Numerical Investigations of Active Flow Control for Low-Pressure Turbine Blades; Final rept. 1 Mar 2005-31 Dec 2007

摘要

The low-pressure turbine (LPT) produces the bulk net power in many jet engines. Changes in LPT efficiency can significantly affect overall engine efficiency. Modern LPTs have to drive larger fans at lower fan speeds and at the same time be less complex and lighter. The goal is a reduction in stage solidity without compromising performance. LPTs must operate efficiently over a large range of Reynolds numbers (1,000,000 during takeoff and 25,000 during high altitude cruise). Low Reynolds number operating conditions in combination with aggressive designs can lead to laminar separation, which can cause significant reductions in turbine and overall engine performance. In fact, Sharma (1998) reported increases of the loss coefficient as high as 300%. Prediction and control of suction side separation, without sacrificing the benefits of higher loading, is therefore, crucial for improved engine designs. It was recognized several years ago that active flow control (AFC) applied to LPT blades can counter such unfavorable conditions, and that AFC could lead to considerable performance improvements as well as to a reduction in component weight. In a broadly based experimental research program at the Air Force Research Laboratory (AFRL) at Wright-Patterson Air Force Base, Rivir and co- workers systematically explored the potential advantages of AFO using vortex generator jets (VGJs), both steady and pulsed, for LPT separation control. Pulsed blowing was shown to be much more efficient, requiring only a small fraction of the mass flow rate compared to the steady VGJs. Experimental observations show how steady angled injection results in the generation of streamwise vortices leading to free-stream momentum entrainment. These vortices maintain their coherence over a larger downstream distance than the counter rotating vortices generated by vertical injection and, therefore, facilitate more free-stream entrainment.