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Research Papers: Flows in Complex Systems

Contributions of Tip Leakage and Inlet Diffusion on Inducer Backflow

[+] Author and Article Information
D. Tate Fanning

Department of Mechanical Engineering,
Brigham Young University,
Provo, UT 84602
e-mail: tatefanning@byu.edu

Steven E. Gorrell, Daniel Maynes

Department of Mechanical Engineering,
Brigham Young University,
Provo, UT 84602

Kerry Oliphant

Concepts NREC,
White River Junction, VT 05001

1Corresponding author.

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received December 3, 2018; final manuscript received May 7, 2019; published online June 17, 2019. Assoc. Editor: Wayne Strasser.

J. Fluids Eng 141(12), 121102 (Jun 17, 2019) (12 pages) Paper No: FE-18-1814; doi: 10.1115/1.4043770 History: Received December 03, 2018; Revised May 07, 2019

Inducers are used as a first stage in pumps to minimize cavitation and allow the pump to operate at lower inlet head conditions. Inlet flow recirculation or backflow in the inducer occurs at low flow conditions and can lead to instabilities and cavitation-induced head breakdown. Backflow of an inducer with a tip clearance (TC) of τ = 0.32% and with no tip clearance (NTC) is examined with a series of computational fluid dynamics simulations. Removing the TC eliminates tip leakage flow; however, backflow is still observed. In fact, the NTC case showed a 37% increase in the length of the upstream backflow penetration. Tip leakage flow does instigate a smaller secondary leading edge tip vortex that is separate from the much larger backflow structure. A comprehensive analysis of these simulations suggests that blade inlet diffusion, not tip leakage flow, is the fundamental mechanism leading to the formation of backflow.

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References

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Figures

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Fig. 1

Lateral view of impeller illustrating tip leakage flow and the formation of backflow. Adapted from Ref. [1].

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Fig. 2

Midplane section of the TC inducer geometry mesh

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Fig. 3

Tip mesh details of both considered inducer geometries: (a) TC mesh and (b) NTC mesh

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Fig. 4

Nondimensional backflow penetration distance as a function of flow coefficient. The solid line is a fit to the NTC data, while the dashed line is a fit to the TC data. At all ϕ < 0.7, the NTC inducer experiences greater backflow upstream penetration. At ϕ > 0.7, the TC inducer exhibits slightly greater backflow upstream penetration, although the backflow is vanishing for both inducers.

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Fig. 5

Nondimensional backflow mass flow at the leading edge as a function of flow coefficient. The TC inducer exhibits greater backflow mass flows at ϕ > 0.042.

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Fig. 6

Nondimensional axial velocity profiles as a function of percent span (hub to shroud) for axial upstream locations z/Dtip = 0.5 (top), 1.5 (middle), and 3 (bottom): (a) axial upstream location of z/Dtip = 0.5, (b) axial upstream location of z/Dtip = 1.5, and (c) axial upstream location of z/Dtip = 3

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Fig. 7

Contour plots of axial velocity magnitude in a cross-sectional view of both inducer geometries operating at ϕ = 0.042. Axial velocity is colored with a red-blue color bar. Backflow extends further upstream in the NTC case: (a) NTC inducer and (b) TC inducer.

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Fig. 8

Streamlines plotted just upstream of the leading edge of the TC inducer operating at ϕ = 0.042. λ2 criterion is colored with a red-blue color bar. Regions where λ2 < 0 can be interpreted as vortex regions. Regions where λ2 ≥ 0 have no physical interpretation. This plot differentiates the tip vortex from the backflow. The NTC inducer has been truncated at r/R =0.995 to allow for better visualization of the tip vortex: (a) NTC inducer and (b) TC inducer.

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Fig. 9

Normalized tip vortex volume as a function of flow coefficient for both inducers. At all flow coefficients, the NTC tip vortex is larger than the TC tip vortex.

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Fig. 10

Normalized tip vortex circulation as a function of flow coefficient for both inducers explored. At all flow coefficients, the TC tip vortex has greater circulation than the NTC tip vortex.

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Fig. 11

Pressure recovery plotted as a function of flow coefficient. The NTC inducer experiences a greater pressure recovery, at all considered flow coefficients.

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Fig. 12

Nondimensional performance curves for both inducer geometries. The solid line is a fit to the NTC data, while the dashed line is a fit to the TC data. At all ϕ, the NTC inducer experiences greater head production than the TC inducer.

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Fig. 13

Nondimensional backflow penetration as a function of AR for all simulated flow coefficients. Polynomial curve fits for each data set are shown. The curve fit for the NTC data has an R2 = 0.9998, while the TC data fit has an R2 = 0.9999.

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Fig. 14

Nondimensional backflow mass flow as a function of AR for all simulated flow coefficients. Linear curve fits for each data set are shown. The curve fit for the NTC data has an R2 = 0.9982, while the TC data fit has an R2 = 0.9981.

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Fig. 15

Nondimensional backflow penetration as a function of ARc. A solid line is fit through all the data for which R2 = 0.9431.

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Fig. 16

ARc as a function of cavitation number. A solid line is fit through the data for each flow coefficient.

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Fig. 17

Isosurface views of ϕvapor = 0.1 for both inducer geometries. Cavity volumes affect upstream flow conditions and introduce error into ARc calculations: (a) NTC inducer, AR = 2.9, ARc = 2.9, σ = 0.025; (b) NTC inducer, AR = 2.9, ARc = 2.9, σ = 0.157; (c) TC inducer, AR = 2.9, ARc = 2.9, σ = 0.062; and (d) TC inducer, AR = 2.9, ARc = 2.9, σ = 0.222.

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