Research Papers

Accounting for Eccentricity in Compressor Performance Predictions

[+] Author and Article Information
Anna M. Young

Whittle Laboratory,
University of Cambridge,
Cambridge CB3 0DY, UK
e-mail: amy21@cam.ac.uk

Teng Cao

Whittle Laboratory,
University of Cambridge,
Cambridge CB3 0DY, UK
e-mail: tc367@cam.ac.uk

Ivor J. Day

Whittle Laboratory,
University of Cambridge,
Cambridge CB3 0DY, UK
e-mail: ijd1000@cam.ac.uk

John P. Longley

Whittle Laboratory,
University of Cambridge,
Cambridge CB3 0DY, UK
e-mail: jpl1000@cam.ac.uk

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received January 23, 2017; final manuscript received February 24, 2017; published online April 19, 2017. Editor: Kenneth Hall.

J. Turbomach 139(9), 091008 (Apr 19, 2017) (10 pages) Paper No: TURBO-17-1014; doi: 10.1115/1.4036201 History: Received January 23, 2017; Revised February 24, 2017

In this paper, experiments and numerical modeling are used to quantify the effects of clearance and eccentricity on compressor performance and to examine the influence of each on flow distribution and stall margin. A change in the size of the tip-clearance gap influences the pressure rise and the stall margin of a compressor. Eccentricity of the tip-clearance gap then further exacerbates the negative effects of increasing tip-clearance. There are few studies in the literature dealing with the combined effect of clearance and eccentricity. There is also little guidance for engine designers, who have traditionally used rules of thumb to quantify these effects. One such rule states that the stall margin of an eccentric machine will be equal to that of a concentric machine with uniform clearance equal to the maximum eccentric clearance. In this paper, this rule of thumb is checked using experimental data and found to be overly pessimistic. In addition, eccentric clearance causes a variation in axial velocity around the circumference of the compressor. The current study uses a three-dimensional model which demonstrates the importance of radial flow gradients in capturing this redistribution. Flow redistribution has been treated analytically in the past, and for this reason, previous modeling has been restricted to two dimensions. The circumferential variation in axial velocity is also examined in terms of the local stability of the flow by considering the stalling flow coefficient of an equivalent axisymmetric compressor with the same local tip-clearance. The large clearance sector of the annulus is found to operate beyond its equivalent axisymmetric stall limit, which means that the small clearance sector of the annulus must be stabilizing the large clearance sector. An improved rule of thumb dealing with the effects of eccentricity is presented.

Copyright © 2017 by ASME
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Fig. 1

Flow coefficient variation around a compressor with eccentric clearance

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Fig. 2

Key parameters of the test compressor

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Fig. 3

Schematic of eccentricity adjustment procedure: rotor remains fixed while casing moves; dial gauge is used to measure casing offset relative to shaft

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Fig. 4

Characteristics with different levels of clearance (concentric), showing stall point and both full- and part-span hysteresis loops

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Fig. 9

Stalling flow coefficient against eccentricity for three mean levels of clearance

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Fig. 8

Stalling flow coefficient against maximum tip-clearance for all configurations tested

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Fig. 7

Effect of eccentricity on compressor characteristic with two levels of average clearance: (a) 1.7% average clearance and (b) 3.3% average clearance

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Fig. 6

Effect of clearance on spanwise rotor exit flow variation (concentric cases used for calibration of IBMSG drag forces)

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Fig. 5

Sketch of control of high drag regions in hub and tip region used for IBMSG calibration

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Fig. 16

Contours of incidence change (incidence minus mean value at that radius) taken from IBMSG simulation, 3.3% clearance, 75% eccentricity

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Fig. 10

Stalling flow coefficient against clearance averaged over a sector of 180 deg centered on maximum clearance point

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Fig. 11

Measured and modeled flow coefficient variations with 1.7% clearance and 75% eccentricity (at two operating points): (a) comparison of experiments with Graf model and (b) comparison of experiments with IBMSG results

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Fig. 12

Measured and modeled tip flow coefficient variation in a compressor with 3.3% clearance and 75% eccentricity at two different flow coefficients

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Fig. 13

Comparison of amplitude of tip flowfield redistribution from experimental data and IBMSG model for a range of cases with different clearance sizes and eccentricity levels (ϕ¯  = 0.5)

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Fig. 14

Comparison of spanwise variation in rotor exit flow coefficient from experimental data and IBMSG model in the maximum and minimum gap of an eccentric compressor (3.3% clearance, 75% eccentricity, ϕ¯  = 0.5)

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Fig. 15

Contour plot of rotor inlet flow coefficient near stall taken from IBMSG simulation, 3.3% clearance, 75% eccentricity

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Fig. 17

Comparison of local inlet flow coefficient (IBMSG simulations at tip) and stall point for three levels of clearance: (a) 1.7% average clearance, (b) 2.4% average clearance, and (c) 3.3% average clearance

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Fig. 19

Comparison of raw flow coefficient measurements with the distribution reconstructed from the first spatial harmonic

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Fig. 18

Unstable sector size at stall against average clearance




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