Effect of Blade Profile Contouring on Endwall Flow Structure in a High-Lift Low-Pressure Turbine Cascade

2016 ◽  
Vol 139 (2) ◽  
Author(s):  
Keith Sangston ◽  
Jesse Little ◽  
M. Eric Lyall ◽  
Rolf Sondergaard
Keyword(s):  
Total Pressure ◽  
Three Dimensional ◽  
Low Pressure ◽  
Stereoscopic Particle Image Velocimetry ◽  
Loss Reduction ◽  
Mean Flow ◽  
High Lift ◽  
Computational Tools ◽  
Design Modification ◽  
Low Pressure Turbine

Previous work has shown that low-stagger contouring near the endwall of a nominally high-lift and high-stagger angle front-loaded low-pressure turbine (LPT) airfoil is successful in reducing endwall loss by limiting the development and migration of low momentum fluid associated with secondary flow structures. The design modification that leads to loss reduction in that study was determined from an intuitive approach based on the premise that reducing flow separation near the endwall will lead to reduced loss production. Those authors also relied heavily upon Reynolds-averaged Navier–Stokes (RANS) based computational tools. Due to uncertainties inherent in computational fluid dynamics (CFD) predictions, there is little confidence that the authors actually achieved true minimum loss. Despite recent advances in computing capability, turbulence modeling remains a shortcoming of modern design tools. As a contribution to overcoming this problem, this paper offers a three-dimensional (3D) view of the developing mean flow, total pressure, and turbulence fields that gave rise to the loss reduction of the airfoil mentioned above. Experiments are conducted in a linear cascade with aspect ratio of 3.5 and Re = 100,000. The results are derived from stereoscopic particle image velocimetry (PIV) and total pressure measurements inside the passage. Overall, the loss reduction correlates strongly with reduced turbulence production. The aim of this paper is to provide readers with a realistic view of mean flow and turbulence development that include all the components of the Reynolds stress tensor to assess, at least qualitatively, the validity of high fidelity computational tools used to calculate turbine flows.

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

2014 ◽  
Vol 136 (8) ◽  
Author(s):  
Keith Sangston ◽  
Jesse Little ◽  
M. Eric Lyall ◽  
Rolf Sondergaard
Keyword(s):  
Flow Field ◽  
Low Pressure ◽  
Stereoscopic Particle Image Velocimetry ◽  
Loss Reduction ◽  
High Lift ◽  
Low Pressure Turbine ◽  
Wall Loss ◽  
Wall Flow ◽  
End Wall ◽  
Stagger Angle

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.

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

2013 ◽  
Author(s):  
Keith Sangston ◽  
Jesse Little ◽  
M. Eric Lyall ◽  
Rolf Sondergaard
Keyword(s):  
Flow Field ◽  
Suction Side ◽  
Boundary Layer Separation ◽  
Horseshoe Vortex ◽  
Low Pressure ◽  
Stereoscopic Particle Image Velocimetry ◽  
Loss Reduction ◽  
High Lift ◽  
Low Pressure Turbine ◽  
Stagger Angle

The hypothesis, posed in Part I [1], that excessive endwall loss of high lift low pressure turbine (LPT) airfoils is due to the influence of high stagger angles on the endwall 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 endwall to reduce the stagger angle while maintaining the front loading. The contouring process effectively generates a fillet at the endwall, so the resulting airfoil is referred to as L2F-EF (Endwall 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 endwall loss. Total pressure loss measurements downstream of the blade row indicate that the use of the lower stagger angle at the endwall reduces mixed out mass averaged endwall 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 endwall 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 endwall 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 endwall 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 endwall loss. Low stagger profiling is successful in reducing endwall loss by limiting the development and migration of the low momentum fluid associated with the SHV and PV interaction.

Passive Boundary Layer Control on a Highly Loaded Low Pressure Turbine Cascade

2010 ◽  
Author(s):  
Markus Martinstetter ◽  
Reinhard Niehuis ◽  
Matthias Franke
Keyword(s):  
Total Pressure ◽  
Three Dimensional ◽  
Reynolds Numbers ◽  
Low Pressure ◽  
Boundary Layer Control ◽  
Loss Reduction ◽  
Suction Surface ◽  
Low Pressure Turbine ◽  
Layer Control ◽  
Highly Loaded

The objective of this work is to investigate the performance of a highly loaded Low Pressure Turbine (LPT)-Cascade over a wide range of turbomachinery realistic Reynolds numbers, while maintaining a constant real engine Mach number. Fundamental investigations show a strong increase in total pressure loss at low Reynolds number due to an enlarged separation region on the suction surface. The aim of this paper is to reduce total pressure loss through the application of passive turbulators on the suction surface. Both, steady inflow conditions, as well as effects of rotor-stator-interaction, are considered. To identify the importance of turbulator geometry on loss reduction, three dimensional turbulator elements have been compared to a two dimensional trip. Both passive devices were located on the suction surface and the three dimensional elements performed better at high Reynolds numbers. Based on the experimental investigations, the use of passive boundary layer control can be recommended as a promising approach for loss reduction on highly loaded low pressure turbine profiles.

Evaluation of RANS Transition Modeling for High Lift LPT Flows at Low Reynolds Number

2013 ◽  
Author(s):  
Kevin Keadle ◽  
Mark McQuilling
Keyword(s):  
Reynolds Number ◽  
Total Pressure ◽  
Current Model ◽  
Pressure Coefficient ◽  
Low Pressure ◽  
Two Dimensional ◽  
High Lift ◽  
Low Pressure Turbine ◽  
Low Reynolds ◽  
Turbine Airfoils

High lift low pressure turbine airfoils have complex flow features that can require advanced modeling capabilities for accurate flow predictions. These features include separated flows and the transition from laminar to turbulent boundary layers. Recent applications of computational fluid dynamics based on the Reynolds-averaged Navier-Stokes formulation have included modeling for attached and separated flow transition mechanisms in the form of empirical correlations and two- or three-equation eddy viscosity models. This study uses the three-equation model of Walters and Cokljat [1] to simulate the flow around the Pack B and L2F low pressure turbine airfoils in a two-dimensional cascade arrangement at a Reynolds number of 25,000. This model includes a third equation for the development of pre-transitional laminar kinetic energy (LKE), and is an updated version of the Walters and Leylek [2] model. The aft-loaded Pack B has a nominal Zweifel loading coefficient of 1.13, and the front-loaded L2F has a nominal loading coefficient of 1.59. Results show the updated LKE model improves predicted accuracy of pressure coefficient and velocity profiles over its previous version as well as two-equation RANS models developed for separated and transitional flows. Transition onset behavior also compares favorably with experiment. However, the current model is not found suitable for wake total pressure loss predictions in two-dimensional simulations at extremely low Reynolds numbers due to the predicted coherency of suction side vortices generated in the separated shear layers which cause a local gain in wake total pressure.

Detailed Experimental Measurement and RANS Simulation of a Low Pressure Turbine With High Lift Blading

2019 ◽  
Author(s):  
Ethan Perez ◽  
John T. Schmitz ◽  
Nicholas A. Jaffa ◽  
Aleksandar Jemcov ◽  
Joshua D. Cameron ◽  
...  
Keyword(s):  
Total Pressure ◽  
Low Pressure ◽  
Aerodynamic Characteristics ◽  
Expansion Ratio ◽  
High Lift ◽  
Time Resolved ◽  
Low Pressure Turbine ◽  
Hot Wires ◽  
Simulation Results ◽  
Improved Performance

Abstract The aerodynamic characteristics of high–lift airfoil designs is of interest for improved performance and reduced blade count in Low–Pressure Turbine (LPT) design. The present paper presents both experimental measurements as well as numerical simulation results from a single-stage LPT. The airfoils were designed for an embedded stage with a total pressure expansion ratio of 1.75 and a rotor Zweifel coefficient of 1.35. The measurement program was highly unique in that detailed measurements were obtained using a variety of different probe types, including time–resolved total pressure and hot–wires. Agreement between various measurement types was generally good, but differences beyond typically stated uncertainty bounds were noted. The computations were done using RANS and a mixing model via commercially available software. The numerical results were evaluated to determine the efficacy of this type of model for prediction and design of high–lift airfoils. The computations agreed very well with the experimental results in the midspan region, but losses were over–predicted in the lower 40% span near the hub. A basic description and understanding of the flow physics in the LPT stage are presented based on the relative agreement between the experiments and computations.

Endwall Loss Reduction of High Lift Low Pressure Turbine Airfoils Using Profile Contouring—Part I: Airfoil Design

2014 ◽  
Vol 136 (8) ◽  
Author(s):  
M. Eric Lyall ◽  
Paul I. King ◽  
John P. Clark ◽  
Rolf Sondergaard
Keyword(s):  
Design Process ◽  
Low Pressure ◽  
Loss Reduction ◽  
High Lift ◽  
Linear Cascade ◽  
Low Pressure Turbine ◽  
Turbine Airfoils ◽  
Stagger Angle

This paper presents the reasoning for and the design process of contouring a high lift front-loaded low pressure turbine (LPT) airfoil near the endwall to reduce the endwall loss. The test airfoil, L2F, was designed to the approximate gas angles with 38% larger pitchwise spacing than the widely studied Pack B airfoil. Being more front-loaded with a higher stagger angle, L2F is shown to produce more endwall losses than Pack B. It is suggested that the high endwall loss of L2F is due to the high stagger angle, not front-loading, as usually suggested in the literature. A procedure is presented to approximate the front-loading and stall resistance of L2F and obtain a low stagger version of that airfoil, designated as L2F-LS. A contoured airfoil is then designed by transitioning L2F into L2F-LS at the endwall to obtain a benefit from the reduced stagger angle at the endwall. Due to the contouring process generating a fillet, the contoured airfoil is referred to as L2F-EF (“endwall fillet”). Predictions in this paper suggest endwall loss reductions between 17% and 24% at Re = 100,000. Linear cascade experiments in Part II of this paper indicate that L2F-EF reduces endwall losses more than 20% compared to L2F. The overall conclusion is that the stagger angle has a significant effect on endwall loss and should be considered for designing high lift LPT airfoils at the endwall.

Separation Control on a Very High Lift Low Pressure Turbine Airfoil Using Pulsed Vortex Generator Jets

2011 ◽  
Vol 133 (4) ◽  
Author(s):  
Ralph J. Volino ◽  
Olga Kartuzova ◽  
Mounir B. Ibrahim
Keyword(s):  
Boundary Layer ◽  
Total Pressure ◽  
Vortex Generator ◽  
Reynolds Numbers ◽  
Low Pressure ◽  
Separation Control ◽  
Suction Surface ◽  
High Lift ◽  
Low Pressure Turbine ◽  
Very High

Boundary layer separation control has been studied using vortex generator jets (VGJs) on a very high lift, low-pressure turbine airfoil. Experiments were done under high (4%) freestream turbulence conditions on a linear cascade in a low speed wind tunnel. Pressure surveys on the airfoil surface and downstream total pressure loss surveys were documented. Instantaneous velocity profile measurements were acquired in the suction surface boundary layer. Cases were considered at Reynolds numbers (based on the suction surface length and the nominal exit velocity from the cascade) of 25,000 and 50,000. Jet pulsing frequency, duty cycle, and blowing ratio were all varied. Computational results from a large eddy simulation of one case showed reattachment in agreement with the experiment. In cases without flow control, the boundary layer separated and did not reattach. With the VGJs, separation control was possible even at the lowest Reynolds number. Pulsed VGJs were more effective than steady jets. At sufficiently high pulsing frequencies, separation control was possible even with low jet velocities and low duty cycles. At lower frequencies, higher jet velocity was required, particularly at low Reynolds numbers. Effective separation control resulted in an increase in lift and a reduction in total pressure losses. Phase averaged velocity profiles and wavelet spectra of the velocity show the VGJ disturbance causes the boundary layer to reattach, but that it can reseparate between disturbances. When the disturbances occur at high enough frequency, the time available for separation is reduced, and the separation bubble remains closed at all times.

Secondary Flow Loss Reduction Through Blowing for a High-Lift Front-Loaded Low Pressure Turbine Cascade

2012 ◽  
Vol 135 (2) ◽  
Author(s):  
Stuart I. Benton ◽  
Jeffrey P. Bons ◽  
Rolf Sondergaard
Keyword(s):  
Reynolds Number ◽  
Pressure Loss ◽  
Total Pressure ◽  
Control Method ◽  
Low Pressure ◽  
Total Pressure Loss ◽  
Suction Surface ◽  
High Lift ◽  
Blade Loading ◽  
Low Pressure Turbine

Efforts to increase individual blade loading in the low pressure turbine have resulted in blade geometries optimized for midspan performance. Many researchers have shown that increased blade loading and a front-loaded pressure distribution each separately contribute to increased losses in the endwall region. A detailed investigation of the baseline endwall flow of the L2F profile, which is a high-lift front loaded profile, is performed. In-plane velocity vectors and total pressure loss maps are obtained in five planes oriented normal to the blade surface for three Reynolds numbers. A row of pitched and skewed jets are introduced near the endwall on the suction surface of the blade. The flow control method is evaluated for four momentum coefficients at the high Reynolds number, with a maximum reduction of 42% in the area averaged total pressure loss coefficient. The same blade is also fitted with midspan vortex-generator jets and is tested at a Reynolds number of 20,000, resulting in a 21% reduction in the area averaged total pressure loss.

Application of Nonaxisymmetric Endwall Contouring to Conventional and High-Lift Turbine Airfoils

2013 ◽  
Vol 135 (6) ◽  
Author(s):  
T. J. Praisner ◽  
E. Allen-Bradley ◽  
E. A. Grover ◽  
D. C. Knezevici ◽  
S. A. Sjolander
Keyword(s):  
Design Methodology ◽  
Three Dimensional ◽  
Low Pressure ◽  
Free Form ◽  
Published Data ◽  
High Lift ◽  
Flow Solver ◽  
Low Pressure Turbine ◽  
Gradient Based ◽  
Endwall Contouring

Here, we report on the application of nonaxisymmetric endwall contouring to mitigate the endwall losses of one conventional and two high-lift low-pressure turbine airfoil designs. The design methodology presented combines a gradient-based optimization algorithm with a three-dimensional computational fluid dynamics (CFD) flow solver to systematically vary a free-form parameterization of the endwall. The ability of the CFD solver employed in this work to predict endwall loss modifications resulting from nonaxisymmetric contouring is demonstrated with previously published data. Based on the validated trend accuracy of the solver for predicting the effects of endwall contouring, the magnitude of predicted viscous losses forms the objective function for the endwall design methodology. This system has subsequently been employed to optimize contours for the conventional-lift Pack B and high-lift Pack D-F and Pack D-A low-pressure turbine airfoil designs. Comparisons between the predicted and measured loss benefits associated with the contouring for Pack D-F design are shown to be in reasonable agreement. Additionally, the predictions and data demonstrate that the Pack D-F endwall contour is effective at reducing losses primarily associated with the passage vortex. However, some deficiencies in predictive capabilities demonstrated here highlight the need for a better understanding of the physics of endwall loss-generation and improved predictive capabilities.

scholarly journals Large-Eddy Simulation of Low-Pressure Turbine Cascade with Unsteady Wakes

Aerospace ◽  
2021 ◽  
Vol 8 (7) ◽  
pp. 184
Author(s):  
Zachary Robison ◽  
Andreas Gross
Keyword(s):  
Flow Field ◽  
Flow Structure ◽  
Three Dimensional ◽  
Suction Side ◽  
Low Pressure ◽  
Flow Fields ◽  
Mean Flow ◽  
Reaction Stage ◽  
Low Pressure Turbine ◽  
Large Eddy

To better understand the wake effects at low Reynolds numbers, large-eddy simulations of a 50% reaction low-pressure turbine stage and a linear cascade with two different bar wake generators were carried out for a chord Reynolds number of 50,000. For the chosen front-loaded high-lift airfoil, the endwall structures are stronger than for more traditional mid-loaded moderate-lift airfoils. By comparing the 50% reaction stage results with the bar wake generator results, insight is gained into the effect of the three-dimensional wake components on the downstream flow field.For the cases with bar wake generator, the endwall boundary layer is growing faster because of the relative motion of the endwall with respect to the freestream. The half-width of the wake is approximately matched for the larger one of the two considered bar wake generators. To improve the quality of the phase-averaged flow fields, the proper orthogonal decomposition was employed as a filter to remove the low-energy unsteady flow field content. Both the mean flow and filtered phase-averaged flow fields were analyzed in detail. Visualizations of the phase-averaged flow field reveal a periodic suppression of the laminar suction side separation from the downstream airfoil even for the smaller bar wake generator. The passage vortex is entirely suppressed for the 50% reaction stage and for the larger bar wake generator. Furthermore, the phase-averaged data for the 50% reaction stage reveal a new longitudinal flow structure that is traced back to near-wall wake vorticity. This flow structure is missing for the bar wake generator cases.

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