Numerical Investigations Into the Oil Capture Efficiency of a Curve-Bladed Scoop System

Abstract

Abstract In civil aero-engines, the shafts are supported by oil-lubricated bearings. These bearings require sufficient lubrication to provide cooling and ensure the reliability of the engine. In some engine configurations, it is not possible to supply oil to the bearings via the traditional direct jet injection method. An oil scoop delivery system is one of the solutions proposed to solve this problem by directing oil from a nozzle towards a rotating plurality of scoops that have internal conduits to direct the captured oil to an under-race location. The performance of an oil scoop can be quantified by using its capture efficiency. The capture efficiency is defined as the ratio of oil delivered by the scoop system to the destination, to that supplied by the feed jet. In this numerical study, the performance of an oil scoop with a curved geometry was investigated in depth. This numerical study was carried out using the commercial computational fluid dynamics (CFD) code, ANSYS Fluent. The simulation adopted the Volume of Fluid (VOF) method for multiphase modelling and the model for turbulence modelling. The study investigated the effects of varying the rotational speed and jet angle on the capture efficiency of a curve-bladed oil scoop with a geometry closely resembling one taken from an existing patent. These findings were then compared with published results of a straight-bladed oil scoop for the same operating conditions and matched scoop tip radius. Both the curved and straight bladed oil scoops show similar trends in capture efficiency when rotational speed is increased. However, the capture efficiency of the curve-bladed oil scoop is higher than that of a straight-bladed oil scoop by 4% to 12% depending on the rotational speed. Comparison of the volume fraction contours shows that the amount of oil lost during the slicing event of the oil jet was lower using the curve bladed scoop as compared to the straight-bladed scoop. With regards to varying the jet angle it was observed that the capture efficiency of the curve-bladed scoop decreases as the jet becomes less tangential. However, it is found that more tangential oil jets can give lower capture efficiencies in straight bladed scoops. Results also indicated that the core flow pattern in the domain is affected by the blade geometry such that the capture efficiency is a function of the blade geometry. The vortex shedding characteristics for this specific geometry in single phase air were also investigated in depth in order to compare with published work on a straight bladed scoop. It was found that at the same rotational speed, the curve-bladed scoop has a slightly higher vortex shedding frequency than that of the investigated straight bladed scoop. Two different geometries which combined elements from the straight and curved blades were also assessed to determine their performance. It was found that both geometries had lower capture efficiencies than that of a fully curve-bladed scoop due to significant reduction in the oil momentum as it flows towards the outlet of the scoop leading to a higher proportion eventually being centrifuged out and lost from the scoop tip.