bluuebunny/crossref_metadata_2025 huggingface dataset:
The Thermodynamics of Wake Blade Interaction in Axial Flow Turbines: Combined Experimental and Computational Study
Authors: Rose Martin , Schu¨pbach Peter , Mansour Michel
Published: February 2010

This paper reports on insights into the detailed thermodynamics of axial turbine NGV wakes as they interact with the rotor blades. The evidence presented is both computational and experimental. Unsteady RANS simulations are used to compare the experimental observations with theoretical predictions. Output processing with both Eulerian and Lagrangian approaches is used to track the property variation of the fluid particles. The wake is found to be hot and loses heat to the surrounding fluid. The Lagrangian output processing shows that the entropy of the wake will fall due to heat loss as it passes through the rotor and this is corroborated experimentally. The experimental vehicle is a one-and-a-half stage shroudless turbine with modest Mach numbers of 0.5 and high response instrumentation. The entropy reduction of the wake is determined to be about four times the average entropy rise of the whole flow across the rotor. The results show that the work done by the wake fluid on the rotor is approximately 24% lower than that of the free-stream. The apparent experimental efficiency of the wake fluid is 114% but the overall efficiency of the turbine at mid-height is around 95%. It is concluded that intra-fluid heat transfer has a strong impact on the loss distribution even in a nominally adiabatic turbine with moderate row exit Mach numbers of 0.5.
An Active Auxiliary Bearing Control Strategy to Reduce the Onset of Asynchronous Periodic Contact Modes in Rotor/Magnetic Bearing Systems
Authors: Cade Iain S. , Sahinkaya M. Necip , Burrows Clifford R. , Keogh Patrick S.
Published: February 2010

To prevent rotor/stator contact in a rotor/magnetic bearing system, auxiliary bearings may be located along the shaft and at the magnetic bearings. Rotor responses after a contact event may include periodic trapped modes where repeated contact may lead to highly localized thermal stresses. This paper considers an active auxiliary bearing system with a control strategy designed to limit the trapped contact modes in a rotor/magnetic bearing system that are induced by rotor unbalance. The controller is evaluated from a system model and its responses to short duration contact events. An active auxiliary bearing model is introduced to the system where the dynamic response of the bearing is dependent on the controller. From a harmonic decomposition of rotor/bearing contact, dynamic controllers are sought which limit the numbers of possible periodic solutions for a given rotor unbalance and operating speed. A case study is performed considering a simple two degree of freedom system with passive and active auxiliary bearings. Recovery of a rotor trapped in an asynchronous contact mode is shown with variation of the auxiliary bearing controller parameters.
Large Eddy Simulation of a Controlled-Diffusion Cascade Blade at Varying Flow Inlet Angles
Authors: McMullan W. Andrew , Page Gary J.
Published: February 2010

A Controlled Diffusion cascade stator blade has been studied numerically using Large Eddy Simulation (LES). The aim of the study is to assess the performance of Large Eddy Simulation in predicting flow features on a highly-loaded blade, including leading-edge separation, transition and turbulent reattachment, particularly at off-design conditions. The need for LES to be performed on high resolution grids is highlighted by preliminary simulations on a mesh typically used in Reynolds-Averaged approaches. On a fine grid, the unsteady flow features captured by time-dependent simulation yield an improvement in surface pressure distributions and boundary layer profiles, although some weaknesses are apparent in the prediction of pressure-side boundary layer properties and wake profiles. The computed loss coefficients show potential for LES to be used to obtain loss-loop data over a wide range of incidence angles.
The Influence of Orifice Restriction on the Stability of Rigid Rotor-Aerostatic Bearing System
Authors: Chen Cheng-Hsien , Yang Ding-Wen , Kang Yuan , Hwang Ren-Ming , Shyr Shrh-Shyong
Published: February 2010

This paper has studied the influence of the restriction effect on the stability of a rigid rotor in rotation supported by double-row, orifice compensated aerostatic bearings. The air which is assumed to be perfect gas, passes through orifice restrictor into the bearing clearance undergoing the adiabatic process is governed by Reynolds equation including the coupled effects of wedge due to spindle rotation and squeezed film due to journal oscillation. The Ph-method is used to analyze Reynolds equation and which is then solved by the finite difference method and numerical integration to yield static and dynamic characteristics of air film. The motion equation of the rotor-bearing system is obtained by using the perturbation method and the eigensolution method is used to determine the stability threshold and critical whirl ratio. The variations of stability threshold of both critical inertial force and critical whirl ratio with restriction parameters are analyzed for various whirl ratios, speed of journal rotation and eccentricity ratios.
Unsteady Pressure Measurements in a Single Stage Low Pressure Axial Compressor: Tip Vortex Flow and Stall Inception
Authors: Brouckaert J. F. , Van de Wyer N. , Farkas B. , Ullmann F. , Desset J. , de Laborderie J. , Chome´ M. , Hiernaux S.
Published: February 2010

The experimental investigation of the unsteady flow field in a single stage low pressure axial compressor designed for a counter-rotating turbofan engine architecture is presented in this paper. The rotor casing was instrumented with fast response pressure transducers to perform a detailed survey of the tip flow features during stable operation, near stall and during stall. Tests were performed at two different Reynolds numbers representative of cruise and take-off conditions in the VKI-R4 closed loop compressor test rig. Simultaneous time-resolved measurements with a miniature fast response total pressure probe were performed by radial traverses at the rotor exit to support the tip flow field investigation. The casing measurements allow to map the direction and extension of the tip leakage vortex. The flow path measurements show its extension at the exit of the rotor blade passage and its evolution as throttling is increased towards the compressor stability limit. These experimental results are discussed and compared to CFD simulations, showing good agreement. Stall inception and rotating stall patterns are investigated as well and described in this paper. They are based both on hot wire measurements and on the casing unsteady pressure measurements.
Detailed Studies on Separated Boundary Layers Over Low-Pressure Turbine Airfoils Under Several High Lift Conditions: Effect of Freesteam Turbulence
Authors: Funazaki Ken-ichi , Yamada Kazutoyo , Tanaka Nozomi , Chiba Yasuhiro
Published: February 2010

This paper deals with experimental investigation on the interaction between inlet freestream turbulence and boundary layers with separation bubble on a low-pressure turbine airfoil under several High Lift conditions. Solidity of the cascade can be reduced by increasing the airfoil pitch by 25%, while maintaining the throat in the blade-to-blade passage. Reynolds number examined is 57000, based on chord length and averaged exit velocity. Freestream turbulence intensity at the inlet is varied from 0.80% (no grid condition) to 2.1% by use of turbulence grid. Hot-wire probe measurements of the boundary layer on the suction surface for Low Pressure (LP) turbines rotor are carried out to obtain time-averaged and time-resolved characteristics of the boundary layers under the influence of the freestream turbulence. Frequency analysis extracts some important features of the unsteady behaviors of the boundary layer, including vortex formation and shedding. Numerical analysis based on high resolution Large Eddy Simulation is also executed to enhance the understanding on the flow field around the highly loaded turbine airfoils. Standard Smagorinsky model is employed as subgrid scale model. Emphasis of the simulation is placed on the relationship of inherent instability of the shear layer of the separation bubble and the freestream turbulence.
Application of Topology-Free Optimization to Manage Cooled Turbine Tip Heat Load
Authors: Dawes W. N. , Kellar W. P. , Richardson G. A.
Published: February 2010

The application of automated design optimization to real-world, complex geometry problems is a significant challenge—especially if the topology is not known a-priori like in turbine internal cooling. The long term goal of our work is to focus on an end-to-end integration of the whole CFD Process, from solid model through meshing, solving and post-processing to enable this type of design optimization to become viable & practical. In recent papers we have reported the integration of a Level Set based geometry kernel with an octree-based cut-Cartesian mesh generator, RANS flow solver, post-processing & geometry editing all within a single piece of software—and all implemented in parallel with commodity PC clusters as the target. The cut-cells which characterize the approach are eliminated by exporting a body-conformal mesh, with a viscous layer, guided by the underpinning Level Set. This paper develops this work further with a simple, generic study showing how the basic functionality can be scripted and then used as the infrastructure for automated optimization applied to an example in turbine cooling: managing blade tip heat lead.
CFD Predicted Discharge Coefficients of a Single Cylindrical Hole With Compressible External Cross-Flow
Authors: Guo L. , Yan Y. Y. , Maltson J. D.
Published: February 2010

A computational investigation on discharge coefficient (Cd) of a single cylindrical hole is presented in this paper. The numerical calculations are carried out on a 3-D compressible model. The Shear-Stress Transport (SST) k–ω model is used to simulate the turbulence in the flow. The inclination angle (α) of the film cooling hole varies from 20° to 30°, 45° and 90°, respectively. The diameter of the hole is fixed at 10mm, but different coolant to mainstream pressure ratios (ptc/pm) are examined. The coolant Mach number (Mac) is set at a constant value of 0.3 and the mainstream flow Mach number (Mam) varies from 0.3 to 1.4. The effects of Mam and α on the Cd value as well as the static pressure distribution at the jet exit are investigated. The numerical results show an acceptable agreement in the trend of the Cd variation compare with the available experimental data. It has been predicted that the static pressure distribution in the vicinity of the jet exit is influenced by a number of factors including the mainstream flow Mach number, shock wave, jet inclination angle and the pressure ratio of the coolant to the mainstream flow. And then the static pressure field near the hole can further give strong influence on the discharge coefficient.
Turbocharger Dynamic Analysis Using First Order System Step Response
Authors: Venson Giuliano Gardolinski , Barros Jose´ Eduardo Mautone
Published: February 2010

This work presents the dynamic modeling of an automotive turbocharger in a hot gas test stand. The objective is to develop a methodology to determine the main turbocharger dynamic properties as moment of inertia, response time, static gain constant, frequency gain amplitude and phase shift. The turbocharger used is the Master Power APL-240 set. The moment of inertia is obtained through the deceleration curve from an instantaneous fuel cut-off in the combustion chamber. The response time and static gain constant, as well the frequency gain amplitude and phase shift curves in function of a signal frequency, are obtained through a step variation. The turbocharger is modeled as a first order system. It is also presented a turbocharger sine excitation by the combustion chamber, generating a rotational speed sine signal output that simulates an engine intermittent acceleration. The rotational speed signal frequency gain and phase shift are compared to the values obtained in the step curves. The rotational speed frequency gain amplitude and phase shift modeled through the step test presents deviation of 16% and 13%, respectively, from the values from sine test.
Combined Effects of Forward Sweep and Tip Clearance on the Performance of Axial Flow Compressor Stage
Authors: Ramakrishna P. V. , Govardhan M.
Published: February 2010

There are a number of performance indices for a turbomachine on the basis of which its strength is evaluated. In the case of axial compressors, pressure ratio, efficiency and stall margin are few such indices which are of major concern in the design phase as well as in the evaluation of performance of the machine. In the process of improving the blade design, 3D blade stacking, where the aerofoil sections constituting the blade are moved in relation to the flow. Tilting the blade sections to the flow direction (blade sweep) would increase the operating range of an axial compressor due to modifications in the pressure and velocity fields on the suction surface. On the other hand, blade tip gap, though finite, has great influence on the performance of a turbomachine. The present work investigates the combined effect of these two factors on various flow characteristics in a low speed axial flow compressor. The objective of the present paper is thereby confined to study the collective effects of sweep and tip clearance without attempting to suggest an outright new design. In the present numerical work, the performance of Tip Chordline Sweeping (TCS) and Axial Sweeping (AXS) of low speed axial compressor rotor blades are studied. For this, 15 computational domains were modeled for five rotor sweep configurations and three different clearance levels for each rotor. Through the results, 20°AXS rotor is found to be distinctive among all the rotors with highest pressure rise, higher operating range and less tip clearance loss characteristics. TCS rotors produced improved total pressure rise at the low flow coefficients when the tip gap is increased. Hence there is a chance that an “optimum” tip gap exists for the TCS rotors in terms of total pressure coefficient and operating range, while AXS rotors are at their best with the minimum possible clearance.
A Study of Stall Inceptions in a Low-Speed Axial-Flow Compressor With Various Radial Loadings
Authors: Zhang J. , Lin F. , Chen J. , Nie C.
Published: February 2010

In this paper, the stall inceptions in a single-stage axial flow compressor with different high loading positions generated by various radial distortions are experimentally and numerically investigated. The results indicate that the stall limit varies with the radial position of the distortion significantly. The closer the position of distortion to the blade tip, the more unstable the compressor becomes. In addition, the results demonstrate that stall inception varies with radial distortion accordingly. While with the hub distortion, the compressor exhibits modal-like disturbances prior to the stall onset, the stall is triggered by the spike-like disturbances directly with the center and tip distortions. The flow mechanism is then further explored with numerical simulations. It is shown that in the hub distortion case, the separation region caused by local high loading in large flow rate can migrate to the tip region along the span as the compressor is throttled to the stall limit. This spanwise migration plays an important role in the formation of the modal-like disturbances. Compared to the hub distortion, the modal-like disturbance in the uniform inlet flow appears in a shorter period of time because it takes less time to initiate stall cell when the separation occurs. In the tip distortion case, the separation at tip dominates so strongly that no modal-like disturbances are found before the stall onset. A discussion is given at the end of this paper to explain why in some compressors, a modal inception emerges first and the stall is triggered by the spike later.
Boundary Layer Development on a High-Lift LP Turbine Profile Under Passing-Wakes Conditions
Authors: Satta Francesca , Simoni Daniele , Ubaldi Marina , Zunino Pietro , Bertini Francesco
Published: February 2010

The boundary layer development on the suction side of a high-lift LP turbine profile has been experimentally investigated under steady and unsteady flow conditions in the range of Reynolds numbers between 70000 and 300000. Upstream wake periodic perturbations are generated by means of a tangential wheel of radial rods. The paper reports the results of the investigations performed for both steady and unsteady inflow cases (reduced frequency f+ = 0.62) for Re = 300000 and Re = 70000, representative of nominal and reduced Reynolds number operating conditions, respectively. A phase-locked ensemble-averaging technique has been employed to reconstruct the phase-averaged velocity and unresolved unsteadiness boundary layer profiles from the hotwire instantaneous velocities. Phase sequences of the boundary layer development, as well as time-space plots of velocity and unresolved unsteadiness in normal and streamwise directions highlight the complex wake/boundary layer interaction mechanism. While at the larger test Reynolds number the wake/boundary layer interaction does not substantially influence the transition process, at the lower test Reynolds number the boundary layer wake receptivity triggers the transition process, strongly attenuating the large separation bubble occurring at steady conditions.
Probabilistic HCF-Investigation of Compressor Blades
Authors: Heinze Kay , Friedl Winfried-Hagen , Vogeler Konrad , Voigt Matthias
Published: February 2010

During the design process of compressor blades predominantly deterministic models are used for High Cycle Fatigue (HCF) strength investigations. The scatter of HCF that results e.g. through abrasion of the production machines [1] or inhomogeneities of the blade material, is accounted by safety factors and conservative assumptions. A more realistic approach to consider these uncertainties is the application of probabilistic methods. Therefore, further information about HCF and eigenfrequency scatter of the really produced blades can be used for a robust design during the design process. Within a measurement campaign 400 blades of a Rolls-Royce High Pressure Compressor were randomly selected and scanned using an automated process that applies the optical measurement technique of strip projection. The measurement data of the airfoil were subdivided into constant spanwise profile slices. Geometric airfoil parameters were determined on each of the profile slices [2]. Due to the large number of scanned blades each geometric airfoil parameter can be described as a distribution function with corresponding parameters. These distribution functions are the input parameters for the probabilistic investigation — the Monte-Carlo-Simulation (MCS). Within the MCS an automatically transfer process varies at first the profile slices of a CAD-airfoil and in a second step morphs an existent 3D finite element mesh applying the meshmorphing tool of the FE preprocessor Hypermesh. The HCF and eigenfrequency scatter of all blades were calculated with the interpretation of the MCS results and parameters were detected with the largest influence on HCF-strength and eigenfrequencies. A detailed interpretation of the HCF-strength at one example shows the power of the probabilistic investigation. The interpretation helps the engineer to understand the entire system and to design a robust blade.
Effect of Static/Dynamic Coupling on the Forced Response of Turbine Bladed Disks With Underplatform Dampers
Authors: Firrone Christian M. , Zucca Stefano , Gola Muzio
Published: February 2010

Friction contacts are often used in turbomachinery design as passive damping systems. In particular underplatform dampers are mechanical devices used to decrease the vibration amplitudes of bladed disks. Numerical codes are used to optimize during design the underplatform damper parameters in order to limit the resonant stress level of the blades. In such codes the contact model plays the most relevant role in the calculation of the dissipated energy at friction interfaces. One of the most important contact parameters is the static normal load acting at the contact, since its value strongly affects the area of the hysteresis loop of the tangential force and therefore the amount of dissipation. A common procedure to estimate the static normal loads acting on underplatform dampers consists in decoupling the static and the dynamic balance of the damper. A preliminary static analysis of the contact is performed in order to get the static contact/gap status to use in the calculation, assuming that it does not change when vibration occurs. In this paper a novel approach is proposed. The static and the dynamic displacements of the system (bladed disk + underplatform dampers) are coupled together during the forced response calculation. Static loads acting at the contacts follow from static displacements and no preliminary static analysis of the system is necessary. The proposed method is applied to a numerical test case representing a simplified bladed disk with underplatform dampers. Results are compared with those obtained with the classical approach.
Aerodynamic Investigation of the Tip Leakage Flow for Blades With Different Tip Squealer Geometries at Transonic Conditions
Authors: Hofer Toma´sˇ , Arts Tony
Published: February 2010

Modern high pressure turbines operate at high velocity and high temperature conditions. The gap existing above a turbine rotor blade is responsible for an undesirable tip leakage flow. It is a source of high aerodynamic losses and high heat transfer rates. A better understanding of the tip flow behaviour is needed to provide a more efficient cooling design in this region. The objective of this paper is to investigate the tip leakage flow for a blade with two different squealer tips and film-cooling applied on the pressure side and through tip dust holes in a non-rotating, linear cascade arrangement. The experiments were performed in the VKI Light Piston Compression Tube facility, CT-2. The tip gap flow was investigated by oil flow visualisations and by wall static and total pressure measurements. Two geometries were tested — a full squealer and a partial suction side squealer. The measurements were performed in the blade tip region, including the squealer rim and on the corresponding end-wall for engine representative values of outlet Reynolds and Mach numbers. The main flow structures in the cavity were put in evidence. Positive influence of the coolant on the tip gap flow and on the aerodynamic losses was found for the full squealer tip case: increasing the coolant mass-flow increased the tip gap flow resistance. The flow through the clearance therefore slows down, the tip gap mass-flow and the heat transfer respectively decreases. No such effect of cooling was found in the case of the partial suction side squealer geometry. The absence of a pressure side squealer rim resulted in a totally different tip gap flow topology, indifferent to cooling. The influence of cooling on the overall mass-weighted thermodynamic loss coefficient, which takes into account the different energies of the mainstream and coolant flows was found marginal for both geometries. Finally the overall loss coefficient was found to be higher for the partial suction side squealer tip than for the full squealer tip.
Blade Forcing Function and Aerodynamic Work Measurements in a High Speed Centrifugal Compressor With Inlet Distortion
Authors: Kammerer Albert , Abhari Reza S.
Published: February 2010

Centrifugal compressors operating at varying rotational speeds, such as in helicopters or turbochargers, can experience forced response failure modes. The response of the compressors can be triggered by aerodynamic flow non-uniformities, such as with diffuser-impeller interaction or with inlet distortions. The work presented here addresses experimental investigations of forced response in centrifugal compressors with inlet distortions. This research is part of an ongoing effort to develop related experimental techniques and to provide data for validation of computational tools. In this work measurements of blade surface pressure and aerodynamic work distribution were addressed. A series of pressure sensors were designed and installed on rotating impeller blades and simultaneous measurements with blade-mounted strain gauges were performed under engine representative conditions. To the best knowledge of the authors, this is the first publication which presents comprehensive experimental unsteady pressure measurements during forced response for highspeed radial compressors. Experimental data were obtained for both resonance and off-resonance conditions with uniquely tailored inlet distortion. This paper covers aspects relating to the design of fast response pressure sensors and their installation on thin impeller blades. Additionally, sensor properties are outlined with a focus on calibration and measurement uncertainty estimations. The second part of this paper presents unsteady pressure results taken for a number of inlet distortion cases. It will be shown that the intended excitation order due to inlet flow distortion is of comparable magnitude to the second and third harmonics which are consistently observed in all measurements. Finally, an experimental method will be outlined that enables the measurement aerodynamic work on the blade surface during resonant crossing. This approach quantifies the energy exchange between the blade and the flow in terms of cyclic work along the blade surface. The phase angle between the unsteady pressure and the blade movement will be shown to determine the direction of energy transfer between the blade and the fluid.
The Effect of Rotor Tip Clearance Size Onto the Separated Flow Through a Super-Aggressive S-Shaped Intermediate Turbine Duct Downstream of a Transonic Turbine Stage
Authors: Marn A. , Go¨ttlich E. , Malzacher F. , Pirker H. P.
Published: February 2010

The demand of further increased bypass ratio of aero engines will lead to low pressure turbines with larger diameters, which rotate at lower speed. Therefore, it is necessary to guide the flow leaving the high pressure turbine to the low pressure turbine at a larger diameter without any loss generating separation or flow disturbances. Due to costs and weight this intermediate turbine duct (ITD) has to be as short as possible. This leads to an aggressive (high diffusion) and further to a super-aggressive s-shaped duct geometry. In order to investigate the influence of the blade tip gap size on such a high diffusion duct flow a detailed test arrangement under engine representative conditions is necessary. Therefore, the continuously operating Transonic Test Turbine Facility (TTTF) at Graz University of Technology has been adapted: An super-aggressive intermediate duct is arranged downstream of a transonic HP-turbine stage providing an exit Mach number of about 0.6 and a swirl angle of −15 degrees. A second LP-vane row is located at the end of the duct and represents the counter rotating low pressure turbine at a larger diameter. A following deswirler and a diffuser are the connection to the exhaust casing of the facility. In order to determine the influence of the blade tip gap size on the flow through such a super-aggressive s-shaped turbine duct measurements were conducted with two different tip gap sizes, 1.5% span (0.8 mm) and 2.4% span (1.3 mm). The aerodynamic design of the HP-turbine stage, ITD, LP-vane and the de-swirler was done by MTU Aero engines. In 2007 at ASME Turbo Expo the influence of the rotor clearance size onto the flow through an aggressive ITD was presented. For the present investigation this aggressive duct has been further shortened by 20% (super-aggressive ITD) that the flow at the outer duct wall is fully separated. This paper shows the influence of the rotor tip clearance size onto this separation. The flow through this intermediate turbine duct was investigated by means of five-hole-probes, static pressure taps, boundary layer rakes and oil flow visualisation. The oil flow visualisation showed the existence of vortical structures within the separation where they seem to be imposed by the upstream HP-vanes. This work is part of the EU-project AIDA (Aggressive Intermediate Duct Aerodynamics, Contract: AST3-CT-2003-502836).
Development and Validation of a New Universal Through Flow Method for Axial Compressors
Authors: Petrovic Milan V. , Wiedermann Alexander , Banjac Milan B.
Published: February 2010

This paper describes the development of a new through flow method for the analysis of axial multistage compressors. The method is based on a stream function approach and a finite element solution procedure. It includes a high-fidelity loss and deviation model with improved correlations and endwall boundary layer calculation. A radial distribution model of losses and a new spanwise mixing model are applied to simulate 3D flow effects. The calibration of the models is made by calculation a number of test cases with different configurations with the aim of achieving high accuracy and optimum robustness for each of the test cases considered. The code was applied to flow analysis and performance prediction of a newly developed gas turbine compressor. Comparison of the predicted results and measured test data for the overall compressor performance and a number of parameters under different operating conditions showed good agreement. The results of the validation confirm that this method based on calibrated correlations can be applied as a reliable tool for flow analysis and parameter variation during the design phase for a wide range of compressor configurations.
Insert Damping of Hollow Airfoils
Authors: Borufka Hans Peter , Arrieta Hernan Victor , Hartung Andreas
Published: February 2010

This paper describes a new mechanical damping device for hollow airfoils. The concept combines cooling and damping systems in one single element. Damping is achieved by frictional contact between an insert and specially selected contact locations on internal airfoil surfaces. The mechanical damping system is optimized for the first torsion mode of a shroud-less turbine blade and leads to very high damping effectiveness. In parallel an acceptable damping of the first flap mode was observed. In addition studies indicating the role of main parameters such as friction coefficient, excitation amplitude, geometry and location of damper elements are presented.
Active Flow Control to Improve the Aerodynamic Performance of Axial Turbomachines
Authors: Neuhaus Lars , Wiederhold Olaf , Neise Wolfgang , Enghardt Lars , King Rudibert , Swoboda Marius
Published: February 2010

Axial turbomachines have a radial gap between the casing and the rotor blades. The static pressure difference between the suction and the pressure side of the impeller blades produces a secondary flow over the tip of the rotor blades. This tip clearance flow is important for the aerodynamic performance of the fan. Fan pressure and efficiency drop, and the usable range of the performance characteristics is diminished as the rotor flow is stalled at low flow rates. Previous investigations have shown that one method for increasing the aerodynamic performance is to control the flow in the tip clearance gap via air injection into the gap. The goal of this paper is to compare the different effects of steady and unsteady air injection on the aerodynamic performance curves and to implement various closed-loop extremum-seeking control algorithms. The main purpose of these active flow control methods is to stabilize the flow at operating points, where it is stalled otherwise. To compare the effect of steady and unsteady air injection, the aerodynamic performance curves (fan pressure rise and efficiency) were measured for different sets of frequencies with the air injection rate held constant. To control the air injection rate automatically and to find optimal actuation parameters, a SISO-extremum-seeking control algorithm was applied. For the improvement of the control performance, the controller was extended by a slope-seeker. Moreover, an extended Kalman filter was used to speed up the control via a faster slope detection to accelerate the estimation of the local gradient of the static input-output map of the process. This new approach led to an almost fivefold increase in closed-loop control speed.
2D Elementary Geometric Decomposition to Study Flutter Motion of a Space Turbine Blisk
Authors: Ferria Hakim , Pacull Franc¸ois , Aubert Ste´phane , Ferrand Pascal , Aknouche Se´bastien , Pouffary Benoiˆt
Published: February 2010

Within the framework of aerospace turbines, an isolated integral bladed disk is examined. The blisk presents very high eigenfrequencies with complex deformations of the blades. A 3D steady RANS computation is at first performed with the aim of characterizing the subsonic flow. Then a simplified 2D approach is considered in order to study fluid-structure interaction and to evaluate the stability: the blades have been found stable. In addition, an original method to better understand the level of stability is presented: the blade vibration is decomposed into elementary geometric movements in order to dissociate the different mechanisms that participate to reach stability. With the assumption of first-order behaviors, the linearized RANS unsteady fields corresponding to both the full deformation and the elementary movements are calculated. Some elementary relative blade movements appear to be either stabilizing or destabilizing when looking at the aerodynamic damping coefficient. The coupling between these elementary movements has also been investigated. Either constructive or destructive interference have been observed. The conclusions that emerge from this study are in line with the results of the classical bending-torsion flutter theory and lead to the improvement of the structure in terms of stability by modifying its mode shape.
Reliable and Accurate Prediction of Three-Dimensional Separation in Asymmetric Diffusers Using Large-Eddy Simulation
Authors: Schneider Hayder , von Terzi Dominic , Bauer Hans-Jo¨rg , Rodi Wolfgang
Published: February 2010

Reynolds-Averaged Navier-Stokes (RANS) calculations and Large-Eddy Simulations (LES) of the flow in two asymmetric three-dimensional diffusers were performed. The numerical setup was chosen to be in compliance with previous experiments. The aim of the present study is to find the least expensive method to compute reliably and accurately the impact of geometric sensitivity on the flow. RANS calculations fail to predict both the extent and location of the three-dimensional separation bubble. In contrast, LES is able to determine the amount of reverse flow and the pressure coefficient within the accuracy of experimental data.
Non-Deterministic CFD Simulation of a Transonic Compressor Rotor
Authors: Gopinathrao Naveen Prasad , Bagshaw David , Mabilat Christophe , Alizadeh Sohail
Published: February 2010

The compressor is one of the most sensitive components in a gas turbine. Small variations in geometry or operating conditions can have a detrimental effect on component performance, efficiency and life. During the past few years, significant effort has been invested in modelling the propagation of input uncertainties for CFD simulations using stochastic methods. Due to the large number of variables involved in typical industrial applications, problems often involve intensive computations, making the use of stochastic methods impractical. Therefore in addition to accuracy issues, the desire to reduce the computational overhead is also a key consideration in industrial applications. This work investigates the propagation of uncertainty within a transonic compressor rotor (NASA Rotor-37), using a Non Intrusive Polynomial Chaos methodology. Extensive computational research of this geometry has previously been undertaken and provides comparative data sets. The Non Intrusive Polynomial Chaos methodology is an inexpensive approach based on the spectral representation of the uncertainty parameters. The polynomial coefficients are evaluated using the Probabilistic Collocation method providing an exponential convergence for arbitrary probability distributions. Results will be shown for variations in inlet total pressure. The resulting performance parameters, including total pressure ratio and adiabatic efficiency, are presented along with their uncertainties. The paper serves as a description of the application of the Polynomial Chaos methodology, within a general purpose CFD software, to a gas turbine aerodynamics problem.
Centrifugal Compressor Stability Prediction Using a New Physics Based Approach
Authors: Moore J. Jeffrey , Ransom David L.
Published: February 2010

The accurate prediction of centrifugal compressor stability continues to be an important area of interest in the oil and gas industry. Ensuring stability is critical to the cost-effective installation and operation of these machines in remote environments, where field stability problems are much more expensive to diagnose and correct. Current industry standards and tools for the prediction of impeller destabilizing forces are based on empirical methods that, to date, have served fairly well for systems with reasonable stability margins. However, as stability margins are decreased, use of a modeling method that is more physics based and can better represent the observed trends in machine behavior at low stability margins is required. Furthermore, the development of mega-class Liquefied Natural Gas (LNG) compressors and ultra-high pressure reinjection compressors provides further motivation to improve accuracy. In this paper, a new physics based expression for the prediction of impeller cross-coupling, previously described by Moore, et al. [1] is further investigated by analyzing several classes and scale factors of impellers ranging from 2-D designs used in reinjection to full 3-D impellers typically used in LNG. The new expression is based on both Computational Fluid Dynamics (CFD) simulation and experimental test data from a known instability. These results are then applied to two case studies of marginally stable and unstable compressors in the field that were studied by the authors’ company. For each case study, the system stability is evaluated using both the new physics based expression as well as the more traditional empirical approaches. Comparisons are made for overall stability prediction as well as sensitivity to system changes. Conclusions are made regarding the applicability and limits of this new approach.
A Robust Design Optimization of a Mixed-Flow Compressor Impeller
Authors: C¸evik Mert , Uzol Og˘uz , Yavrucuk I˙lkay
Published: February 2010

This paper presents the results of a design and optimization study for the impeller of a small mixed-flow compressor. The objective of the optimization is to obtain an impeller geometry that could minimize a cost function based on the specific thrust and the thrust specific fuel consumption (TSFC) of a small jet engine. The design methodology is based on generating a configurational database for various compressor geometries using ID design codes and a commercial turbomachinery design/analysis software, application of Design of Experiments to properly sample the design space, representing the cost function variations within the design space through a neural-network and then finding the optimum configuration that minimizes the cost function. A sample design optimization study showed that the designed compressor as well as the engine that it’s designed for have improved performance characteristics when compared to a typical small turbojet engine and its small radial compressor.
Rotordynamic Testing and Evaluation of a Large Centrifugal Compressor Using a Magnetic Bearing Exciter
Authors: Soulas Thomas A. , Kuzdzal Mark J.
Published: February 2010

Large centrifugal compressors have at times shown to be challenging applications from a rotordynamic standpoint. Inherent rotordynamic stability and robust synchronous response to unbalance are critical for the compressor to function in the field in a reliable manner. State-of-the-art analytical prediction techniques and comprehensive test verification procedures are necessary to ensure the acceptability of the design of such machinery. A full-scale test vehicle was developed and assembled at the authors’ company’s test facility in order to duplicate the design of typical large multi-stage centrifugal compressors. The machine features a number of unique attributes that enable the comprehensive study and evaluation of its aero-thermodynamic as well as rotordynamic performance under different test conditions. In particular, a large Magnetic Bearing Exciter is mounted on one end of the compressor shaft and is used to introduce forces of varying magnitude and frequency to assess the response of the rotor system as well as its rotordynamic stability. Test data (including vibration signals recorded by proximity probes located at the mid-span of the rotor) are compared with analytical predictions for critical speed location, amplification factor and logarithmic decrement. Good correlation between measurements and calculations was achieved, as well as compliance with the standard American Petroleum Institute (API) requirements.
A Multi-Objective Adaptive Controller for Magnetic Bearing Systems
Authors: Sahinkaya M. Necip , Abulrub Abdul-Hadi G. , Burrows Clifford R. , Keogh Patrick S.
Published: February 2010

The paper considers three issues in flexible rotor and magnetic bearing systems, namely control of rotor vibration, control of transmitted forces, and prevention of rotor contact with auxiliary bearings. An adaptive multi-objective optimization method is developed to tackle these issues simultaneously using a modified recursive open loop adaptive controller. The proposed method involves automatic tuning of the weighting parameters in accordance with performance specifications. A two-stage weighting strategy is implemented involving base weightings, calculated from a singular value decomposition of the system’s receptance matrices, and two adjustable weighting parameters to shift the balance between the three objective functions. The receptance matrices are functions of rotational speed and they are estimated in situ. The whole process does not require prior knowledge of the system parameters. Real-time implementation of the proposed controller is explained and tested by using an experimental flexible rotor magnetic bearing system. The rotor displacements were measured relative to the base frame using four pairs of eddy current displacement transducers. System stability is ensured through local PID controllers. The proposed adaptive controller is implemented in parallel and the effectiveness of the weighting parameters in changing the balance between the transmitted forces and rotor vibrations is demonstrated experimentally.
Friction Factor Jump in Honeycomb Seals Explained by Flow-Acoustic Interactions
Authors: Keith Graeme M. , Bhattacharjee Pratik , Nielsen Kenny K. , Ha Tae W. , Childs Dara W. , Platt John P.
Published: February 2010

In a series of experiments over flat plate honeycomb seal surfaces with deep cavities, Ha and Childs [1] have reported, under certain conditions, sudden “jumps” in the friction factor, which possibly could seriously impair honeycomb seal rotordynamic performance. This phenomenon was never fully explained before. In this study, the results have been investigated both analytically and computationally in the light of the Strouhal number dependency, and it was found that the friction factor jump phenomenon is nearly uniquely correlated with the Strouhal number of the flow. This lends strong credence to the hypothesis that the friction factor jump is due to a flow-acoustic interaction. Indeed, the measured friction factors were found to jump at the exact Strouhal Numbers where the energy transfer rate from the flow mode to the acoustic mode were at a maximum. CFD direct numerical simulations at the configurations of friction factor jump also demonstrate a lock-in between the acoustic and vortex shedding frequencies.
Role of Quantitative NDE Techniques in Probabilistic Design and Life Management of Gas Turbine Components: Part II
Authors: Gorelik Michael , Peralta-Duran Alonso , Singh Surendra , Moody Jonathan , Enright Michael
Published: February 2010

A number of earlier publications discussed the benefits of probabilistic lifing in application to critical rotating engine components. One of the key random variables in both probabilistic and deterministic damage tolerant analysis is a probability of detection (POD) that represents a quantified Non-Destructive Evaluation (NDE) capability. The importance of having accurate POD information and its use in the probabilistic design and life management process is further promoted by the changing customer and regulatory requirements. New and existing FAA / EASA regulations and Air Force standards require damage tolerance analysis to be performed on critical engine components. POD for an NDE inspection is recognized as an important input for damage tolerance analysis. This information is needed for establishing and optimizing inspection schedules, calibrating life predictions, calculating fleet risk etc. Sensitivity of the above results to the accuracy of the POD curve, and to its approximation with a single point value (e.g. 90/50 POD = value of 90% probability with 50% confidence) will be discussed and illustrated with numerical examples. A brief overview of the methods for generating POD curves, and recent advancements in this area will be presented as well.
Tribological Properties of Cast Al6061-Si3N4 Composite
Authors: Ramesh C. S. , Keshavamurthy R. , Channabasappa B. H.
Published: February 2010

Si3N4 reinforced Al6061 composite is fabricated by liquid metallurgy technique. Si3N4 particles are nickel coated prior to addition to molten metal to improve its wettability and to ensure excellent bond between matrix and the reinforcement. Metallographic studies, friction and wear tests were carried out using pin on disc type machine. Coefficient of friction and wear rate were measured at loads varying from 20–100N and sliding velocities from 0.314–1.574m/s. It is observed that Al6061-4wt%Si3N4 composites exhibited lower wear rate and lower coefficient of friction when compared with matrix alloy under all the test conditions studied.
Contact Area Variation in Elastic Indentation Problems
Authors: Riznychuk Roman
Published: February 2010

Contact problem of the frictionless indentation of elastic half-space by smooth rigid punch of curved profile is investigated. An exact expression of the contact pressure distribution for a curved profile punch in terms of integral involving the pressure distribution for sequence of flat punches is derived. The method is illustrated and validated by comparison with some well-known analytical solutions.