Effect of Pressure and Oil Mist on Windage Power Loss of a Shrouded Spiral Bevel Gear

Abstract

In many aeroengines the accessory power offtake is achieved using a spiral bevel gear set running off one of the main shafts. The crown and bevel gears are housed in an internal gearbox and there is significant heat generation within this chamber, some of which is attributed to windage power loss (WPL) generated by the gear. Over the past few years the University of Nottingham Technology Centre (UTC) in Gas Turbine Transmissions has been researching spiral bevel gear windage power loss, both computationally and experimentally, using a purpose-built test rig at the UTC. In this study the test rig has been adapted such that chamber pressures up to 8 bar can be generated. Test data has been obtained that shows the effect on the WPL of chamber pressure, advancing understanding of the relationship between data obtained at ambient and pressurized conditions. Three configurations have been studied: unshrouded gear, a gear with 360 deg shroud and a shrouded crown and pinion meshing pair. Furthermore, the effect of oil mist within the chamber on the WPL has been studied. An oil mist generation system was developed for introducing a fine mist into the chamber and results are presented for varying mist flowrates. A mist measurement system was developed to sample mist mass fraction within the chamber and the data obtained is used to calculate an effective (oil/gas mixture) chamber density. Increasing chamber pressure increases the Reynolds number, moving the system behavior further along the moment coefficient-Reynolds number correlation. The Cm-Re correlation is similar in form to that for a shrouded cone, showing transitional behavior around Re = 2 × 106. Beyond this transition, Cm decreases with increasing Re. Introducing an oil spray has two effects: a reduction in chamber temperature and an increase in the effective density of the chamber fluid. Both effects can be accounted for by calculating Re and Cm based on mixture properties but it seems highly likely that the properties of the fluid under the shroud differ from those of the fluid in the external chamber.