Experimental Study on the Effect of Clocking on the Propagation of Inflow Pressure Distortions in a Low Pressure Turbine Stage

Abstract The design of modern aero engines enhances the interaction between components and facilitates the propagation of circumferential distortions of total pressure and temperature. As a consequence, the inlet conditions of a real turbine have significant spatial non-uniformities, which have direct consequences on both its aerodynamic and vibration characteristics. This work presents the results of an experimental study on the effects of different inlet total pressure distortion-stator clocking positions on the propagation of total pressure inflow disturbances through a low pressure turbine stage, with a particular focus on both the aerodynamic and aeroelastic performance. Measurements at a stable engine relevant operating condition and during transient operation were carried out in a one and a half stage subsonic turbine test facility at the Institute of Thermal Turbomachinery and Machine Dynamics at Graz University of Technology. A localised total pressure distortion was generated upstream of the stage in three different azimuthal positions relative to the stator vanes. The locations were chosen in order to align the distortion directly with a vane leading edge, suction side and pressure side. Additionally, a setup with clean inflow was used as reference. Steady and unsteady aerodynamic measurements were taken downstream of the investigated stage by means of a five-hole-probe (5HP) and a fast response aerodynamic pressure probe (FRAPP) respectively. Strain gauges applied on different blades were used in combination with a telemetry system to acquire the rotor vibration data. The aerodynamic interactions between the stator and rotor rows and the circumferential perturbation were studied through the identification of the main structures constituting the flow field. This showed that the steady and unsteady alterations created by the distortion in the flow field lead to modifications of the rotor vibration characteristics. Moreover, the importance of the impact that the pressure distortion azimuthal position has on the LPT stage aerodynamics and vibrations was highlighted.

Download Full-text

Investigation of Variated Unsteady Inflow Boundary Conditions on the Transition Behavior of a Low Pressure Turbine Cascade Family

Volume 2C: Turbomachinery ◽

10.1115/gt2014-25891 ◽

2014 ◽

Author(s):

Franz F. Blaim ◽

Roland E. Brachmanski ◽

Reinhard Niehuis

Keyword(s):

Boundary Condition ◽

Transport Equation ◽

Flow Separation ◽

Total Pressure ◽

Low Pressure ◽

Test Facility ◽

Number Range ◽

Pressure Losses ◽

Low Pressure Turbine ◽

Transition Behavior

The objective of this paper is to analyze the influence of incoming periodic wakes, considering the variable width, on the integral total pressure loss for two low pressure turbine (LPT) airfoils. In order to reduce the overall weight of a LPT, the pitch to chord ratio was continuously increased, during the past decades. However, this increase encourages the development of the transition phenomena or even flow separation on the suction side of the blade. At low Reynolds numbers, large separation bubbles can occur there, which are linked with high total pressure losses. The incoming wakes of the upstream blades are known to trigger early transition, leading to a reduced risk of flow separation and hence minor integral total pressure losses caused by separation. For the further investigation of these effects, different widths of the incoming wakes will be examined in detail, here. This variation is carried out by using the numerical Unsteady Reynolds Averaged Solver TRACE developed by the DLR Cologne in collaboration with MTU Aero Engines AG. For the variation of the width of the wakes, a variable boundary condition was modeled, which includes the wake vorticity parameters. The width of the incoming wakes was used as the relevant variable parameter. The implemented boundary condition models the unsteady behavior of the incoming wakes by the variation of the velocity profile, using a prescribed frequenc. TRACE can use two different transition models; the main focus here is set to the γ–Reθt transition model, which uses local variables in a transport equation, to trigger the transition within the turbulence transport equation system. The experimental results were conducted at the high speed cascade open loop test facility at the Institute for Jet Propulsion at the University of the German Federal Armed Forces in Munich. For the investigation presented here, two LPT profiles — which were designed with a similar inlet angle, turning, and pitch are analyzed. However, with a common exit Mach number and a similar Reynolds number range between 40k and 400k, one profile is front loaded and the other one is aft loaded. Numerical unsteady results are in good agreement with the conducted measurements. The influence of the width of the wake on the time resolved transition behavior, represented by friction coefficient plots and momentum loss thickness will be analyzed in this paper.

Download Full-text

Unsteady Wet Steam Flow Field Measurements in the Last Stage of Low Pressure Steam Turbine

Volume 8: Microturbines, Turbochargers and Small Turbomachines; Steam Turbines ◽

10.1115/gt2015-43504 ◽

2015 ◽

Cited By ~ 2

Author(s):

Ilias Bosdas ◽

Michel Mansour ◽

Anestis I. Kalfas ◽

Reza S. Abhari ◽

Shigeki Senoo

Keyword(s):

Flow Field ◽

Steam Turbine ◽

Total Pressure ◽

Low Pressure ◽

Operating Conditions ◽

Test Facility ◽

Flow Structures ◽

Wet Steam ◽

Operating Condition ◽

Last Stage

Modern steam turbines need to operate efficiently and safely over a wide range of operating conditions. This paper presents a unique unprecedented set of time-resolved steam flowfield measurements from the exit of the last two stages of a low pressure (LP) steam turbine under various volumetric massflow conditions. The measurements were performed in the steam turbine test facility in Hitachi city in Japan. A newly developed fast response probe equipped with a heated tip to operate in wet steam flows was used. The probe tip is heated through an active control system using a miniature high-power cartridge heater developed in-house. Three different operating points, including two reduced massflow conditions, are compared and a detailed analysis of the unsteady flow structures under various blade loads and wetness mass fractions is presented. The measurements show that at the exit of the second to last stage the flow field is highly three dimensional. The measurements also show that the secondary flow structures at the tip region (shroud leakage and tip passage vortices) are the predominant sources of unsteadiness at 85% span. The high massflow operating condition exhibits the highest level of periodical total pressure fluctuation compared to the reduced massflow conditions at the inlet of the last stage. In contrast at the exit of the last stage, the reduced massflow operating condition exhibits the largest aerodynamic losses near the tip. This is due to the onset of the ventilation process at the exit of the LP steam turbine. This phenomenon results in 3 times larger levels of relative total pressure unsteadiness at 93% span, compared to the high massflow condition. This implies that at low volumetric flow conditions the blades will be subjected to higher dynamic load fluctuations at the tip region.

Download Full-text

Blade–Row Interactions in a Low Pressure Turbine at Design and Strong Off-Design Operation

Journal of Turbomachinery ◽

10.1115/1.4028213 ◽

2014 ◽

Vol 136 (11) ◽

Cited By ~ 1

Author(s):

Martin Lipfert ◽

Jan Habermann ◽

Martin G. Rose ◽

Stephan Staudacher ◽

Yavuz Guendogdu

Keyword(s):

Experimental Data ◽

Flow Field ◽

Three Dimensional ◽

Suction Side ◽

Low Pressure ◽

Test Facility ◽

Time Resolved ◽

Joint Project ◽

Low Pressure Turbine ◽

Blade Row

In a joint project between the Institute of Aircraft Propulsion Systems (ILA) and MTU Aero Engines, a two-stage low pressure turbine is tested at design and strong off-design conditions. The experimental data taken in the Altitude Test Facility (ATF) aims to study the effect of positive and negative incidence of the second stator vane. A detailed insight and understanding of the blade row interactions at these regimes is sought. Steady and time-resolved pressure measurements on the airfoil as well as inlet and outlet hot-film traverses at identical Reynolds number are performed for the midspan streamline. The results are compared with unsteady multistage computational fluid dynamics (CFD) predictions. Simulations agree well with the experimental data and allow detailed insights in the time-resolved flow-field. Airfoil pressure field responses are found to increase with positive incidence whereas at negative incidence the magnitude remains unchanged. Different pressure to suction side (SS) phasing is observed for the studied regimes. The assessment of unsteady blade forces reveals that changes in unsteady lift are minor compared to changes in axial force components. These increase with increasing positive incidence. The wake-interactions are predominating the blade responses in all regimes. For the positive incidence conditions, vane 1 passage vortex fluid is involved in the midspan passage interaction, leading to a more distorted three-dimensional (3D) flow field.

Download Full-text

Experimental Investigation of Total Pressure Loss Development in a Highly Loaded Low-Pressure Turbine Cascade

Journal of Turbomachinery ◽

10.1115/1.4038413 ◽

2017 ◽

Vol 140 (3) ◽

Cited By ~ 16

Author(s):

Philip Bear ◽

Mitch Wolff ◽

Andreas Gross ◽

Christopher R. Marks ◽

Rolf Sondergaard

Keyword(s):

Flow Field ◽

Secondary Flow ◽

Pressure Loss ◽

Total Pressure ◽

Low Pressure ◽

Total Pressure Loss ◽

Low Pressure Turbine ◽

Pressure Transport ◽

Secondary Flow Field ◽

Highly Loaded

Improvements in turbine design methods have resulted in the development of blade profiles with both high lift and good Reynolds lapse characteristics. An increase in aerodynamic loading of blades in the low-pressure turbine (LPT) section of aircraft gas turbine engines has the potential to reduce engine weight or increase power extraction. Increased blade loading means larger pressure gradients and increased secondary losses near the endwall. Prior work has emphasized the importance of reducing these losses if highly loaded blades are to be utilized. The present study analyzes the secondary flow field of the front-loaded low-pressure turbine blade designated L2F with and without blade profile contouring at the junction of the blade and endwall. The current work explores the loss production mechanisms inside the LPT cascade. Stereoscopic particle image velocimetry (SPIV) data and total pressure loss data are used to describe the secondary flow field. The flow is analyzed in terms of total pressure loss, vorticity, Q-Criterion, turbulent kinetic energy, and turbulence production. The flow description is then expanded upon using an implicit large eddy simulation (ILES) of the flow field. The Reynolds-averaged Navier–Stokes (RANS) momentum equations contain terms with pressure derivatives. With some manipulation, these equations can be rearranged to form an equation for the change in total pressure along a streamline as a function of velocity only. After simplifying for the flow field in question, the equation can be interpreted as the total pressure transport along a streamline. A comparison of the total pressure transport calculated from the velocity components and the total pressure loss is presented and discussed. Peak values of total pressure transport overlap peak values of total pressure loss through and downstream of the passage suggesting that the total pressure transport is a useful tool for localizing and predicting loss origins and loss development using velocity data which can be obtained nonintrusively.

Download Full-text

Impact of Passive Tip-Injection on Tip-Leakage Flow in Axial Low Pressure Turbine Stage

Volume 2A: Turbomachinery ◽

10.1115/gt2015-42226 ◽

2015 ◽

Cited By ~ 1

Author(s):

Pouya Ghaffari ◽

Reinhard Willinger ◽

Sabine Bauinger ◽

Andreas Marn

Keyword(s):

Aerodynamic Resistance ◽

Low Pressure ◽

Leakage Flow ◽

Turbine Stage ◽

Tip Leakage Flow ◽

Tip Leakage ◽

Low Pressure Turbine ◽

Blade Tip ◽

Tip Injection ◽

The Impact

In addition to geometrical modifications of the blade tip for reducing tip-leakage mass flow rate the method of passive tip-injection serves as an aerodynamic resistance towards the tip-leakage flow. The impact of this method has been investigated thoroughly at unshrouded blades in linear cascades. Furthermore combinations of shrouded blades with passive tip-injection have been investigated analytically as well as via numerical simulations for incompressible flow in linear cascades. The objective of this paper is to consider a real uncooled low pressure turbine stage with shrouded blades and to investigate the effect of passive tip-injection on various operational characteristics. CFD calculations have been carried out in a rotational frame taking into consideration compressible flow and serve for evaluating the method of passive tip-injection in the given turbine stage. Experimental data obtained from the machine without tip-injection serve as boundary conditions for the CFD calculations.

Download Full-text

On the Influence of an Acoustically Optimized Turbine Exit Casing Onto the Unsteady Flow Field Downstream of a Low Pressure Turbine Rotor

Volume 2A: Turbomachinery ◽

10.1115/gt2018-76725 ◽

2018 ◽

Author(s):

L. Simonassi ◽

M. Zenz ◽

S. Zerobin ◽

F. Heitmeir ◽

A. Marn ◽

...

Keyword(s):

Flow Field ◽

Unsteady Flow ◽

Low Pressure ◽

Turbine Rotor ◽

Low Noise ◽

Test Facility ◽

Pressure Probe ◽

Guide Vanes ◽

Low Pressure Turbine ◽

Aerodynamic Pressure

Modern low pressure turbines (LPT) are designed in order to fulfil a various number of requirements such as high endurance, low noise, high efficiency, low weight and low fuel consumption. Regarding the reduction of the emitted noise, different designs of low pressure turbine exit guide vanes (aerodynamically and/or acoustically optimized) of the turbine exit casing (TEC) were tested and their noise reduction capabilities and aerodynamic performance were evaluated. In particular, measurements of TEC-losses were performed and differences in the losses were reported. Measurements were carried out in a one and a half stage subsonic turbine test facility at the engine relevant operating point approach. This work focuses on the study of the unsteady flow field downstream of an unshrouded low pressure turbine rotor. The influence on the upstream flow field of a TEC design including acoustically optimized vanes (Inverse cut-off TEC) is investigated and compared with a second TEC configuration without vanes (Vaneless TEC), by means of fast response aerodynamic pressure probe measurements. The second configuration served as a reference concerning the influence of TEGVs onto the upstream located LPT rotor. The interactions between the stator and rotor wakes, secondary flows and the turbine exit guide vanes potential effect are identified via modal decomposition according to the theory of Tyler and Sofrin. The main structures constituting the unsteady flow field are detected and the role of the major interaction effects in the loss generation mechanism and in the acoustic emission is analysed. This study based on the modal analysis of the unsteady flow field offers new insight into the main interaction mechanisms and their importance in the assessment of the aerodynamic and aeroelastic performance of modern low pressure turbine exit casings.

Download Full-text

Reducing Secondary Flow Losses in Low-Pressure Turbines With Blade Fences: Part II — Experimental Validation on Linear Cascades

Volume 2B: Turbomachinery ◽

10.1115/gt2019-91284 ◽

2019 ◽

Author(s):

Matteo Giovannini ◽

Filippo Rubechini ◽

Giorgio Amato ◽

Andrea Arnone ◽

Daniele Simoni ◽

...

Keyword(s):

Flow Field ◽

Large Scale ◽

Passive Control ◽

Low Pressure ◽

Secondary Flows ◽

Speed Test ◽

Flow Losses ◽

Low Pressure Turbine ◽

Beneficial Impact ◽

The Impact

Abstract This paper deals with the design of passive control devices for reducing the impact of secondary flows on the aerodynamics of low-pressure turbine (LPT) stages. A novel kind of device is introduced which consists of shelf-like fences to be added to the blade surface. Such a device is intended to contrast the development of secondary flows, thus reducing losses and flow turning deviation with respect to the straight blade. In this second part, an experimental campaign on a linear cascade is presented which is aimed at proving the beneficial impact of the blade fences. Experiments were carried out on a low-speed test-rig, equipped with a large scale blade representative of the stators of the engine-like environment considered in part I. Measurements are mainly focused on the stator losses and on the flow field at the stator exit. The performance of the blade fences was evaluated by comparing the straight cascade and the fenced ones. The measurements highlighted the impact of the blade fences on the development of the secondary flows, affecting both the stator losses and the non-uniformity of the flow field over the exit plane, which, in the actual stage environment, impacts the operation of the downstream blade row. Moreover, the comparison between CFD and experiments proved the accuracy of the CFD setup, thus suggesting its reliability in predicting the stage performance in the engine-like configuration.

Download Full-text

Experimental Investigation of Total Pressure Loss Development in a Highly Loaded Low Pressure Turbine Cascade

Volume 2A: Turbomachinery ◽

10.1115/gt2017-64684 ◽

2017 ◽

Cited By ~ 2

Author(s):

Philip Bear ◽

Mitch Wolff ◽

Andreas Gross ◽

Christopher R. Marks ◽

Rolf Sondergaard

Keyword(s):

Flow Field ◽

Pressure Loss ◽

Total Pressure ◽

Low Pressure ◽

Total Pressure Loss ◽

Turbine Cascade ◽

Low Pressure Turbine ◽

Pressure Transport ◽

Secondary Flow Field ◽

Highly Loaded

Improvements in turbine design methods have resulted in the development of blade profiles with both high lift and good Reynolds lapse characteristics. An increase in aerodynamic loading of blades in the low pressure turbine section of aircraft gas turbine engines has the potential to reduce engine weight or increase power extraction. Increased blade loading means larger pressure gradients and increased secondary losses near the endwall. Prior work has emphasized the importance of reducing these losses if highly loaded blades are to be utilized. The present study analyzes the secondary flow field of the front-loaded low-pressure turbine blade designated L2F with and without blade profile contouring at the junction of the blade and endwall. The current work explores the loss production mechanisms inside the low pressure turbine cascade. Stereoscopic particle image velocimetry data and total pressure loss data are used to describe the secondary flow field. The flow is analyzed in terms of total pressure loss, vorticity, Q-Criterion, turbulent kinetic energy and turbulence production. The flow description is then expanded upon using an Implicit Large Eddy Simulation of the flow field. The RANS momentum equations contain terms with pressure derivatives. With some manipulation these equations can be rearranged to form an equation for the change in total pressure along a streamline as a function of velocity only. After simplifying for the flow field in question the equation can be interpreted as the total pressure transport along a streamline. A comparison of the total pressure transport calculated from the velocity components and the total pressure loss is presented and discussed. Peak values of total pressure transport overlap peak values of total pressure loss through and downstream of the passage suggesting that total pressure transport is a useful tool for localizing and predicting loss origins and loss development using velocity data which can be obtained non-intrusively.

Download Full-text

A Low Pressure Turbine at Extreme Off-Design Operation

Journal of Turbomachinery ◽

10.1115/1.4025592 ◽

2013 ◽

Vol 136 (3) ◽

Cited By ~ 4

Author(s):

Martin Lipfert ◽

Martin Marx ◽

Martin G. Rose ◽

Stephan Staudacher ◽

Inga Mahle ◽

...

Keyword(s):

Reynolds Number ◽

Flow Field ◽

Three Dimensional ◽

Leading Edge ◽

Low Pressure ◽

Test Facility ◽

Low Pressure Turbine ◽

Second Stage ◽

Wide Range ◽

High Incidence

In a cooperative project between the Institute of Aircraft Propulsion Systems and MTU Aero Engines GmbH, a two-stage low pressure turbine with integrated 3D airfoil and endwall contouring is tested. The experimental data taken in the altitude test-facility study the effect of high incidence in off-design operation. Steady measurements are covering a wide range of Reynolds numbers between 40,000 and 180,000. The results are compared with steady multistage CFD predictions with a focus on the stator rows. A first unsteady simulation is taken into account as well. The CFD simulations include leakage flow paths with disk cavities modeled. Compared to design operation the extreme off-design high-incidence conditions lead to a different flow-field Reynolds number sensitivity. Airfoil lift data reveals changing incidence with Reynolds number of the second stage. Increased leading edge loading of the second vane indicates a strong cross channel pressure gradient in the second stage leading to larger secondary flow regions and a more three-dimensional flow-field. Global characteristics and area traverse data of the second vane are discussed. The unsteady CFD approach indicates improvement in the numerical prediction of the predominating flow-field.

Download Full-text