An experimental investigation has been carried out to determine the aerodynamic
performance of an annular S-shaped duct representative of that used to connect the
compressor spools of aircraft gas turbine engines. Measurements of both the mean
flow and turbulent structure have been obtained using both 5 hole pressure probes and
a3 component Laser Doppler Anemometry (LDA) system. The measurements indicate
that development of the flow within the duct is complex and significantly influenced
by the combined effects of streamwise pressure gradients and flow curvature.
For inlet conditions in which boundary layers are developed along an upstream
entry length the static pressure, shear stress and velocity distributions are presented.
The data shows that as a result of flow curvature significant streamwise pressure
gradients exist within the duct, with this curvature also affecting the generation and
suppression of turbulence. The stagnation pressure loss within the duct is also assessed
and is consistent with the measured distributions of shear stress. More engine
representative conditions are provided by locating a single stage compressor at inlet to
the duct. Relative to the naturally developed inlet conditions the flow within the duct is
less likely to separate, but mixing out of the compressor blade wakes increases the
measured duct loss. With both types of inlet conditions the effect of a radial strut, such
as that used for carrying loads and engine services, is also described both in terms of
the static pressure distribution along the strut and its contribution to overall loss.
The effects of inlet swirl on the flow field that develops within an annular
S-shaped duct have also been investigated. By removing the outlet guide vanes from an
upstream single stage compressor swirl angles in excess of 30° were generated. Results
show that within the S-shaped duct tangential momentum is conserved, leading to
increasing swirl velocities through the duct as its radius decreases. Furthermore, this
component influences the streamwise velocity as pressure gradients are established to
ensure the mean flow follows the duct curvature. Consequently in the critical region
adjacent to the inner casing, where separation is most likely to occur, higher
streamwise velocities are observed. Within the duct substantial changes also occur to
the turbulence field which results in an increased stagnation pressure loss between duct
inlet and exit. Data is also presented showing the increasing swirl angles through the
duct which has consequences both for the design of the downstream compressor spool
and of any radial struts which may be located within the duct.
A Doctoral Thesis. Submitted in partial fulfillment of the requirements for the award of Doctor of Philosophy of Loughborough University.