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.

A Low Reynolds Number Experimental Evaluation of Tubercles on a Low-Pressure Turbine Cascade

2019 ◽  
Author(s):  
Stephen A. Pym ◽  
Asad Asghar ◽  
William D. E. Allan ◽  
John P. Clark
Keyword(s):  
Boundary Layer ◽  
Turbine Blade ◽  
Reynolds Numbers ◽  
Low Pressure ◽  
Blade Profile ◽  
Suction Surface ◽  
Low Reynolds Numbers ◽  
Low Pressure Turbine ◽  
Low Reynolds ◽  
Highly Loaded

Abstract Aircraft are operating at increasingly high-altitudes, where decreased air density and engine power settings have led to increasingly low Reynolds numbers in the low-pressure turbine portion of modern-day aeroengines. These operating conditions, in parallel with highly-loaded blade profiles, result in non-reattaching laminar boundary layer separation along the blade suction surface, increasing loss and decreasing engine performance. This work presents an experimental investigation into the potential for integrated leading-edge tubercles to improve blade performance in this operating regime. A turn-table cascade test-section was constructed and commissioned to test a purpose-designed, forward-loaded, low-pressure turbine blade profile at various incidences and Reynolds numbers. Baseline and tubercled blades were tested at axial chord Reynolds numbers at and between 15 000 and 60 000, and angles of incidence ranging from −5° to +10°. Experimental data collection included blade surface pressure measurements, total pressure loss in the blade wakes, hot-wire anemometry, surface hot-film measurements, and surface flow visualization using tufts. Test results showed that the implementation of tubercles did not lead to a performance enhancement. However, useful conclusions were drawn regarding the ability of tubercles to generate stream-wise vortices at ultra-low Reynolds numbers. Additional observations helped to characterize the suction surface boundary layer over the highly-loaded, low-pressure turbine blade profile when at off-design conditions. Recommendations were made for future work.

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.

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.

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

2010 ◽  
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 re-separate 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.

The Effect of Pulsed Blowing on the Boundary Layer of a Highly Loaded Low Pressure Turbine Blade

2013 ◽  
Author(s):  
Marion Mack ◽  
Roland Brachmanski ◽  
Reinhard Niehuis
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Mach Number ◽  
Suction Side ◽  
Reynolds Numbers ◽  
Low Pressure ◽  
Trailing Edge ◽  
Low Pressure Turbine ◽  
Wide Range ◽  
Highly Loaded

The performance of the low pressure turbine (LPT) can vary appreciably, because this component operates under a wide range of Reynolds numbers. At higher Reynolds numbers, mid and aft loaded profiles have the advantage that transition of suction side boundary layer happens further downstream than at front loaded profiles, resulting in lower profile loss. At lower Reynolds numbers, aft loading of the blade can mean that if a suction side separation exists, it may remain open up to the trailing edge. This is especially the case when blade lift is increased via increased pitch to chord ratio. There is a trend in research towards exploring the effect of coupling boundary layer control with highly loaded turbine blades, in order to maximize performance over the full relevant Reynolds number range. In an earlier work, pulsed blowing with fluidic oscillators was shown to be effective in reducing the extent of the separated flow region and to significantly decrease the profile losses caused by separation over a wide range of Reynolds numbers. These experiments were carried out in the High-Speed Cascade Wind Tunnel of the German Federal Armed Forces University Munich, Germany, which allows to capture the effects of pulsed blowing at engine relevant conditions. The assumed control mechanism was the triggering of boundary layer transition by excitation of the Tollmien-Schlichting waves. The current work aims to gain further insight into the effects of pulsed blowing. It investigates the effect of a highly efficient configuration of pulsed blowing at a frequency of 9.5 kHz on the boundary layer at a Reynolds number of 70000 and exit Mach number of 0.6. The boundary layer profiles were measured at five positions between peak Mach number and the trailing edge with hot wire anemometry and pneumatic probes. Experiments were conducted with and without actuation under steady as well as periodically unsteady inflow conditions. The results show the development of the boundary layer and its interaction with incoming wakes. It is shown that pulsed blowing accelerates transition over the separation bubble and drastically reduces the boundary layer thickness.

An Experimental Investigation of the Effect of Freestream Turbulence on the Wake of a Separated Low-Pressure Turbine Blade at Low Reynolds Numbers

1999 ◽  
Vol 122 (2) ◽  
pp. 431-433 ◽  
Author(s):  
C. G. Murawski ◽  
K. Vafai
Keyword(s):  
Reynolds Number ◽  
Turbine Blade ◽  
Turbulence Intensity ◽  
Reynolds Numbers ◽  
Low Pressure ◽  
Freestream Turbulence ◽  
Suction Surface ◽  
Low Reynolds Numbers ◽  
Low Pressure Turbine ◽  
Flow Reynolds Number

An experimental study was conducted in a two-dimensional linear cascade, focusing on the suction surface of a low pressure turbine blade. Flow Reynolds numbers, based on exit velocity and suction length, have been varied from 50,000 to 300,000. The freestream turbulence intensity was varied from 1.1 to 8.1 percent. Separation was observed at all test Reynolds numbers. Increasing the flow Reynolds number, without changing freestream turbulence, resulted in a rearward movement of the onset of separation and shrinkage of the separation zone. Increasing the freestream turbulence intensity, without changing Reynolds number, resulted in shrinkage of the separation region on the suction surface. The influences on the blade’s wake from altering freestream turbulence and Reynolds number are also documented. It is shown that width of the wake and velocity defect rise with a decrease in either turbulence level or chord Reynolds number. [S0098-2202(00)00202-9]

Parametric Optimization of Unsteady Endwall Blowing on a Highly Loaded LPT

2013 ◽  
Author(s):  
Stuart I. Benton ◽  
Chiara Bernardini ◽  
Jeffrey P. Bons ◽  
Rolf Sondergaard
Keyword(s):  
Large Scale ◽  
Boundary Layer Separation ◽  
Low Pressure ◽  
Suction Surface ◽  
Low Pressure Turbine ◽  
Pressure Distributions ◽  
Image Velocimetry ◽  
Stereo Particle Image Velocimetry ◽  
Turbine Airfoils ◽  
Highly Loaded

Efforts to reduce blade count and avoid boundary layer separation have led to low-pressure turbine airfoils with significant increases in loading as well as front-loaded pressure distributions. These features have been independently shown to increase losses within the secondary flow field at the endwall. Compound angle blowing from discrete jets on the blade suction surface near the endwall has been shown to be effective in reducing these increased losses and enabling the efficient use of highly loaded blade designs. In this study, experiments are performed on the front loaded L2F low-pressure turbine airfoil in a linear cascade. The required mass flow is reduced by decreasing hole count from previous configurations and from the introduction of unsteady blowing. The effects of pulsing frequency and duty cycle are investigated using phase-locked stereo particle image velocimetry to demonstrate the large scale movement and hysteresis behavior of the passage vortex interacting with the pulsed jets. Total pressure loss contours at the cascade outlet demonstrate that the efficiency benefit is maintained with the use of unsteady forcing.

Experimental Investigation of Total Pressure Loss Development in a Highly Loaded Low-Pressure Turbine Cascade

2017 ◽  
Vol 140 (3) ◽  
Author(s):  
Philip Bear ◽  
Mitch Wolff ◽  
Andreas Gross ◽  
Christopher R. Marks ◽  
Rolf Sondergaard
Keyword(s):  
Flow Field ◽  
Secondary Flow ◽  
Pressure Loss ◽  
Total Pressure ◽  
Low Pressure ◽  
Total Pressure Loss ◽  
Low Pressure Turbine ◽  
Pressure Transport ◽  
Secondary Flow Field ◽  
Highly Loaded

Improvements in turbine design methods have resulted in the development of blade profiles with both high lift and good Reynolds lapse characteristics. An increase in aerodynamic loading of blades in the low-pressure turbine (LPT) section of aircraft gas turbine engines has the potential to reduce engine weight or increase power extraction. Increased blade loading means larger pressure gradients and increased secondary losses near the endwall. Prior work has emphasized the importance of reducing these losses if highly loaded blades are to be utilized. The present study analyzes the secondary flow field of the front-loaded low-pressure turbine blade designated L2F with and without blade profile contouring at the junction of the blade and endwall. The current work explores the loss production mechanisms inside the LPT cascade. Stereoscopic particle image velocimetry (SPIV) data and total pressure loss data are used to describe the secondary flow field. The flow is analyzed in terms of total pressure loss, vorticity, Q-Criterion, turbulent kinetic energy, and turbulence production. The flow description is then expanded upon using an implicit large eddy simulation (ILES) of the flow field. The Reynolds-averaged Navier–Stokes (RANS) momentum equations contain terms with pressure derivatives. With some manipulation, these equations can be rearranged to form an equation for the change in total pressure along a streamline as a function of velocity only. After simplifying for the flow field in question, the equation can be interpreted as the total pressure transport along a streamline. A comparison of the total pressure transport calculated from the velocity components and the total pressure loss is presented and discussed. Peak values of total pressure transport overlap peak values of total pressure loss through and downstream of the passage suggesting that the total pressure transport is a useful tool for localizing and predicting loss origins and loss development using velocity data which can be obtained nonintrusively.

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