The tip clearance leakage flow is known to cause a high loss in efficiency within the turbine stages of a gas-turbine engine. The tip clearance gap that exists between the wall casing and the blade tip is necessitated because of blade rotation and thermal expansion. The presence of the tip gap allows high temperature, high pressure air to ‘escape’ from the main flow path between adjacent turbine blades and leak through the tip clearance gap.
The tip clearance gap flow allows the greatest single loss within a turbomachine, compared to other secondary flows, though the gap flow only contains a small fraction of the total mass flow through the stage. The gap flow is the cause of a number of unwanted aerodynamic effects including: an overturning or underturning of the casing boundary layer, the creation of a flow discontinuity between the gap flow and main passage flow at the gap exit, vibration and flutter on the downstream turbine stages, near-tip blade unloading, blade tip degradation, downstream pressure field and boundary layer unsteadiness, and the formation of a coherent tip clearance vortex.
This research addresses three specific aims. They are to understand:
It is hoped that active flow control coupled with local tip geometry modifications will decrease or destroy leakage vortex formation, with the goal of increasing efficiency. This research problem is being investigated by using:
These methods are being used to document the formation of the leakage vortex, the loss that occurs from the tip gap flow, and the extent to which the tip clearance and other secondary vortices are responsive to active flow control.
Here, the active flow control method being used is a blade tip mounted weakly ionized single dielectric barrier discharge plasma actuator. Using a low pressure linear turbine cascade, it has been shown that the location of the tip leakage vortex can be seen in the near-tip blade surface pressure measurements. Also, unsteady forcing of the local tip gap flowfield using plasma actuation on the blade tip surface has been able to affect the downstream leakage vortex loss. It remains to be seen how the flowfield responds to variations in the blade tip geometry with actuation and how further iterations in the tip actuator configuration are able to alter the tip gap flow.
This research is being funded by a grant from the Air Force Office of Scientific Research and the National Defense Science and Engineering Graduate Fellowship from the American Society of Engineering Education.