The Impact of Thermal Load and Turbulent Flow on Minimum Film Thickness of a Large Tilting-Pad Journal Bearing

Abstract

Thermal loads induced by internal fluid friction significantly influence the operating behavior of high-speed journal bearings. These loads increase on average if critical Reynolds numbers are exceeded. This paper presents a detailed thermo-elasto-hydrodynamic (TEHD) analysis of a large five-pad tilting-pad journal bearing operating up to surface speeds of 94 m/s. The structure mechanical analysis is conducted by a FE-Code that is implemented as a sub-module of the bearing code to improve computational efficiency. The non-linear structure model includes the pads, their aligning ring and the housing while the journal's thermal expansion is approximated analytically. The contact of pads and ring is modeled with a contact algorithm. Validation with test data from literature indicates the relevance of structure model's complexity. Besides the pad deflection, in particular, the consideration of the ring and housing thermal expansion is decisive to predict the effective clearance properly. Furthermore, comparisons between measurement and simulation show that the experimentally identified non-linear speed dependent characteristic of minimum film thickness that features an increase due to turbulent flow is well predicted by the theoretical model. Generally, results provide an insight on the impact of the different effects dominating the behavior of the bearing in the respective speed ranges, and therefore, improve the understanding of complex bearing systems. Finally, the quality of results of approximation formulas that are easier to implement and more time efficient than the complex FE-code is investigated.