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Research Papers

Compressor Efficiency Variation With Rotor Tip Gap From Vanishing to Large Clearance

[+] Author and Article Information
S. Sakulkaew

e-mail: sitanun@mit.edu

C. S. Tan

e-mail: choon@mit.edu
MIT Gas Turbine Laboratory,
Cambridge, MA 02139

E. Donahoo

Siemens Energy, Inc.,
Orlando, FL 32828
e-mail: eric.donahoo@siemens.com

C. Cornelius

Siemens AG,
Mülheim an der Ruhr,
45473Germany
e-mail: christian.cornelius@siemens.com

M. Montgomery

Siemens Energy, Inc.,
Orlando, FL 32828 
e-mail: montgomery.matthew@siemens.com

For example, representative values for hub-to-tip ratio is ∼0.9, aspect ratio in the range of ∼1 to 2, solidity ∼1 to 1.3, and blade passage pressure rise coefficient ∼0.3 to 0.5, etc.

APNASA is a three-dimensional, steady-state, time-averaged computational tool for multistage turbomachinery flows developed by Adamczyk and his colleagues at NASA Glenn Research Center.

The entropy dissipation method tends to underestimate the accumulative entropy generated compared to that based on the entropy flux method unless the grid resolution is high enough (see Zlatinov et al. [18]); however, the trend from either methods is similar.

A. Wadia (personal communication. September 2012) stated that tests on multistage compressors reported by Wisler [28] for which the rotor tip loading is aft-shifted (i.e. tip region that unloads the leading edge and loads the trailing edge) relative to the baseline rotor showed a 0.24 point improvement in efficiency at design point.

The content of this paper is copyrighted by Siemens Energy, Inc. and is licensed to ASME for publication and distribution only. Any inquiries regarding permission to use the content of this paper, in whole or in part, for any purpose must be addressed to Siemens Energy, Inc. directly.

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the Journal of Turbomachinery. Manuscript received July 5, 2012; final manuscript received July 24, 2012; published online March 25, 2013. Editor: David Wisler.

J. Turbomach 135(3), 031030 (Mar 25, 2013) (10 pages) Paper No: TURBO-12-1132; doi: 10.1115/1.4007547 History: Received July 05, 2012; Revised July 24, 2012

Compressor efficiency variation with rotor tip gap is assessed using numerical simulations on an embedded stage representative of that in a large industrial gas turbine with Reynolds number ∼ 2 × 106 to 7 × 106. The results reveal three distinct behaviors of efficiency variation with tip gap. For relatively small tip gap (less than 0.8% span), the change in efficiency with tip gap is nonmonotonic with an optimum tip gap for maximum efficiency. The optimum tip gap is set by two competing flow processes: decreasing tip leakage mixing loss and increasing viscous shear loss at the casing with decreasing tip gap. An optimum tip gap scaling is established and shown to satisfactorily quantify the optimal gap value. For medium tip gap (0.8%–3.4% span), the efficiency decreases approximately on a linear basis with increasing tip clearance. However, for tip gap beyond a threshold value (3.4% span for this rotor), the efficiency becomes less sensitive to tip gap as the blade tip becomes more aft-loaded thus reducing tip flow mixing loss in the rotor passage. The threshold value is set by the competing effects between increasing tip leakage flow and decreasing tip flow induced mixing loss with increasing tip gap. Thus, to desensitize compressor performance variation with blade gap, rotor should be tip aft-loaded and hub fore-loaded while stator should be tip fore-loaded and hub aft-loaded as much as feasible. This reduces the opportunity for clearance flow mixing loss and maximizes the benefits of reversible work from unsteady effects in attenuating the clearance flow through the downstream blade-row. The net effect can be an overall compressor performance enhancement in terms of efficiency, pressure rise capability, robustness to end gap variation, and potentially useful operable range broadening.

FIGURES IN THIS ARTICLE
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References

Figures

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Fig. 1

An embedded compressor stage representative of that in a large IGT compressor

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Fig. 2

Efficiency variation with tip clearance for medium tip gap

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Fig. 3

Loss variation with tip gap in rotor passage based on Denton's leakage mixing model for tip clearance between 0.8% to 5% span

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Fig. 4

Flow blockage variation along axial chord for different tip clearances. (Midspan LE: 0% midspan chord (y-axis) and midspan TE: 100% midspan chord (red dashed line); see online version for color.)

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Fig. 5

Streamlines of two particles released in the tip gap near blade leading edge showing double leakage flow paths

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Fig. 6

Rotor efficiency variation with tip clearance for small tip clearances at three different operating points

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Fig. 7

Rotor efficiency variation with tip clearance for tip clearance larger than 3.4% span

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Fig. 8

Tip leakage mass flow per unit area

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Fig. 9

Rotor blade loading at the tip for different tip clearances

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Fig. 10

Entropy profile near the casing

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Fig. 11

Local entropy generation rate

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Fig. 12

Instantaneous vorticity disturbance due to tip leakage flow in the stator passage

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Fig. 13

Schematic model of recovery associated with streamwise vorticity components in the incoming tip vortex. (Vortex lines: red and blue arrows, velocities: green and black arrows; see online version for color) [26].

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Fig. 14

Reversible attenuation in stator passage [26]

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