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Unsteady aerodynamics in an axial flow compressor

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Hughes, J. D.(Jeremy David) (2001) Unsteady aerodynamics in an axial flow compressor. PhD thesis, University of Tasmania.

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Abstract

Current methods for designing compressor blades in axial turbomachines assume that
the flow through each blade row is steady. However, interactions associated with the
relative motion of neighbouring blade rows are known to produce a disturbance fleld
with both random and periodic components. Despite a growing amount of research
into the influence of these disturbances on the downstream row in a rotor-stator or
stator-rotor blade row pair, these efiects axe still not generally accounted for at the
design stage. Recent advances in Low Pressure Turbine blade design have shown that
incorporating unsteady effects can lead to increases in blade loading beyond the loading
limits inferred from steady flow calculations.
The current experimental work investigates the unsteady flow behaviour in the
neighbourhood of the outlet stator in a 1.5 stage axial flow compressor using thermal
anemometry. The aim is to provide a base for more accurate unsteady modelling, and
facilitate the development of compressor blade designs which gain maximum beneflt
from unsteady effects. High-speed data acquisition with synchronised sampling was
used to acquire data ensembles for a specific set of rotor wakes, and estimate the
periodic and random components of the stator inflow disturbance field. The stator
inflowdisturbance flow field was altered by clockingof the inlet guide vane row relative
to the stator row, and by changing the rotor-stator axial blade row spacing.
The interaction of inlet guide vane and rotor waJkes was examined using hot-wire
measurements downstream of the rotor row. The interaction process was shown to
restrict the relative motion of rotor wake fluid and produce local accumulations of low
energy fluid on the suction side of the inlet guide vane wakes. Significant circumferential
variations in both time-mean velocity and the periodic disturbance component were
observed. Clocking of the downstream stator row relative to the inlet guide vanes
altered the mid-span stator blade boundary layer behaviour. Hot-wire measurements
performed downstream of the stator were used to evaluate the influence of blade row
clocking on the stator mid-span viscous losses. The magnitude of periodic fluctuations
in ensemble-average stator wake thickness was significantly influenced by IGV wakerotor
wake interaction effects. The changes in time-mean stator losses were marginal
The periodically unsteady laminar to turbulent transition of the stator blade bound
ary layer was examined using a hot-film surface array mounted on both the suction and
pressure surfaces. Observations were made for stator blade loading or incidence cases
near stall, design and maximum flow, and for a range of relative axial and circum
ferential blade row positions. Ensemble average plots of turbulent intermittency and
relaxation factor (extent of calmed flow following the passage of a turbulent spot) are
presented for a range of inflow disturbance cases. These show the strength of peri
odic wake-induced transition phenomena to be significantly influenced by incidence. clocking and blade row spacing effects. The periodic, wake-induced, transition in sepa
ration bubbles was also altered by changes in blade row spacing. Significant differences
between suction and pressure surface transition behaviour were observed, particularly
with regard to the strength and extent of calming.
Subsequent collaborative work (not reported within) evaluated the quasi-steady
application of conventional transition correlation to predict unsteady transition onset
on the blading of an embedded axial compressor stage. The viscous/inviscid interaction
code MISES was used to calculate the blade surface pressure distributions and boundary
layer development. Predictions of the temporal variation in transition onset based on
the measured temporal variation of inflow turbulence were compared with the transition
onset observations from the compressor stator. Computations for both natural and
bypass transition modes indicated that the natural transition mode predicted by a
modified method tended to dominate on the compressor blades. The success of
the MISES transition onset predictions provided strong circumstantial evidence for the
importance of natural transition mechanisms in strongly decelerating flow and provided
the impetus further experimental investigation.
Transitional flow data from the surface film gauges were also analysed using wavelet
techniques. The primary use of the wavelet analysis was to facilitate identification of
transient instability phenomena in the complex periodic transitional flows present in
the stator blade boundary layer. Wavelet analysis and high-pass filtering techniques
revealed significant wave packet activity in the unstable laminar flow regions. An
algorithm was developed to identify instability waves within the Tolhnien-Schlichting
(T-S) frequency range. This was combined with a turbulent intermittency detection
routine to produce space~time diagrams showing the probability of instability wave
occurrence prior to regions of turbulent flow. The implications for transition prediction
in decelerating flow regions on axial turbomachine blades are discussed.

Item Type: Thesis (PhD)
Keywords: Axial flow compressors, Axial flow compressors, Compressors
Copyright Holders: The Author
Copyright Information:

Copyright 2001 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (Ph.D.)--University of Tasmania, 2001. Includes bibliographical references

Date Deposited: 19 Dec 2014 02:41
Last Modified: 22 Aug 2016 01:19
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