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

Abstract

Scoops are used in aero-engines to capture oil and direct the oil flow for the lubrication of bearings, where a direct oil injection is not effective or possible. The design of scoops focuses on the capture of oil to aim for the highest capture efficiency. The operating conditions are usually chosen such that splashing of the oil jet against the outer side of scoops is avoided. In this paper it is shown that some degree of splashing may be beneficial and results in an increase of the scoop capture efficiency. An analytical approach is presented to describe the range of operating conditions at which the jet is aimed to hit a specific fixed point on the outer or inner scoop contour. These operating conditions are introduced as splashing and capture conditions, respectively, and described in terms of a dimensionless velocity ratio and the jet angle. A “splashing criterion” is introduced to describe the operating conditions at which the jet is aimed to hit the rear side of the scoop. A “capture criterion” is introduced to describe the operating conditions at which the jet is aimed to hit the scoop tip. The basic assumption in this paper is that the best scoop capture efficiency is correlated to the splashing criterion and not to the capture criterion. The correlation is confirmed by a series of experiments on a scoop, performed at different jet angles and a range of velocity ratios. From the match of the correlation curve with the experiments, it is concluded that the best scoop efficiency is obtained when the jet is aimed to splash on the outer scoop contour at a point near the rear side. It could be proven that this splash point does not change with the jet angle. The fact that this point of “best efficiency” remains a fixed splashing point on the scoop contour may be helpful in the design of scoops.