End Wall Loss Reduction of High Lift Low Pressure Turbine Airfoils Using Profile Contouring—Part II: Validation

Abstract

The hypothesis, posed in Part I, that excessive end wall loss of high lift low pressure turbine (LPT) airfoils is due to the influence of high stagger angles on the end wall pressure distribution and not front loading is evaluated in a linear cascade at Re = 100,000 using both experimental and computational studies. A nominally high lift and high stagger angle front-loaded profile (L2F) with aspect ratio 3.5 is contoured at the end wall to reduce the stagger angle while maintaining the front loading. The contouring process effectively generates a fillet at the end wall, so the resulting airfoil is referred to as L2F-EF (end wall fillet). Although referred to as a fillet, this profile contouring process is novel in that it is designed to isolate the effect of stagger angle on end wall loss. Total pressure loss measurements downstream of the blade row indicate that the use of the lower stagger angle at the end wall reduces mixed out mass averaged end wall and passage losses approximately 23% and 10%, respectively. This is in good agreement with computational results used to design the contour which predict 18% and 7% loss reductions. The end wall flow field of the L2F and L2F-EF models is measured using stereoscopic particle image velocimetry (PIV) in the passage. These data are used to quantify changes in the end wall flow field due to the contouring. PIV results show that this loss reduction is characterized by reduced inlet boundary layer separation as well as a change in strength and location of the suction side horseshoe vortex (SHV) and passage vortex (PV). The end wall profile contouring also produces a reduction in all terms of the Reynolds stress tensor consistent with a decrease in deformation work and overall flow unsteadiness. These results confirm that the stagger angle has a significant effect on high-lift front-loaded LPT end wall loss. Low stagger profiling is successful in reducing end wall loss by limiting the development and migration of the low momentum fluid associated with the SHV and PV interaction.

Comparison of Endwall Loss Reduction Techniques in a High-Lift Turbine Passage

“…Numerous planes of Q contours throughout the passage of the cascade for both the baseline flow case and EF profile were compared in Refs. [11,22]. Both studies found a decrease in inlet boundary layer separation and a weaker PV that had shifted forward in the passage when the EF profile was used, in agreement with the present oil flow visualization.…”

“…While the basic mean secondary flow model has been known for years, see for example the review paper of Langston [4], there is on-going computational and experimental research by several groups working toward better physical understanding and modeling of high-lift LPT aerodynamics. Various loss reduction strategies for the secondary flow region have been developed including passive techniques such as endwall surface shaping or contouring [5][6][7], profile contouring near the blade/endwall junction [8][9][10][11][12], and endwall fences [13,14]. These approaches are typically optimized for a single flow condition.…”

Boundary layer separation control of low-pressure turbine blades based on oscillating jet

“…The boundary layer control of low-pressure turbine has two control methods, active control and passive control. Passive control methods mainly include: leading edge variants (Sangston, Little et al 2014), passive vortex generators , Zhang, Vera et al 2005, grooves , etc. The disadvantages of passive control methods are unable to adapt to other variable working conditions.…”

Effects of Input Voltage and Freestream Velocity on Active Flow Control of Passage Vortex in a Linear Turbine Cascade Using Dielectric Barrier Discharge Plasma Actuator

“…To reduce the secondary flow and the associated losses, various passive and active flow control applications in turbines are discussed in the literature [12]. Passive flow control initiatives, non-axisymmetric endwall contouring [13], leading-edge fillets/bulbs [14], a boundary layer endwall fence [15], tubercles [16], undulated blade [17], etc., have all been pursued with varying degrees of success, however, they have the disadvantage of being permanent; thus, they produce an unwanted penalty loss at different operating conditions. As the active flow control initiatives, air suction [18], steady jet blowing [19], and pulsed jet blowing [20], have been demonstrated for secondary flow reduction.…”