Turbine Hub Cavity Modes and Their Impact on Efficiency

2021 ◽  
pp. 1-47
Author(s):  
Vahid Iranidokht ◽  
Naman Purwar ◽  
Anestis I. Kalfas ◽  
Reza S. Abhari ◽  
Shigeki Senoo
Keyword(s):  
Fast Response ◽  
Test Facility ◽  
Turbine Stage ◽  
Characteristic Parameters ◽  
Reference Case ◽  
Reduced Pressure ◽  
Pressure Transducers ◽  
Unsteady Pressure ◽  
Flow Parameters ◽  
Cavity Modes

Abstract Non-synchronous pressure and temperature fluctuations at the hub cavity of a turbine stage are the main focus of this study. Cavity modes (CMs) are unsteady fluctuations generated at the cavity exit due to instabilities in this region. The CMs carried into the main flow impose an unsteady flow field in the rotor passages which varies the passage-wise flow parameters considerably. A two-stage axial turbine was designed and tested in the “LISA” test facility at ETH Zurich. A reference case with baseline geometry and a modified case with an axial deflector at the hub cavity exit were tested. Comprehensive unsteady pressure and temperature measurements were performed using Fast Response Aerodynamic (FRAP) and Entropy Probes (FENT), respectively. In addition, 12 fast response unsteady pressure transducers were mounted on the stationary wall of the cavity exit to measure the main characteristic parameters of the CMs. Full annular unsteady simulations were also carried out for both cases to support the experiments. CFD successfully predicted the CMs effect both in frequency and amplitude of the fluctuations. The CMs indicated fluctuation amplitudes up to 8 times of the blade passing fluctuations at the cavity exit. The analysis shows that the convected CMs alter the efficiency of different rotor passages by redistributing the mass flow and the losses resulting in a drop in overall efficiency. This work suggests that implementing a small axial deflector at the hub cavity exit would completely eliminate the CMs leading to a reduced pressure unsteadiness and enhanced efficiency.

scholarly journals Sensitivity analysis on the impact of geometrical and operational variations on turbine hub cavity modes and practical methods to control them

2021 ◽  
Vol 5 ◽  
pp. 66-78
Author(s):  
Vahid Iranidokht ◽  
Anestis Kalfas ◽  
Reza Abhari ◽  
Shigeki Senoo ◽  
Kazuhiro Momma
Keyword(s):  
Experimental Investigation ◽  
Pressure Ratio ◽  
Fast Response ◽  
Test Facility ◽  
Flow Structures ◽  
Characteristic Parameters ◽  
Pressure Transducers ◽  
Cavity Modes ◽  
Layer Velocity ◽  
The Impact

This paper presents an experimental investigation on the impact of different design and operational variations on the instabilities induced at the hub cavity outlet of a turbine. The experiments were conducted at the “LISA” test facility at ETH Zurich. The axial gap at the 2nd stage hub cavity exit was varied, and also three different flow deflectors were implemented at the cavity exit to control the cavity modes (CMs). Furthermore, the turbine pressure ratio was altered to mimic the off-design condition and study the sensitivity of the CMs to this parameter. Measurements were performed using pneumatic, and Fast Response Aerodynamic Probes (FRAP) at stator and rotor exit. In addition, unsteady pressure transducers were installed at the cavity exit wall to measure the characteristic parameters of the CMs. For the small axial gap, distinct and strong CMs were generated, which actively interacted with stator and rotor hub flow structures. Increasing the gap damped the fluctuations; however, a broader range of frequencies was amplified. The flow deflectors successfully suppressed the CMs by manipulating the shear layer velocity profile and blocking the growing instabilities. Eventually, the increase in the turbine pressure ratio strengthened the CMs and vice versa.

Experimental Investigation of the Aerodynamic Stability of an Annular Compressor Cascade Performing Tuned Pitching Oscillations in Transonic Flow

1999 ◽  
Author(s):  
H. Hennings ◽  
J. Belz
Keyword(s):  
Transonic Flow ◽  
Suction Side ◽  
Test Facility ◽  
Feedback Signal ◽  
Detailed Knowledge ◽  
Compressor Cascade ◽  
Aerodynamic Stability ◽  
Pressure Transducers ◽  
Unsteady Pressure ◽  
Unsteady Aerodynamic

A prerequisite for aeroelastic stability investigations on vibrating compressor cascades is the detailed knowledge of the unsteady aerodynamic loads acting on the blades. In order to obtain precise insight into the aerodynamic damping of a vibrating blade assembly, a basic experiment was performed where unsteady pressure distributions were measured for subsonic and transonic flow conditions. The experiments were performed on a non-rotating, two-dimensional section of a compressor cascade in an annular test facility. The cascade consists of 20 blades (NACA3506 profile) mounted on elastic spring suspensions. In order to measure the unsteady pressure distribution, the cascade was set to tuned pitching oscillations (traveling wave modes). Each blade was driven to controlled harmonic torsional motions around midchord by a magnetic excitation system and by inductive displacement probes which measure the feedback signal of the motion. Steady and unsteady pressures were measured by steady pressure taps and piezo-electric pressure transducers, respectively. The measurement of the unsteady aerodynamic response to a shock vibrating on the suction side of the blades was enabled by a dense spacing of transducers in this region. The global aerodynamic stability is assessed by a damping coefficient evaluated from the out-of-phase parts of the unsteady moment coefficients and by the contributions from the local work coefficient, using the measured pressure data.

On High-Resolution Pressure Amplitude and Phase Measurements Comparing Fast-Response Pressure Transducers and Unsteady Pressure-Sensitive Paint

2020 ◽  
Author(s):  
Martin Bitter ◽  
Stephan Stotz ◽  
Reinhard Niehuis
Keyword(s):  
Fast Response ◽  
Pressure Amplitude ◽  
Pressure Sensitive Paint ◽  
Simultaneous Application ◽  
Pressure Transducers ◽  
Unsteady Pressure ◽  
Flow Sensing ◽  
Pressure Sensitive ◽  
Perturbation Frequency ◽  
Response Pressure

Abstract This paper presents the simultaneous application of fast-response pressure transducers and unsteady pressure-sensitive paint (unsteady PSP) for the precise determination of pressure amplitudes and phases up to 3,000 Hz. These experiments have been carried out on a low-pressure turbine blade cascade under engine-relevant conditions (Re, Ma, Tu) in the High-Speed Cascade Wind Tunnel. Periodic blade/vane interactions were simulated at the inlet to the cascade using a wake generator operating at a constant perturbation frequency of 500 Hz. The main goal of this paper is the detailed comparison of amplitude and phase distributions between both flow sensing techniques at least up to the second harmonic of the wake generator’s fundamental perturbation frequency (i.e. 1,000 Hz). Therefore, a careful assessment of the key drivers for relative deviations between measurement results as well as a detailed discussion of the data processing is presented for both measurement techniques. This discussion outlines the mandatory steps which were essential to achieve the quality as presented down to pressure amplitudes of several pascal even under challenging experimental conditions. Apart from the remarkable consistency of the results, this paper reveals the potential of (unsteady) PSP as a future key flow sensing technique in turbomachinery research, especially for cascade testing. The results demonstrate that PSP was able to successfully sense pressure dynamics with very low fluctuation amplitudes down to 8 Pa.

Past Developments and Current Advancements in Unsteady Pressure Measurements in Turbomachines

2018 ◽  
Vol 140 (11) ◽  
Author(s):  
Jan Lepicovsky ◽  
David Simurda
Keyword(s):  
Pressure Measurement ◽  
High Speed ◽  
Measurement Techniques ◽  
Fast Response ◽  
Future Research ◽  
Flow Measurements ◽  
Time Resolved ◽  
Turbine Engine ◽  
Pressure Transducers ◽  
Unsteady Pressure

The aim of this paper is to review, summarize, and record long-term experience with development and application of aerodynamic probes with built-in miniature pressure transducers for unsteady pressure measurement and industrial research in turbomachine components. The focus of the first half of the paper is on the work performed at VZLU Prague, Czech Republic (Secs. 3–8). The latest development in unsteady pressure measurement techniques and data reduction methodology suitable for future research in highly loaded, high-speed turbine engine components performed at NASA GRC Cleveland, OH, is reported in Secs. 8–15 of this paper. Excellent reviews of similar activities at ETH Zürich, Switzerland by Kupferschmied, et al. (2000, “Time-Resolved Flow Measurements With Fast-Response Aerodynamic Probes in Turbomachines,” Meas. Sci. Technol., 11(7), pp. 1036–1054) and at VKI Rhode-Sain-Genèse, Belgium by Sieverding, et al. (2000, “Measurement Techniques for Unsteady Flows in Turbomachines,” Exp. Fluids, 28(4), pp. 285–321) were already reported and are acknowledged here. A short list of reported accomplishments achieved by other researchers at various laboratories is also reported for completeness. The authors apologize to those whose contributions are not reported here. It is just an unfortunate oversight, not an intentional omission.

scholarly journals Unsteady Pressure Measurement in a High Temperature Environment Using Water Cooled Fast Response Pressure Transducers

1995 ◽  
Author(s):  
D. G. Ferguson ◽  
P. C. Ivey
Keyword(s):  
High Temperature ◽  
Elevated Temperatures ◽  
Thermal Protection ◽  
Dynamic Pressure ◽  
Fast Response ◽  
Ambient Conditions ◽  
Pressure Transducers ◽  
Unsteady Pressure ◽  
Temperature Environment ◽  
High Temperature Environment

Measurement of unsteady pressure is a requirement in many proposed aero-engine active control systems. In the high temperature environment associated with the engine, thermally unprotected transducers may not measure accurately or even survive. This paper reports an examination of two water cooled, commercially available unsteady pressure transducers, which assesses the ability of the transducer to accurately measure unsteady pressure when mounted in a water cooling adapter and the effectiveness of the thermal protection at high temperatures. Mounting the transducer in a cooling adapter was shown to have no adverse effect upon its ability to measure dynamic pressure. Deliberately recessing the adapter back from the flow provided the most stable and predictable output at all flow conditions tested. Thermal protection allowed the transducer to survive at flow temperatures of up to 500°C with a potential to survive at higher temperatures. No reduction in performance is shown at elevated temperatures relative to performance at ambient conditions.

Unsteady Pressure Characteristics in the Mainstream/Disc Cavity of a Turbine-Stage

2017 ◽  
Author(s):  
A. V. Mirzamoghadam ◽  
J. Balasubramanian ◽  
M. Michael ◽  
R. P. Roy
Keyword(s):  
Flow Rate ◽  
Static Pressure ◽  
Cfd Simulation ◽  
Air Flow ◽  
Fast Response ◽  
Pressure Measurements ◽  
Turbine Stage ◽  
Unsteady Pressure ◽  
Purge Flow ◽  
The Difference

The interaction between the mainstream and disc cavity purge flows in a turbine stage is an unsteady 360° phenomenon. Most of the current rotating rigs have used steady pressure transducers to measure the mainstream annulus pressure distributions as well as the pressure distribution in the disc cavity. Unsteady static pressure measurements in these regions using fast-response transducers have also been reported but to a much lesser degree, mainly at ASU, OSU, VKI, and ETH. To gain better insight into the prevailing unsteady flow phenomena, and to assess the difference between steady and time-averaged unsteady pressure data, new unsteady static pressure measurements were recently carried out at three locations in an ASU-Honeywell turbine stage, namely, in the main gas path on the outer shroud near vane trailing edge as well as on the vane platform lip, and on the stator surface rim seal. They are reported in this paper along with the comparative results of the corresponding URANS CFD simulation reported in an earlier publication. Experiments were carried out at five different purge air flow conditions for each of the two mainstream air flow rate and rotor speed combinations. The current unsteady measurements indicate that the rim cavity pressure frequency is governed by the blade passage frequency. The unsteadiness amplitude increases with purge flow in the main gas path, but decreases with increase in purge flow for the rim cavity where the sensitivity to change in purge flow is smaller at the lower mainstream flow rate. The difference in the ambient-corrected time-averaged static pressures between those evaluated from the current unsteady measurements and the previously published steady measurements are found to be within the measurement uncertainties.

scholarly journals Dynamic-stall measurements using time-resolved pressure-sensitive paint on double-swept rotor blades

2021 ◽  
Vol 63 (1) ◽  
Author(s):  
Armin Weiss ◽  
Reinhard Geisler ◽  
Martin M. Müller ◽  
Christian Klein ◽  
Ulrich Henne ◽  
...  
Keyword(s):  
Measurement System ◽  
Transient Flow ◽  
Fast Response ◽  
Rotor Blades ◽  
Dynamic Stall ◽  
Test Facility ◽  
Pressure Sensitive Paint ◽  
Detached Eddy Simulation ◽  
Pressure Transducers ◽  
Pressure Sensitive

Abstract The study presents an optimized pressure-sensitive paint (PSP) measurement system that was applied to investigate unsteady surface pressures on recently developed double-swept rotor blades in the rotor test facility at the German Aerospace Center (DLR) in Göttingen. The measurement system featured an improved version of a double-shutter camera that was designed to reduce image blur in PSP measurements on fast rotating blades. It also comprised DLR’s PSP sensor, developed to capture transient flow phenomena (iPSP). Unsteady surface pressures were acquired across the outer 65% of the rotor blade with iPSP and at several radial blade sections by fast-response pressure transducers at blade-tip Mach and Reynolds numbers of $$\mathrm {M}_\mathrm{tip} = 0.282-0.285$$ M tip = 0.282 - 0.285 and $$\mathrm {Re}_\mathrm{tip}= 5.84-5.95 \times 10^5$$ Re tip = 5.84 - 5.95 × 10 5 . The unique experimental setup allowed for scanning surface pressures across the entire pitch cycle at a phase resolution of $${0.225}\,{\mathrm{deg}}$$ 0.225 deg azimuth for different collective and cyclic-pitch settings. Experimental results of both investigated cyclic-pitch settings are compared in detail to a delayed detached eddy simulation using the flow solver FLOWer and to flow visualizations from unsteady Reynolds-averaged Navier–Stokes (URANS) computations with DLR’s TAU code. The findings reveal a detailed and yet unseen insight into the pressure footprint of double-swept rotor blades undergoing dynamic stall and allow for deducing “stall maps”, where confined areas of stalled flow on the blade are identifiable as a function of the pitch phase. Graphical abstract

Wake, Shock, and Potential Field Interactions in a 1.5 Stage Turbine—Part I: Vane-Rotor and Rotor-Vane Interaction

2003 ◽  
Vol 125 (1) ◽  
pp. 33-39 ◽  
Author(s):  
R. J. Miller ◽  
R. W. Moss ◽  
R. W. Ainsworth ◽  
N. W. Harvey
Keyword(s):  
High Pressure ◽  
Potential Field ◽  
Pressure Field ◽  
Fast Response ◽  
Operating Conditions ◽  
Rotor Blades ◽  
Time Resolved ◽  
Pressure Transducers ◽  
Unsteady Pressure ◽  
The Individual

The composition of the time-resolved surface pressure field around a high-pressure rotor blade caused by the presence of neighboring blade rows is investigated, with the individual effects of wake, shock and potential field interaction being determined. Two test geometries are considered: first, a high-pressure turbine stage coupled with a swan-necked diffuser exit duct; secondly, the same high-pressure stage but with a vane located in the downstream duct. Both tests were conducted at engine-representative Mach and Reynolds numbers, and experimental data was acquired using fast-response pressure transducers mounted on the mid-height streamline of the HP rotor blades. The results are compared to time-resolved computational predictions of the flowfield in order to aid interpretation of experimental results and to determine the accuracy with which the computation predicts blade interaction. The paper is split into two parts: the first investigating the effect of the upstream vane on the unsteady pressure field around the rotor (vane-rotor interaction), and the second investigating the effect of the downstream vane on the unsteady pressure field around the rotor (rotor-vane interaction). The paper shows that at typical design operating conditions shock interaction from the upstream blade row is an order of magnitude greater than wake interaction and that with the design vane-rotor inter-blade gap the presence of the rotor causes a periodic increase in the strength of the vane trailing edge shock. The presence of the potential field of the downstream vane is found to affect significantly the rotor pressure field downstream of the Mach one surface within each rotor passage.

Modulation and Radial Migration of Turbine Hub Cavity Modes by the Rim Seal Purge Flow

2016 ◽  
Vol 139 (1) ◽  
Author(s):  
R. Schädler ◽  
A. I. Kalfas ◽  
R. S. Abhari ◽  
G. Schmid ◽  
S. Voelker
Keyword(s):  
Gas Turbines ◽  
Fast Response ◽  
Loss Mechanism ◽  
Axial Turbine ◽  
Pressure Transducers ◽  
Aerodynamic Loss ◽  
Numerical Predictions ◽  
Cavity Modes ◽  
Annulus Flow ◽  
Purge Flow

In the present paper, the results of an experimental and numerical investigation of the hub cavity modes and their migration into the main annulus flow are presented. A one-and-a-half stage, unshrouded and highly loaded axial turbine configuration with three-dimensionally shaped blades and cylindrical end walls has been tested in an axial turbine facility. Both the blade design and the rim seal purge flow path are representative to modern high-pressure gas turbines. The unsteady flow field at the hub cavity exit region has been measured with the fast-response aerodynamic probe (FRAP) for two different rim seal purge flow rates. Furthermore, fast-response wall-mounted pressure transducers have been installed inside the cavity. Unsteady full-annular computational fluid dynamics (CFD) simulations have been employed in order to complement the experimental work. The time-resolved pressure measurements inside the hub cavity reveal clear cavity modes, which show a strong dependency on the injected amount of rim seal purge flow. The numerical predictions provide information on the origin of these modes and relate them to pronounced ingestion spots around the circumference. The unsteady probe measurements at the rim seal interface show that the signature of the hub cavity induced modes migrates into the main annulus flow up to 30% blade span. Based on that, an aerodynamic loss mechanism has been found, showing that the benefit in loss reduction by decreasing the rim seal purge flow rate is weakened by the presence of turbine hub cavity modes.

Experimental and Numerical Assessment of the Aeroelastic Stability of Blade Pair Packages

2014 ◽  
Author(s):  
Almudena Vega ◽  
Roque Corral ◽  
Achim Zanker ◽  
Peter Ott
Keyword(s):  
High Speed ◽  
Suction Side ◽  
Travelling Wave ◽  
Shielding Effect ◽  
Test Facility ◽  
Wave Form ◽  
Electromagnetic Excitation ◽  
Pressure Transducers ◽  
Unsteady Pressure ◽  
Stabilizing Effect

The stabilizing effect of grouping rotor blades in pairs has been assessed both, numerically and experimentally. The bending and torsion modes of a low aspect ratio high speed turbine cascade tested in the non-rotating test facility at EPFL (Ecole Polytechnique Fédérale de Lausanne) have been chosen as the case study. The controlled vibration of 20 blades in travelling wave form was performed by means of an electromagnetic excitation system, enabling the adjustement of the vibration amplitude and inter blade phase at a given frequency. Unsteady pressure transducers located along the blade mid-section were used to obtain the modulus and phase of the unsteady pressure caused by the airfoil motion. The stabilizing effect of the torsion mode was clearly observed both in the experiments and the simulations, however the effect of grouping the blades in pairs in the minimum damping at the tested frequency was marginal in the bending mode. A numerical tool was validated using the available experimental data and then used to extend the results at lower and more relevant reduced frequencies. It is shown that the stabilizing effect exists for the bending and torsion modes in the frequency range typical of low-pressure turbines. It is concluded that the stabilizing effect of this configuration is due to the shielding effect of the pressure side of the airfoil that defines the passage of the pair on the suction side of the same passage, since the relative motion between both is null. This effect is observed both in the experiments and simulations.

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