the effects of elliptical leading-edge of diffuser vane in a high … · 2019. 5. 18. · diffuser...

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The Effects of Elliptical Leading-edge of Diffuser Vane in a high-pressure ratio

centrifugal compressor

Dr. Justin Jongsik Oh

5/18/2019 TurboAeroDesign.com 1

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01Motives & Objectives

In general, most of the channel-wedge diffuser vanehas been designed with a circular leading-edge incentrifugal compressors.

It is presumed that due to quite a small leading-edgethickness, the benefit of changing from a circular to anelliptic edge might not be significant.

In transonic centrifugal compressors, however, thestory might be different as found in axial compressorexperiences.

For the same transonic centrifugal compressor asprevious studies in series[1][2], three shapes ofelliptical edges were designed with the aspect ratiofrom 2 to 4, in addition to the circular edge of theoriginal design.

Through the same CFD numerical analysis, the effectson aerodynamic performance were investigated.


Fig.1 Diffuser vane leading-edge shapes

Circular 4ER1

Elliptic (2:1) 4ER2

Elliptic (3:1) 4ER3

Elliptic (4:1) 4ER4

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01Transonic Centrifugal Compressor

Modified design of a 6:1 pressure ratio centrifugal compressor

Impeller d2 = 161 mm

Impeller b2 = 5.16 mm

Specific speed (Ns) = 98

Channel-wedge diffuser


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01Numerical Methods

CNSTURBO, turbomachinery CFD code developed by author since 1996Time marching 3D steady-state FVM

K-omega turbulence model

Artificial dissipation of 2nd/4th-order

Multi-grid acceleration

Residual smoothing

Rotor tip clearances

Mixing plane interface

Structured multi-block grids


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01Compressor Overall Performance (at 100% design speed)

More elliptical it goes, better performance of pressure ratio and efficiency, but weaker in surge margin.

Very similar trend to the effects of the impeller tip clearance

Switching to a 4:1 elliptical leading-edge shows potential to achieve a peak isentropic efficiency of 81.7% at pressure ratio of 6.2.


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01Compressor Overall Performance (at 80% design speed)

Subsonic inlet

More elliptical it goes, better performance of pressure ratio and peak efficiency, but weaker in surge margin.

More dramatic drops of efficiency over lower flows at more elliptical edges

Even at subsonic inlet, significant performance changes were brought by vane edge shapes.


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01Diffuser Performance – Throat Blockage

At subsonic inlet, the incidence at minimum throat blockage changes according to vane edge shapes.

At transonic inlet, the change gets insensitive.

At subsonic inlet, peak efficiencies of all elliptical edges were at minimum throat blockages.

At transonic inlet, not likely.

At transonic inlet, all elliptical edges brought much smaller blockages at vane throat over the range.

At subsonic inlet, not likely.

At subsonic inlet, compressor operability is dramatically affected by the edge shape.


100% design speed 80% design speed

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01Diffuser Performance – ∆p in semi-vaneless space

At subsonic inlet, CP23thsurgemore depends on vane edge shapes dropping to 0.3.

At transonic inlet, it gets insensitive approaching 0.4.



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01Diffuser Performance – ∆po in semi-vaneless space

Irrespective of inlet Mach, more elliptical edges brought less total pressure drops in the space of 2-3th.

However, less elliptical edges provided wider operability.

Highly elliptical edges get weaker in operability at subsonic inlet.



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01Diffuser Performance – ∆po in diffuser passage

Irrespective of inlet Mach, more elliptical edges brought less min. total pressure drops in the diffuser passage.

Pretty much similar to the loss bucket of axial cascade



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01Mach contours near design flow - Midspan


Design flow at design speed at midspan

Case 4ER1

Case 4ER2

Case 4ER3

Case 4ER4

Significantly larger boundary layer blockages at throat in 4ER1, compared to other cases

1.2

0

Mach

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01Mach contours near design flow – Near shroud


Design flow at design speed near shroud

Case 4ER1

Case 4ER2

Case 4ER3

Case 4ER4

No noticeable changes in throat blockages near the shroud in all cases

1.2

0

Mach

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01Mach contours near design flow – Near hub


Design flow at design speed near hub

Case 4ER1

Case 4ER2

Case 4ER3

Case 4ER4

Significantly larger boundary layer developments toward the hub in all cases

Higher incidence happens toward the hub from centrifugal impeller discharge

Highly skewed flow angles along the span at the impeller exit

Obviously delayed blockages at throat in all elliptical edges, compared to 4ER1

1.2

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Mach

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01Mach contours at 80% speed near 0.65 kg/s flow – Near hub


Case 4ER1

Case 4ER2

Case 4ER3

Case 4ER4

Significantly larger boundary layer developments toward the hub in all cases

Higher incidence happens toward the hub from centrifugal impeller discharge

Highly skewed flow angles along the span at the impeller exit

Obviously delayed blockages at throat in all elliptical edges, compared to 4ER1

1.2

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Mach

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01Summary

Irrespective of inlet Mach levels, the leading-edge shape of the channel-wedge diffuser broughtconsiderable changes in aerodynamic performance in a high-pressure ratio centrifugal compressor.

More elliptical edge it goes with, a higher pressure ratio and a higher peak efficiency it provides. However,compressor operability drops accordingly.

At subsonic inlet, compressor operability is more significantly affected by the vane edge shape.Conventional circular edge would be preferred if the surge margin is of higher priority. Controlled by CP2-3th approaching 0.3.

In particular, suppressed throat blockages thanks to elliptical edges are dominant near the hub because ofrelatively higher incidences transferred from the impeller discharge.

Overall compressor map changes are very similar to those by the effects of the impeller tip clearance.


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01References

[1]

[2]

[3]


/Paper/Online/ of TurboAeroDesign.com

the effects of elliptical leading-edge of diffuser vane in a high … · 2019. 5. 18. · diffuser...

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