Research Papers

Area Schedule Based Design of High-Pressure Recovery Radial Diffusion Systems

[+] Author and Article Information
Ruhou Gao

Gas Turbine Laboratory,
Massachusetts Institute of Technology,
Cambridge, MA 02139
e-mail: ruhou@mit.edu

Zoltán Spakovszky

Fellow ASME
Gas Turbine Laboratory,
Massachusetts Institute of Technology,
Cambridge, MA 02139
e-mail: zolti@mit.edu

Daniel Rusch

Compressor Development,
ABB Turbo Systems, Ltd.,
Baden 5400, Switzerland
e-mail: daniel.rusch@ch.abb.com

René Hunziker

Compressor Development,
ABB Turbo Systems, Ltd.,
Baden 5400, Switzerland
e-mail: rene.hunziker@ch.abb.com

1Corresponding author.

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received August 1, 2016; final manuscript received August 9, 2016; published online September 27, 2016. Editor: Kenneth Hall.

J. Turbomach 139(1), 011012 (Sep 27, 2016) (9 pages) Paper No: TURBO-16-1179; doi: 10.1115/1.4034488 History: Received August 01, 2016; Revised August 09, 2016

High-pressure ratio centrifugal compressors require advanced diffusion systems to achieve enhanced efficiencies set by future turbocharger applications. To address the shortcomings of the commonly used channel diffuser and airfoil cascade design perspectives, a streamtube based area schedule is adopted paying special attention to the diffuser entry region. It is shown that the diffusion in the semivaneless space, controlled chiefly by inlet flow angle and the vane suction side geometry, plays a key role in improving diffuser performance. Removing excess thickness from the suction side eliminates flow overspeed, increases effective diffusion length, and leads to higher pressure recovery at reduced stagnation pressure loss. The pressure side thickness distribution controls the channel area schedule. Thin leading edges (LEs) ensure smooth flow area transition into the channel and reduce the vane upstream influence, mitigating high-cycle fatigue related mechanical issues.

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Everitt, J. , Spakovskzy, Z. , Rusch, D. , and Schiffmann, J. , 2016, “ The Role of Impeller Outflow Conditions on the Performance and Stability of Airfoil Vaned Radial Diffusers,” ASME Paper No. GT2016-56168.
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Gao, R. , 2015, “ Area-Schedule Based Design of High Pressure Recovery Radial Diffusion Systems,” Master's thesis, Department of Aeronautics and Astronautics, Massachusetts Institute of Technology, Cambridge, MA.
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Fig. 1

Meridional view of centrifugal compressor

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Fig. 2

Illustration of a typical vaned diffuser and its subcomponents (top) and flow area distribution as a function of radius (bottom)—note flow acceleration and therefore reduced diffusion in semivaneless space (SVLS)

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Fig. 3

Aft-thickened vaned diffuser geometry defined by an ideal area schedule and high loading in the semivaneless space

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Fig. 4

For a given impeller geometry and fixed throat area, the diffuser area schedule is altered via changes in diffuser inlet and outlet radius ratios and turning

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Fig. 5

Static pressure recovery Cp of camberline diffusers with r3/r2=1.15. Highest pressure recovery observed for 13 deg turning.

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Fig. 6

Stagnation pressure loss Cp,t of camberline diffusers with  r3/r2=1.15. Diffusers with short diffusion path and moderate area ratio yield lowest loss.

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Fig. 7

Performance parameter Cp/Cp,t  of camberline diffusers with  r3/r2=1.15

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Fig. 8

Strong dependence of diffuser performance parameter Cp/Cp,t  on diffuser inlet radius ratio governed by diffusion in SVLS

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Fig. 9

A closer-spaced semivaneless space improves diffusion and achieves the desired area ratio at lower exit radius ratio

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Fig. 10

Exit Mach number of camberline diffusers with  r3/r2=1.15. Minimum exit Mach number coincides with maximum Cp in Fig. 5.

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Fig. 11

Mixed-out volute inlet swirl parameter λ of camberline diffusers with  r3/r2=1.15. Lowest exit swirl parameter coincides with highest Cp in Fig. 5.

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Fig. 12

Top: normalized profile thickness distribution, front versus aft-thickened vanes—the aft-thickened profile distribution removes thickness in the semivaneless space and instead of a thick leading edge thin, elliptical leading edges are used (not shown here). Bottom: front versus aft-thickened vane geometries—controlled diffusion versus Gaussian diffusion (G-series).

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Fig. 13

Comparison of area schedule of the front versus aft-thickened vanes

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Fig. 14

Performance parameter Cp/Cp,t  of front and aft-thickened diffuser compared to the baseline and the ideal camberline diffusers

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Fig. 15

Mach number contours at midspan at design mass flow: baseline (top) versus aft-thickened G-series diffuser (bottom)

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Fig. 16

Comparison of diffuser geometries with different leading edge thicknesses

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Fig. 17

Impact of diffuser leading edge thickness on diffuser effectiveness. Thicker leading edges lead to more pronounced area reduction at diffuser channel inlet, reducing diffuser effectiveness.

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Fig. 18

Illustration of common diffuser measurement plate instrumentation used to assess diffuser subcomponent performance

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Fig. 19

Measured diffuser channel static pressure rise for baseline, G5, and G7 diffusers. The SVLS of the G5 diffuser provides 12% improvement of overall pressure recovery relative to the baseline diffuser.

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Fig. 20

Comparison of baseline versus G5 diffuser performance parameter Cp/Cp,t : the baseline RANS calculation overpredicts separation and therefore underpredicts Cp/Cp,t

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Fig. 21

Pitchwise static pressure variation near baseline diffuser inlet. Pressure tap locations are marked by thick circles. Linear interpolation used for missing pressure tap (marked by solid dot).

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Fig. 22

Pitchwise static pressure variation near G5 diffuser inlet

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Fig. 23

The G5 diffuser yields a 0.8% point improvement in impeller efficiency relative to the baseline diffuser at design condition

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Fig. 24

The G5 diffuser achieves a 0.74% point improvement in stage efficiency relative to the baseline diffuser at design condition

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Fig. 25

Stage efficiency compared to diffusion system efficiency (volute included) at design condition: the higher exit Mach number of the G5 diffuser increases volute stagnation pressure loss

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Fig. 26

Work coefficient and system total pressure ratio at design condition



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