This research examines the use of passive and active blade-mounted flow control to reduce the unwanted losses associated with the blade tip clearance flow in a stationary, open-return, rectilinear turbine cascade at one atmosphere.
Traditional flow control techniques have focused on passive methods to improve the aerodynamics in the tip region. However passive methods can create increased heat transfer coefficients on the blade tip and clearance endwall, leading to blade degradation. To improve on these methods, various active flow control methods were designed and tested. The active control was designed to improve the clearance flow aerodynamics without introducing negative heat transfer effects. The flow control methods implemented were single dielectric barrier discharge plasma actuators of various designs and a passive partial suction-side squealer design. The passive squealer was used to benchmark the active designs against a known favorable device.
The tip clearance flow was investigated over Reynolds numbers ranging from 53,000 to 104,000 at clearance heights between one and four percent of axial blade chord. The tip clearance flow was documented using flow visualization and pressure measurements on the blade and endwall surfaces, inlet endwall boundary layer surveys, and wake pressure measurements downstream of the blade. These were carried out in order to understand the receptivity of the tip clearance flow to various types of flow control and the applicable range over which the flow control was effective.
The plasma actuator designs caused a reduction in the downstream total pressure loss coefficient ranging between 2% to 12%, depending on Reynolds number, while the passive squealer showed a change of approximately 40%. The results show that the plasma actuator was able to favorably mitigate the adverse effects of the tip clearance flow in a similar manner as the squealer tip, without the drawbacks of the passive method. Plasma actuation was demonstrated as a suitable as a means of reducing the tip clearance flow loss.