Research Papers

Loss and Deviation in Windmilling Fans

[+] Author and Article Information
Ewan J. Gunn

Whittle Laboratory,
University of Cambridge,
1 JJ Thomson Avenue,
Cambridge CB3 0DY, UK
e-mail: ejg55@cam.ac.uk

Cesare A. Hall

Whittle Laboratory,
University of Cambridge,
1 JJ Thomson Avenue,
Cambridge CB3 0DY, UK
e-mail: cah1003@cam.ac.uk

1Corresponding author.

Manuscript received July 24, 2015; final manuscript received March 16, 2016; published online April 26, 2016. Assoc. Editor: Li He.

J. Turbomach 138(10), 101002 (Apr 26, 2016) (9 pages) Paper No: TURBO-15-1168; doi: 10.1115/1.4033163 History: Received July 24, 2015; Revised March 16, 2016

For an unpowered turbofan in flight, the airflow through the engine causes the fan to freewheel. This paper considers the flow field through a fan operating in this mode, with emphasis on the effects of blade row losses and deviation. A control volume analysis is used to show that windmilling fans operate at a fixed flow coefficient which depends on the blade metal and deviation angles, while the blade row losses are shown to determine the fan mass flow rate. Experimental and numerical results are used to understand how the loss and deviation differ from the design condition due to the flow physics encountered at windmill. Results are presented from an experimental study of a windmilling low-speed rig fan, including detailed area traverses downstream of the rotor and stator. Three-dimensional computational fluid dynamics (CFD) calculations of the fan rig and a representative transonic fan windmilling at a cruise flight condition have also been completed. The rig test results confirm that in the windmilling condition, the flow through the fan stator separates from the pressure surface over most of the span. This generates high loss, and the resulting blockage changes the rotor work profile leading to modified rotational speed. In the engine fan rotor, a vortex forms at the pressure surface near the tip and further loss results from a hub separation caused by blockage from the downstream core and splitter.

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Fig. 1

Velocity triangles for a windmilling fan section

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Fig. 2

Capture streamtube used as the control volume for a windmilling turbofan

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Fig. 3

Fan nondimensional speed at windmill as given by Eqs. (1)(7): (a) variation with flight Mach number and (b) variation with nondimensional mass flow

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Fig. 4

Scale meridional views of the fan rig and the engine fan computational domain: (a) low-speed fan rig experiment + CFD rig schematic and (b) transonic research fan CFD only computational domain

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Fig. 5

Loss breakdowns at design and windmill

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Fig. 6

Spanwise variations in the fan. Experiment: lines with symbols; CFD: solid lines—(a) stagnation pressure, (b) axial velocity, and (c) angular momentum.

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Fig. 7

Stator exit axial velocity contours: (a) design point (experiment), (b) windmill (experiment), and (c) windmill (CFD)

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Fig. 8

Instantaneous contours of unsteady computed velocity at midspan

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Fig. 9

Spanwise variations of inlet and exit rotor relative angles and blade metal angles: (a) rotor inlet and (b) rotor exit

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Fig. 10

Spanwise variations in the engine fan at windmill: (a) axial velocity, (b) angular momentum, (c) relative flow angles, and (d) entropy

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Fig. 11

Contours of entropy and streamwise vorticity at rotor exit (station 3) and axial velocity at OGV exit (station 4) in the engine fan at windmill. (a) Entropy (rotor exit), (b) streamwise vorticity (rotor exit), and (c) axial velocity (OGV exit).

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Fig. 12

Computed flow visualizations in the engine fan rotor: (a) suction surface flow, (b) pressure surface flow, and (c) 3D rotor streamlines (view of pressure surface)




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