Unsteady Wake-Induced Boundary Layer Transition in High Lift LP Turbines

Abstract

The development of the unsteady suction side boundary layer of a highly loaded LP turbine blade has been investigated in a rectilinear cascade experiment. Upstream rotor wakes were simulated with a moving-bar wake generator. A variety of cases with different wake-passing frequencies, different wake strength, and different Reynolds numbers were tested. Boundary layer surveys have been obtained with a single hotwire probe. Wall shear stress has been investigated with surface-mounted hot-film gages. Losses have been measured. The suction surface boundary layer development of a modern highly loaded LP turbine blade is shown to be dominated by effects associated with unsteady wake-passing. Whereas without wakes the boundary layer features a large separation bubble at a typical cruise Reynolds number, the bubble was largely suppressed if subjected to unsteady wake-passing at a typical frequency and wake strength. Transitional patches and becalmed regions, induced by the wake, dominated the boundary layer development. The becalmed regions inhibited transition and separation and are shown to reduce the loss of the wake-affected boundary layer. An optimum wake-passing frequency exists at cruise Reynolds numbers. For a selected wake-passing frequency and wake strength, the profile loss is almost independent of Reynolds number. This demonstrates a potential to design highly loaded LP turbine profiles without suffering large losses at low Reynolds numbers.

Effect of Reynolds Number and Periodic Unsteady Wake Flow Condition on Boundary Layer Development, Separation, and Intermittency Behavior Along the Suction Surface of a Low Pressure Turbine Blade

“…For this purpose, Schobeiri et al [9] utilized a conceptually different type wake generator, which is also used for the investigation presented in this paper. Fottner and his coworkers [ 13,14] and Schulte and Hodson [ 15] used the same wake generating concept for the investigations on the influence of the unsteady wake flow on the LPT-boundary layer. Kaszeta, Simon, and Ashpis [ 16] experimentally investigated the laminar-turbulent transition within a channel with the two curved walls resembling the suction and pressure surfaces of an LPT-blade using a retractable wake generator.…”

An assessment of the laminar kinetic energy concept for the prediction of high-lift, low-Reynolds number cascade flows

“…Many studies demonstrate how high-lift [1,2] and ultra-high-lift [3,4] airfoils can be operated with loss control by taking advantage of wake-induced transition in LPT low-Reynolds, number flows. The study of wake-induced transition in LP turbines has thus led to improvements in performance of presentgeneration turbomachinery [5,6]. With a reduction of the suction-side separation effects, now recognized as the basic mechanism providing increased blade *Corresponding author: Department of Energy Engineering, University of Florence, via di Santa Marta 3, 50139, Firenze, Italy.…”

Pressure and Suction Surfaces Redesign for High Lift Low Pressure Turbines

“…To achieve a realistic simulation of wake-blade interactions, several parameters must be correctly matched [1,12]. The correct kinematics of the interaction is achieved by matching the velocity triangles using 50% stage reaction.…”