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Research Papers: Flows in Complex Systems

Flow Separation Control in an Annular to Conical Diffuser Using Two-Dimensional and Three-Dimensional Wall Steps

[+] Author and Article Information
Kin Pong Lo

e-mail: kinpong.lo@gmail.com

Christopher J. Elkins

e-mail: celkins@stanford.edu

John K. Eaton

e-mail: eatonj@stanford.edu
Department of Mechanical Engineering,
Stanford University,
488 Escondido Mall,
Building 500,
Stanford, CA 94305

Cases VIII and IX were tested at ReDh = 61,000 as opposed to 64,000 for all other cases.

Manuscript received June 4, 2012; final manuscript received January 9, 2013; published online March 21, 2013. Assoc. Editor: Michael G. Olsen.

J. Fluids Eng 135(4), 041102 (Mar 21, 2013) (11 pages) Paper No: FE-12-1275; doi: 10.1115/1.4023648 History: Received June 04, 2012; Revised January 09, 2013

Conical diffusers are often installed downstream of a turbomachine with a central hub. Previous studies showed that nonstreamlined hubs had extended separated wakes that reduced the adverse pressure gradient in the diffuser. Active flow control techniques can rapidly close the central separation bubble, but this restores the adverse pressure gradient, which can cause the outer wall boundary layer to separate. The present study focuses on the use of a step-wall diffuser to stabilize the wall boundary layer separation in the presence of core flow control. Three-component mean velocity data for a set of conical diffusers were acquired using magnetic resonance velocimetry. The results showed the step-wall diffuser stabilized the wall boundary layer separation by fixing its location. An axisymmetric step separation bubble was formed. A step with a periodically varying height reduced the reattachment length of the step separation and allowed the diffuser to be shortened. The step-wall diffuser was found to be robust in a range of core flow velocity profiles. The minimum distance between the core flow control mechanism and the step-wall diffuser as well as the minimum length of the step were determined.

Copyright © 2013 by ASME
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References

Figures

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Fig. 1

Flow loop schematic for MRV experiments

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Fig. 2

Detailed dimensions of the center body section and the baseline diffuser section. All dimensions are in millimeters.

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Fig. 3

Cutaway drawing of the diffuser model with a straight section with length LSS and an axisymmetric step-wall diffuser with step height h and step length l. Subfigure shows a 3D wavy wall step option.

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Fig. 15

Reverse flow area ratio ARrev for cases X (BR = 0, ReDh = 64,000) and XI (BR = 1.0, ReDh = 64,000)

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Fig. 16

Streamwise velocity contours for case XI. BR = 1.0. ReDh = 64,000. Contour values are circumferentially averaged among the trough locations of the step edge and normalized by the bulk average velocity at the annulus.

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Fig. 14

Streamwise velocity contours on a cylindrical surface at a radial distance of 38 mm or 1.1 Dcb from the diffuser axis for case X. Contour values are normalized by the bulk average velocity at the annulus.

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Fig. 13

Streamwise velocity contours on a cylindrical surface at a radial distance of 38 mm or 1.1 Dcb from the diffuser axis for case VII. Contour values are normalized by the bulk average velocity at the annulus.

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Fig. 12

Streamwise velocity contours for case X. BR = 0. ReDh = 64,000. Contour values are circumferentially averaged among the peak locations of the step edge and normalized by the bulk average velocity at the annulus.

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Fig. 11

Streamwise velocity contours for case X. BR = 0. ReDh = 64,000. Contour values are circumferentially averaged among the trough locations of the step edge and normalized by the bulk average velocity at the annulus.

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Fig. 10

Circumferentially averaged streamwise velocity profiles at the diffuser exit for cases VII (BR = 0, ReDh = 64,000), VIII (BR = 1.0, ReDh = 61,000), and IX (BR = 1.3, ReDh = 61,000)

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Fig. 9

Circumferentially averaged streamwise velocity contours for a step-wall diffuser with a 66.5 mm long straight section at BR = 1.0 (case VIII). Contour values are normalized by the bulk average velocity at the annulus.

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Fig. 8

Reverse flow area ratio ARrev for cases VII, VIII, and IX

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Fig. 7

Circumferentially averaged streamwise velocity contours for a step-wall diffuser with a 66.5 mm long straight section at BR = 0 (case VII). Contour values are normalized by the bulk average velocity at the annulus.

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Fig. 6

Normalized streamwise velocity contours for step-wall diffuser with a 66.5 mm long straight section at BR = 0 (case VII). Contours are shown on a y-z plane one step-height downstream of the step. Contour values are normalized by the bulk average velocity at the annulus.

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Fig. 5

Cross-section view of the hardware for cases VII, VIII, and IX with indication of the x/Dcb values for some key features

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Fig. 4

Normalized streamwise velocity contours for baseline straight-wall diffuser at BR = 1.0 (case III). Contours are shown on a y-z plane at the end of the diffuser section. Contour values are normalized by the bulk average velocity at the annulus.

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Fig. 17

Streamwise velocity contours for case XI. BR = 1.0. REDh = 64,000. Contour values are circumferentially averaged among the peak locations of the step edge and normalized by the bulk average velocity at the annulus.

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Fig. 18

Circumferentially averaged streamwise velocity profiles at the diffuser exit for cases VII (BR = 0, ReDh = 64,000), X (BR = 0, ReDh = 64,000), and XI (BR = 1.0, ReDh = 64,000)

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Fig. 19

Normalized streamwise velocity contours for case X (BR = 0, ReDh = 64,000). Contours are shown on a y-z plane one step-height downstream of the step (x/Dcb = 5.4). Contour values are normalized by the bulk average velocity at the annulus.

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Fig. 20

Detailed view of the upper left corner of Fig. 19 with velocity vectors projection overlaid

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Fig. 21

Streamwise velocity contours for case XII. BR = 0. ReDh = 64,000. Contour values are circumferentially averaged among the peak locations of the step edge and normalized by the bulk average velocity at the annulus.

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Fig. 22

Streamwise velocity contours for case XIII. BR = 1.0. ReDh = 64,000. Contour values are circumferentially averaged among the peak locations of the step edge and normalized by the bulk average velocity at the annulus.

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Fig. 23

Contours of the circumferentially averaged mean radial flux of streamwise momentum for case X with step length l = 9.5 h. BR = 0. ReDh = 64,000.

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Fig. 24

Contours of the circumferentially averaged mean radial flux of streamwise momentum for case XII with step length l = 5 h. BR = 0. ReDh = 64,000.

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Fig. 25

Circumferentially averaged streamwise velocity profiles at the diffuser exit for cases X (BR = 0, l = 9.5 h), XI (BR = 1.0, l = 9.5 h), XII (BR = 0, l = 5 h), and XIII (BR = 1.0, l = 5 h)

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Fig. 26

Reverse flow area ratio ARrev for cases XVI (0.8 Dcb straight section), XVIII (1.6 Dcb straight section), XIII (2.0 Dcb straight section), and XX (2.4 Dcb straight section). The curves show the ARrev values between the start of the step xstep and the end of the diffuser.

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Fig. 27

Streamwise velocity contours for case XXI. BR = 0.8. ReDh = 64,000. Contour values are circumferentially averaged among the peak locations of the step edge and normalized by the bulk average velocity at the annulus.

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Fig. 31

Streamwise velocity contours for case XXV. BR = 1.3. ReDh = 64,000. Contour values are circumferentially averaged among the peak locations of the step edge and normalized by the bulk average velocity at the annulus.

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Fig. 32

Volume flow rate within a region of interest as a function of streamwise position. Qroi is the volume flow rate within a cylindrical volume with a diameter of 1 Dcb. Qjet is the volume flow rate of the Coanda jet. Qtotal is the total volume flow rate in the diffuser model.

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Fig. 28

Streamwise velocity contours for case XXII. BR = 0.9. ReDh = 64,000. Contour values are circumferentially averaged among the peak locations of the step edge and normalized by the bulk average velocity at the annulus.

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Fig. 29

Streamwise velocity contours for case XXIII. BR = 1.1. ReDh = 64,000. Contour values are circumferentially averaged among the peak locations of the step edge and normalized by the bulk average velocity at the annulus.

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Fig. 30

Streamwise velocity contours for case XXIV. BR = 1.2. ReDh = 64,000. Contour values are circumferentially averaged among the peak locations of the step edge and normalized by the bulk average velocity at the annulus.

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