Active flow control: The Tip Gap Flow of Low Pressure Turbine Blades in Near Sonic Flow

It is of interest to increase the efficiency of turbine and compressor blades for jet engines. A primary source of loss in turbines is the tip gap vortex, caused by flow leaking over the blade tip from the pressure to suction-side in the gap between the blade and the casing. There are a number of ways to affect the behavior of this vortex. Common methods include various changes to the blade tip geometry and are called passive flow control techniques. These methods, while reducing the effect of the tip gap vortex on the flow, have downsides including increased weight and increased wear on the engine (increased maintenence costs). Two of these methods include squealers (shown left), and winglets (shown right).

This research aims to understand the behavior of the tip gap vortex, including the effects of flow speed and gap size. Additionally, we are investigating the effects some of the commonly used passive control techniques have on the flow, and why they are effective at reducing the losses due to the tip gap vortex. The ultimate goal is to use this knowledge to design a plasma actuator to be used as an active flow control technique to reduce losses due to the tip gap vortex without the increased weight and wear on the engine.

To do this we have designed and constructed a 3-blade low pressure turbine cascade. It is in the main lab at the Hessert laboratory. A five-hole pitot probe mounted to a traverse is used to study the wake downstream of the blades. Pressure taps on the blade surface and endwall are also used to document the flow behavior. Also, flow visualization is used to study the streamlines of the flow on the surfaces around the gap. The gap is varied from between 0 and 8% of axial blade chord. The Reynolds number varies between 100,000 and 500,000, although most of the studies focus on the high speed case. Winglets and partial suction-side squealers are employed as passive flow control strategies. Finally, plasma actuators, both blade and wall mounted, are employed and their effects documented.

Traversed wake region showing the tip-gap vortex via pressure contours, and a variety of wake measurements

The plasma actuator is two electrodes that are separated by a dielectric material. One electrode is exposed to air, while the second is completely covered by the dialectric. When a high voltage a.c. amplitude is supplied to the electrodes the air ionizes in the region of largest electric potential, generally located at the edge of the exposed electrode. The presence of an electric field gradient causes the ionized air to produce a body force on the ambient air. One actuator investigated by this research is shaped like the partial suction side squealer. It is milled out of copper clad circuit board.