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Turbulent flow phenomena and boundary layer transition at the circular arc leading edge of an axial compressor stator blade


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Perkins, SC 2012 , 'Turbulent flow phenomena and boundary layer transition at the circular arc leading edge of an axial compressor stator blade', PhD thesis, University of Tasmania.

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As fuel prices rise and environmental awareness becomes an increasingly important topic,
the efficiency of engines used for power production and transport must be increased whilst
decreasing exhaust gas emissions and noise levels. From results obtained during this research
project, in combination with work being produced at other research facilities, it is
hoped that a greater understanding of how the leading edge region of compressor blades
react to changes in engine operating points in a steady and unsteady environment is gained.
This thesis investigates the boundary layer development at the leading edge of a controlled
diffusion stator blade with a circular arc leading edge profile. Steady flow measurements
were made inside a large scale 2D compressor cascade at Reynolds numbers of 260, 000
and 400, 000 over a range of inlet flow angles corresponding to both positive and negative
incidence at a level of freestream turbulence similar to that seen in an embedded stage of
industrial axial flow compressor.
The instrumented blade of a large scale 2D cascade contained a series of very high resolution
static pressure tappings and an array of hot-film sensors in the first 10% of surface
length from the leading edge. Detailed static pressure measurements in the leading edge region
show the time-mean boundary layer development through the velocity over-speed and
following region of accelerating flow on the suction surface. The formation of separation
bubbles at the leading edge of the pressure and suction surfaces trigger the boundary layer
to undergo an initial and rapid transition to turbulence. On the pressure surface the bubble
forms at all values of incidence tested, whereas on the suction surface a bubble only forms
for incidence greater than design. In all cases the bubble length was reduced significantly
as Reynolds number was increased. These trends are supported by the qualitative analysis
of surface flow visualisation images.
Quasi-wall shear stress measurements from hot-film sensors were interpreted using a hybrid
threshold peak-valley-counting algorithm to yield time-averaged turbulent intermittency
on the blade’s suction surface. These results, in combination with raw quasi-wall
shear stress traces show evidence of boundary layer relaminarisation on the suction surface
downstream of the leading edge velocity over-speed in the favorable pressure gradient
leading to peak suction. The relaminarisation process is observed to become less effective
as Reynolds number and inlet flow angle are increased.

The boundary layer development is shown to have a large influence on the blade total pressure
loss. Initial observations were made without unsteady wakes and at negative incidence
loss was seen to increase as the Reynolds number was decreased and, in contrast, at positive
incidence the opposite trend was displayed. The cascade’s rotating bar mechanism was
used for unsteady tests where the influence of changing reduced frequency was investigated
and compared to the performance of the cascade in steady operation. Results showed
that increasing the stator reduced frequency brought about an increase in total blade pressure
loss. The proportion of total loss generated by the suction surface increased linearly as
the reduced frequency was increased from 0.47

Item Type: Thesis - PhD
Authors/Creators:Perkins, SC
Keywords: Compressor - Turbulence - Transistion - Separation - Relaminarisation - Loss Generation
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Copyright 2012 the Author

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