Session: 23-04, Hydrodynamic Bearings

Paper Number: 124990

124990 - Large Axial Vibrations in Turbomachines: Non-Linear Behaviour of Double Sided Lubricated Thrust Bearing 

Even if a not common phenomenon, large axial vibrations are a critical concern in turbomachinery, particularly when dealing with double-sided, oil-lubricated thrust bearings in compressors and large gas turbines. This phenomenon, known as axial Sub-Synchronous Vibration (SSV), manifests as high-amplitude vibrations that can extend throughout the entire machine clearance. Consequently, the structural integrity of the machinery is a paramount issue, as it can lead to detrimental effects such as seal abrasion and fretting on bearing pads.

The issue occurs both in compressors and large gas turbines featured by double-sided and oil-lubricated thrust bearing. The development of the vibration is often unpredictable, marked by sudden jumps in vibration amplitude tied to various operational parameters of the machine, like: active power, clearance adjustments, oil temperature and flow rate. Extensive discussions in the literature have explored the root causes and correlations between vibration levels and these operating parameters. A prevalent theory links the vibration occurrences to pad flutter on the reverse side of the bearing, but it remains unverified and doesn't account for all case studies.

In this paper, we delve into the coupled fluid dynamics of the two-sided thrust bearing by solving the 2D-Reynolds equation for the fluid film, with a THD model. Key global properties of the bearing are extracted, including pressure distribution, film thickness, oil temperature, Reynolds number, and integral acting load. Furthermore, we analyze how these parameters respond to external disturbances, such as changes in clearance, inlet oil temperature, flow rate, and axial load. The results show that no significant anomalies occur within the bearing itself.

These bearing simulation outcomes serve as a foundation for further investigation. The unpredictable behavior of axial SSV and the abrupt amplitude jumps can be attributed to the non-linear axial response of the bearing. We calculate the relationship between rotor position and bearing reaction force while considering the properties of the oil film. To model this phenomenon, we perform dynamic analysis to simulate a real machine, accounting for the rotor and the axial flexibility of the bearing casing. We develop a non-linear system with one and two degrees of freedom, integrated in the time domain. It reveals high-amplitude vibrations arising from the bi-stability condition of the non-linear oscillator. Jump phenomena are primarily influenced by the static thrust load acting on the bearing.

Presenting Author: Ludovico Dassi Politecnico di Milano, Dipartimento di Meccanica

Presenting Author Biography: Ludovico Dassi is a Ph.D candidate in mechanical engineering at Politecnico di Milano. The main research activity is focused on Rotordynamics and diagnostics of gas turbines. Bearing design, seals and mechanical integrity are other important fields of interest.

Authors:

Ludovico Dassi Politecnico di Milano, Dipartimento di Meccanica
Steven Chatterton Politecnico di Milano
Edoardo Gheller Politecnico di Milano
Paolo Pennacchi Politecnico di Milano