CFD Study of Journal Bearing Flow Physics and the Influence of Oil Feed Condition Within a Planetary Gearbox

Increasingly strict emission regulations and continuous pressure on cost-per-flying-hour require further reductions in fuel consumption of modern gas-turbines in the aviation industry. One promising technology is the geared turbofan, enabling for a larger bypass ratio for better thrust efficiency. The required gearbox may be a planetary gear box including journal bearing for the planet gears to cope with the high torques and centrifugal forces. The flow physics in such bearings is usually described as a Taylor-Couette flow, a two-dimensional simplification of the Navier-Stokes equations. However, during testing it has been observed that the feed condition, w.r.t. the positioning of oil feed bores and the flow split among these, has significant three-dimensional influence on measurements. Within the scope of this paper, the journal-bearing-flow physics is assessed based on a journal bearing which has a static inner, and a rotating outer cylinder, as well as its axis rotating around a centre axis. A full three-dimensional CFD is performed which includes centrifugal force, thermal conduction to the metal parts, two-phase flow (air/oil), cavitation from oil to oil-vapor, as well as evaporation of dissolved air within the oil phase. This assessment shows how the feed condition and feed temperature affect the three-dimensional gap flow along the circumference of a journal bearing. Furthermore, contour plots of flow variables within the lubrication gap show the presence of a recirculation zone just before the smallest gap which is not described by the traditionally applied Reynolds equation.