bluuebunny/crossref_metadata_2025 huggingface dataset:
Compressor Leading Edge Spikes: A New Performance Criterion
Authors: Goodhand Martin N. , Miller Robert J.
Published: February 2010

Compressor blades often have a small ‘spike’ in the surface pressure distribution at the leading edge. This may result from blade erosion, manufacture defects or compromises made in the original design process. This paper investigates the effect of these spikes on profile loss and presents a criterion to ensure they are not detrimental to compressor performance. In the first part of the paper two geometries of leading edge are tested. One has a small spike, typical of those found on modern compressors; the other has no spike, characteristic of an idealized leading edge. Testing was undertaken on the stator of a single-stage low speed compressor. The time-resolved boundary layer was measured using a hot-wire micro traversing system. It is shown that the presence of the small spike changes the time resolved transition process on the suction surface, but that this results in no net increase in loss. In the second part of the paper, spike height is systematically changed using a range of leading edge geometries. It is shown that below a critical spike height the profile loss is constant. If the critical spike height is exceeded, the leading edge separates and profile loss rises by 30%. Finally a criterion is developed, based on the total diffusion across the spike. Three different leading edge design philosophies are investigated. It is shown that if the spike diffusion factor is kept below 0.1 over the blade’s incidence range, performance is unaffected by leading edge geometry.
Enhancing the Stability of Subsonic Compressors Using Casing Grooves
Authors: Houghton Tim , Day Ivor
Published: February 2010

Casing grooves are known to increase the stable operating range of axial compressors. The mechanism by which this stability enhancement occurs is poorly understood. This paper develops a better understanding of the behaviour of grooves through analysis of new data. An experimental parametric study is used to demonstrate the effect of varying the axial location of a single casing groove on the stability and efficiency of the compressor. The effect that the groove has on rotor outflow blockage, blade loading and the near-casing flow field is then studied using both experimental and computational methods. It is found that the interaction of the groove with the flow field is different when the groove is positioned forward or aft relative to the blade. The interaction of the groove with the flow in the tip region in both of these positions is presented in detail. Finally, the implications of these findings for the design of casing grooves of different depths are discussed.
Compressor Design From Specification to Validation: Application of a Fast and Reliable Process
Authors: Micheli Marco , Kappis Wolfgang , Guidati Gianfranco , Felderhoff Markus
Published: February 2010

To support customer requirements for existing power plants inside the Service business Alstom did develop over the years a fast and reliable design process for compressors. This has meanwhile been proven with several engines. Besides high-end technologies, main focus is given on the interdisciplinary alignment. The application for an upgraded product for the mature fleet will be briefly shown. The blading re-design is based on Controlled Diffusion Airfoil (CDA) technology, individually optimized for every single blade row. Together with mechanically optimized thin blades this results in high compressor efficiency and surge margin. The CDA blading technology has been introduced in Alstom during 1990s. Therefore extended experience on this technology is available and has been manifested in design rules & criteria. The interdisciplinary blading concept and optimization is done with in-house 2D & 3D CFD codes for aerodynamics and FEM codes for mechanical integrity. To speed-up the design process, task-optimized tools are used for specific topics. All tools are linked into a design system and have an interface to a CAM system. Aerodynamic and FEM codes have been tuned/well-calibrated according to the past experience. A review process with experts is applied at certain milestones for technical and commercial issues. During blading design usually several disciplines are involved. This causes a challenge to organize the corresponding resources. Alstom uses a different approach: tasks are partly executed independent from the disciplines with standard tools. Nevertheless the final release stays inside the ownership of the expert discipline. This means e.g. that simplified tools are developed for pre-checks to be done by any discipline and the final check with high-end tools is done by the experts. This ensures a highly flexible and fast interdisciplinary approach. To validate the design and operation range of the upgraded compressors special attention is paid on testing. This reveals usually in a pre-test for the baseline engine and a post-test for the upgraded engine. For validation purposes a full compressor mapping under load and idle is done for both test series. Additional measurements for pressure rise inside the engine and validation of natural frequencies and stresses to certain blades is applied. The evaluation of the measurement data is used to confirm the improved compressor design and to check achievement of guaranteed performance figures. In addition the surge margin is measured to confirm that the upgraded compressor can be safely operated under all condition.
Numerical Studies on Fan Casing Vibration and Noise Radiation
Authors: Cai Jian-Cheng , Qi Da-Tong , Lu Fu-An
Published: February 2010

This paper presents a method for predicting casing vibration and noise radiation of a centrifugal fan due to internal fluctuating pressure fields at the blade passing frequency (BPF). Using computational fluid dynamics (CFD) technique based on the finite volume method (FVM), a three-dimensional unsteady simulation of internal flow was carried out by solving the Reynolds averaged Navier-Stokes equations. The oscillating pressure distribution at the volute casing was taken as the excitation of the finite element analysis (FEA) model of the casing structure. Time-domain nodal forces on the casing were obtained by integrating the pressure and viscous stress over the element surfaces. Fast Fourier transform (FFT) was applied to these time series of nodal forces in order to extract the BPF components. Harmonic response analysis was carried out to the casing structure. Sound radiation was finally calculated by the indirect boundary element method (IBEM) with the vibration results as velocity boundary condition. This study shows that it is feasible to use fluid-structure weakly coupled simulations for the prediction of flow-induced casing vibration and noise radiation of centrifugal turbomachinary.
Concurrent Aerodynamic-Aeromechanic Design Optimization for Turbomachinery Blades Using Adjoint Method
Authors: Wang D. X. , He L.
Published: February 2010

Presented in this paper is the development and application of the adjoint method to efficient concurrent blading aerodynamic and aeromechanic design optimizations. A nonlinear harmonic phase solution method is adopted to solve the unsteady Reynolds Averaged Navier-Stokes equations for blading aerodynamic and aeromechanic performance evaluations. This unsteady flow solution method has a high computational efficiency and provides a concise and convenient basis for formulating the corresponding adjoint equations. The adjoint equations for the unsteady flow system are solved effectively by a relatively simple extension of the methods and techniques previously developed for a steady flow adjoint solver. As a result, the sensitivities of both the steady (time-mean) flow loss and the aerodynamic damping/forcing to detailed blade geometry changes can be very efficiently obtained by solving equivalently 3 steady adjoint equations. Two case studies are provided to illustrate the appropriate implementation and the effectiveness of these methodologies.
Low Cycle Fatigue Crack Initiation of Rotating Structures Under Biaxial Stress States
Authors: Zhang Chao
Published: February 2010

Rotating structures can experience biaxial stress states with a wide range of biaxiality ratios on structure surfaces. Low cycle fatigue (LCF) crack initiation in such conditions demonstrates different fatigue characteristics in terms of crack orientation, fatigue life, etc. The biaxial stress states can be categorized into two types: in-phase and out-of-phase under which fatigue characteristics can be significantly different according to rig test results. This paper presents an investigation of LCF crack initiation under in-phase and out-of-phase biaxial stress states based on rig test results of a nickel alloy. The crack orientations are reviewed and discussed at different stress states. Relations of biaxial LCF life debit factor vs biaxiality ratio are derived (the debit factor is defined as a ratio of the LCF life at a biaxial stress state to the LCF life at corresponding uniaxial stress state which has same cyclic and mean stresses as the primary cyclic and mean stressees of the biaxial stress state). The rig test results showed that the crack orientation is usually normal to the primary stress vector under in-phase biaxial stress states but is inclined to the primary stress vector under out-of-phase stress states. As per the derived biaxial LCF life debit factors, the LCF life was found to be slightly reduced with increasing biaxiality ratios under in-phase biaxial stress states but significantly reduced under out-of-phase biaxial stress states compared with corresponding uniaxial primary stress states. The equivalent cyclic stress fatigue criterion is also employed to theoretically model the biaxial LCF life debit factor under in-phase biaxial stress states. The hydrostatic cyclic stress is included in the equivalent cyclic stress in order to take into account the hydrostatic cyclic pressure effects. The equivalent cyclic stress in the criterion can physically reflect the materials’ ductility reduction under in-phase multiaxial stress states.
Designing a High Fidelity Wake Simulator for Research Using Linear Cascades
Authors: Pluim Jonathon , Memory Curtis , Bons Jeffrey , Chen Jen-Ping
Published: February 2010

Owing to the extensive use of wake generators in the study of turbine and compressor airfoils in linear cascades, a study was undertaken to determine the most accurate model for the wakes generated by upstream blade rows. Velocity (PIV) measurements were taken to compare wake properties of several bluff bodies with different cross sections to the wake of an ultra high lift low pressure turbine profile. These measurements were taken at two Reynolds numbers, a low and a high one to simulate a separated and attached wake, respectively, for both the blade and two of the shape configurations. The L1A turbine blade profile was determined to shed a wake typical of high lift turbine blade profiles. It is shown that the wake of the turbine blade is highly dependent on Reynolds number. In order to make an appropriate comparison, all bluff body data were extracted along a plane parallel to the equivalent inlet plane of a rotor stage in the stationary frame of reference. It was found that no single rod shape matched all of the blade wake characteristics. From among the shapes used in this study, a 30° isosceles wedge placed 6 diameters upstream of the cascade inlet in the axial direction and skewed 15° from the rod relative flow was found to yield the closest match for the low Re case due to the asymmetry in the velocity and Reynolds shear stresses in this wake compared with the wake of the low pressure turbine blade. The same configuration placed 10 diameters upstream yielded the best comparison to a higher Re, more attached L1A wake. Large Eddy Simulations of various shapes largely corroborate the experimental findings.
Thermomechanical Fatigue of a Directionally-Solidified Ni-Base Superalloy in the Presence of Stress Concentrations
Authors: Kupkovits Robert A. , Neu Richard W.
Published: February 2010

Directionally-solidified (DS) Ni-base superalloys are used in high temperature gas turbines because of their excellent properties in the most aggressive mechanical, thermal, and environmental operating conditions. Complex thermomechanical loading of turbine blades is induced by repeated engine start-up, firing, and shut-down transients. These histories make life prediction for such components difficult and subjective. In addition, accurate techniques need to account for the presence of cooling hole stress concentrations, time-dependent dwells, thermal gradients, and anisotropic material properties. In working towards such an analytical life model, this paper describes the cyclic deformation response and damage mechanisms resulting from thermomechanical fatigue (TMF) of directionally-solidified CM247LC DS. Experimental LCF tests consisted of linear in-phase (IP) and out-of-phase (OP) TMF cycles performed on smooth and notched round-bar specimens in both longitudinal and transverse grain orientations. Results take into consideration anisotropy, time-dependent deformation, and notch effects in addition to the waveform and temperature cycle characteristics. The active damage mechanisms are identified as a function of these parameters. Conclusions are drawn in light of fractography, microscopy, and finite element analysis conducted to evaluate geometric and microstructural influences on material behavior.
Optimized Short Bearing Theory for Squeeze Film Dampers
Authors: Edney Stephen L.
Published: February 2010

It is well established that classical short bearing theory can be applied to assess squeeze film dampers whirling in circular centered orbits. This theory yields accurate values for the stiffness and damping coefficients for designs with small length-to-diameter (L/D) ratios (typically less than 0.5) whirling at amplitudes of less than half the damper radial clearance. For L/D ratio designs above 0.5 and/or whirling amplitudes approaching the damper radial clearance, the short bearing theory increasingly overestimates the stiffness and damping coefficients that stretch its applicability for some designs. There are two limitations with the classical theory that compromise the solution at high L/D ratios and large whirling amplitudes. The first is that as the L/D ratio increases, the unrestricted end flow assumption that forms the basis of the short bearing theory introduces increasingly larger errors. The second is that as the whirling amplitude approaches the damper radial clearance, the stiffness and damping coefficients approach infinity much more rapidly than those from a full solution of the governing lubrication equation. The ideal method for determining more exact values is to numerically solve the full lubrication equation, although not everyone has access to such a code. An alternative approach is to use the expressions presented in this paper that are derived from an optimized solution of the short bearing theory that appreciably reduces the errors introduced at high L/D ratios and whirling amplitudes approaching the damper radial clearance. The optimized solution yields a simple closed form correction factor based on Galerkin’s method that minimizes these errors over the positive pressure region of the oil film. This analytic correction factor increases the accuracy of the short bearing theory for all whirling amplitudes and extends the applicability of the closed form solution to larger L/D ratio damper designs. The simple closed form expressions presented herein apply to a damper whirling in a circular centered orbit for both a partial pi-film cavitated model and a full-film uncaviated model. Examples are given that demonstrate the optimized solution yields stiffness and damping values that are significantly closer to the numerical solution for L/D ratio designs up to 1.0 and/or whirling amplitudes approaching the damper radial clearance.
Determination of Quasi-Steady Characteristics for an Oscillating Airfoil at Varied Reduced Frequency and Reynolds Number
Authors: Sharma Deepakkumar M. , Poddar Kamal
Published: February 2010

Wind tunnel experiments were conducted on NACA-0015 airfoil model to investigate the effect of the reduced frequency (κ) & Reynolds Number (Re) on the aerodynamic characteristics and hysteresis behavior associated with the pitching motion of the airfoil. Pressure measurements were conducted on the mid span of the airfoil for quantitative results. The hysteresis behavior was observed in aerodynamic characteristics as strong function of reduced frequency in form of a loop which gets enlarged with the increase in the value of reduced frequency. The present investigation of oscillating sinusoidal motion of airfoil, α = 10° + 15° sin(ωt) showed that, under the operating conditions studied, the airfoil was allowed to execute oscillating motion from almost steady (quasi-steady) conditions to unsteadiness with the incremental increase in the reduce frequency. Static conditions show distinct characteristics as compared to dynamic mode. But at extremely low reduced frequency of oscillation even in the dynamic mode the quasi-steady conditions do prevail. This results in a value of reduced frequency upto which the quasi-steady characteristics are preserved. Normal force defect is derived from the surface pressure distribution and the value of reduced frequency within 5% of normal force defect is obtained for varied range of Re to determine and validate the quasi-steady conditions. Re effect is dominant at very low reduced frequency. Also as reduced frequency increases causes decrease in the Re effect and increase in the normal force defect upto certain limiting value. Further increase in the reduced frequency reduces the normal force defect. Effect of Leading edge contamination is also depicted to cause incremental shift in the static stall angle.
Numerical and Experimental Analysis of the Flow in an Aggressive Intermediate Turbine Duct
Authors: Johansson Peter B. V. , Axelsson Lars-Uno
Published: February 2010

Multi-spool turbofan engines have intermediate ducts connecting the high-pressure turbine with the low-pressure turbine. The demands for more efficient and environmentally friendly jet engines force future turbofan engines to have higher by-pass ratios. This leads to that the radial off-set between the turbine stages increases and therefore the radial off-set of the intermediate duct has to increase. In order to save fuel consumption it is desirable to decrease engine weight and thus the duct should be made as short as possible. The flow in the intermediate duct is complex, so safely shortening the duct requires a thorough understanding of the flow physics in order to be able to accomplish this in a controlled manner. The present investigation focus on a joint experimental and numerical investigation of an intermediate duct configuration. The main purpose of the numerical simulation is to obtain an insight into the current capabilities of numerical simulations for these kinds of complex flow. The conclusion is that the numerical simulations manage fairly well to capture the total pressure and swirl angle profile from about 20% to 80% span. Also, the hub and shroud pressure profiles are well predicted while the total pressure loss is overpredicted in the numerical simulations.
Experimental Investigation of the Flow in the Pipe Diffuser of a Centrifugal Compressor Stage Under Selected Parameter Variations
Authors: Zachau Uwe , Niehuis Reinhard , Hoenen Herwart , Wisler David C.
Published: February 2010

On a centrifugal compressor test facility various experimental investigations have been carried out contributing a valuable gain in knowledge on the fundamental flow physics within passage type diffusers. An extensive measurement series using various steady, unsteady and laser optical measurement techniques has been performed to detect the unsteady, highly three dimensional diffuser flow under various realistic operating conditions. Zachau et al. [1] presented the test facility and the results gathered under nominal conditions. As a follow-up the results of investigated parameter variations are now presented, covering bleed variations, impeller tip clearance and impeller-diffuser misalignment studies. The data is compared to the benchmark created from the nominal baseline data sets and evaluated with respect to the compressor stage performance. Zachau et al. [1] found that under nominal conditions the flow in the pipe diffuser separates on the pressure side in the first half of the pipe. In the last 30% of the pipe hardly any deceleration of the flow takes place. From this, special attention is given to the investigated parameter variations regarding a first proposal for a diffuser design change, which consists in shortening the diffuser. The results for each parameter variation are evaluated in detail in direct comparison to the nominal baseline configuration underlining the conclusion made earlier that the diffuser flow always separates on the pressure side with negligible deceleration in the last third of the diffusing pipe.
Reduced Order Model for a Two Stage Gas Turbine Including Mistuned Bladed Disks and Shaft Interaction
Authors: Boulton Luis A. , Casanova Euro
Published: February 2010

A number of previous works have suggested that in some cases the interaction between shaft and bladed disk modes could significantly modify the dynamics of the whole assembly i.e. the bladed disks mounted on a flexible shaft. This paper presents the application of a previously published reduced-order modeling technique to the dynamical modeling of a real two stage gas turbine, including the bladed disks and the shaft. In the resulting reduce order model, mistuning is included in the bladed disk models and the shaft is modeled using beam finite elements according to the classical rotordynamic approach. Generation of finite element parent model for the real turbine is presented and discussed as well as simplifications used in order to generate the reduced order model. Comparisons are made between the reduced model and the full finite element solution for free response frequencies and mode shapes in order to assess the methodology and to evaluate the impact of simplifying hypothesis considered in model generation. Finally, this work also shows interaction between shaft modes and bladed disk modes, therefore confirming that stage independent analysis might not be adequate for predicting the global dynamic response of some turbomachinery rotors.
Riblets in Turbulent Flow Regimes of 2-D Compressor Blades
Authors: Klumpp Stephan , Meinke Matthias , Schro¨der Wolfgang , Feldhaus Bjo¨rn , Klocke Fritz
Published: February 2010

The application of riblets to compressor blades is investigated. A rolling technique is developed which is able to produce riblets on the surface of titanium alloy workpieces and possesses several advantages concerning the durability of the final blade. The rolling process generates a strain hardening of the rim zone and a deformation of the micro structure which improves the durability of the riblets. Furthermore, compressive residual stresses are induced below the riblet valleys reducing or even compensating the extra tensile stresses caused by the notch effect during bending stress. The reduction of skin friction due to the produced riblet surface is determined by numerical flow simulations in zero-pressure gradient flow. Based on the data of a controlled diffusion airfoil (CDA) strategies to distribute riblets along the blade surface are discussed under consideration of the local shear stress. A new strategy following the idea of stepwise adapted riblets along the surface is presented. The amount of total drag reduction which could be gained by this novel strategy is estimated.
Experimental Investigation of Two-Phase Flow Phenomena in Transonic Compressor Cascades
Authors: Eisfeld Tjark , Joos Franz
Published: February 2010

The subject of the experimental investigations presented in this paper is the behaviour of a two phase air and water droplets flow in a transonic compressor rotor blade passage carried out to understand the detailed processes in wet compression operation. Non-intrusive measurements were taken to investigate the droplet airflow interactions as well as the droplet blade surface collision. The qualitative results presented in this paper concentrate on the phenomena close to the blade and on the blade surface. Water formations and collision mechanisms on the surface are shown as well as the deformation of the droplets due to the acceleration of the flow in the blade passage. The quantitative results provide the velocity distributions of dry airflow and the flow fields of different droplet classes in two phase flow at the same operation point. Results show that the liquid phase has little impact on the airflow. The droplet impingement on the leading edge causes a splitting into a large amount of small droplets due to splashing. Droplets of larger diameter originate from the trailing edge of the blade as a consequence of the water mass accumulating on the blade surface.
A Physical Interpretation of Stagnation Pressure and Enthalpy Changes in Unsteady Flow
Authors: Hodson H. P. , Hynes T. P. , Greitzer E. M. , Tan C. S.
Published: February 2010

This paper provides a physical interpretation of the mechanism of stagnation enthalpy and stagnation pressure changes in turbomachines due to unsteady flow, the agency for all work transfer between a turbomachine and an inviscid fluid. Examples are first given to illustrate the direct link between the time variation of static pressure seen by a given fluid particle and the rate of change of stagnation enthalpy for that particle. These include absolute stagnation temperature rises in turbine rotor tip leakage flow, wake transport through downstream blade rows, the influence on mixing losses of turbine wake behavior in downstream blade rows, and effects of wake phasing on compressor work input. Fluid dynamic situations are then constructed to explain the effect of unsteadiness, including a physical interpretation of how stagnation pressure variations are created by temporal variations in static pressure; in this it is shown that the unsteady static pressure plays the role of a time-dependent body force potential. It is further shown that when the unsteadiness is due to a spatial nonuniformity translating at constant speed, as in a turbomachine, the unsteady pressure variation can be viewed as a local power input per unit mass from this body force to the fluid particle at that point.
Advanced Non-Axisymmetric Endwall Contouring for Axial Compressors by Generating an Aerodynamic Separator—Part II: Experimental and Numerical Cascade Investigation
Authors: Hergt Alexander , Dorfner Christian , Steinert Wolfgang , Nicke Eberhard , Schreiber Heinz-Adolf
Published: February 2010

Modern methods for axial compressor design are capable of shaping the blade surfaces in a three dimensional way. Linking these methods with automated optimization techniques provides a major benefit to the design process. The application of non-axisymmetric contoured endwalls is considered to be very successful in turbine rotors and vanes. Concerning axial compressors non-axisymmetric endwalls are still a field of research. This two-part paper presents the recent development of a novel endwall design. A vortex created by a nonaxisymmetric endwall groove acts as an aerodynamic separator, preventing the passage vortex from interacting with the suction side boundary layer. This major impact on the secondary flow results in a significant loss reduction by means of load redistribution, reduction of recirculation areas and suppressed corner separation. Part I of this paper deals with the endwall design and its compressor application. The resulting flow phenomena and physics are described and analysed in detail. The second paper presents the detailed experimental and numerical investigation of the developed endwall groove. The measurements carried out at the transonic cascade wind tunnel of DLR in Cologne, demonstrated a considerable influence on the cascade performance. A loss reduction and redistribution of the cascade loading were achieved at the aerodynamic design point as well as near the stall condition of the cascade. This behaviour is well predicted by the numerical simulation. The combined analysis of experimental and numerical flow patterns allows a detailed interpretation and description of the resulting flow phenomena. In this context high fidelity 3D-RANS flow simulations are required to analyse the complex blade and endwall boundary layer interaction.
Controlling Tip Leakage Flow Over a Shrouded Turbine Rotor Using an Air-Curtain
Authors: Curtis Eric M. , Denton John D. , Longley John P. , Rosic Budimir
Published: February 2010

This paper describes an experimental and computational investigation into the performance of an air-curtain seal used to control the leakage flow over the tip shroud of a turbine rotor. The results show that a seal of this type has the potential to reduce or eliminate shroud leakage whilst having a practical level of clearance between the stationary and moving components. The experimental measurements were undertaken using a single-stage low-speed air turbine equipped with a continuous circumferential nozzle in the casing to deliver an axisymmetric jet into the cavity over the rotor shroud. The jet was angled at 45° to the axial direction so that its momentum opposed the shroud leakage flow. In this arrangement the air-curtain was able to sustain the pressure difference between the inlet and outlet of the rotor blade row without any leakage. The test facility had comprehensive instrumentation for obtaining accurate measurements of turbine efficiency that were corrected for the externally supplied additional flow required for the air-curtain. Measurements were obtained for a range of jet flows and show the change in the turbine efficiency as the jet flow is increased. The measurements have been compared with calculations.
Fully Parametric High-Fidelity CFD Model for the Design Optimisation of the Cyclic Stagger Pattern of a Set of Fan Outlet Guide Vanes
Authors: Milli Andrea , Bron Olivier
Published: February 2010

The present paper deals with the redesign of cyclic variation of a set of fan outlet guide vanes by means of high-fidelity full-annulus CFD. The necessity for the aerodynamic redesign originated from a change to the original project requirement, when the customer requested an increase in specific thrust above the original engine specification. The main objectives of this paper are: 1) make use of 3D CFD simulations to accurately model the flow field and identify high-loss regions; 2) elaborate an effective optimisation strategy using engineering judgement in order to define realistic objectives, constraints and design variables; 3) emphasise the importance of parametric geometry modelling and meshing for automatic design optimisation of complex turbomachinery configurations; 4) illustrate that the combination of advanced optimisation algorithms and aerodynamic expertise can lead to successful optimisations of complex turbomachinery components within practical time and costs constrains. The current design optimisation exercise was carried out using an in-house set of software tools to mesh, resolve, analyse and optimise turbomachinery components by means of Reynolds-averaged Navier-Stokes simulations. The original configuration was analysed using the 3D CFD model and thereafter assessed against experimental data and flow visualisations. The main objective of this phase was to acquire a deep insight of the aerodynamics and the loss mechanisms. This was important to appropriately limit the design scope and to drive the optimisation in the desirable direction with a limited number of design variables. A mesh sensitivity study was performed in order to minimise computational costs. Partially converged CFD solutions with restart and response surface models were used to speed up the optimisation loop. Finally, the single-point optimised circumferential stagger pattern was manually adjusted to increase the robustness of the design at other flight operating conditions. Overall, the optimisation resulted in a major loss reduction and increased operating range. Most important, it provided the project with an alternative and improved design within the time schedule requested and demonstrated that CFD tools can be used effectively not only for the analysis but also to provide new design solutions as a matter of routine even for very complex geometry configurations.
Investigation of Skewed Slot Casing on Transonic Axial-Flow Fan Stage
Authors: Lu Jialing , Chu Wuli , Wu Yanhui
Published: February 2010

In this paper, a highly loaded transonic fan stage, which was designed and tested by Wennerstrom in the 70’s was selected to study the functions of skewed slot casing treatments. Three casings, each different in axial locations and slot directions, were designed and numerically studied by this research. Experience on optimum slot location and direction was summarized. The simulation was run by commercial CFD software Fine/Turbo. Un-steady simulations were performed on the casing treated rotors to depict the real nature of unsteadiness in a slotted casing. The rotor exit boundary conditions were set according to the parameters extracted from the stage simulation. With this condition, computational expense was much saved, and the attention was made solely focused on the slots and rotor interaction. Performance simulation at partial design speed (70% design speed) was also conducted. Unexpected performance improvement with the treated casing was observed at this speed. This unusual phenomenon was analyzed, with a corresponding explanation provided.
Effects of Endwall Profiling on Axial Flow Compressor Stage
Authors: Lu Jialing , Chu Wuli , Wu Yanhui
Published: February 2010

In recent years endwall profiling has been well validated as a major new engineering design tool for the reduction of secondary loss in turbines. However, its application on compressors have been rarely performed and reported. This paper documents the findings of the analysis for diminishing compressor stator corner separation using endwall profiling; In the study, novel profiled endwalls were designed and numerically studied on a subsonic axial-flow compressor stage. The compressor stator endwalls were profiled on both axial and azimuthal directions. The results showed, the stator corner separation was significantly suppressed under all the operating conditions by implementing this profiled endwall. Significant improvements on stage pressure ratios and stage efficiency were observed. Detailed flow field changes, as well as endwall profiling methods are provided in the paper, so that the results of this research can be referenced to other compressor designs.
Examination of Rotating Stall Inception in a Small High Speed Axial Compressor
Authors: Bennett L. G. N. , Allan W. D. E.
Published: February 2010

Rotating stall is an internal aerodynamic disturbance that limits the performance and operating life of a compressor. It has been studied with the aim of developing techniques for its prediction and prevention. To further the understanding of rotating stall inception, a test rig was constructed with the axial stages of a Rolls Royce Model 250-C20B small, high speed axial compressor as the test article. A gasoline engine was used to power the compressor and airflow was throttled through a pneumatically controlled valve. Simultaneous static pressure measurements were taken with seven high speed transducers arranged in two configurations: distributed both axially and circumferentially around the compressor casing. The compressor characteristic was mapped and detailed pressure measurements were taken between normal and surge operating conditions. Previous studies of high speed multi-stage compressors have shown both modal and spike type stall inception at different compressor stages. Other examinations of the Model 250 compressor have shown stall inception occurring at the first stage of the compressor. Similar results were observed in this study and an analysis was conducted using a variety of signal processing techniques including pressure trace inspection and discrete spatial Fourier decomposition.
Experimental Investigation on Stochastic Crack Growth Model of GH4133B at Fatigue-Creep
Authors: Hu Dianyin , Wang Rongqiao
Published: February 2010

Experiments on the fatigue crack growth have shown great dispersancy. Study on stochastic crack growth of material at room temperature has been widely performed. However, probabilistic model for crack growth at fatigue-creep has been little investigated due to the complexity of the deterministic model for crack growth at fatigue-creep as well as the time-consuming and the difficulty of the experiments. Traditional crack measurement such as direct current and alternating current electrical potential technique, compliance method is limited for circuit interference at large crack, especially when the temperature is higher than 500°C. Experimental system to achieve real-time FCCG detection at high temperature is established by introducing a long-distance microscope with high magnification and resolution from distances of 15cm to 35cm. The experimental setup consists of a dynamic testing machine, a machine controller, a temperature controlled box, a long-distance microscope and a high temperature furnace from room temperature to 1000°C. Then the fatigue-creep crack growth (FCCG) rate tests on thirty compact tension (CT) specimens made of GH4133B material at 600°C are carried out. The reason for choosing the GH4133B Ni-based superalloy is owing to its popularity in use for the turbine disc of the aero-engine. The tests are conducted on a 100KN capacity servo-hydraulic closed-loop machine employed trapezoidal load with hold time at upon peak load. Based on the crack growth models used for room temperature, the deterministic model for FCCG rate considering the parameters including temperature, hold time is established through comparison of the analytical results with the experimental data. Then the stochastic FCCG model for GH4133B is proposed and the probability of random to reach a specified crack size can be obtained as well as the distribution function of crack size at the service time. Through comparison between the analytical and experimental results, it’s found that the probabilistic FCCG model can fit the experimental data well. Once the stochastic FCCG model is established, it can be used for the probabilistic damage tolerance design of the turbine components made of GH4133B material.
A Procedure to Predict Influence of Acceleration and Damping of Blades Passing Through Critical Speeds on Fatigue Life
Authors: Rao J. S. , Ratnakar Rejin , Suresh S. , Narayan R.
Published: February 2010

Turbine Blades suffer fatigue damage when they cross over a critical speed during start up and shut down conditions. The stress response is usually determined from quasi-steady analysis through resonance with an assumed damping. This response above fatigue limit can be divided into several steps to reach the peak value at critical speed and then fall after passing the critical. For a given acceleration of the rotor, one can then determine the number of cycles at each of these stress levels and assess cumulative damage for one crossing. In this paper, the effect of acceleration and damping on the magnitude of peak stress and where it occurs in the vicinity of critical speed is included in determining the damage suffered by a blade while passing through the critical speed.
Influence of Labyrinth Seal Leakage on Centrifugal Compressor Performance
Authors: Mischo Bob , Ribi Beat , Seebass-Linggi Christof , Mauri Sebastiano
Published: February 2010

The focus of this paper lies on the leakage flow across the shroud of a centrifugal compressor impeller. It is common practice to use shrouded impellers in multi stage compressors featuring a single shaft. The rotating impeller then has to be sealed against the higher pressure in the downstream diffuser by means of labyrinths. The relative amount of leakage is higher for stages designed for low flow, meaning that the associated losses gain in relevance. In addition to this loss source, the injection of the leakage flow has a serious influence on the main flow in a region where it is prone to separation, i.e. at the suction side of the impeller blades close to the shroud, where the highest relative velocities are found. The present paper discusses the numerical results of several geometrical arrangements where the leakage flow was mixed with the main flow in different ways. The distance between the location of injection and the leading edge of the impeller as well as the orientation of the injected flow showed a distinct influence on the performance of the entire stage, mainly on stability.
Experimental Investigations of the Disturbed Inlet-Flow Structure Caused by Mixing Geometries and Its Influence on the Performance of a Turbocharger Centrifugal Compressor
Authors: Brune K.-H. , Schiffer H.-P. , Christmann R. , Gnewikow M.
Published: February 2010

One solution for reducing the nitrogen oxide emissions of diesel engines is the Low-Pressure-Exhaust-Gas-Recirculation method. This solution involves gathering exhaust gas from behind the turbine and mixing it with the aspirated fresh air upstream of the turbocharger compressor. This in turn tends to result in an inhomogeneous inflow into the compressor affecting the distribution of the velocity, the total pressure and the total temperature. The efficiency of the centrifugal compressor is influenced by this strongly disturbed inflow. In this paper the consequences of different mixer geometries developed for this application are examined. Experimental investigations are carried out in order to understand the flow processes of the disturbed inlet-flow structure and the associated effects on the efficiency of the compressor. These different mixer geometries are investigated for different mass flow ratios. These inflow conditions are then compared to a straight, undisturbed inflow and an elbow inlet. The aim is to decide which one of these mixing geometries is most appropriate.
Cracked Blade Detection From Bladed Disk Forced Response
Authors: Kharyton Vsevolod , Laine Jean-Pierre , Thouverez Fabrice , Kucher Olexiy
Published: February 2010

The primary task of this study is to offer reliable and accurate model of a bladed disk containing cracked blade. This model allows simulation of bladed disk dynamic behavior for various crack positions and lengths. Due to absence of cyclic symmetry caused by crack presence in the disk, a reduction procedure was implemented to simulate full bladed disk. It is proposed to use crack location as an interface between two substructures for subsequent fixed-interface method application. Harmonic balance method was applied to take into account crack nonlinear behavior under periodically varying loads. The method implementation considers contact interaction between crack sides at the crack being closed. The contact force is calculated using penalty method of contact force calculation. Relative vertical displacements between nodes in contact were used as nonlinear degrees of freedom (DOFs). Developed bladed disk model is able to take into account external excitation forces phase lag caused by difference between number of rotor and stator blades. Also presence of mistuning was considered. It was shown that certain level of mistuning can directly affect cracked blade detectability. Cracked blade dynamic behavior localization plays here very important role. Absence of cracked blade localization results in impossibility to separate cracked blade response at any mistuning level. Validity of zig-zag diagram for structures with disrupted symmetry is shown using developed bladed disk model with presence of certain level of mistuning.
Prediction of Engine Mounting Loads in Transient Dynamic Response Under Blade Shedding Unbalance
Authors: Herran Mathieu , Chalons Herve´ , Ne´lias Daniel , Ortiz Roland
Published: February 2010

For the TURBOMECA new generation turboshaft engines, overspeed electronic protection of the free turbine is coupled with a mechanical protection called “Blade Shedding”. This mechanical protection is provided by the rupture of all the turbine blades and the retention of the generated debris by a dedicated containment shield. During Blade Shedding, significant transient loads are transmitted to the helicopter frame through the engine mountings. The prediction of the transient loads within the turbine engine itself and those transmitted through the engine mountings is of first importance in order to plan and reduce the number of development engine tests. These loads come on one hand from impacts on the blade housing, and on the other hand from the rotor unbalance. The effects of the rotor dynamics were first implemented in the finite element code Europlexus (a Computer Program for the Finite Element Simulation of Fluid-Structure Systems under Transient Dynamic Loading) to take into account gyroscopic effects. Then the analysis of an academical test case demonstrates the relevance of the method. Finally a blades-off sequence is simulated on an industrial whole engine model and the mounting loads due to unbalance are investigated. The various assumptions made in this simulation are then discussed and the results are compared to test results.
Generalised Single-Passage Multi-Bladerow Method for Turbomachinery Forced Response
Authors: Mata Sanjay , Di Mare Luca , Mehmet Imregun , Green Jeffrey
Published: February 2010

This paper describes the formulation and validation of a novel computational method for a quick and accurate assessment of multi-bladerow turbomachinery forced response using 3D, viscous, time-domain, unsteady aerodynamics. Phase-lagged boundary conditions are applied at the periodic boundaries and information is exchanged using a 0-thickness whole-annulus representation of the bladerow boundary. The proposed method is general in terms of rotor/stator blade numbers and preserves the accuracy of an equivalent multi-bladerow whole-annulus time-domain computation. The method is first validated on simple wave propagation cases for cylinders with known exact solutions. A turbine configuration is studied next, and results from the single-passage multi-bladerow method are checked against those from equivalent whole-annulus computations. The two sets of results are found to be in excellent agreement. The computational requirements of the two methods are compared and the advantages of the generalized single-passage method over its whole-annulus counterpart are highlighted in terms of CPU effort and memory storage. Implementation details, such as convergence, solution acceleration and the effect of non-reflecting boundary condition treatments at the inter-bladerow boundaries, are also discussed.
Forced Response of Mistuned Bladed Discs in Gas Flow: A Comparative Study of Predictions and Full-Scale Experimental Results
Authors: Petrov Evgeny , Di Mare Luca , Hennings Holger , Elliott Robert
Published: February 2010

An integrated experimental-numerical study of forced response for a mistuned bladed disc has been performed. A full chain for the predictive forced response analysis has been developed including data exchange between the CFD code and a code for the prediction of the nonlinear forced response for a bladed disc. The experimental measurements are performed at a full-scale single stage test rig with excitation by aerodynamic forces from gas flow. Numerical modelling approaches and the test rig setup are discussed. Comparison of experimentally measured and predicted values of blade resonance frequencies and response levels for a mistuned bladed disc without dampers is performed. A good correspondence between frequencies at which individual blades have maximum response levels is achieved. The effects of structural damping and underplatform damper parameters on amplitudes and resonance frequencies of the bladed disc are explored. It is shown that the underplatform damper significantly reduces scatters in values of the individual blade frequencies and maximum forced response levels.