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Research Papers

An Experimental Assessment of the Effects of Stator Vane Tip Clearance Location and Back Swept Blading on an Automotive Variable Geometry Turbocharger

[+] Author and Article Information
Jason Walkingshaw

Queen's University Belfast,
Belfast BT9 5AH, UK
e-mail: jwalkingshaw01@qub.ac.uk

Stephen Spence

Queen's University Belfast,
Belfast BT9 5AH, UK
e-mail: s.w.spence@qub.ac.uk

Jan Ehrhard

IHI Charging Systems International,
Heidelberg 69126, Germany
e-mail: j.ehrhard@ihi-csi.de

David Thornhill

Queen's University Belfast,
Belfast BT9 5AH, UK
e-mail: d.thornhill@qub.ac.uk

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received June 29, 2012; final manuscript received August 8, 2012; published online November 8, 2013. Editor: David Wisler.

J. Turbomach 136(6), 061001 (Nov 08, 2013) (9 pages) Paper No: TURBO-12-1099; doi: 10.1115/1.4007517 History: Received June 29, 2012; Revised August 08, 2012

Off-design performance is of key importance now in the design of automotive turbocharger turbines. Due to automotive drive cycles, a turbine that can extract more energy at high pressure ratios and lower rotational speeds is desirable. Typically a radial turbine provides peak efficiency at U/C values of 0.7, but at high pressure ratios and low rotational speeds, the U/C value will be low and the rotor will experience high values of positive incidence at the inlet. The positive incidence causes high blade loading resulting in additional tip leakage flow in the rotor as well as flow separation on the suction surface of the blade. An experimental assessment has been performed on a scaled automotive VGS (variable geometry system). Three different stator vane positions have been analyzed: minimum, 25%, and maximum flow position. The first tests were to establish whether positioning the endwall clearance on the hub or shroud side of the stator vanes produced a different impact on turbine efficiency. Following this, a back swept rotor was tested to establish the potential gains to be achieved during off-design operation. A single passage CFD model of the test rig was developed and used to provide information on the flow features affecting performance in both the stator vanes and turbine. It was seen that off-design performance was improved by implementing clearance on the hub side of the stator vanes rather than on the shroud side. Through CFD analysis and tests, it was seen that two leakage vortices form, one at the leading edge and one after the spindle of the stator vane. The vortices affect the flow angle at the inlet to the rotor, in the hub region. The flow angle is shifted to more negative values of incidence, which is beneficial at the off-design conditions but detrimental at the design point. The back swept rotor was tested with the hub side stator vane clearance configuration. The efficiency and MFR were increased at the minimum and 25% stator vane position. At the design point, the efficiency and MFR were decreased. The CFD investigation showed that the incidence angle was improved at the off-design conditions for the back swept rotor. This reduction in the positive incidence angle, along with the improvement caused by the stator vane tip leakage flow, reduced flow separation on the suction surface of the rotor. At the design point, both the tip leakage flow of the stator vanes and the back swept blade angle caused flow separation on the pressure surface of the rotor. This resulted in additional blockage at the throat of the rotor reducing MFR and efficiency.

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References

Figures

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Fig. 1

Naming convention used for inlet blade angle study

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Fig. 2

Stator vane designs: (a) automotive VGS stator vane (b) test rig/CFD stator vane

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Fig. 3

Location of static pressure measurements at stator vane exit

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Fig. 4

Location of static pressure measurements at rotor shroud: (a) hole positions relative to stator vanes (b) axial distribution of holes

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Fig. 5

Tip leakage vortices found in stator domain: (a) test rig (b) CFD

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Fig. 6

Incidence angle plot at rotor inlet for hub side stator vane clearance

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Fig. 7

Stage efficiency against PR: (a) minimum stator vane (b) 25% stator vane (c) maximum stator vane

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Fig. 8

Static pressure measurements taken on rotor shroud at minimum stator vane position for baseline rotor

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Fig. 9

Velocity contour plot at 10% span (U/C = 0.35): (a) back swept rotor (b) baseline rotor

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Fig. 10

VGS efficiency against U/C: (a) minimum stator vane (b) 25% stator vane (c) maximum stator vane

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Fig. 11

VGS efficiency against PR: (a) minimum stator vane (b) 25% stator vane (c) maximum stator vane

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Fig. 12

Velocity vector plot at 25% span (U/C = 0.64) (a) back swept rotor

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Fig. 13

Velocity streamlines of recirculation caused by back swept blading (U/C of 0.64)

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Fig. 14

Static pressure measurements at the shroud of the rotor for the minimum stator vane position

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Fig. 15

Static pressure measurements at the shroud of the rotor for the maximum stator vane position

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Fig. 16

Flow visualization of trailing edge wake on a stator vane in the minimum MFR position: (a) test rig (b) CFD entropy plot

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