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Research Papers

The Inner Workings of Axial Casing Grooves in a One and a Half Stage Axial Compressor With a Large Rotor Tip Gap: Changes in Stall Margin and Efficiency

[+] Author and Article Information
Chunill Hah

NASA Glenn Research Center,
Cleveland, OH MS 5-10
e-mail: Chunill.hah-1@nasa.gov

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received July 10, 2018; final manuscript received August 6, 2018; published online October 17, 2018. Editor: Kenneth Hall. This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States. Approved for public release; distribution is unlimited.

J. Turbomach 141(1), 011001 (Oct 17, 2018) (10 pages) Paper No: TURBO-18-1152; doi: 10.1115/1.4041133 History: Received July 10, 2018; Revised August 06, 2018

Effects of axial casing grooves (ACGs) on the stall margin and efficiency of a one and a half stage low-speed axial compressor with a large rotor tip gap are investigated in detail. The primary focus of the current paper is to identify the flow mechanisms behind the changes in stall margin and on the efficiency of the compressor stage with a large rotor tip gap. Semicircular axial grooves installed in the rotor's leading edge area are investigated. A large eddy simulation (LES) is applied to calculate the unsteady flow field in a compressor stage with ACGs. The calculated flow fields are first validated with previously reported flow visualizations and stereo particle image velocimetry (SPIV) measurements. An in-depth examination of the calculated flow field indicates that the primary mechanism of the ACG is the prevention of full tip leakage vortex (TLV) formation when the rotor blade passes under the axial grooves periodically. The TLV is formed when the incoming main flow boundary layer collides with the tip clearance flow boundary layer coming from the opposite direction near the casing and rolls up around the rotor tip vortex. When the rotor passes directly under the axial groove, the tip clearance flow boundary layer on the casing moves into the ACGs and no roll-up of the incoming main flow boundary layer can occur. Consequently, the full TLV is not formed periodically as the rotor passes under the open casing of the axial grooves. Axial grooves prevent the formation of the full TLV. This periodic prevention of the full TLV generation is the main mechanism explaining how the ACGs extend the compressor stall margin by reducing the total blockage near the rotor tip area. Flows coming out from the front of the grooves affect the overall performance as it increases the flow incidence near the leading edge and the blade loading with the current ACGs. The primary flow mechanism of the ACGs is periodic prevention of the full TLV formation. Lower efficiency and reduced pressure rise at higher flow rates for the current casing groove configuration are due to additional mixing between the main passage flow and the flow from the grooves. At higher flow rates, blockage generation due to this additional mixing is larger than any removal of the flow blockage by the grooves. Furthermore, stronger double-leakage tip clearance flow is generated with this additional mixing with the ACGs at a higher flow rate than that of the smooth wall.

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References

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Figures

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Fig. 7

Vortex generation when rotor is located between grooves, 15% axial chord downstream, flow coefficient = 0.25 (instantaneous vectors superimposed on color contours of vorticity distribution)

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Fig. 6

Changes in TLV at different relative rotor positions, instantaneous pressure distribution at rotor tip superimposed on instantaneous velocity at ACG entrance, flow coefficient = 0.25

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Fig. 5

Changes in TLV at different relative rotor positions, cavitation image, flow visualization, flow coefficient = 0.25 from Chen et al. [13]

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Fig. 8

Vortex generation when rotor is located under a groove, 15% axial chord downstream, instantaneous vectors and vorticity distribution, flow coefficient = 0.25 (instantaneous vectors superimposed on color contours of vorticity distribution)

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Fig. 9

Comparison of time-averaged axial-velocity/rotor-tip-speed at rotor trailing edge plane, flow coefficient = 0.25: (a) smooth casing and (b) casing with ACGs

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Fig. 10

Comparison of radial velocity contours at rotor tip, flow coefficient = 0.25: (a) ensemble-averaged PIV and (b) instantaneous LES

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Fig. 4

Comparison of TLV, smooth casing, flow coefficient = 0.25: (a) flow visualization (with cavitation) and (b) LES (with instantaneous pressure distribution)

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Fig. 18

Instantaneous radial velocity contours at groove entrance, LES, flow coefficient = 0.38

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Fig. 19

Comparison of instantaneous pressure distribution at rotor tip, LES, flow coefficient = 0.38: (a) smooth casing and (b) casing with ACGs

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Fig. 3

Comparison of pressure rise characteristics of the compressor, measurement from Chen et al. [13]

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Fig. 2

Axial casing groove configuration

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Fig. 1

One and a half stage test compressor

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Fig. 12

Time-averaged nondimensional flow rate going into the groove (radial-flow-rate/main-flowrate), flow coefficient = 0.25

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Fig. 13

Comparison of relative flow angle at rotor tip, flow coefficient = 0.25: (a) ensemble-averaged PIV and (b) instantaneous LES

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Fig. 11

Instantaneous radial velocity contours at groove entrance, LES, flow coefficient = 0.25

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Fig. 17

Distribution of time-averaged axial-velocity/rotor-tip-speed at rotor tip, flow coefficient = 0.38: (a) smooth casing and (b) casing with ACGs

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Fig. 14

Comparison of relative flow angle at rotor inlet, flow coefficient = 0.25, LES

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Fig. 15

Changes in TLV at different relative rotor positions, cavitation image, flow coefficient = 0.35 from Chen et al. [13]

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Fig. 16

Instantaneous pressure distribution at rotor tip superimposed on instantaneous velocity at ACG entrance, flow coefficient = 0.38

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Fig. 20

Time-averaged nondimensional flow rate going through the groove (radial-flow-rate/main-flowrate), flow coefficient = 0.38

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Fig. 21

Comparison of flow angle at rotor inlet, flow coefficient = 0.38, LES

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Fig. 22

Comparison of total pressure rise across rotor (nondimensional with inlet total pressure), flow coefficient = 0.38, LES

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Fig. 23

Radial distribution of total pressure ratio, flow coefficient = 0.38, LES

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