Effect of Vortex Shedding on the Performance of Scoop Based Lubrication Devices

Abstract

In civil aircraft aeroengine bearing chambers it is sometimes difficult to feed oil to bearings using the traditional under-race or targeted jet approaches. In such situations one proposed solution is that of a scoop delivery system. Published experimental investigations into scoop performance show that scoop collection efficiency (the percentage of oil delivered by the scoop system to its destination compared to that supplied by the feed jet) is a function of many operational and geometric parameters. However even with high speed imaging it is impossible to experimentally determine in detail the factors that most contribute to reduction in collection efficiency and it is here particularly that a computational fluid dynamics (CFD) investigation has value. In the work reported here a commercial CFD code (ANSYS Fluent) is used to investigate vortex formation at the scoop tips and the effect these structures have on scoop collection efficiency. The computational domain, a 2D slice through the chosen scoop system, is discretized utilizing ANSYS Meshing. A Volume of fluid (VOF) method is used to model the multiphase flow of oil and air in the system and the RNG k-ε turbulence model is employed. The results obtained show that the formation of vortices from the tip of the rotating scoops leads to a reduction in pressure in the region near the tip of the oil jet, subsequently causing part of the jet to divert upwards away from the scoop creating a plumed tip. The pluming effect reduces capture efficiency because the oil plume moves outwards under centrifugal effects and this oil is not captured. The frequency of vortex shedding from the scooped rotor was investigated and the Strouhal numbers obtained were around 0.132. This compares well to 0.15 for an inclined flat plate. Two potential methods to reduce the jet pluming effect are investigated one in which the sharp tip of the scoop is blunted and the other in which the jet direction is reversed. The blunt tip increased capture efficiency by almost 2%. Reversing the jet orientation reduces jet pluming but also significantly reduces capture efficiency; it was found to be 10% lower for the case investigated.

Numerical and Experimental Investigations to Assess the Impact of an Oil Jet Nozzle with Double Orifices on the Oil Capture Performance of a Radial Oil Scoop

“…Korsukova and Morvan have compared semi-3D and two-dimensional numerical simulations, and their results show that both models can reflect typical two-phase flow characteristics inside an under-race lubrication system [13]. In addition, some studies have also proved that two-dimensional numerical simulations can reasonably predict the internal flow and oil collection performance of an under-race lubrication system [6][7][8][9]11]. To improve the calculation efficiency while ensuring prediction accuracy, a two-dimensional numerical calculation model was established for the middle section containing the main structural features of the oil jet nozzle and the radial oil scoop, as is shown in Figure 2.…”

“…Prabhakar [11] found that the flow capacity of the oil capture passage increases as the outlet pressure decreases. When the pressure difference is 0.18 bar, the oil capture efficiency increases by 7%.…”

Operating under jet splashing conditions can increase the capture efficiency of scoops

“…Prabhakar et al [19] evaluated the capture efficiency using conventional CFD. Two parameters were investigated in an attempt to reduce the plume formation and to improve the capture efficiency.…”

Flow Characteristics of Liquid Jet in Transverse Shear Crossflow

“…Paloma [6] researched on the capability of a scoop system to capture and retain delivered oil. Prabhakar and Abakr [7] demonstrated that oil loss due to the centrifugal effect is influenced by the location of the oil jet impacting the inner surface of the blade. Korsukova and Morvan [8] studied the radial under-race lubrication structure and found that oil collection efficiency decreases significantly when the oil impinges on the outer surface profile near the blade tip but improves when it impacts the outer surface contour near the blade root.…”