bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental Characterization of Vaneless Diffuser Rotating Stall: Part V — Influence of Diffuser Geometry on Stall Inception and Performance (3rd Impeller Tested)
Authors: Ferrara G. , Ferrari L. , Baldassarre L.
Published: September 2008

Vaneless diffuser rotating stall is a serious problem for centrifugal compressors, since it limits their working range. In the literature some good correlations can be found for predicting stall inception but they have limited coverage of last stage configuration case, especially for very low blade-outlet-width-to-impeller-radius-ratio impellers typically used in high-pressure applications. In addition, stall inception is strictly bounded to diffuser geometry (for example, diffuser width and diffusion ratio). As a part of a wide range activity on rotating stall, a stage with a blade-outlet-width-to-impeller-radius-ratio of 0.1 has been tested. The stage configuration is made up by a 1:1 geometrical scale stage with a return channel upstream, a 2D impeller with a vaneless diffuser and a volute with a constant cross sectional area downstream. Diffusers with three different widths and two diffusion ratios were tested in order to find their influence on stage stability and performance. Results obtained for impellers with lower blade-outlet-width-to-impeller-radius-ratios have been published in previous papers. The purpose of this paper is to comment the obtained results and increase the amount of experimental data available on vaneless diffuser rotating stall behaviour.
Assessing Convergence in Predictions of Periodic-Unsteady Flowfields
Authors: Clark J. P. , Grover E. A.
Published: September 2008

Predictions of time-resolved flowfields are now commonplace within the gas-turbine industry, and the results of such simulations are often used to make design decisions during the development of new products. Hence it is necessary for design engineers to have a robust method to determine the level of convergence in design predictions. Here we report on a method developed to determine the level of convergence in a predicted flowfield that is characterized by periodic-unsteadiness. The method relies on fundamental concepts from digital signal processing including the discrete Fourier transform, cross-correlation, and Parseval’s theorem. Often in predictions of vane-blade interaction in turbomachines, the period of the unsteady fluctuations is expected. In this method, the development of time-mean quantities. Fourier components (both magnitude and phase), cross-correlations, and integrated signal power are tracked at locations of interest from one period to the next as the solution progresses. Each of these separate quantities yields some relative measure of convergence that is subsequently processed to form a fuzzy set. Thus the overall level of convergence in the solution is given by the intersection of these sets. Examples of the application of this technique to several predictions of unsteady flows from two separate solvers are given. These include a prediction of hot-streak migration as well as more typical cases. It is shown that the method yields a robust determination of convergence. Also, the results of the technique can guide further analysis and/or post-processing of the flowfield. Finally, the method is useful for the detection of inherent unsteadiness in the flowfield, and as such it can be used to prevent design escapes.
Boundary Layer Separation Control With Fluidic Oscillators
Authors: Cerretelli Ciro , Kirtley Kevin
Published: September 2008

Fluidic oscillating valves have been used in order to apply unsteady boundary layer injection to repair the separated flow of a model diffuser, where the hump pressure gradient represents that of the suction surface of a highly loaded stator vane. The fluidic actuators employed in this study consist of a fluidic oscillator that has no moving parts or temperature limitations and therefore is more attractive for implementation on production turbomachinery. The fluidic oscillators developed in this study generate an unsteady velocity with amplitudes up to 60% RMS of the average operating at non-dimensional blowing frequencies (F+) in the range 0.6 < F+ < 6. These actuators are able to fully reattach the flow and achieve maximum pressure recovery with a 60% reduction of injection momentum required and a 30% reduction in blowing power compared to optimal steady blowing. PIV velocity and vorticity measurements have been performed that show no large-scale unsteadiness in the controlled boundary layer flow.
CFD Aerodynamic Investigation of Air-Water Trajectories on Rotor-Stator Blade of an Axial Compressor for Online Washing
Authors: Mustafa Zainol , Pilidis Pericles , Amaral Teixeira Joao A. , Ahmad Kamarul Arifin
Published: September 2008

In multistage axial-flow compressors, the droplet size distribution of the injected fluid depends upon the entry conditions. This paper presents the numerical approach used in the development of a preliminary analytical tool to investigate droplet flow pattern during fluid injection into an axial compressor. An investigation was carried out using the commercial CFD code CFX-TASCflow on the aerodynamics of an axial-flow compressor designed for operation with air while operating with an air-fluid droplet mixture during online washing. A test matrix involving different initial droplet diameters was investigated with the machine rotating at constant shaft speed. The range of droplet sizes used in these simulations is representative of the droplet dimensions used during online washing. The liquid employed in this study was water. The main objective of the investigation is to gain added physical insight into the washing process, through the assembly of a realistic model, describing the movement of liquid phase in an industrial axial compressor. The results are presented to show how varying droplet sizes affect the flow pattern of a representative 260MW Gas Turbines axial compressor during online washing. Furthermore, results are presented for the amount of water collected by the blades due to these tests as well as the quantity of water evaporated. This study shows that the droplet dispersion and flow pattern inside the compressor is strongly influenced by the initial conditions of the droplets during online compressor washing.
Automated Blade Optimization and 3D CFD Analysis for an Axial Multistage GT Compressor Redesign
Authors: Moberg Lars , Guidati Gianfranco , Savic Sasha
Published: September 2008

This paper focuses on (1) the basic compressor layout based on meridional through flow analysis and (2) the re-design of blades and vanes using sophisticated automated design optimization methods. All tools and processes are integrated into a consistent Compressor Design System, which runs on a powerful Linux cluster. This design system allows designing, analyzing and documenting blade design in mostly automated way. This frees the engineer from repetitive tasks and allows him to concentrate on a physical understanding and improvement of the compressor. The tools and methods are illustrated on the basis of an actual ALSTOM compressor. The main objectives of this upgrade are a modest increase in mass flow and an efficiency improvement. The latter is to be achieved through the replacement of NACA blades by modern Controlled Diffusion Airfoils (CDA). Results are presented including a CFD analysis of the front stages of the baseline and upgrade compressor.
Investigation of 3D Unsteady Flows in a Two Stage Shrouded Axial Turbine Using Stereoscopic PIV and FRAP: Part I — Interstage Flow Interactions
Authors: Porreca L. , Yun Y. I. , Kalfas A. I. , Song S. J. , Abhari R. S.
Published: September 2008

A detailed flow analysis has been carried out in a two-stage shrouded axial turbine by means of intrusive and non-intrusive measurement techniques. Multi-sensor Fast Response Aerodynamic Probe (FRAP) and 3D-PIV system were applied at two locations downstream of the first and second rotors. Several radial planes were measured focusing on the blade tip region in order to obtain a unique set of steady and unsteady velocity data. The investigation deals with the aerodynamics and kinematics of flow structures downstream of the first and second rotors and their interaction with the main flow in a partially shrouded turbine typical of industrial application. The first part of this work is focused on the flow field downstream of the first rotor while the second part studies the leakage flow in the cavity of the second rotor and its interaction with the main stream. The interstage region is characterized by interactions between the tip passage vortex and a vortex caused by the recessed shroud platform design. Flow coming from the blade passage suddenly expands and migrates radially in the cavity region causing a localized total pressure drop. The time evolution of these vortical structures and the associated downstream unsteady loss generation are analyzed. The partial shroud design adopted in this geometry is beneficial in terms of blade stress and thermal load; however flow field downstream of the first rotor is highly three dimensional due to the intense interaction between cavity and main streams. A flow interpretation is provided and suggestions for improved design are finally addressed based on the steady and unsteady flow analysis.
Effect of the Leakage Flows and the Upstream Platform Geometry on the Endwall Flows of a Turbine Cascade
Authors: de la Rosa Blanco E. , Hodson H. P. , Vazquez R.
Published: September 2008

This work describes the effect that the injection of leakage flow from a cavity into the mainstream has on the endwall flows and their interaction with a large pressure surface separation bubble in a low-pressure turbine. The effect of a step in hub diameter ahead of the blade row is also simulated. The blade profile under consideration is a typical design of modern low-pressure turbines. The tests are conducted in a low speed linear cascade. These are complemented by numerical simulations. Two different step geometries are investigated, i.e., a backward-facing step and a forward-facing step. The leakage tangential velocity and the leakage mass flow rate are also modified. It was found that the injection of leakage mass flow gives rise to a strengthening of the endwall flows independently of the leakage mass flow rate and the leakage tangential velocity. The experimental results have shown that below a critical value of the leakage tangential velocity, the net mixed-out endwall losses are not significantly altered by a change in the leakage tangential velocity. For these cases, the effect of the leakage mass flow is confined to the wall, as the inlet endwall boundary layer is pushed further away from the wall by the leakage flow. However, for values of the leakage tangential velocity around 100% of the wheelspeed, there is a large increase in losses due to a stronger interaction between the endwall flows and the leakage mass flow. This gives rise to a change in the endwall flows structure. In all cases, the presence of a forward-facing step produces a strengthening of the endwall flows and an increase of the net mixed-out endwall losses when compared with a backward-facing step. This is because of a strong interaction with the pressure surface separation bubble.
Modeling and Analysis of Main Flow–Shroud Leakage Flow Interaction in LP Turbines
Authors: Gier Jochen , Engel Karl , Stubert Bertram , Wittmaack Ralf
Published: September 2008

Endwall losses significantly contribute to the overall losses in modern turbomachinery, especially when aerodynamic load and pressure ratios are increased. In turbines with shrouded airfoils a large portion of these losses are generated by the leakage flow across the shroud clearance. For the design of modern jet engine turbines it becomes increasingly important to include the impact of shroud leakage flows in the aerodynamic design. There are two main aspects connected to this issue. The first aspect is to optimize the cavity flow and its interaction with the main flow. The second aspect is to perform the airfoil design with boundary conditions, which include the shroud leakage flow effects. In comparison to the simplified approach of neglecting the real endwall geometry and leakage flow this should enable the designer to produce improved airfoils for the entire span. In order to address the second aspect of supporting the airfoil design with improved shroud leakage consideration within the airfoil design process, an efficient procedure for modeling the shroud leakage flow has been implemented into the design Navier-Stokes code. The intention is to model the major leakage flow phenomena without the necessity of pre-defining all details of the shroud geometry. In the paper the results of this model are compared to conventional computations, computations with mesh-resolved cavities and experimental data. The differences are discussed and the impact of certain configuration aspects are analyzed.
Steady and Unsteady Characteristics of Two Corner Separations in a Decelerating Square Channel Flow
Authors: Kinoue Yoichi , Setoguchi Toshiaki , Shiomi Norimasa , Kaneko Kenji
Published: September 2008

The three-dimensional separation in a decelerating channel flow generated by the suction through a porous side wall had been investigated experimentally. Unsteady characteristics of two corner separations were especially focused on. The profiles of time averaged static pressure are two-dimensional in the height direction even in the case with large regions of corner separation. The contour of the turbulence level of the mainstream velocity shows two peak values which correspond to two corner separations. The value of cross-correlation gets large negative values without any lag and the value of coherence gets peak value at around 0.5–2 Hz. The corner separations near the top wall and near the bottom wall are not independent and keep coherent structure both with negative value of cross correlation function and with the frequency of around 0.5–2 Hz.
Time-Space Evolution of Secondary Flow Structures in a Two-Stage Low-Speed Turbine
Authors: Puddu Pierpaolo , Palomba Chiara , Nurzia Franco
Published: September 2008

The aim of this work is to highlight the unsteady effects related to wake-blade and blade rows interactions, but also the time-space evolution of secondary flow structures in a two-stage low-speed turbine model designed and constructed to perform unsteady measurements with different techniques [1]. In this case attention has been addressed to the analysis of the flow field in the first stage of the turbine model. Measurements are performed with aerodynamic probes downstream of the first stator and using a single slanted hot-wire anemometer downstream of the first rotor. Time-dependent relative flow field downstream of the first rotor (obtained from phase-locked averaging technique) have been reconstructed for different relative positions between stator and rotor blades. From these results the time-dependent secondary flow vectors have been obtained as well. The mean reference flow used to determine the secondary flow structure has been evaluated for each frame by mass-averaged technique. The evolution of the secondary flow structure due to the influence of the upstream and downstream stators on the first rotor has been investigated. The main unsteady effects put in evidence the variation of the intensity and spatial extension of the vortex flow structure.
Movement of Deposited Water on Turbomachinery Rotor Blade Surfaces
Authors: Williams John , Young John B.
Published: September 2008

A theoretical approach for calculating the movement of liquid water following deposition onto a turbomachine rotor blade is described. Such a situation can occur during operation of an aero-engine in rain. The equation of motion of the deposited water is developed on an arbitrarily oriented plane triangular surface facet. By dividing the blade surface into a large number of facets and calculating the water trajectory over each one crossed in turn, the overall trajectory can be constructed. Apart from the centrifugal and Coriolis inertia effects, the forces acting on the water arise from the blade surface friction, and the aerodynamic shear and pressure gradient. Non-dimensionalisation of the equations of motion provides considerable insight and a detailed study of water flow on a flat rotating plate set at different stagger angles demonstrates the paramount importance of blade surface friction. The extreme cases of low and high blade friction are examined and it is concluded that the latter (which allows considerable mathematical generalisation) is the most likely in practice. It is also shown that the aerodynamic shear force, but not the pressure force, may influence the water motion. Calculations of water movement on a low-speed compressor blade and the fan blade of a high bypass ratio aero-engine suggest that in low rotational speed situations most of the deposited water is centrifuged rapidly to the blade tip region.
Influence of the Radial and Axial Gap of the Shroud Cavities on the Flowfield in a 2-Stage Turbine
Authors: Bohn Dieter , Krewinkel Robert , Tu¨mmers Christian , Sell Michael
Published: September 2008

An important goal in the development of turbine bladings is to improve their efficiency for an optimized usage of energy resources. This requires a detailed insight into the complex 3D-flow phenomena in multi-stage turbines. In order to investigate the flow characteristics of modern highly loaded turbine profiles a test rig with a two stage axial turbine has been set up at the Institute of Steam and Gas Turbines, RWTH Aachen University. The test rig is especially designed to investigate the influence of different cavity sizes. In order to analyze the influence of the cavity size on the secondary flow and to discuss the effects of the blade loading, the 3D flow through the 2-stage turbine with shrouded blades is investigated numerically, using the steady Navier-Stokes inhouse computer code, CHT-Flow. The turbine blading is designed to concentrate the mass flow in the middle of the passage in order to keep the main flow away from the secondary flow regions at the endwalls of the blade. The simulations include a comparison of a configuration without cavities (design case) and two configurations, where the axial gap between the shroud and the endwalls is about 5 mm and the radial gap between the shroud and the endwall is varied between 0.8 mm (open radial gap) and radial gaps “near zero” (closed radial gap). The investigations are done with focus on the secondary flow phenomena in the second guide vane. For a detailed analysis of the blade load the design point and an off-design point are simulated for each blading. The flow conditions are taken from experimental investigations performed at the Institute of Steam and Gas Turbines, Aachen University. In the experimental setup, the turbine is operated at a low pressure ratio of 1.4 with an inlet pressure of 3.2·105 Pa. The numerical results will also be compared to the corresponding experimental data at the outlet of the second stage.
A Turbine Cascade Facility for Secondary Flow Research
Authors: Bagshaw D. A. , Gregory-Smith D. G. , Ingram G. L. , Stokes M. R.
Published: September 2008

This paper outlines some recent major changes to the so-called Durham Cascade that will be of interest to other experimenters working on low speed linear cascades. The Durham Cascade is a popular CFD test case and has also been used extensively for profiled endwall research (Hartland et al. [1]) and more recently for the study of reverse compound lean (Bagshaw et al. [2]). The cascade has recently undergone a complete rebuild, which enables it to provide experimental results of lasting significance for a number of years to come. The improvements described in this paper include, good quality symmetrical inlet flow, a new three-dimensional traverse system and an improved blade cartridge system allowing arbitrary three-dimensional geometries to be tested.
Interaction of Wheelspace Coolant and Main Flow in a New Aeroderivative LPT
Authors: Montomoli Francesco , Massini Michela , Maceli Nicola , Cirri Massimiliano , Lombardi Luca , Ciani Alessandro , D’Ercole Michele , De Prosperis Roberto
Published: September 2008

Increased computational capabilities make available for the aero/thermal designers new powerful tools to include more geometrical details, improving the accuracy of the simulations, and reducing design costs and time. In the present work, a low-pressure turbine was analyzed, modeling the rotor-stator including the wheel space region. Attention was focused on the interaction between the coolant and the main flow in order to obtain a more detailed understanding of the behavior of the angel wings, to evaluate the wall heat flux distribution, and to prevent hot gas ingestion. Issues of component reliability related to thermal stress require accurate modeling of the turbulence and unsteadiness of the flow field. To satisfy this accuracy requirement, a full 3D URANS simulation was carried out. A reduced count ratio technique was applied in order to decrease numerical simulation costs. The study was carried out to investigate a new two-stage Low Pressure Turbine from GE Infrastructure Oil&Gas to be coupled to a new aeroderivative gas generator, the LM2500+G4, developed by GE Infrastructure, Aviation.
Aerodynamic Characteristics of Supercritical Outlet Guide Vanes at Low Reynolds Number Conditions
Authors: Sonoda Toyotaka , Schreiber Heinz-Adolf
Published: September 2008

As a part of an innovative aerodynamic design concept for a single stage low pressure turbine, a high turning outlet guide vane is required to remove the swirl from the hot gas. The airfoil of the vane is a highly loaded compressor airfoil that has to operate at very low Reynolds numbers (Re ∼ 120,000). Recently published numerical design studies and experimental analysis on alternatively designed airfoils showed that blade profiles with an extreme front loaded pressure distribution are advantageous for low Reynolds number conditions. The advantage even holds true for an increased inlet Mach number at which the peak Mach number on the airfoils reaches and exceeds the critical conditions (Mss > 1.0). This paper discusses the effect of the inlet Mach number and Reynolds number on the cascade performance for both a controlled diffusion airfoil (CDA) (called baseline) and a numerically optimized front loaded airfoil. The results show that it is advantageous to design the profile with a fairly steep pressure gradient immediately at the front part in order to promote early transition or to prevent too large laminar — even shock induced — separations with the risk of a bubble burst. Profile Mach number distributions and wake traverse data are presented for design and off-design conditions. The discussion of Mach number distributions and boundary layer behavior is supported by numerical results obtained from the blade-to-blade flow solver MISES.
Direct Numerical Simulation of By-Pass and Separation-Induced Transition in a Linear Compressor Cascade
Authors: Zaki Tamer , Durbin Paul , Wissink Jan , Rodi Wolfgang
Published: September 2008

Direct Numerical Simulation (DNS) of flow through a linear compressor cascade with incoming free-stream turbulence was performed. On the pressure side, the boundary layer flow is found to undergo by-pass transition: The incident vortical disturbances trigger the formation of elongated boundary layer perturbation jets (or streaks) with amplitudes on the order of 10% of the mean flow. The inception of turbulent spots, which leads to breakdown, is triggered on the backward perturbation jets (negative u-fluctuations). The turbulent patches spread and finally merge into the downstream, fully turbulent region. The suction surface boundary layer is initially subject to a Favorable Pressure Gradient (FPG), followed by a strong Adverse Pressure Gradient (APG). The FPG suppresses the formation of boundary layer streaks. The result is a stabilized boundary layer that does not undergo transition. Farther downstream, the strong APG causes the laminar boundary layer to separate, which is followed by turbulent reattachment.
The Effect of Wake Induced Structures on Compressor Boundary-Layers
Authors: Wheeler Andrew P. S. , Miller Robert J. , Hodson Howard P.
Published: September 2008

The interaction of a convected wake with a compressor blade boundary-layer was investigated. Measurements within a single-stage compressor were made using an endoscopic PIV system, a surface mounted pressure transducer, hotfilms and hotwire traverses, along with CFD simulations. The wake/leading-edge interaction was shown to lead to the formation of a thickened laminar boundary-layer, within which turbulent spots formed close to the leading-edge. The thickened boundary-layer became turbulent and propagated down the blade surface, giving rise to pressure perturbations of 7% of the inlet dynamic head in magnitude. The results indicate that wake/leading-edge interactions have a crucial role to play in the performance of compressor blades in the presence of wakes.
On the Coupling of Inverse Design and Optimization Techniques for Turbomachinery Blade Design
Authors: Bonaiuti Duccio , Zangeneh Mehrdad
Published: September 2008

Optimization strategies have been used in recent years for the aerodynamic and mechanical design of turbomachine components. One crucial aspect in the use of such methodologies is the choice of the geometrical parameterization, which determines the complexity of the objective function to be optimized. In the present paper, an optimization strategy for the aerodynamic design of turbomachines is presented, where the blade parameterization is based on the use of a three-dimensional inverse design method. The blade geometry is described by means of aerodynamic parameters, like the blade loading, which are closely related to the aerodynamic performance to be optimized, thus leading to a simple shape of the optimization function. On the basis of this consideration, it is possible to use simple approximation functions for describing the correlations between the input design parameters and the performance ones. The Response Surface Methodology coupled with the Design of Experiments (DOE) technique was used for this purpose. CFD analyses were run to evaluate the configurations required by the DOE to generate the database. Optimization algorithms were then applied to the approximated functions in order to determine the optimal configuration or the set of optimal ones (Pareto front). The method was applied for the aerodynamic redesign of two different turbomachine components: a centrifugal compressor stage and a single-stage axial compressor. In both cases, both design and off-design operating conditions were analyzed and optimized.
Experimental Investigation During Stall and Surge in a Transonic Fan Stage and Rotor-Only Configuration
Authors: Gannon A. J. , Hobson G. V. , Shreeve R. P. , Villescas I. J.
Published: September 2008

High-speed pressure measurements of a transonic compressor rotor-stator stage and rotor-only configuration during stall and surge are presented. Rotational speed data showed the difference between the rotor-only case and rotor-stator stage. The rotor-only case stalled and remained stalled until the control throttle was opened. In the rotor-stator stage the compressor surged entering a cyclical stalling and then un-stalling pattern. An array of pressure probes was mounted in the case wall over the rotor for both configurations of the machine. The fast response probes were sampled at 196 608 Hz as the rotor was driven into stall. Inspection of the raw data signal allowed the size and speed of the stall cell during its growth to be investigated. Post-processing of the simultaneous signals of the casing pressure showed the development of the stall cell from the point of inception and allowed the structure of the stall cell to be viewed.
Exploring Multidisciplinary Trade-Offs in Turbomachinery Design
Authors: Bell Theo A. , Jarrett Jerome P. , Clarkson P. John
Published: September 2008

The complexity of the modern aeroengine has led to a fragmented, modular, and evolutionary design process which, due to the lack of methods for generating understanding about the design space, results in a potentially holistically suboptimal design. Technological maturity, environmental pressures and changing business models are among the challenges facing the industry today; thus, the need to understand and exploit the trade-offs between the economics of production, maintenance, and operating costs and understand their effects during the design process is increasingly important. The first steps toward increased design space exploration and understanding can be achieved by integrating multidisciplinary engine design tools. By coupling engine design tools via SignPosting, a design process management and optimisation technique with a hierarchical database to manage multiple fidelity data and confidences, we have developed an integrated engine design model (IED) with which “better” designs and multidisciplinary trade-offs can be explored and visualized. As aeroengine aerodynamics mature and business models change, the benefit of focusing on other performance variables increases and the definition of what constitutes “better” changes. Accordingly, we have used the IED to minimize the weight of an intermediate pressure compressor and uncooled turbine spool while maintaining aerodynamic performance. By taking three different approaches to the optimisation and using weight objectives of different resolutions (complete spool weight, turbine and compressor weight, and both blade and disc weights for the turbine and compressor separately), the implications of a more traditional approach, which uses the turbine and compressor weights, can be visualized. We also present an analysis of how multidisciplinary non-linearities are exploited to reduce spool weight.
Numerical Investigation of Effects of Incidence Angle on Aerodynamic Performance of Ultra-Highly Loaded Turbine Cascade
Authors: Tsujita Hoshio , Mizuki Shimpei , Yamamoto Atsumasa
Published: September 2008

An increase in turbine blade loading is a useful means to improve the performance characteristics of gas turbines. This paper describes the results of numerical investigation for the internal flow within a low speed linear ultra highly loaded turbine cascade (UHLTC) at the off design condition. The present UHLTC has the design inlet flow, angle of 80 degree and the blade turning angle of 160 degree. The computations were made for the incidence angles from −30.0 to +7.5 degree relative to the design incidence. The two dimensional computations were carried out for eight incidence angles in order to reveal the effects of incidence on the profile loss of UHLTC. Subsequently, the three dimensional computations were performed for the several incidence angles to clarify the sensitivity of secondary flow and the associated loss generation mechanisms to the change of incidence angle. The influences of incidence variation on the blade loading were also examined. The computed results showed that the loss generation and the strength and the structure of secondary flows were much sensitive to the increase of incidence angle from the design incidence. On the other hand, the decrease of the incidence from the design one did not give the strong effect for the loss generation and the blade loading.
The Control of Shroud Leakage Loss by Reducing Circumferential Mixing
Authors: Rosic Budimir , Denton John D.
Published: September 2008

Shroud leakage flow undergoes little change in the tangential velocity as it passes over the shroud. Mixing due to the difference in tangential velocity between the main stream flow and the leakage flow creates a significant proportion of the total loss associated with shroud leakage flow. The unturned leakage flow also causes negative incidence and intensifies the secondary flows in the downstream blade row. This paper describes the experimental results of a concept to turn the rotor shroud leakage flow in the direction of the main blade passage flow in order to reduce the aerodynamic mixing losses. A three-stage air model turbine with low aspect ratio blading was used in this study. A series of different stationary turning vane geometries placed into the rotor shroud exit cavity downstream of each rotor blade row was tested. A significant improvement in flow angle and loss in the downstream stator blade rows was measured together with an increase in turbine brake efficiency of 0.4%.
Computer Aided Aero-Thermodynamic Design for Impellers
Authors: Al-Zubaidy Sarim N.
Published: September 2008

The complete design of radial flow turbomachinery rotors invariably requires the aero-thermodynamic, structural and the manufacturing criteria to be satisfied simultaneously. In recent years, a number of computer aided design and geometry description procedures for centrifugal impellers have been developed. Essentially the detailed geometry generation in most of these codes operates in an iterative mode. The main drawback of such approaches is that the decision whether or not a particular step produces best results tends to be made arbitrarily. Moreover, it is not a simple matter to know what changes in the geometry would produce the desired results. This paper employs an inverse approach, which is based on a prescribed mean streamline deceleration schedule to generate the discrete geometry of centrifugal impellers for given performance requirements. The approach does not claim to offer a comprehensive solution of the aero-thermodynamic design problem of impellers but rather attempt to advance a methodology that is capable of generating designs that can theoretically produce maximum attainable total-to-total pressure ratio and improved efficiency with reduced design time. The systematic deployment of the proposed approach can also be utilized as a tool to explain differences in performance of centrifugal impellers that seemed alike under careful qualitative inspection.
An Investigation of the IGV/Rotor Interaction in a Low Speed Axial Compressor Using DPIV
Authors: Gong Zhiqiang , Li Zhiping , Li Maoyi , Lu Yajun
Published: September 2008

IGV/rotor interaction phenomenon in axial compressors is important, because different matching states of IGV and rotor can result in significant differences in performance of the compressors. An experimental investigation of IGV/rotor interaction is performed on a one stage low speed axial compressor. The performance of the compressor is measured with the number of IGV varied within a wide range. Different IGV results in very different performance of the compressor, and the performance does not change with the number of IGV monotonously. Flow field around a rotor blade is measured using 2D Digital Particle Image Velocimetry (DPIV) and dynamic pressure probes, without IGV and with the IGV which brings the largest stall margin to the compressor. Comparison of flow fields reveals that the IGV wakes change the flow field around the rotor blade significantly. The wake of the rotor blade is weakened and its structure is changed. The rotor exit total pressure is elevated throughout the entire span. The tip leakage flow is suppressed, so relevant blockage is reduced and consequently the stable operating range of the compressor is extended. The relatively high turbulence intensity and periodic changes in flow velocity and direction brought by IGV wakes to the rotor may account for some of the observed changes in the flow field structure and the compressor performance. The flow instability and receptivity theory must be included to explain all the experimental results, and to utilize the rotor/stator interaction phenomena during the compressors design process.
Response Surface-Based Transition Duct Shape Optimization
Authors: Wallin Fredrik , Eriksson Lars-Erik
Published: September 2008

Demands on improved efficiency and reduced noise levels cause a strive toward very high by-pass ratio (BPR) turbo-fan-engines resulting in large fans and small high-pressure-ratio cores. The trend of increasing the radial offset between low- and high-pressure systems has made the design of intermediate transition ducts an area of growing importance. Shape optimization techniques for turbo-machinery applications have become a powerful aero-design tool and thanks to the rapid development of computer technology, computational fluid dynamics (CFD) may be used for optimization purposes. Surrogate model-based optimization has in recent years become a good alternative to the gradient-based search algorithms. One well-known surrogate model-based approach is response surface methodology (RSM), which has been used in the present work. RSM used together with design of experiments (DOE) can be a very efficient and robust method for CFD-based optimization. Optimization of two different transition ducts has been performed and evaluated; one 2D axi-symmetric turbine duct and one 3D compressor duct. The optimization objective was to minimize the total pressure loss. For the turbine duct both a high-Re and a low-Re turbulence closure were evaluated. The influence of design space size on the turbine duct optimization was investigated and for the 3D compressor duct the effects of adding an outflow constraint were examined. It has been found that end-wall optimization has the potential to reduce duct losses significantly. An early rapid diffusion and a stream-wise curvature shift toward the outlet seem to be important mechanisms for reducing transition duct losses.
Influence of the Leakage Flow Tangential Velocity on the Loss Generation and Leakage Flow Kinematics in Shrouded Axial Compressor Cascades
Authors: Sohn D. W. , Kim T. , Song S. J.
Published: September 2008

Although compressor blades have long been shrouded for aerodynamic and structural reasons, the impact of the leakage flow in the shroud cavities on passage flows has only recently been investigated. Furthermore, the tangential velocity of the leakage flow, set by the blading and the relative motion between rotating and stationary surfaces, has a strong influence on the passage flow. Yet the influence of the tangential velocity variation on the kinematics and dynamics (loss) of the leakage flow (from its ingress to egress) in the shrouded cavity and main flow in the blade passage are unknown. Therefore, this paper reports on an experimental investigation of the axial evolution of loss generation in the blade passage and behavior of the leakage flow in the seal cavity in shrouded axial compressor cascades subject to the variation of leakage tangential velocity. The newly found results are as follows. First, increasing tangential velocity of the leakage flow reduces loss at 10% and 50% chordwise locations in the passage. However, most of the blockage and loss reductions occurs in the aft half chord and downstream of the blade passage. Second, the increasing tangential velocity spreads the loss core, which is originally concentrated in the suction side hub corner, in the pitchwise direction. Thus, the loss core becomes more two-dimensional, and the region’s radial extent is reduced. Third, increasing tangential velocity of the leakage flow makes the near hub passage flow more radially uniform. Consequently, the shear and resultant mixing loss between the passage and leakage flows are reduced near the hub, reducing the overall loss. Finally, the leakage flow is ingested through the downstream cavity and makes an abrupt turn at the seal tooth. Thus, two distinct flow regions — downstream and upstream of the single-tooth seal — are found. Before the leakage flow rejoins the mainstream via the upstream cavity trench, the leakage flow circumferentially migrates in the direction of rotation. The magnitude of the circumferential shift depends strongly on the leakage tangential velocity.
Inner Workings of Shrouded and Un-Shrouded Transonic Fan Blades
Authors: Wadia A. R. , Szucs P. N.
Published: September 2008

This paper reports on the numerical assessment of the differences in aerodynamic performance between part span shrouded and un-shrouded fan blades generally found in the first stage of multi-stage fans in low bypass ratio aircraft engines. Rotor flow fields for both blade designs were investigated at two operating conditions using a three-dimensional viscous flow analysis. Although designed to the same radius ratio, aspect ratio and solidity, the un-shrouded fan rotor had a slightly increased tip speed (+3%) and somewhat lower pressure ratio (-3.2%) due to engine cycle requirements. Even when allowing for these small differences, the analysis reveals interesting differences in the level and in the radial distribution of efficiency between these two rotors. The reason for the improved performance of the shrouded rotor in part can be attributed to the shroud blocking off the radial migration of boundary layer fluid centrifuged from the hub on the suction side. As a result, the shock boundary layer interaction seems to be improved on the shrouded blade. At the cruise condition, the efficiency is the same for both rotors. The slightly better efficiency of the shrouded blade in the outer panel is nullified by the large efficiency penalty in the vicinity of the shroud. As there is no significant radial migration of fluid in the suction side boundary layer as indicated by the analysis at this condition relative to the design speed case; the benefit due to the shroud is greatly reduced. At this speed and at lower speeds the shroud becomes a net additional loss for the blade. Also of interest from the numerical results, is the indication that significant blade ruggedization penalties to performance can be reduced in the case of the un-shrouded blade through custom tailoring of its mean camber line.
Boundary Layer Transition on the High Lift T106A LP Turbine Blade With an Oscillating Downstream Pressure Field
Authors: Opoka Maciej M. , Thomas Richard L. , Hodson Howard P.
Published: September 2008

This paper presents the results of an experimental study of the interaction between the suction surface boundary layer of a cascade of LP turbine blades and a fluctuating downstream potential field. A linear cascade equipped with a set of T106 LP turbine blades was subjected to a periodic variation of the downstream pressure field by means of a moving bar system at low-speed conditions. Measurements were taken in the suction surface boundary layer using 2D Laser Doppler Anemometry, flush mounted unsteady pressure transducers and surface shear stress sensors. The Reynolds number, based on the chord and exit conditions, was 1.6×105. The measurements revealed that the magnitudes of the suction surface pressure variations induced by the oscillating downstream pressure field, just downstream of the suction peak, were approximately equal to those measured in earlier studies involving upstream wakes. These pressure field oscillations induced a periodic variation of the transition onset location in the boundary layer. Two turbulence levels were investigated. At a low level of inlet freestream turbulence of 0.5%, a separation bubble formed on the rear part of the suction surface. Unsteady measurements of the surface pressure revealed the presence of high frequency oscillations occurring near the start of the pressure recovery region. The amplitude of these fluctuations was of the order of 7–8% of exit dynamic pressure, and inspection of the velocity field revealed the presence of Kelvin-Helmholtz type shear layer vortices in the separated free shear layer. The frequency of these shear layer vortices was approximately one order of magnitude greater than the frequency of the downstream passing bars. At a higher inlet freestream turbulence level of 4.0%, which is more representative of real engine environments, separation was prevented by an earlier onset of transition. Oscillations were still observed in suction surface shear stress measurements at a frequency matching the period of the downstream bar, indicating a continued influence on the boundary layer from the oscillating pressure field. However, the shear layer vortices seen in the lower turbulence intensity case were not so clearly observed and the maximum amplitude of suction surface pressure fluctuations was reduced.
Performance Margins of Non-Reflecting Slotted Walls in a Transonic Linear Cascade Tunnel
Authors: Rona A. , Gostelow J. P.
Published: September 2008

An experimental investigation was conducted into the endwall interference from a low-pressure turbine nozzle blade profile tested in linear cascade at an isentropic discharge Mach number of 1.27. This was above the profile design point of Mach 0.995. This highlighted that inviscid flow phenomena, at the cascade pitchwise boundaries, are the main cause of spurious end-wall interference at these test conditions. Specifically, fish-tail shocks from the profile trailing edges reflect at these boundaries disturbing the interior flow. An appreciable reduction of such interference was obtained through the use of a slotted tailboard downstream of the outmost blade trailing edge, with void ratio and pitch optimized to operate at Mach 1.27 in this cascade wind tunnel. Cascade tests in the Mach number range 1.20 to 1.32 show that the tailboard at off-design conditions gives a more pitchwise periodic discharge than without a tailboard and a test section open jet boundary. Reducing the tailboard pitch from its design value of 64° to 62° does not further improve the flow pitchwise periodicity over the Mach number range 1.18 to 1.31. Over this range, the slotted tailboard at its design pitch angle retains an appreciable performance margin over the other two end-wall configurations tested.
The Effect of Coolant Injection on the Endwall Flow of a High Pressure Turbine
Authors: Ong Jonathan H. P. , Miller Robert J. , Uchida Sumiu
Published: September 2008

This paper presents a study of the effects of two types of hub coolant injection on the rotor of a high pressure gas turbine stage. The first involves the leakage flow from the hub cavity into the mainstream. The second involves a deliberate injection of coolant from a row of angled holes from the edge of the stator hub. The aim of this study is to improve the distribution of the injected coolant on the rotor hub wall. To achieve this, it is necessary to understand how the coolant and leakage flows interact with the rotor secondary flows. The first part of the paper shows that the hub leakage flow is entrained into the rotor hub secondary flow and the negative incidence of the leakage strengthens the secondary flow and increases its penetration depth. Three dimensional unsteady calculations were found to agree with fast response pressure probe measurements at the rotor exit of a low speed test turbine. The second part of the paper shows that increasing the injected coolant swirl angle reduced the secondary flow penetration depth, improves the coolant distribution on the rotor hub and improves stage efficiency. Most of the coolant however, was still found to be entrained into the rotor secondary flow.