree of confidence. Based on this research, model-based method was utilized to analyze real vibration signals extracted from the historical data of this unit. Fault identification result was an effective basis for conditioned based maintenance.
Full Design of a Highly Loaded and Compact Contra-Rotating Fan Using Multidisciplinary Evolutionary Optimization
Authors: Joly Michael , Verstraete Tom , Paniagua Guillermo
Published: November 2013

Contra-rotation enables one to produce low-weight and high-load fans suitable for high-speed propulsion systems, such as Air Turbo Rocket engines. This paper presents a Multidisciplinary Design Optimization (MDO) methodology to achieve the full-design of highly-loaded and compact contra-rotating fans.

It utilizes a multi-objective optimization at every step of the design. Performances of the two-stage machine is first evaluated by a through-flow model to determine an optimal flow path configuration. A novel parameterization based on span-wise distributions is used to smooth the transition between the preliminary design phase and the detailed three-dimensional shape optimization. High-fidelity aero-mechanical performances are then considered to generate the detailed design of the rotors, including the section profiles along the span, as well as lean and sweep. The multi-objective optimization algorithm treats simultaneously Computational Fluid Dynamics (CFD) and Computational Structural Mechanics (CSM) performances of both rotors. The designed rotors satisfy a 20% safety margin to the yielding strength of titanium. A pressure ratio of 3.07 is achieved with an overall efficiency of 72.4%. A comparison between the through-flow model and the CFD-based optimized shape is made, illustrating a very close match in velocity distributions and losses. It shows that the through-flow code is able to identify an optimal configuration for highly-loaded turbomachines and only needs a small refinement in the subsequent CFD-based optimization. The developed methodology allows to produce innovative configurations at a reduced time-to-market cost compared to traditional designs.
Control of a Supercritical CO2 Recompression Brayton Cycle Demonstration Loop
Authors: Conboy T. , Pasch J. , Fleming D.
Published: November 2013

The US Department of Energy is currently focused on the development of next-generation nuclear power reactors, with an eye towards improved efficiency and reduced capital cost. To this end, reactors using a closed-Brayton power conversion cycle have been proposed as an attractive alternative to steam turbines. The supercritical-CO2 recompression cycle has been identified as a leading candidate for this application as it can achieve high efficiency at relatively low operating temperatures with extremely compact turbomachinery.

Sandia National Laboratories has been a leader in hardware and component development for the supercritical-CO2 cycle. With contractor Barber-Nichols Inc, Sandia has constructed a megawatt-class S-CO2 cycle test-loop to investigate the key areas of technological uncertainty for this power cycle, and to confirm model estimates of advantageous thermodynamic performance. Until recently, much of the work has centered on the simple S-CO2 cycle — a recuperated Brayton loop with a single turbine and compressor. However work has recently progressed to a recompression cycle with split-shaft turbo-alternator-compressors, unlocking the potential for much greater efficiency power conversion, but introducing greater complexity in control operations.

The following sections use testing experience to frame control actions made by test loop operators in bringing the recompression cycle from cold startup conditions through transition to power generation on both turbines, to the desired test conditions, and finally to a safe shutdown. During this process, considerations regarding turbocompressor thrust state, CO2 thermodynamic state at the compressor inlet, compressor surge and stall, turbine u/c ratio, and numerous other factors must be taken into account. The development of these procedures on the Sandia test facility has greatly reduced the risk to industry in commercial development of the S-CO2 power cycle.
Evolution of Pressure Signature Dominated by Unsteady Tip Leakage Flow
Authors: Geng Shaojuan , Zhang Xiaoyu , Li Jichao , Zhao Long , Zhang Hongwu , Nie Chaoqun
Published: November 2013

This paper deals with the detailed numerical analyses of diverse manifestations of unsteady features induced by periodical oscillation of tip leakage flow under different operating points for the cases with uniform and hub distorted inlet conditions. The characteristics evolutions of pressure signature near rotor tip region during compressor throttling process are studied and compared with the experimental results. Monitors circumferentially arranged and aligned with blade chord are imposed to collect static pressure signals. Analysis methods of coordinate transformation between the rotor relative and absolute stationary reference frames, fast Fourier transform and frequency band pass filter are used. The modulated frequency features along blade chord in two reference frames are analyzed. Typically for the dominated frequency components, the circumferential propagation characteristics are studied, such as propagation speed and mode orders. And the unified evolution trends of modulated frequency relation for dominant components between two reference frames and circumferential propagation features during throttling process are summarized. A critical mass flow point is found to distinguish the different change trend of the characteristics of tip leakage flow unsteadiness. Based on the different speeds between circumferential propagation of tip leakage flow unsteadiness and revolution of compressor rotor, the fluid dynamic reason for the decrease of autocorrelation coefficient of pressure signals from transducer mounted on compressor casing is elucidated. All the results are helpful to further unveil the initiation mechanism of stall inception.
Flutter Susceptibility Approximation via Curve Fitting and MAC-Analysis in an Automated Optimization Design Process
Authors: Blocher Michael , Aulich Anne-Laure
Published: November 2013

At present, several institutes of the German Aerospace Center (DLR) jointly work on the design of a counter rotating fan made of carbon fiber reinforced material. To generate blades with superior aerodynamic properties, a multidimensional optimization tool developed by the Institute of Propulsion Technology is used. Since aeroelastic problems often arise during late design stages and are inevitably very expensive to solve, the goal is to include aeroelastic estimations into this optimization tool which is used in the early design phase.

Assuming that rotors generally show a relatively sinusoidal behavior of the damping-over-IBPA-diagram at operating points sufficiently far away from the surge line, not all IBPAs need to be investigated in order to determine the lowest aerodynamic damping.

An algorithm was implemented that predicts the IBPA with the lowest damping value from three already calculated IBPAs through which a sine curve is fitted. It was established that this algorithm is valid for damping-vs-IBPA-curves that possess a roughly sinusoidal shape.

A further investigation of the possibilities of time reduction in the automated design process revealed that flow solutions and structural properties do not change significantly during one optimization run. Consequently, a method was found to measure the similarity of two different cases. For very similar cases it can be assumed that the aeroelastic properties do not change significantly. Therefore, just the IBPA with the predictably lowest aerodynamic damping value needs to be calculated.

An introduction to the theory of these approaches is presented and examples arising during the design of the counter rotating fan are given.
Non-Destructive Ceramic Matrix Composite Impact Modeling Validation
Authors: Xue Yibin , Abdi Frank , Morscher Gregory N. , Choi Sung
Published: November 2013

Ceramic matrix composite (CMC) materials technology is of fundamental importance to gas turbine engine application. FOD (foreign Object Damage) in CMC components can result in component localized damage and a loss of post-impact performance. CMC impact generates a varying degree of damage from localized surface damage to complete penetration depending on the severity of impact events. Ceramic Composite equivalent electrical properties are computed based on simplified Multi-scale micromechanics equations. Electrical resistance and/or conductivity are computed utilizing the constituent material properties, effective medium, and percolation theories. Ceramic composite electrical properties simulation requires the algorithm development that combines the effective medium and percolation theories. A physically based percolation model is implemented to characterize the effective electrical conductivity of heterogeneous composites by means of the combination of effective medium (EM) and percolation equations with universal exponents. It is shown that the present model correlates well with the experimental electrical resistivity and acoustic emission data. The change in electrical resistivity after impact is compared with test data of a SA-SiC fiber reinforced SiC matrix composite. The predicted damage after impact and the trend of damage volume correlated well with experimental observations of damage shape and reduction in electrical resistance. Thus, an empirical relationship between damage volume and mechanisms and electrical resistance are developed and presented.
Aeroelastic Investigation of a Transonic Research Compressor
Authors: Leichtfuß S. , Holzinger F. , Brandstetter C. , Wartzek F. , Schiffer H. P.
Published: November 2013

The trend in modern compressor design is towards higher stage loading and less structural damping, resulting in increased flutter risk. The understanding of the underlying aeroelastic effects, especially at highly loaded BLISK rotors, is small. This paper reports on the analysis of flutter phenomena in a modern transonic compressor.

The geometry examined here is the one-and-a-half stage transonic research compressor operated by Technische Universität Darmstadt. High blade deflections recorded during throttling measurements point to an aerodynamic excitation. Therefore, numerical investigations are carried out using the CFD-Code TRACE developed at the German Aerospace Center (DLR). Simulations are compared to measured compressor speed lines to validate the steady state results. The open source Finite Element code CalculiX is used to simulate the rotor blade eigenmodes and -frequencies. The results are then used in time-linearized calculations to determine the onset of flutter. These calculations confirm that there is an aerodynamic excitation of the first torsional eigenmode and blade flutter is at risk.

A sensitivity study is carried out to further investigate the aerodynamic conditions under which structural vibrations become unstable and to identify influencing factors.
A Review of the AFRL Turbine Research Facility
Authors: Anthony Richard J. , Clark John P.
Published: November 2013

The Air Force Research Laboratory Turbine Research Facility is a transient blowdown facility that allows simultaneous measurement of unsteady heat transfer and aerodynamics on full scale engine hardware. It is unique for its size and consequent blowdown duration. Compared to engine validation testing, full scale, short duration turbine-rig testing is able to provide very large amounts of rotating turbine flowfield information using far less energy, orders of magnitude lower cost, and greater instrumentation selection.

This paper provides an updated review of the facility’s history, operation, and enhancements since its initial construction over two decades ago. Historical connections to pioneering work in short-duration turbine heat transfer testing are highlighted, and an overview of past developments, features, and capabilities is given. More recent experimental and computational integration is described using a suite of in-house developed CFD design and analysis tools. Example test programs include a non-proprietary 1+ 1/2 stage research turbine rig, which is the most heavily instrumented high pressure turbine tested in the facility to date.. Recent data illustrates the character of unsteady airfoil shock interactions that may lead to large levels of resonant stress or turbine high cycle fatigue. The paper ends with a brief discussion of future work.
Flow Field and Vibration Behavior of Straight Transonic Compressor Cascade due to External Acoustic Excitation
Authors: Li Manlu , Hou Anping , Yang Xiaodong , Zhang Mingming , Wang Peng
Published: November 2013

A fluid-structure coupled approach is utilized to study the influence of external acoustic excitation on straight compressor cascade flow field and blade vibration behavior. Interaction between fluid and structure are dealt with in a coupled manner, based on the interface exchange of information between the aerodynamic and structural model. The computation fluid mesh is updated at every time step with an improved algebraic method. The flow field of cascade with/without external acoustic excitation is carried out using a 3D unsteady CFD model based on moving boundary way, as well as some experimental studies based on transonic wind tunnel. Then coupled with blade FE model, mode shapes, frequencies, vibration stress and the structural deformations of blade are identified. The performance of the cascade is obtained by computational and experimental ways, consistency of numerical and test results shows that the numerical model is suitable. The numerical results show that acoustic excitation has a greater impact on negative and designed attack angle in contrast to high positive attack angle. The cascade wake and blade surface pressure frequency characteristic are changed and the main frequency is almost the same as the acoustic excitation frequency. Compared results with no excitation, the vibration characteristics of the blade is changed, also the vibration behavior is sensitive to the excitation amplitude and frequency.
Performance Estimation of a Turbine Under Partial-Admission and Flow Pulsation Conditions at Inlet
Authors: Ni Ron Ho , Humber William , Ni Michael , Sondergaard Rolf , Ooten Mike
Published: November 2013

This paper describes a procedure for assessing the efficiency of a turbine operating under unsteady periodic flow at the inlet. Time-resolved flow simulations of the modified Garrett turbine case A3K7 are conducted with time-varying inlet conditions upstream of the turbine. A mass-weighted moving average is applied to the instantaneous data over a period corresponding to the pulsing frequency of the time-varying inlet conditions. The resulting time averaged data is then used to assess the convergence of the numerical solution and evaluate the turbine performance.

Three methods of computing the time-averaged turbine efficiency are presented. The first method, referred to as the TP method, is based upon the use of mass-weighted average of the total temperature (Tt) and total pressure (Pt). The second method, the TS method, uses the mass-weighted average of total temperature (Tt) and entropy (S). The third method, the WS method, employs the moving average of specific work output (Ws) and work lost due to entropy (S) increase.

A comparison of time-averaged specific work output, stage efficiency, and rotor efficiency is made for three turbine operating configurations: partial admission, synchronized pulsation, and sequential pulsing flow conditions of the sector inlets. Results show that for the tested rotor speed, the efficiency increases as the inlet pulsing frequency is reduced, and that when inlet sectors are not opened simultaneously, a large drop in efficiency occurs due to spillage of high energy fluid between the open and closed sectors. Additionally, it is shown that due to the non-convective nature of total pressure, the TP method for efficiency is not reliable while the TS and WS methods are able to produce consistent results.
Evaluation of Commercial and Next Generation Alumina-Forming Austenitic Foil for Advanced Recuperators
Authors: Pint Bruce A. , Dryepondt Sebastien , Brady Michael P. , Yamamoto Yukinori
Published: November 2013

Alumina-forming austenitic (AFA) steels represent a new class of corrosion- and creep-resistance austenitic steels to enable higher temperature recuperators. Several commercial batches of the first AFA composition have been produced with different thicknesses and widths over 39cm. This commercial AFA foil was successfully folded by two manufacturers. Creep and laboratory oxidation results at 650°-800°C are presented to compare to conventional recuperator alloy performance. While this initial effort was successful, concerns with cost and ease of production suggested that a leaner AFA composition with a lower final annealing temperature would be more attractive for commercial applications. Therefore, several new AFA compositions are being evaluated in laboratory trials and compared to the initial material for down selection of a better AFA composition for commercialization.
Development and Application of a Simulation Tool for Vertical and Horizontal Axis Wind Turbines
Authors: Marten David , Wendler Juliane , Pechlivanoglou Georgios , Nayeri Christian Navid , Paschereit Christian Oliver
Published: November 2013

A double-multiple-streamtube vertical axis wind turbine simulation and design module has been integrated within the open-source wind turbine simulator QBlade. QBlade also contains the XFOIL airfoil analysis functionalities, which makes the software a single tool that comprises all functionality needed for the design and simulation of vertical or horizontal axis wind turbines. The functionality includes two dimensional airfoil design and analysis, lift and drag polar extrapolation, rotor blade design and wind turbine performance simulation. The QBlade software also inherits a generator module, pitch and rotational speed controllers, geometry export functionality and the simulation of rotor characteristics maps. Besides that, QBlade serves as a tool to compare different blade designs and their performance and to thoroughly investigate the distribution of all relevant variables along the rotor in an included post processor. The benefits of this code will be illustrated with two different case studies. The first case deals with the effect of stall delaying vortex generators on a vertical axis wind turbine rotor. The second case outlines the impact of helical blades and blade number on the time varying loads of a vertical axis wind turbine.
Investigation of Efficient CFD Methods for the Prediction of Blade Damping
Authors: Elder Robin , Woods Ian , Patil Sunil , Holmes William , Steed Robin , Hutchinson Brad
Published: November 2013

Accurate and efficient prediction of blade damping is one essential element in the engineering of durable and reliable compressors and turbines. Over the years, a variety of empirical and linearized methods have been developed and used, and have served well. Recently, the development of efficient unsteady CFD methods combined with an expansion in available and affordable computing power has enabled CFD analysis of blade damping. This paper looks at the prediction of aerodynamic blade damping using some recently developed CFD methods.

Unsteady CFD methods are used to predict the fluid flow in a transonic fan rotor, with tip Mach number of about 1.4. Deformation of the blade is determined from a mechanical pre-stressed modal analysis. In this investigation, blade motion for the first bending moments is prescribed in the CFD code, for a range of nodal diameters. After periodic unsteady solutions are obtained, damping coefficients are calculated based on the predicted blade forces and the specified blade motion.

Traditional unsteady CFD methods require the simulation of many blades in a given row, depending on the nodal diameter. For instance, for a nodal diameter of four, a wheel with 22 blades would require simulation of eleven blades. Computational methods have been developed which now enable simulation of only a few (1 or 2) blades per row yet yield the full sector solution, thus providing considerable savings in computing time and machine resources. The properties of the available methods vary, but one method, the Fourier Transformation method, has the property that it is frequency preserving, and hence suitable for the present task.

Fourier Transformation predictions, for a variety of nodal diameters, are compared with full sector predictions. Positive damping was predicted for this range of nodal diameters at design speed near peak efficiency operating condition indicating a stable system. The Fourier Transformation predictions for blade aerodynamic damping match very closely the reference full sector solutions. The Fourier transformation methods also provide solutions 3.5 times faster than average periodic reference cases.
Student Perceptions on the New Teaching Methods for Engineers: The Influence of Podcasting on Education
Authors: Besem Fanny , Lennie Matthew , Chávez Carlos
Published: November 2013

This paper presents the perceptions of engineering students who followed podcasted courses during their higher education. Podcasting is widely used in remote education, but it also benefits on-campus students because it supports flexible and personalized teaching. The first three generations of students from an international Master’s degree program participated in a survey to give their impressions about the use of podcasting in their program. This preliminary survey targeted three aspects of podcasting: the format, the effect on learning experience, and the effect on student isolation. Results showed a majority of students considered recorded lectures a very helpful tool to support traditional on-campus lectures. The students appreciated the opportunity to pause and re-watch the videos to learn at their own pace. However, few students would consider a purely remote education; they felt less engaged in their education because of the lack of direct contact with teachers and peers. This highlights the importance of the social networking that happens on-site. In conclusion, the main advantage of podcasting is to compensate for the lack of individually adapted teaching in higher education. However, it will not completely replace traditional lectures without the development of both new tools to facilitate the professor-student interactions, and of teaching techniques to keep students engaged in their studies.
Experimental Investigation on Aerodynamic Unsteadiness in a Full Scale Gas Turbine Midframe Sector
Authors: Golsen Matthew J. , Hossain Jahed , Bravato Anthony , Harrington John , Bernstein Joshua , Mahadevan Srikrishna , Kapat J. S. , Rodriguez José
Published: November 2013

Aerodynamic unsteadiness generated upstream of the combustor basket via the complicated geometry of a modern gas turbine can lead to incomplete combustion, reduced efficiency, greater pressure drop, flashback, and reduced part life. The MidFrame section encompasses the main gas path from the compressor exit to the turbine inlet. Diffuser performance, support struts, transition pieces, and other flow obstructing geometries can lead to flow unsteadiness which can reduce performance. This study uses a combination of thermal anemometry, pressure microphone, and wall mounted accelerometer measurements to determine the primary unsteadiness frequencies and target their source. Diffuser performance is shown to have a significant impact on the downstream flow behavior. Inlet conditions are modified to provide a separated bottom wall and a fully attached compressor exit diffuser (CED) condition at an area average inlet Mach number of 0.26. Unsteadiness levels are seen to increase as a result of the separated inlet condition while the mean flow characteristics are slightly altered due to the varying exit trajectory of the main core from the CED, nevertheless the overall level of unsteadiness/turbulence is low for such a complex flow field (8 to 11 %). Results of this study can help diagnose and prevent the aforementioned issues for complicated geometries where simple flow experiments fall short.
A Study of the Heat Transfer in Winglet and Squealer Rotor Tip Configurations for a Non-Cooled HPT Blade Based on CFD Calculations
Authors: da Silva Lucilene Moraes , Tomita Jesuino Takachi
Published: November 2013

HPT operate at high pressure and temperatures. One of the most important loss sources is the tip leakage flow on the rotor tip region. The flow that leaks in this region does not participate in the energy transfer process between the hot gas and rotor blade row. Hence, the main flow suffers a penalty to maintain the energy conservation. To try decreasing this mass flow leakage some techniques can be applied. The most common are the winglet and squealer rotor tip configuration. These techniques improve the turbine performance, but some attention should be taken into account because the temperature distribution changes on this region for different tip configurations. In this work, the winglet and squealer tip geometries are compared with the common flat tip configuration. The analysis was performed for design and off-design conditions. The HPT developed in the E3 program was used as baseline turbine to explore the differences of the flowfield on the rotor tip region. The results are compared and discussed in detail.
Inlet Duct Treatment for Stability Improvement on a High Pressure Ratio Centrifugal Compressor
Authors: Yang Mingyang , Martinez-Botas Ricardo , Zhang Yangjun
Published: November 2013

The operating range of a centrifugal compressor, determined by surge and choke flow rate, is a key issue for turbocharging since a vehicle internal combustion engine (ICE) is usually operated across a wide range. In this paper a new flow control method is developed and validated numerically, in which an array of circumferentially distributed holes is designed in the inner wall of the inlet duct of a high pressure ratio centrifugal compressor of a turbocharger. Firstly the numerical method is validated by experimental results of the original turbocharging centrifugal compressor with a pressure ratio of 4. Next the validated method is used to investigate the new flow control method and its effect on the compressor’s performance. Results show that the method can enhance the compressor stability and widen the operating range effectively at all investigated speeds. At meantime, the choke flow reduces slightly. The flow details in the compressor are further analysed according to the CFD results. It is found that the flow near the blade tip at inlet is pre-swirled by the method as the conventional IGV does while the flow in the middle span or near the hub remains in an axial direction. As a result, the stability of the compressor is enhanced by the pre-swirl effect at the tip while minimally sacrificing the choke flow rate, thus the map is extended effectively by the flow control method.
Unsteady Simulation and Analysis on Centrifugal Impeller With Self-Adaptive Casing Treatment
Authors: Wang Tong , Wang Wei , Gu Chuangang
Published: November 2013

A self-adaptive casing treatment for unshrouded centrifugal compressor was proposed in our previous studies. It is a kind of passive control techniques. The experimental results proved that the stable working range of the compressor was extended greatly with the technique. As for the stability mechanism, there is no convinced explanation. Many researchers believe that the unsteady flow could be one of the key points. In the paper, steady and unsteady numerical simulations were carried out to get the performances of the centrifugal impeller by ANSYS CFX software. The numerical method was validated by comparing with the experimental results. It was found that there were two types of flow pattern in the bleeding-recirculation passages by the numerical simulation with the self-adaptive casing treatment. One was the recirculation flow at the smaller flowrate working conditions and the other was bypass flow at the larger flowrate working conditions. The pressure at the bleeding ports was more than that at the recirculation port at the smaller flowrate. It would result in the recirculation flow in the bleeding-recirculation passages. Otherwise, it would result in the bypass flow in the bleeding-recirculation passages. The numerical results of each bleeding-recirculation passage provided the variation of mass flowrate in it with the pressure difference. The relation of the pressure drop coefficient and Reynolds number based on the bleeding hole was fitted. It was different for the recirculation flow and bypass flow. It is helpful to decide the position of the bleeding ports during the centrifugal compressor design process. Moreover, an unsteady numerical simulation method with the increasing back pressure boundary condition was proposed to investigate the unsteady process approaching to the numerical stall point or unstable flow. The dynamic pressure data in impeller and diffuser were recorded. The amplitudes of the data were picked up to compare the time dependent process. The dynamic pressure at the inlet of diffuser fluctuated more strongly than those at the other positions while the back pressure was increased to the numerical stall point. The experimental data provided the similar phenomena. It suggested that the unstable flow tendency could be caught up by the unsteady simulation process with the increasing back pressure boundary condition. Furthermore, the time dependent flow fields at the blade tip region were compared on the conditions with and without the self-adaptive casing treatment. The effect of the self-adaptive casing treatment was proved by unsteady numerical method with the increasing back pressure boundary condition. The stability mechanism of the self-adaptive casing treatment was explained to some extent.
Enhanced Groove Geometry for Herringbone Grooved Journal Bearings
Authors: Schiffmann J.
Published: November 2013

Although gas lubricated Herringbone Grooved Journal Bearings (HGJB) are known for high rotordynamic stability thresholds, small clearance to diameter ratios are required for stable rotor operation. Tight clearances not only increase bearing losses but also yield challenging manufacturing and assembly tolerances, which ultimately translate into cost. Traditionally, the grooves of HGJB are of helical nature with constant cross-section and pitch. The current paper aims at increasing the clearance to diameter ratio and the stability threshold of grooved bearings by introducing enhanced groove geometries. The axial evolution of groove width, depth and local pitch are described by individual 3rd order polynomials with four interpolation points. The expression for the smooth pressure distribution resulting from the narrow groove theory is modified to enable the calculation of bearing properties with modified groove patterns. The reduced order bearing model is coupled to a linear rigid body rotordynamic model for predicting the whirl speed map and the corresponding stability. By introducing a critical mass parameter as a measure for stability, a criterion for the instability onset is proposed. The optimum groove geometry is found by coupling the gas bearing supported rotor model with a multi-objective optimizer. By maximizing both the clearance to diameter ratio and the rotordynamic stability it is shown that with optimal groove geometry, which deviates from helicoids with constant pitch and cross-section, the critical mass parameter can be improved by more than one order of magnitude compared to traditional HGJB geometries. The clearance to diameter ratio can be increased by up to 80% while keeping the same stability margin, thus reducing both losses and manufacturing constraints. The optimum groove pattern distributions (width ratio, angle and depth) are summarized for a variety of L/D ratios and for different compressibility numbers in a first attempt to set up general design guidelines for enhanced gas lubricated HGJB.