numerical benchmarking of tip vortex breakdown in axial turbines

Numerical Benchmarking of Tip Numerical Benchmarking of Tip Vortex Breakdown in Axial TurbinesVortex Breakdown in Axial Turbines

Eunice Allen-BradleyEunice Allen-Bradley

April 22, 2009April 22, 2009

TVB Cascade Tip Vortex – TVB Cascade Tip Vortex – Overview & IntroductionOverview & Introduction

• Tip leakage losses have been studied since the 1950s:– Rains (1954) was the first to experimentally measure tip vortex in compressor cascade test;– Further studies focused on tip leakage losses in compressor and fan cascades in the 1960s,

1970s, & 1980s;• Lakshminarayana et al. (1962), Lewis et al. (1977), Pandya et al. (1983), Inoue et al. (1989).

– Tip leakage loss studies in turbine cascades conducted in 1980s,1990s, & 2000s;• Booth et al. (1982), Sjolander et al. (1987), Moore et al. (1988), Morphis et al. (1988), Yamamoto

(1989), Dishart et al. (1990), Yaras et al. (1992), Chan et al. (1994), Govardhan et al. (1994), Sondak et al. (1999)

– Tip desensitization studies in turbine & compressor cascades conducted in 1990s & 2000s.• Hamik et al. (2000), Schabowski et al. (2007), Shavalikul et al. (2008), Van Ness et al. (2008).

• Tip vortex breakdown studies (published) have been limited to external body applications:

– General definition given by Hall (1972) as any abrupt change in vortex core behavior.– Delta wing tip vortex formation, unsteady effects, far field wake effects:

• Sarpkaya (1971), El-Ramly (1972).

• Studies are lacking in which the event of tip vortex breakdown occurs in turbomachines:

– Is it possible to adequately predict tip vortex breakdown in turbomachines with the current computational tools available?

– Current study will focus on prediction capability in axial turbine simulations, using RANS CFD.

TVB Tip Vortex Methodology & Procedure –TVB Tip Vortex Methodology & Procedure –Design of ExperimentsDesign of Experiments

• Using the geometry and boundary conditions of an existing cascade facility, model tip leakage with RANS CFD.

• Alter boundary conditions until tip vortex breakdown is predicted;– Tip clearance, exit Mach number, inlet flow angle.

• Confirm results with several turbulence models for benchmarking and possible cascade testing for validation.

TVB Cascade Design Conditions

Cascade Span (H) 2.4”

Axial Chord (BX) 1.0”

Inlet flow angle (1) 63o

Exit flow angle (2) 26o

Exit Mach number (M2) 0.8

TVB Tip Vortex Results & Discussion –TVB Tip Vortex Results & Discussion –Tip Clearance EffectTip Clearance Effect

• Sjolander and Cao (1994) varied the size of the tip gap in their tip leakage flow study, and showed that cascade loss increased with an increase in tip clearance. These results are also consistent with the results of the Govardhan et al. (1994) study.

Loss Generation

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Axial Location (in)

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0.004" TC

0.006" TC

0.010" TC

0.013" TC

The CFD predicted tip vortex breakdown for the 0.004” & 0.006” tip gap at design

conditions.

The size of the tip vortex grows with the increase in tip gap.

TVB Tip Vortex Results & Discussion –TVB Tip Vortex Results & Discussion –Inlet Boundary Layer EffectInlet Boundary Layer Effect

• Chan et al. (1994) concluded in their turbine cascade tip leakage study that the inlet boundary layer has no effect on the cascade performance. The CFD results suggest, however, that this may also be a function of the measurement location.

Loss Generation

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The CFD did not predict tip vortex breakdown for any of the inlet boundary layer thickness for design conditions and tip gap of 0.010”.

The size of the tip vortex did not change, but the intensity of the core does.

TVB Tip Vortex Results & Discussion –TVB Tip Vortex Results & Discussion –Exit Mach Number EffectExit Mach Number Effect

• The tip vortex core size and subsequent losses can be directly linked to the discharge coefficient (the ratio of actual gap mass flow rate to the mass flow rate at 1D flow conditions).

Loss Generation

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Axial Location

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M2 1.00

M2 0.90

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The CFD did not predict tip vortex breakdown with a tip gap of 0.010” at the design inlet

flow angle.

The size of the tip vortex grows with the increase in exit Mach number.

TVB Tip Vortex Results & Discussion –TVB Tip Vortex Results & Discussion –Inlet Flow Angle effectInlet Flow Angle effect

• As Willinger and Haselbacher (2004) showed in their tip leakage flow study, the cascade performance loss increases as the inlet flow angle is set to more positive incidences. However, as tip vortex breakdown was predicted in the simulations for the similar incidence change, they did not report the event of vortex breakdown in their study.

Loss Generation

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Axial Location

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B1 43

B1 53

B1 63

B1 73

B1 83

The CFD predicted tip vortex breakdown for 1 = 43o & 1 = 53o at all exit Mach number

and tip gap conditions.

TVB Cascade Tip Vortex Results & Discussion – TVB Cascade Tip Vortex Results & Discussion – Benchmarking ConditionsBenchmarking Conditions

ON - Tip CLR = 0.010”; MON - Tip CLR = 0.010”; M22 = 0.8; = 0.8; 11 = 53 = 53oo OFF - Tip CLR = 0.010”; MOFF - Tip CLR = 0.010”; M22 = 0.8; = 0.8; 11 = 63 = 63oo

• Recall that the axial chord of the TVB cascade is 1.0”:– The size of potential measurement probes may be larger than

the core of the predicted tip vortex;– Furthermore, the presence of potential measurement probes

may artificially induce tip vortex to breakdown.– An alternate method for confirmation of tip vortex breakdown is

needed.

The suction side streamlines of the TVB cascade serve as further The suction side streamlines of the TVB cascade serve as further visual confirmation of predicted tip vortex breakdown. visual confirmation of predicted tip vortex breakdown.


Direction of Flow

These are the result from the Baldwin-Lomax fully turbulent predictionThese are the result from the Baldwin-Lomax fully turbulent prediction

TVB Cascade Tip VortexTVB Cascade Tip VortexContour of Total PressureContour of Total Pressure

Axial Location = 1.50*BxAxial Location = 1.50*Bx

ON ON OFF OFF

Spanwise Loss Plot*Spanwise Loss Plot*

*Mass averaged results from Baldwin-Lomax fully turbulent model*Mass averaged results from Baldwin-Lomax fully turbulent model



Direction of Flow

These are the result from the kThese are the result from the k fully turbulent prediction fully turbulent prediction



ON ON OFF OFF


*Mass averaged results from k*Mass averaged results from k fully turbulent model fully turbulent model



Direction of Flow

These are the result from the kThese are the result from the k transitional prediction transitional prediction



ON ON OFF OFF


*Mass averaged results from k*Mass averaged results from k transitional model transitional model

The suction side streamlines of the TVB cascade show NO visual The suction side streamlines of the TVB cascade show NO visual confirmation of predicted tip vortex breakdown. confirmation of predicted tip vortex breakdown.


Direction of Flow

These are the result from the SST kThese are the result from the SST k fully turbulent prediction fully turbulent prediction

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D Loss

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an

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*Mass averaged results from SST k*Mass averaged results from SST k fully turbulent model fully turbulent model

The The DDLoss generation plots between four different models show Loss generation plots between four different models show the same trend through the cascade passage.the same trend through the cascade passage.


Loss Generation

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Axial Location

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Loss Generation

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Baldwin-Lomax fully turbulentBaldwin-Lomax fully turbulent

kk transitional transitional

kk fully turbulent fully turbulent

Loss Generation

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Loss Generation

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SST kSST k fully turbulent fully turbulent

Performance Comparison -- Mass Averaged Results

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Baldw in-Lomax kw Fully Turbulent kw Transitional SST kw fully turbulent

Simulation Model

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The performance comparison of The performance comparison of DDloss for the various models run loss for the various models run to date suggest solid confirmation of tip vortex breakdown to date suggest solid confirmation of tip vortex breakdown

prediction .prediction .

Average Average DD Loss Loss between 4 modelsbetween 4 models

Tip vortex breakdown is predicted for three out of the four models shown Tip vortex breakdown is predicted for three out of the four models shown above.above.

0.44%0.44%

ConclusionsConclusions

• 3D steady CFD models are trend accurate to performance, and capture many of flow features that have been measured experimentally.

• Confirmation of the vortex breakdown prediction is demonstrated with several different turbulence models: – The CFD calculations predict an average delta cascade loss of 0.44% between

the models used to simulate the cascade. • Results suggest that resulting vortex breakdown phenomenon is not the

driving cause of the increased loss through the cascade:– This is supported by the accompanying spanwise loss plots.

• Future Work:– Cascade measurements taken based on benchmarking conditions;– Explore influence of relative rotation of outer endwall.

• Back up slides

TVB Cascade Tip Vortex: Contour of Total PressureTVB Cascade Tip Vortex: Contour of Total PressureBaldwin Lomax fully turbulent modelBaldwin Lomax fully turbulent model

ON - Tip CLR = 0.010”; MON - Tip CLR = 0.010”; M22 = 0.8; = 0.8; 11 = 53 = 53oo OFF -- Tip CLR = 0.010”; MOFF -- Tip CLR = 0.010”; M22 = 0.8; = 0.8; 11 = 63 = 63oo

Axial Location = 0.75*BxAxial Location = 0.75*Bx Axial Location = 0.95*BxAxial Location = 0.95*Bx


TVB Cascade Tip Vortex: Contour of Total PressureTVB Cascade Tip Vortex: Contour of Total Pressurekw fully turbulent resultskw fully turbulent results




TVB Cascade Tip Vortex: Contour of Total PressureTVB Cascade Tip Vortex: Contour of Total Pressurekw transitional resultskw transitional results




TVB Cascade Tip Vortex: Contour of Total PressureTVB Cascade Tip Vortex: Contour of Total PressureSST kw fully turbulent resultsSST kw fully turbulent results




numerical benchmarking of tip vortex breakdown in axial turbines

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