Gas Turbines: Aircraft Engine

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

[+] Author and Article Information
Kathy Simmons, Graham Johnson

 University of Nottingham Technology Centre in Gas Turbine Transmission Systems, Nottingham, UK

Nikolas Wiedemann

 Rolls-Royce Plc, Derby, UK

The crown shroud geometry in this case was different to that of the 360 deg shrouded case.

With the open gear the whole chamber flow will be highly rotating so it is likely the mass loading near the gear will differ from that in the case of the shrouded gear. This assumption is, at best, questionable.

J. Eng. Gas Turbines Power 134(8), 081202 (Jun 29, 2012) (7 pages) doi:10.1115/1.4005984 History: Received September 12, 2011; Revised November 01, 2011; Published June 27, 2012; Online June 29, 2012

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.

Copyright © 2012 by American Society of Mechanical Engineers
Topics: Pressure , Gears , Sprays
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Figure 1

Schematic representation of oil spray and chamber scavenge system

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Figure 2

Schematic diagram of oil mass fraction measurement system

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Figure 3

Moment coefficient for unshrouded gear

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Figure 4

Dimensions of gear shroud tested

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Figure 5

Moment coefficient for shrouded gear

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Figure 6

Yamada and Ito [9] data for enclosed cone with throughflow

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Figure 7

Moment coefficient for shrouded gear pair; effect of chamber pressure and shaft speed

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Figure 8

Short extract from time history of chamber air temperature

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Figure 9

Relationship between moment coefficient and spray rate for the single unshrouded gear at 12,500 rpm: (a) Cm based on gas density under shroud, and(b) Cm based on mixture fluid density at shroud inlet

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Figure 10

Relationship between spray flow rate and moment coefficient for shrouded crown gear at 12,500 rpm

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Figure 11

Relationship between mixture moment coefficient and mixture Reynolds number for 360 deg shrouded crown gear at 12,500 rpm with different spray rates

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Figure 12

Relationship between mixture moment coefficient and mixture Reynolds number for unshrouded crown gear at 12,500 rpm with different spray rates

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Figure 13

Effect of spray flow rate and chamber pressure on moment coefficient for meshing gear pair at 12,500 rpm




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