Low pressure turbine (LPT) blades must be carefully designed in order to perform well under demanding operating conditions. They require a very aggressive turning angle (Figure 1-1a) to provide maximum power during take-off and sufficient power for auxiliary systems during flight. At very high altitudes, the flight Reynolds number drops significantly, which can produce a laminar boundary layer along the surface of the blade. This laminar boundary layer is susceptible to separation, a condition that greatly diminishes the efficiency of the turbine (Figure 1-1b). For example, Sharma et al.1 reported an increase in blade loss coefficient of 300% as a result of the boundary layer separation.
Much research has been conducted in an effort to find a method of reducing the laminar boundary layer separation in LPTs. Lake et al.2 determined that a modification of the actual blade surface (for example, by adding dimples) successfully decreased the separation-induced losses. Volino3 experimented with 2D spanwise bars that act as boundary layer trips and thus prevent suction surface separation. However, passive methods of flow control such as these may inhibit proper turbine performance at design conditions and/or increase blade heat transfer. Active methods of flow control, although more difficult to implement, are preferred as they can be adapted in-flight according to the current flight conditions.
One such method of active flow control is the use of vortex generator jets (VGJs) to inject a small amount of airflow into the separating boundary layer (Figure 1-1c). It has been suggested that VGJs cause the separated boundary layer to reattach by mixing high momentum freestream fluid with low momentum boundary layer fluid4. This process of freestream entrainment effectively reenergizes the flow and reduces the incurred losses. For example, Sondergaard et al.5 determined that the use of VGJs reduced the separation induced wake losses by up to 60%. However, before VGJs can be successfully implemented in LPT design, a more thorough understanding of the physical mechanisms involved in the reduction of the boundary layer must be obtained.
