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On the choice of impeller axial length of a high-pressure ratio centrifugal compressor

Dr. Justin Jongsik Oh

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-4Motivation & Objectives

In general, the impeller axial length becomes dependent of machine system design restrictions,often requested to be shorter.

However, definitely it is one of aerodynamic design parameters strongly influencing performance,because of the change of passage endwall curvatures in the meridional plane leading to the changeof secondary flow development.

Starting with a well-designed impeller as a baseline, it would be useful in understanding designphilosophy to look into some design variants of the axial length in an identical compressor.

The same transonic centrifugal compressor as previous studies in series [1][2][3][4] was taken in thepresent study where the axial length ratio (L/R2) was 0.61 as a baseline.

Total 3 cases of different axial lengths were considered, while 2 more cases were added at thesmallest length, to investigate how the impeller axial length and others would change aerodynamicperformance.

Through the same CFD numerical analysis, the effects on aerodynamic performance wereinvestigated.

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-4Transonic 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

Corrected flow = 1.033 kg/s

Corrected speed = 68384 rpm

Machrel,1t =1.18

Channel-wedge diffuser

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[5]

Baseline

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-4Numerical 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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-43 Cases of Impeller Axial Length

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R2

L

Case 2F3L/R2 = 0.70Ath = 0.9891

Case 2F2L/R2 = 0.50Ath = 1.0088

BaselineL/R2 = 0.61Ath = 1.0

Case 2F2nL/R2 = 0.50Ath = 1.0516

Case 2F2pL/R2 = 0.50Ath = 1.0353

Impeller throat area is as constant as possible.

1) Larger radius of endwall curvature 1) Larger radius of endwall curvature2) Reduced rake angle

More design changes to Case 2F2

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-43 Cases of Impeller Axial Length

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Total 3 cases of different axial length of impeller of L/R2 = 0.5, 0.61 and 0.7

Impeller throat area and passage area schedule kept unchanged

Blade angle distributions very slightly different but the same at inlet and exit

Meridional hub and shroud curvatures kept unchanged

Resulting in changes of impeller blade rake angle

No changes of impeller blade thickness

No changes of impeller blade count

No changes of impeller tip clearance

No changes of vaned diffuser design

2 more cases were added at L/R2 = 0.5, with the following design changes allowed, because it would be worth investigating them due to many requests of a shorter axial length in the field applications. Case 2F2n and Case 2F2p

Change of meridional hub/shroud curvatures (Case 2F2n and Case 2Fp)

Change of blade angle distributions to reduce rake angles (Case 2F2p)

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-4Compressor & Impeller Overall Performance (at 100% design speed)

For 3 different axial lengths,

The worst performance of both impeller and compressor was found at Case 2F2 (of L/R2 = 0.5), as expected.

Case 2F3 (of L/R2 = 0.7) shows a similar level of compressor pressure ratio and efficiency to Baseline, but an impressively improved surge margin.

The level of Case 2F3 impeller efficiency is however lower than that of Baseline (, but compressor efficiency is then restored).

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-4Flow Angles (at 100% design speed)

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Normal lines = α2

Thicker lines = α3 For 3 different axial lengths,

Smaller absolute flow angle at impeller exit (α2) in Case 2F3, relative to Baseline

Lower Mach level approaching diffuser vane

Wider compressor operability

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-4Compressor & Impeller Overall Performance (at 100% design speed)

For Case 2F2, 2F2n and 2Fp,

Performance crossing between 2F2 and 2Fn (marked by arrows)

Performance jump in 2Fp compared to the other two

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-4Flow Angles (at 100% design speed)

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Normal lines = α2

Thicker lines = α3

For Case 2F2, 2F2n and 2Fp,

Performance crossing between 2F2 and 2Fn when flow angles at impeller exit get larger in 2Fn, implying higher Mach levels and more losses

Performance jump in 2Fp was caused by a significantly lower level of flow angles leaving the impeller, implying boosted diffusion.

Likely thanks to suppressed secondary flow developments by the effects of rake angle

Impeller rake angle looks playing a more critical role in case of a shorter axial length.

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-4Deep Dive into Flow Fields

How the axial length impacts secondary flow developments

Why Case 2F2p shows performance boost

In order to find answers, three of total 6 cross sections were taken to see the development of secondary flows in the impeller passage. The same approach of my previous study [6] was applied using the normalized relative helicity contours which would be the best in understanding the structure of secondary vortices.

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From Reference[6],

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-4Effects of Axial Length on Secondary Flows

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Baseline 2F2

HubSplitter

Shroud

2F3

Suction Pressure

SECTION

(VI)

(III)

(II)

As the axial length decreases (2F3 > Baseline > 2F2), The pressure-side blade vortex expands (at II).

Due to low-momentum flow accumulation toward the pressure surface

The passage vortex shrinks (at III). Implying loss of work

The shroud endwall vortex expands (at VI). Due to meridional curvature effects

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-4Effects of Case 2F2n and 2F2p

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2F2n 2F2p

HubSuction Pressure

Shroud

2F2

Splitter

(VI)

(III)

(II)

SECTION

A release of meridional endwall curvatures (2F2 > 2F2n) brings, when resulting in a large impeller rake angle, Too much load diffusion near shroud (at II). A larger pressure-side blade vortex (at VI).

By reducing the impeller rake angle (2F2n > 2F2p), Vorticity structure returns close to a normal pattern. Performance is boosted relative to 2F2 with a stronger passage vortex and a smaller shroud vortex.

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-4Summary

Case 2F2 (L/R2 = 0.50) showed the worst performance in terms of pressure ratio and efficiency.

Case 2F3 (L/R2 = 0.70) showed an impressive increase of operability, relative to Baseline, with similar levelsof compressor pressure ratio and efficiency.

The increased surge margin of Case 2F3 is attributed to a lower level of impeller exit Mach.

To improve Case 2F2, both a larger radius of meridional endwall curvatures and a reduced impeller rakeangle would be recommended as in Case 2F2p, but performance could be boosted only to be shifted fromBaseline.

As the impeller axial length gets smaller, Pressure-side blade vortex expands,

Passage vortex shrinks,

Shroud endwall vortex at the exit expands.

Case 2F2p performance curves might be further able to be adjusted by controlling throat areas, matchingdesign flow, if a shorter impeller axial length is strongly requested, implying the choice will not be badpotentially.

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-4References

[1]

[2]

[3]

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TurboAeroDesign.com/Paper/Online

[4]

[5]

[6]