Active Flow Control on Low-Pressure Turbine Blades Using Synthetic Jets

Abstract

The operating Reynolds numbers (Re) for a low-pressure turbine (LPT) in an aircraft engine can drop below 25,000 during high-altitude cruise conditions. At these low Reynolds numbers, the boundary layer on the LPT blade is largely laminar, and is susceptible to separation on the aft portion of the blade suction surface. This separation is detrimental and causes a significant loss in the engine efficiency. The objective of the current research is to control this separation, and minimize the associated losses by numerically implementing an active flow control strategy. Unlike passive flow control techniques, active flow control (AFC) techniques can be turned on and off depending on the requirement for flow control. In the present paper, we numerically investigate the flow through an LPT cascade at a chord inlet Reynolds number of 25,000 with active separation control using synthetic jets and synthetic vortex-generator jets (VGJ’s). Synthetic jets hold an advantage over steady or pulsed jets in that they require no net mass flow, i.e., synthetic jets are formed entirely from the working fluid of the flow system in which they are deployed and, thus, can transfer linear momentum to the flow system without net mass injection across the flow boundary. In the LPT environment, this means that no compressor bleed air is required. While LPT separation control using steady and pulsed VGJs has been numerically investigated before, AFC on an LPT blade by synthetic jets and synthetic VGJs has not yet been numerically investigated. The geometrical difference between a synthetic jet and synthetic VGJ is the angle at which the jet enters the main flow. A synthetic jet enters the main flow normal to the surface, and on the other hand, a synthetic VGJ enters at a certain angle to the wall (pitch angle) and at a certain angle to the main flow (skew angle). For the present case, the VGJs are oriented at 30° to the surface and 90° to the main flow. In addition to the angle at which these two jets enter the main flow, these flow control mechanisms differ in the way they delay or avoid separation. Synthetic jets generate turbulent spots which energize the flow, whereas synthetic VGJ’s generate streamwise vortices which enhance mixing. The relative magnitudes of the effects of turbulence and streamwise vortices in enhancing mixing are being investigated. The results for both control mechanisms will be compared to each other, and with experimental data. An MPI-based higher-order accurate, Chimera version of the FDL3DI flow solver developed by the Air Force Research Laboratory at Wright Patterson Air Force Base, is extended for the present turbomachinery application.