Toward Investigation of External Oil Flow From a Journal Bearing in an Epicyclic Gearbox

Abstract

High loads and bearing life requirements make journal bearings the preferred choice for use in high power, epicyclic gearboxes in jet engines. In contrast to conventional, nonorbiting journal bearings in epicyclic star gearboxes, the kinematic conditions in epicyclic planetary arrangements are much more complex. With the planet gears rotating about their own axis and orbiting around the sun gear, centrifugal forces generated by both motions interact with each other and affect the external flow behavior of the oil exiting the journal bearing. This paper presents a literature and state-of-the-art knowledge review to identify existing work performed on cases similar to external journal bearing oil flow. In order to numerically investigate external journal bearing oil flow, an approach to decompose an actual journal bearing into simplified models is proposed. Later, these can be extended in a step-wise manner to allow key underlying physical phenomena to be identified. Preliminary modeling considerations will also be presented. This includes assessing different geometrical inlet conditions with the aim of minimizing computational requirements and different numerical models for near-wall treatment. The correct choice of near-wall treatment models is particularly crucial as it determines the bearing's internal and external thermal behavior and properties. The findings and conclusions are used to create a three dimensional (3D), twocomponent computational fluid dynamic (CFD) sector model with rotationally periodic boundaries of the most simplistic approximation of an actual journal bearing: a non-orbiting representation, rotating about its own axis, with a circumferentially constant, i.e. concentric, lubricating gap. The inlet boundary conditions for simulating the external oil flow are generated by partly simulating the internal oil flow within the lubricating gap. In order to track the phase interface between the oil and the air surrounding the bearing, the Volume of Fluid (VoF) method is used. The quality of the CFD simulations of the domain of interest is not only dependent on the accuracy of the inlet conditions, but is also dependent on the computational mesh type, cell count, cell shape and numerical methods used. External journal bearing oil flow was simulated with a number of different mesh densities and the effect on the flow field behavior will be discussed. Two different operating temperatures, representing low and high viscosity oil, were used and their effect on the flow field behavior will also be assessed.In order to achieve the future objective of creating a design tool for routine use, key areas will be identified in which further progress is required. This includes the need to progressively increase the model fidelity to eventually simulate an orbiting journal bearing in planetary configuration with an eccentric, i.e. convergent-divergent, lubricating gap.

Multiphase Computational Fluid Dynamics Modeling of External Oil Flow From a Journal Bearing

“…However, since the groove usually does not extend over the full length of the bearing, it can be assumed that, at the point where the liquid emerges from the lubricating gap into the external environment, the velocity profiles in the axial and circumferential directions are fully developed. The validity of this assumption has previously been confirmed in [4]. Figure 7 shows a schematic diagram of the boundary condition types applied to the CFD model.…”

“…Numerical models suitable for modeling external oil flow from a journal bearing were previously reported in [4]. The modeling approaches can be categorized according to their kinematic description.…”

“…Effective and efficient flow path management is therefore essential. Failure to design an effective oil scavenge system can lead to a number of issues, all of which have been previously discussed in [4].…”

“…In order to ensure reliable gearbox operation and maximum oil scavenge effectiveness, a comprehensive understanding is required of the oil flow as it exits the journal bearing. An analysis approach, including initial results, is presented in [4].…”

“…A strategy of decomposing a journal bearing in the complex environment of an epicyclic planetary gearbox (Figure 2) into simpler models was proposed in [4]. In its most basic form a journal bearing can be modeled by two concentric cylinders, creating an axially and circumferentially constant lubricating gap height, h, as shown in Figure 4.…”