| Abstract |
A rise of 1% in the efficiency of low-pressure turbines will lead to an increase in the overall
efficiency of a turbofan engine of between 0.7% and 0.9%, with a matching improvement
in specific fuel consumption. Especially the laminar flow separation on the suction side
of the low-pressure turbine aerofoils restrict the efficiency of this component. Generally
this laminar separation occurs at high aerodynamic load and low Reynolds number. A
possibility exists to both reduce this impairment and improve the operational range of
the components by actively manipulating the laminar flow separation. The objective of
this flow manipulation is either to eliminate the boundary layer or to force the flow to
reattach. If the flow separation cannot be entirely eliminated, it should be reduced as
much as possible.
A technology that can achieve this end, giving an active injection method which could
buy its way into the turbomachinery, is the use of pneumatic vortex generator jets. By
means of this technique, a fluid jet can be injected into the impaired flow to create a
strong vortex structure through the interaction of the jets with the main airflow to re-
energize the flow and suppress flow separation. Despite extensive research and interest
in the application of this active technique, gaps remain in our understanding of how
this technology can be applied to reduce flow separation. In particular, the role of the
laminar-turbulent transition and the effect of flow injection upstream the suction peak of
an aerofoil pressure distribution has been, at present, little investigated.
Prior experimental work in this area has used a linear cascade as an experimental ve-
hicle that closely resemble deployed machinery, offering a limited basis for making mea-
surements. In order to gather best quality data, this study developed a test rig, which
superimposes a pressure distribution on a flat plate boundary layer to simulate the suc-
tion side flow of a Mach-number scaled low-pressure turbine aerofoil. The fundamental
measuring technologies used were Hot Wire Anemometry and stereoscopic Particle Image
Velocimetry.
This work has relied on prior research findings and places its focus on the fluid dynamics
of the flow domain related to the active method. The questions, in which way the two ele-
ments of the technique, transition and induced vortex structure, interact with each other,
and how the interaction itself impact the development of the profile loss, are answered.
Moreover, this work led to a number of findings regarding the behaviour of the induced
vortex structure, when injecting upstream the suction peak of pressure distribution. |
