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

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.

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Turbine Hub Cavity Modes and Their Impact on Efficiency

Journal of Turbomachinery ◽

10.1115/1.4050110 ◽

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.

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

Experiments in Fluids ◽

10.1007/s00348-021-03366-6 ◽

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

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

Journal of Turbomachinery ◽

10.1115/1.4034416 ◽

2016 ◽

Vol 139 (1) ◽

Cited By ~ 5

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.

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Instability Characteristic of a Single-Stage Centrifugal Compressor Exposed to Dry and Wet Gas

Volume 8: Turbomachinery, Parts A, B, and C ◽

10.1115/gt2012-69473 ◽

2012 ◽

Cited By ~ 3

Author(s):

Trond G. Grüner ◽

Lars E. Bakken

Keyword(s):

Experimental Investigation ◽

Centrifugal Compressor ◽

Pressure Sensors ◽

Ambient Air ◽

Single Stage ◽

Performance Characteristics ◽

Test Facility ◽

Wet Gas ◽

And Performance ◽

The Impact

An experimental investigation was conducted to determine the instability and performance characteristics of a single-stage centrifugal compressor exposed to wet gas. The compressor was tested at different rotational speeds and low gas mass fractions (GMFs) in an open-loop test facility with ambient air and water. The stage consisted of a shrouded impeller with a vaneless diffuser surrounded by a symmetrical circular volute with increasing cross-sectional area. Liquid was uniformly injected into the impeller eye through multiple nozzles mounted in the inlet pipe. High-response dynamic pressure sensors flush-mounted in the diffuser were used to identify instability inception and evolution. Changes in the instability and pressure ratio characteristics at different GMFs and rotational speeds were revealed. Analysis with reference to dry gas was performed. Visual observation of the wet gas surge process at the inlet is described. Results and analysis obtained from the experimental investigation on wet gas instability are presented. The investigation contributed to knowledge concerning the impact of wet gas on the instability and performance characteristics.

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Developing a Combustor Simulator for Investigating High Pressure Turbine Aerodynamics and Heat Transfer

Volume 3: Turbo Expo 2004 ◽

10.1115/gt2004-53613 ◽

2004 ◽

Cited By ~ 13

Author(s):

M. D. Barringer ◽

K. A. Thole ◽

M. D. Polanka

Keyword(s):

Heat Transfer ◽

High Pressure ◽

Film Cooling ◽

Pressure Ratio ◽

Test Facility ◽

Temperature Profiles ◽

High Pressure Turbine ◽

Blow Down ◽

Turbine Engine ◽

The Impact

Within a gas turbine engine, the high pressure turbine vanes are subjected to very harsh conditions from the highly turbulent and hot gases exiting the combustor. The temperature and pressure fields exiting the combustor dictate the heat transfer and aero losses that occur in the turbine passages. To better understand these effects, the goal of this work is to develop an adjustable combustor exit profile simulator for the Turbine Research Facility (TRF) at the Air Force Research Laboratory (AFRL). The TRF is a high temperature, high pressure, short duration blow-down test facility that is capable of matching several aerodynamic and thermal non-dimensional engine parameters including Reynolds number, Mach number, pressure ratio, corrected mass flow, gas-to-metal temperature ratio, and corrected speed. The research objective was to design, install, and verify a non-reacting simulator device that provides representative combustor exit total pressure and temperature profiles to the inlet of the TRF turbine test section. This required the upstream section of the facility to be redesigned into multiple concentric annuli that serve the purpose of injecting high momentum dilution jets and low momentum film cooling jets into a central annular chamber, similar to a turbine engine combustor. The design of the simulator allows for variations in injection levels to generate turbulence and pressure profiles. It also can vary the dilution and film cooling temperatures to create a variety of temperature profiles consistent with real combustors. To date, the design and construction of the simulator device has been completed. All of the hardware has been trial fitted and the flow control shutter systems have been successfully installed and tested. Currently, verification testing is being performed to investigate the impact of the generated temperature, pressure, and turbulence profiles on turbine heat transfer and secondary flow development.

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

Volume 2B: Turbomachinery ◽

10.1115/gt2016-56661 ◽

2016 ◽

Cited By ~ 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 3-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 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.

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Investigation on the Impact of Circumferential Grooves on Aerodynamic Centrifugal Compressor Performance

Volume 2C: Turbomachinery ◽

10.1115/gt2015-42211 ◽

2015 ◽

Cited By ~ 2

Author(s):

Simon Bareiß ◽

Damian M. Vogt ◽

Elias Chebli

Keyword(s):

Centrifugal Compressor ◽

Pressure Ratio ◽

Pressure Rise ◽

Surface Friction ◽

Detailed Examination ◽

Test Facility ◽

Groove Surface ◽

And Performance ◽

Cfd Analyses ◽

The Impact

Casing treatments are widely used in compressors for increasing range, stability and aerodynamic performance. However, applications in centrifugal compressors, as commonly used in turbochargers, are rare and mostly applied at the inlet region in terms of bleed slots. This paper presents the application of circumferential grooves, which are known to increase stability and performance in axial compressors, to the rear part of the impeller shroud casing in a centrifugal compressor. Experimental and numerical investigations of three different configurations have been performed and compared with the initial geometry. Experiments were conducted on a hot gas test facility where static pressure and temperature measurements up- and downstream of the compressor were acquired. The results indicate only small changes in operating range except for one speedline, where a considerable improvement is observed. Efficiency remains nearly unaffected for all configurations whereas the pressure ratio is increased at some operating points. For detailed examination of the compressor flow field, CFD analyses were conducted using steady-state RANS calculations. Structured meshes with node to node connections were used to suspend any possible influences stemming from interfaces in regions of interest. Validation with test data yields good agreement concerning choke margin and gradient trends. CFD results confirm that the investigated configurations of circumferential grooves have only small impact on efficiency and pressure ratio. Investigations on the mechanism which balances the additional losses due to increased groove surface friction and increases pressure rise are presented.

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