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Research Papers

Effect of FreeStream Velocity Definition on Boundary Layer Thickness and Losses in Centrifugal Compressors

[+] Author and Article Information
Jonna Tiainen

Laboratory of Fluid Dynamics,
LUT School of Energy Systems,
Lappeenranta University of Technology,
Lappeenranta 53851, Finland
e-mail: jonna.tiainen@lut.fi

Ahti Jaatinen-Värri, Aki Grönman, Teemu Turunen-Saaresti, Jari Backman

Laboratory of Fluid Dynamics,
LUT School of Energy Systems,
Lappeenranta University of Technology,
Lappeenranta 53851, Finland

1Corresponding author.

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received October 21, 2017; final manuscript received November 7, 2017; published online February 27, 2018. Editor: Kenneth Hall.

J. Turbomach 140(5), 051003 (Feb 27, 2018) (11 pages) Paper No: TURBO-17-1192; doi: 10.1115/1.4038872 History: Received October 21, 2017; Revised November 07, 2017

The estimation of boundary layer losses requires the accurate specification of the freestream velocity, which is not straightforward in centrifugal compressor blade passages. This challenge stems from the jet-wake flow structure, where the freestream velocity between the blades cannot be clearly specified. In addition, the relative velocity decreases due to adverse pressure gradient. Therefore, the common assumption of a single freestream velocity over the blade surface might not be valid in centrifugal compressors. Generally in turbomachinery, the losses in the blade cascade boundary layers are estimated, e.g., with different loss coefficients, but they often rely on the assumption of a uniform flow field between the blades. To give guidelines for the estimation of the mentioned losses in highly distorted centrifugal compressor flow fields, this paper discusses the difficulties in the calculation of the boundary layer thickness in the compressor blade passages, compares different freestream velocity definitions, and demonstrates their effect on estimated boundary layer losses. Additionally, a hybrid method is proposed to overcome the challenges of defining a boundary layer in centrifugal compressors.

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Figures

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Fig. 1

Schematic presentation of the pitchwise idealized symmetrical and typical centrifugal compressor velocity profiles

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Fig. 2

Dimensionless compressor map for both compressors

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Fig. 3

Normalized relative velocity distribution in the blade passage of the compressor with splitter blades (design point, midspan)

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Fig. 4

Normalized relative velocity distribution in the blade passage of the compressor without splitter blades (peak efficiency point, midspan)

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Fig. 5

Observation locations along the meridional direction from the FB LE to the TE

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Fig. 6

Boundary layer thickness on the FBPS of the compressor with splitter blades (design point, midspan)

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Fig. 7

Boundary layer thickness on the splitter blade pressure side (SBPS) of the compressor with splitter blades (design point, midspan)

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Fig. 8

Boundary layer thickness on the FBPS of the compressor without splitter blades (peak efficiency point, midspan)

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Fig. 9

Boundary layer thickness on the FBSS of the compressor without splitter blades (peak efficiency point, midspan)

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Fig. 10

Relative velocity vectors at the mid-span near the LE of the compressor without splitter blades (peak efficiency point). Boundary layer separation is visible on the blade suction side.

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Fig. 11

Normalized relative velocity distribution in the blade passage of the compressor without splitter blades (peak efficiency point, midspan)

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Fig. 12

Normalized relative velocity distribution in the blade passage of the compressor without splitter blades (peak efficiency point, midspan)

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Fig. 13

Normalized freestream velocity distribution in the meridional direction in the compressor with splitter blades (design point). The freestream velocity is based on the second method.

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Fig. 14

Normalized loss coefficients in the compressor with splitter blades (design point). The normalized loss coefficient based on the second freestream velocity definition equals unity.

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Fig. 15

Normalized loss coefficients in the compressor without splitter blades (peak efficiency point). The normalized loss coefficient based on the second freestream velocity definition equals unity.

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Fig. 16

Normalized change in the specific entropy from the impeller inlet to the boundary layers at the TE (design point). The specific entropy is normalized by the value based on the second method.

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Fig. 17

Specific entropy distribution in the blade passage of the compressor with splitter blades (design point). The blades lie between the discontinuities of the curve.

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Fig. 18

Specific entropy distribution in the blade passage of the compressor without splitter blades (peak efficiency point). The blade lies between the discontinuities of the curve.

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Fig. 19

Effect of the data point density of the observed data points on the sensitivity of the second method on the FBPS of the compressor without splitter blades

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Fig. 20

Effect of the data point density of the observed data points on the sensitivity of the second method on the FBSS of the compressor without splitter blades

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Fig. 21

Boundary layer thickness on the FBPS of the compressor without splitter blades calculated using a hybrid method. When flow does not separate, hybrid method equals to second method.

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Fig. 22

Boundary layer thickness on the FBSS of the compressor without splitter blades calculated using a hybrid method

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Fig. 23

Boundary layer thickness on the FBSS (upper side) and FBPS (lower side) of the compressor with splitter blades calculated using a hybrid method at different operating conditions. Velocity profiles 10% from the FBLE are shown on the left.

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Fig. 24

Relative velocity contours in the compressor with splitter blades. Velocity vectors from the full blade leading edge (FBLE) to 20% of the chord are shown on the left.

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Fig. 25

Boundary layer thickness on the FBSS (upper side) and FBPS (lower side) of the compressor without splitter blades calculated using a hybrid method at different operating conditions. Velocity profiles 10% from the FBLE are shown on the left.

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Fig. 26

Relative velocity contours in the compressor without splitter blades. Velocity vectors from the FBLE to 20% of the chord are shown on the left.

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