Machine Elements, Luleå University of Technology CFD simulation of non isothermal turbulence in LEG Tilting Pad Bearings Philip Croné, Andreas Almqvist & Roland Larsson Machine Elements, Luleå University of Technology

Objectives Modelling a Leading Edge Grooved (LEG) segment of a Tilting Pad Bearing to include thermal and turbulence effects

Objectives Classic Approach [1] This paper Reynolds Equation Approximative Energy Equ. Inlet temperature BC 3D CFD Full Energy Equation State-of-the-art Turbulence model Two methods Leading Edge Groove Geometry Effective inlet temperature BC How does the two approches compare? & [1] Dmochowski, W. K. S. A., Brockwell, K., DeCamillo, S., & Mikula, A. (1993). A study of the thermal characteristics of the leading edge groove and conventional tilting pad journal bearings. Journal of tribology, 115(2), 219-226.

Modelling of a Leading Edge Grooved Tilting Pad segment: Objectives Modelling of a Leading Edge Grooved Tilting Pad segment: Does the LEG cause a higher level of turbulence in the lubricant as sugested by M. He in [2]? Is there a large discrepancy between the temperature predicted by 1) An effective BC calculated with a mixing equation 2) Modelling the actual LEG geometry [2] He, M., Allaire, P. E., Barrett, L., & Nicholas, J. (2002, September). TEHD modeling of leading edge groove tilting pad bearings. In 6th International Conference on Rotor Dynamics (IFTOMM), Sydney, Sept.

Geometry Note that only half of the axial length is included due to symmetry Pad B (LEG) Pad A

Model Mean Velocity and Pressure: SST Turbulence model (1) (2) (3) (4) Reynolds averaged (RANS) two equation eddy viscosity model (1) (2) (3) (4) (5) (6)

Model Mean Temperature: Reynolds averaged Energy equation (7) (8) (9)

Model The tilting angle is found by imposing a moment equilibrium around the pivot Shaft excentricity fixed at ~ 0.5 of Cb A mixing model is used for the leading edge temperature in Pad A

Model Inlet conditions :

Model Inlet temperature:

Simulation The governing equations were solved using the commercial FE software Comsol Multiphysics Operating speed: 3000 rpm Lubricant ISO VG 32 Shaft diameter: 0.5 m Axial length: 0.35 m These conditions results in a shaft surface speed of 79 m/s! Use of a turbulence model is justified, [3]. [3] Gardner, W. W., & Ulschmid, J. G. (1974). Turbulence effects in two journal bearing applications. Journal of Lubrication Technology, 96(1), 15-20.

Simulation-Mesh The outcome of the simulations is very sensitive to the mesh resolution in the near wall region. At most, 12 elements, most of them clustered near the wall, were used across the thickness of the lubricant domain. This was deemed an acceptable trade-off between accuracy and convergence rate Pad B (LEG) mesh Pad B (LEG) zoom in on boundary layer mesh

Simulation- Results

Simulation- Results Comparison between film thickness for the cases studied

Simulation-Results Comparison between surface pressure for the cases studied

Simulation-Results Comparison between surface temperature for the cases studied

Simulation-Results Comparison between the turbulent to molecular dynamic viscosity ratio

How does the dimensions of the LEG portion influence the results? A parameter study was performed where the circumferential length of the LEG portion was diminished. The influence of groove depth was also studied

Simulation-Results 2 Diminishing the LEG portion!

Simulation-Results 2 OG>Geom2>Geom3>Geom4>Geom5>Geom6 Influence on turbulence by shrinking the LEG portion

Simulation-Results 2 Influence on metal temperature by shrinking the LEG portion

Simulation-Results 2 Diminishing the groove depth!

Simulation-Results 2 Groove depth: Geom6>Geom7>Geom8 Influence on turbulence by shrinking the groove depth

Simulation-Results 2 Influence on metal temperature by shrinking the groove depth

Simulation-Results 3 What if we increase the rotational speed? Influence of rotational speed on turbulence level, comparison between Pad B (LEG) and Pad A at a speed ranging from RPM=3000-3400

Conclusions The simulations suggest that NOT including the LEG geometry underestimates pressure, temperature and film thickness. Not modelling the LEG geometry predicts a higher level of turbulence, using the current (SST) model. A nice bonus in including the LEG is that the ram pressure inherently gets included.

Acknowledgements This research was conducted using the resources of High Performance Computing Center North (HPC2N) 5th International Computational on Contact Mechanics 5-7 July 2017, Lecce

Thank you for your attention! Questions? Philip Croné Philip.crone@ltu.se