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Research Papers

Multiphase Computational Fluid Dynamics Modeling of External Oil Flow From a Journal Bearing

[+] Author and Article Information
Martin Berthold

Gas Turbine and Transmissions
Research Centre (G2TRC),
University of Nottingham,
Energy Technology Building,
Nottingham NG7 2TU, UK
e-mail: martin.berthold@nottingham.ac.uk

Hervé Morvan

Gas Turbine and Transmissions
Research Centre (G2TRC),
University of Nottingham,
Energy Technology Building,
Nottingham NG7 2TU, UK
e-mail: herve.morvan@nottingham.ac.uk

Richard Jefferson-Loveday

Gas Turbine and Transmissions
Research Centre (G2TRC),
University of Nottingham,
Coates Building,
Nottingham NG7 2RD, UK
e-mail: richard.jefferson-loveday@nottingham.ac.uk

Colin Young

Rolls-Royce plc,
P.O. Box: 31,
Derby DE24 8BJ, UK
e-mail: colin.young@rolls-royce.com

Benjamin C. Rothwell

Gas Turbine and Transmissions
Research Centre (G2TRC),
University of Nottingham,
Energy Technology Building,
Nottingham NG7 2TU, UK
e-mail: benjamin.rothwell@nottingham.ac.uk

Stephen Ambrose

Gas Turbine and Transmissions
Research Centre (G2TRC),
University of Nottingham,
Energy Technology Building,
Nottingham NG7 2TU, UK
e-mail: stephen.ambrose3@nottingham.ac.uk

Manuscript received August 9, 2018; final manuscript received August 28, 2018; published online November 20, 2018. Editor: Jerzy T. Sawicki.

J. Eng. Gas Turbines Power 141(5), 051002 (Nov 20, 2018) (12 pages) Paper No: GTP-18-1556; doi: 10.1115/1.4041517 History: Received August 09, 2018; Revised August 28, 2018

High loads and bearing life requirements make journal bearings a potential choice for use in high power, epicyclic gearboxes in jet engines. Particularly, in a planetary configuration, the kinematic conditions are complex. With the planet gears rotating about their own axes and orbiting around the sun gear, centrifugal forces generated by both motions interact with each other and affect the external flow behavior of the oil exiting the journal bearing. Computational fluid dynamics (CFD) simulations using the volume of fluid (VoF) method are carried out in ANSYS fluent (ANSYS, 2013, “ANSYS Fluent User's Guide,” ANSYS Inc., Canonsburg, PA) to numerically model the two-phase flow behavior of the oil exiting the bearing and merging into the air surrounding the bearing. This paper presents an investigation of two numerical schemes that are available in ansysfluent to track or capture the air–oil phase interface: the geometric reconstruction scheme and the compressive scheme. Both numerical schemes are used to model the oil outflow behavior in the most simplistic approximation of a journal bearing: a representation, rotating about its own axis, with a circumferentially constant, i.e., concentric, lubricating gap. Based on these simplifications, a three-dimensional (3D) CFD sector model with rotationally periodic boundaries is considered. A comparison of the geometric reconstruction scheme and the compressive scheme is presented with regard to the accuracy of the phase interface reconstruction and the time required to reach steady-state flow-field conditions. The CFD predictions are validated against existing literature data with respect to the flow regime, the direction of the predicted oil flow path, and the oil film thickness. Based on the findings and considerations of industrial requirements, a recommendation is made for the most suitable scheme to be used. With a robust and partially validated CFD model in place, the model fidelity can be enhanced to include journal bearing eccentricity. Due to the convergent-divergent gap and the resultant pressure field within the lubricating oil film, the outflow behavior can be expected to be very different compared to that of a concentric journal bearing. Naturally, the inlet boundary conditions for the oil emerging from the journal bearing into the external environment must be consistent with the outlet conditions from the bearing. The second part of this paper therefore focuses on providing a method to generate appropriate inlet boundary conditions for external oil flow from an eccentric journal bearing.

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References

Townsend, D. P. , 1991, Dudley's Gear Handbook, 2nd ed., McGraw-Hill, New York.
Berthold, M. , Morvan, H. , Young, C. , and Jefferson-Loveday, R. , 2017, “ Toward Investigation of External Oil Flow From a Journal Bearing in an Epicyclic Gearbox,” ASME J. Eng. Gas Turbines Power, 140(6), p. 062501. [CrossRef]
ANSYS Inc., 2013, “ ANSYS Fluent 16.2 User's Guide,” ANSYS Inc., Canonsburg, PA.
Youngs, D. L. , 1982, “ Time-Dependent Multi-Material Flow With Large Fluid Distortion,” Numerical Methods for Fluid Dynamics, Academic Press, Aldermaston, UK.
ANSYS Inc., 2013, “ ANSYS Fluent Theory Guide,” ANSYS Inc., Canonsburg, PA.
Ubbink, O. , 1997, “ Numerical Prediction of Two Fluid Systems With Sharp Interfaces,” Ph.D. thesis, Imperial College of Science, London.
ANSYS Inc., 2014, “ Multiphase Flow Modeling With Free Surfaces Flow,” 2014 Convergence Conference by Jinwon Seo, pp. 1–45. https://www.ansys.com/en-gb/resource-library/presentation/multiphase-flow-modeling-with-free-surfaces-flow
Fraser, R. P. , Dombrowski, N. , and Routley, J. H. , 1963, “ The Filming of Liquids by Spinning Cups,” Chem. Eng. Sci., 18(6), pp. 323–337. [CrossRef]
Hinze, J. O. , and Milborn, H. , 1950, “ Atomization of Liquids by Means of a Rotating Cup,” ASME J. Appl. Mech., 17(2), pp. 145–153.
Kamiya, T. , and Kayano, A. , 1972, “ Film-Type Disintegration by Rotating Disk,” J. Chem. Eng. Jpn., 5(2), pp. 174–182. [CrossRef]
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Glahn, A. , Busam, S. , Blair, M. F. , Allard, K. L. , and Wittig, S. , 2002, “ Droplet Generation by Disintegration of Oil Films at the Rim of a Rotating Disk,” ASME J. Eng. Gas Turbines Power, 124, pp. 117–124. [CrossRef]
Friedrich, M. A. , Lan, H. , Wegener, J. L. , Dralleier, J. A. , and Armaly, B. F. , 2008, “ A Separation Criterion With Experimental Validation for Shear-Driven Films in Separated Flows,” ASME J. Fluids Eng., 130(5), p. 051301.
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Figures

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Fig. 1

Rolls-Royce Ultrafan® engine with epicyclic gearbox1

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Fig. 2

Epicyclic gearbox in planetary configuration

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Fig. 3

Forces acting on planet gear

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Fig. 4

Simple journal bearing model with axially and circumferentially constant lubricating gap height, h, and possible exit flow directions (a, b) according to Ref. [2]

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Fig. 5

Schematic diagram of phase interface reconstruction using the geometric reconstruction scheme with true interface (left), oil volume fractions (middle) and piecewise linear reconstructed interface (right)

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Fig. 6

Schematic diagram of CFD model with boundary condition types

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Fig. 7

Detail A of planet gear (Fig. 4) with possible exit flow directions (a, b) according to Ref. [2]

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Fig. 8

Baseline CFD sector model results for T = 30 °C with boundary conditions and parameter settings as specified in the Appendix Table 4 in full (a) and detail view (b). Displayed iso-surface indicates 25% cell oil volume content.

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Fig. 9

Force balance on oil film control volume upon separation from the lower edge of the gear base, d1

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Fig. 10

Rim disintegration (a) and wave disintegration (b) according to Ref. [8]

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Fig. 11

Oil film thickness measurement location and direction with m˙f = 0.66 m˙f,MTO

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Fig. 12

Geometric (a) and compressive, explicit (b) phase interface reconstruction with Δt = 2 × 10−7 s

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Fig. 13

Compressive, implicit phase interface reconstruction with Δt = 2 × 10−7 s (a) and Δt = 5 × 10−7 s (b)

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Fig. 14

Compressive, implicit phase interface reconstruction with Δt = 1 × 10−6 s (a) and Δt = 5 × 10−6 s (b)

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Fig. 15

Compressive, implicit phase interface reconstruction with Δt = 1 × 10−5 s

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Fig. 16

Schematic diagram of journal bearing

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Fig. 17

Normalized circumferential pressure distribution at bearing midplane with and without cavitation

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Fig. 18

Normalized axial pressure distribution at θ = θpmax with and without cavitation

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Fig. 19

Normalized axial pressure gradient distribution at z/(l/2) = −1 with and without cavitation

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Fig. 20

Normalized mean axial velocity distribution at z/(l/2) = −1 with and without cavitation

Tables

Errata

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