<p>Rising fuel costs and growing
environmental concerns have forced gas turbine engine manufacturers to place
high value on reducing fuel burn. This
trend has pushed compressor technology into new design spaces that are not
represented by historical experience.
Specifically, centrifugal compressor diffusers are trending toward
higher pressure recovery and smaller diameters.
The internal fluid dynamics in these new flow regimes are not well
understood and additional study is necessary.
This work outlines detailed experimental and numerical observations of
the flow field through a vaned diffuser for aeroengine applications.</p>
<p>The experimental data consist
of extensive Laser Doppler velocimetry measurements of the unsteady velocity
field from the impeller trailing edge through the majority of the diffuser
passage. These data were obtained
non-intrusively and yielded all three components of the velocity vector field
at approximately 2,000 geometric points. The correlation between fluctuations
in the three velocity components were also observed at several key locations to
determine the components of the local Reynolds stress tensor.</p>
<p>These data indicated a jet/wake
profile at the impeller exit represented by a consistent velocity deficit
region from hub to shroud adjacent to the suction surface of the passage. This region was more prevalent adjacent to
the splitter blade. The unsteady
fluctuations due to the propagation of the jet and wake through the diffuser
passage persist to 40% downstream of the throat. A complex secondary flow field was also observed
with large axial velocities and a passage-spanning vortex developing through
the diffuser passage. The velocity data
and total-pressure data indicated a region of flow separation developing along
the pressure surface of the vane near the hub due to the unsteady propagation
of the jet and wake flow through the diffuser.
Although this region was stable in time, its development arose due to
unsteady aspects of the flow. Finally,
the strong interconnection between the jet and wake flow, unsteady
fluctuations, secondary velocities, incidence, and flow separation was demonstrated. </p>
<p>Computationally, a
“best-practice” methodology for the modelling of a centrifugal compressor was
developed by a systematic analysis of various turbulence models and many
modelling features. The SST and
BSL-EARSM turbulence models with the inclusion of fillets, surface roughness,
and non-adiabatic walls was determined to yield the best representation of the
detailed flow development through the diffuser in steady (mixing-plane)
simulations. The accurate modelling of
fillets was determined to significantly impact the prediction of flow
separation with the SST turbulence closure model. Additionally, the frozen rotor approach was
shown to not accurately approximate the influence of unsteady effects on the
flow development.</p>
<p>Unsteady simulations were also
compared to the detailed experimental data through the diffuser. The BSL-EARSM turbulence model best matched
the experimentally observed flow field due to the SST model’s prediction of
flow separation in the shroud-pressure side corner of the passage. In general, lower levels of axial velocity
were predicted numerically that resulted in less spanwise mixing between the
endwall and freestream flows.
Additionally, the turbulent kinetic energy levels in the computational
results showed little streamwise variation through the vaneless and
semi-vaneless space. The large variation
observed experimentally indicated that the production and dissipation of
turbulent kinetic energy through this region was not accurately predicted in
the two turbulence models implemented for the unsteady simulations.</p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/12174147 |
| Date | 24 April 2020 |
| Creators | William J Gooding (8747457) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY-NC-SA 4.0 |
| Relation | https://figshare.com/articles/Unsteady_Diffuser_Flow_in_an_Aeroengine_Centrifugal_Compressor/12174147 |
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