The Application of Flow Control to an Aft-Loaded Low Pressure Turbine Cascade With Unsteady Wakes

Abstract

The synchronous application of flow control in the presence of unsteady wakes was studied on a highly-loaded low pressure turbine blade. The L1A blade has a design Zweifel coefficient of 1.34 and a suction peak at 58% axial chord, making it an aft-loaded pressure distribution. Velocity and pressure data were acquired at Rec = 20,000 with 3% incoming freestream turbulence. Unsteady wakes from an upstream vane row are simulated with a moving row of bars at a flow coefficient of 0.76. At this Reynolds number, the blade exhibits a non-reattaching separation bubble beginning at 57% axial chord under steady flow conditions without upstream wakes. The separation zone is modified substantially by the presence of unsteady wakes, producing a smaller separation zone and reducing the area-averaged wake total pressure loss by more than 50%. The wake disturbance accelerates transition in the separated shear layer but stops short of reattaching the flow. Rather, a new time-averaged equilibrium location is established for the separated shear layer, further downstream than without wakes. The focus of this study was the application of pulsed flow control using two spanwise rows of discrete vortex generator jets (VGJs). The VGJs were located at 59% Cx, approximately the peak cp location, and at 72% Cx. The most effective separation control was achieved at the 59% Cx location. Wake total pressure loss decreased 60% from the wake only level and the cp distribution fully recovered its high Reynolds number (attached flow) performance. The VGJ disturbance dominates the dynamics of the separated shear layer, with the wake disturbance assuming a secondary role only. When the pulsed jet actuation (30% duty cycle) was initiated at the 72% Cx location, synchronization with the wake passing frequency (10.6Hz) was key to producing the most effective separation control. A 25% improvement in effectiveness was obtained by aligning the jet actuation between wake events. Evidence suggests that flow control using VGJs will be effective in the highly unsteady LPT environment of an operating gas turbine, provided the VGJ location and amplitude are adapted for the specific blade profile.

Separation control on high lift low-pressure turbine airfoils using pulsed vortex generator jets

“…The L1A has a Zweifel coefficient of 1.35, which corresponds to 10% higher loading than the "ultra-high lift" airfoils described by Zhang and Hodson [7], and 17% higher loading than the Pack B. Because the L1A is highly loaded and aft loaded, it is prone to separation, as documented in Bons et al [16] and Volino [3]. In cases without flow control and low Reynolds numbers, the boundary layer separates and does not reattach, in spite of transition to turbulence in the shear layer over the separation bubble.…”

“…Separation control with VGJs has been demonstrated on the L1A airfoil by Bons et al [16], who considered a case with Re ¼ 50000, background freestream turbulence TI ¼ 3%, and periodic wakes, and by Volino et al [17], who considered cases with TI ¼ 0.6% and documented pressure distributions on the airfoils and total pressure losses. Cases were considered at Reynolds numbers from 25000 to 100000 (10000e40000 based on inlet velocity and axial chord).…”

Active Boundary Layer Control on a Highly Loaded Turbine Exit Case Profile

“…Nevertheless, the high loss reductions for steady inflow conditions suggest that the usage of fluidic actuators will still be very valuable. Bons et al [23] for example reported that the combination of unsteady wakes with flow control by vortex generator jets could reduce total pressure losses by 60% compared to the case with unsteady wakes only.…”

Active Boundary Layer Control on a highly loaded Turbine Exit Case profile

“…Nevertheless, the high loss reductions for steady inflow conditions suggest that the usage of fluidic actuators will still be very valuable. Bons et al (2011) for example reported that the combination of unsteady wakes with flow control by vortex generator jets could reduce total pressure losses by 60% compared to the case with unsteady wakes only.…”