Computational Analysis of Turbulence and Thermal Characteristics in Fluid Film Thrust Bearings
Deng, Xin, Mechanical and Aerospace Engineering - School of Engineering and Applied Science, University of Virginia
Wood, Houston, Mechanical and Aerospace Engineering, University of Virginia
A thrust bearing is a type of rotary bearing that permits rotation between parts and is designed to support a load parallel to the axis of rotation. The film pressure that separates the surfaces is created by the surface's relative motion (rotation) as the lubricant is dragged into the converging wedge between the surfaces. The stationary and rotating surfaces are separated by the thin film of lubricants, such as oil, air, water, or other process fluid. The main performance characteristics of a thrust bearing are film thickness, load capacity, and temperature. Demand for turbo machines to run faster and the increasing use of nontraditional, low viscosity lubricants also demand new analysis tools that properly account for their physical properties. The oil lubrication in high speed or water lubrication is challenging for traditional turbulence modeling in bearing as the high rotating speed or the low viscosity of water produces highly turbulent bearings. Due to the inherent shortcomings, the traditional zero-equation can be improved to enable better prediction accuracy and less empiricism. There is a temperature drop region with the increase in speed. However, the physics causing such temperature drop is not well understood.
A series of studies are conducted to comprehensively analyze turbulence and thermal characteristics in thrust bearings through computational analysis. The finite element model is used to model three methods of modeling wall shear stress. Then sensitivity study of the three methods is performed for water and oil lubrication. Moreover, it compared the results between those three methods. For typical turbulence models, the value of y+ must be within a certain range to maintain accuracy. Preserved the y+ value to make water-lubricated thrust bearing models valid and optimized parameters in the Ng-Pan turbulence model. A new mixed zero-equation and one-equation turbulence model was developed in the new thrust bearing modeling code package “ThrustX”. It is a Thermo-hydrodynamic (THD) code consisting of looping between turbulence equation, Reynolds’ equation, film energy equation, and pad & runner conduction equation. A full fluid-solid CFX model was developed for a center pivot fluid-film thrust-bearing experimental model to study the physics of temperature drop in the transitional region. A novel physics finding causing the temperature drop in the transitional region was proposed, studied, and verified. A rigid verification between the experiment, benchmark FE code, and CFX was performed. The CFX model is verified by using an experimentally measured pad temperature map and by matching the temperature drop shown in the experiment to capture the cooling effect in the transitional region. The full fluid-solid CFX model has four groove regions – inlet and outlet grooves between the pads, a groove at the inner diameter groove, and a groove at the outer diameter.
All modeling results show good agreement with the available test data. The minimum film thickness with method three excluded is consistently larger. For water-lubricated conditions, the influence of the core turbulence region is not as obvious as that for oil-lubricated conditions. Minimum film thickness shows a significantly improved fit with the cross-film element number modified from the benchmark. The proposed modified Ng-Pan turbulence model fits well with the benchmark, showing a significant improvement in fit over the original Ng-Pan model at high Reynolds number cases. The eddy viscosity from the mixed zero-equation and one-equation turbulence model is very close to Eddy Viscosity Transport (EVT) and DNS results. The turbulence model in the mixed zero-equation and one-equation turbulence model is much improved compared to Ng-Pan zero-equation. The SST turbulence model captures the temperature drop in the transitional region and produces a very different thermal picture from the laminar groove models. It is found that the turbulence in the groove is the significant factor that causes the temperature drop, rather than turbulence developing in the film, from traditional understanding. The turbulence in the groove created eddies in the flow in the groove, and such enhanced mixing and conduction in the groove, produce a nearly uniform, reduced temperature at the leading edge of the film. As the groove flow becomes increasingly turbulent, the leading-edge temperature drops due to the increased turbulence in the groove conducting more heat away, with better heat transfer to the surrounding surfaces. The study of temperature drop in the transitional region to the thrust bearing modeling can significantly improve the understanding of predicting the overall thermal performance characteristics and dynamic coefficients for fluid film lubricated thrust bearings.
PHD (Doctor of Philosophy)
Turbulence Modeling, Finite Element Modeling, Computational Fluid Dynamics, Thrust Bearing