Increased Load Carrying Capacity of Large Tilting-Pad Journal Bearings by Injection of Cold Oil

Abstract

In this paper a theoretical study with the aim to achieve higher load capacity of large tilting-pad turbine bearings is presented. The main focus is set on the reduction of thermal gradients inside the pad and thus, of adverse thermomechanical deformations. This allows for the increase of either the load carrying capacity, minimum film thickness hmin, and/or decrease maximum pad temperature Tmax. Subject of the investigation is a 5-pad tilting-pad bearing with rocker pivots. Each pad arc measures 56° and the pivot is positioned at 60 %. By having a 500mm inner diameter the 350mm long bearing features a relative clearance of 1.28% and nominal preload of 0.23. It is shown that the axial pad bending Δh (crowning) has a major impact on film thickness and pressure distributions and thus on the operational safety parameters. In order to reduce this effect, radial bores through the pad supplying pressurized cold oil (Tinj = 50 °C) are simulated. Despite the evident increase in oil film pressure, the primary purpose of the injection is to rinse away the layer of hot oil sticking to the pad surface. The maximum pad temperature and the overall pad temperature gradients are thereby decreased. The code used for simulation solves Reynolds and energy equations and computes thermomechanical deformations simultaneously. However, the simulations are carried out for one single pad only and are therefore supported by boundary conditions taken from experiments. In order to determine the impact of the approach on the static bearing characteristics, diameter and location of the bores are varied (0.3mm ≤ db ≤ 0.5mm). It is shown that pad crowing can be reduced significantly: The axial deviation of the film thickness Δh can be decreased from Δh = 47 μm to Δh = 31 μm, while the maximum temperature Tmax can be decreased by 20 K. Further, the minimum film thickness hmin can be increased by 16 μm. Subsequently, allowing the same limits for hmin and Tmax for the new design, the load capacity can be raised by up to 1.21MPa ≙ 44 %.

Influence of Active Cooling at the Trailing Edge on the Thermal Behavior of a Tilting-Pad Journal Bearing

“…Compared to the characteristic values of the critical operating point of the non-profiled initial form, the profiled bearing can be operated at the same operating point with a notably reduced temperature and increased oil film thickness. Kukla et al [30] and Buchhorn et al [31] present two theoretical approaches to influence and improve the steady-state operating characteristics of a ∅-500 mm TPJB without experimental validation. Both approaches focus on a single pad in their simulation model, which considers additional volume flows through the pad into or out of the lubrication gap.…”

“…Due to the heating and the more homogeneous temperature distribution, the thermomechanical deformations can be reduced. Buchhorn et al [31] inject a small volume flow of cold oil into the lubrication gap. The additional cold oil volume flow effects a kind of flushing away of the warm oil from the running surface and therefore reduces the maximum oil film temperatures and the pad deformation.…”