Using Turbulence Intensity and Reynolds Number to Predict Flow Separation on a Highly Loaded, Low-Pressure Gas Turbine Blade at Low Reynolds Numbers

2018 ◽  
Author(s):  
Kenneth Van Treuren ◽  
Tyler Pharris ◽  
Olivia Hirst
Keyword(s):  
Reynolds Number ◽  
Gas Turbine ◽  
Turbine Blade ◽  
Turbulence Intensity ◽  
Flow Separation ◽  
Reynolds Numbers ◽  
Low Pressure ◽  
High Altitudes ◽  
Low Reynolds Numbers ◽  
Low Pressure Turbine

The low-pressure turbine has become more important in the last few decades because of the increased emphasis on higher overall pressure and bypass ratios. The desire is to increase blade loading to reduce blade counts and stages in the low-pressure turbine of a gas turbine engine. Increased turbine inlet temperatures for newer cycles results in higher temperatures in the low-pressure turbine, especially the latter stages, where cooling technologies are not used. These higher temperatures lead to higher work from the turbine and this, combined with the high loadings, can lead to flow separation. Separation is more likely in engines operating at high altitudes and reduced throttle setting. At the high Reynolds numbers found at takeoff, the flow over a low-pressure turbine blade tends to stay attached. At lower blade Reynolds numbers (25,000 to 200,000), found during cruise at high altitudes, the flow on the suction surface of the low-pressure turbine blades is inclined to separate. This paper is a study on the flow characteristics of the L1A turbine blade at three low Reynolds numbers (60,000, 108,000, and 165,000) and 15 turbulence intensities (1.89% to 19.87%) in a steady flow cascade wind tunnel. With this data, it is possible to examine the impact of Reynolds number and turbulence intensity on the location of the initiation of flow separation, the flow separation zone, and the reattachment location. Quantifying the change in separated flow as a result of varying Reynolds numbers and turbulence intensities will help to characterize the low momentum flow environments in which the low-pressure turbine must operate and how this might impact the operation of the engine. Based on the data presented, it is possible to predict the location and size of the separation as a function of both the Reynolds number and upstream freestream turbulence intensity (FSTI). Being able to predict this flow behavior can lead to more effective blade designs using either passive or active flow control to reduce or eliminate flow separation.

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]

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.

Analysis of Laminar-Turbulent Transition of a Low-Loss Generic Low Pressure Turbine Distribution

2015 ◽  
Author(s):  
Christian Brück ◽  
Christoph Lyko ◽  
Dieter Peitsch ◽  
Christoph Bode ◽  
Jens Friedrichs ◽  
...  
Keyword(s):  
Reynolds Number ◽  
Wind Tunnel ◽  
Pressure Distribution ◽  
Turbulence Intensity ◽  
Suction Side ◽  
Reynolds Numbers ◽  
Low Pressure ◽  
Test Case ◽  
Low Loss ◽  
Low Pressure Turbine

The efficiency of modern Turbofan engines can be significantly increased by using a gearbox between compressor and turbine of the low pressure section. Rotational speed of the low pressure turbine (LPT) in a Geared Turbofan is much higher than in normal LPT’s which lead to necessary adjustments in blade design. This work has investigated the transition behavior of a modified profile geometry for low-loss at engine cruise conditions. Typical LPT conditions have thus been chosen as baseline for the experimental work. A pressure distribution has been created on a flat plate by means of contoured walls in a low speed wind tunnel. The paper will analyze the experimental results and show additionally the numerical predictions of the test case. The experimental part of this paper describe how the blade was Mach number scaled to obtain the geometry of the wind tunnel wall contour. The pressure distribution for the incompressible test case show a very good agreement to the compressible case. Boundary layer (BL) measurements with hot-wire-anemometry have been performed at high spatial resolution under a freestream turbulence of almost 8%. Different Reynolds numbers have been investigated and will be compared with special attention being paid to the transition on the suction side by contour plots (turbulence levels, turbulent intermittency) and integral BL parameters. It was found that the transition on the suction side is not completed for small Reynolds numbers but takes place at higher velocities. In the numerical part studies by means of steady RANS simulations with k-ω – SST turbulence model and γ-Reθ transition model have been conducted. The aim is to validate the RANS solver for the low-loss LPT application. Hence, comparison is made to the measured data and the transitional behavior of the BL. Furthermore, additional parameter variations have been conducted (turbulence intensity and Reynolds number). The numerical investigations show partially a good comparison for the BL development indicating the different transition modi with increasing Reynolds number and turbulence intensity.

Effects of Flow Separation on a Highly Loaded, Low-Pressure Gas Turbine Blade at Low Reynolds Numbers

2015 ◽  
Author(s):  
Tyler M. Pharris ◽  
Olivia E. Hirst ◽  
Kenneth W. Van Treuren
Keyword(s):  
Wind Tunnel ◽  
Gas Turbine ◽  
Flow Separation ◽  
Separation Bubble ◽  
Reynolds Numbers ◽  
Low Pressure ◽  
Low Flow ◽  
Turbine Wheel ◽  
Low Pressure Turbine ◽  
Low Reynolds

Current gas turbine engines experience a loss in performance due to the low Reynolds number flow in the low-pressure turbine. This low flow speed can result in separation of the air from the blade surface, reducing the efficiency of the engine. The Baylor University Cascade wind tunnel (BUC) is being used to study this flow separation. A cascade wind tunnel contains a row of turbine vanes that simulates a turbine wheel. The BUC is capable of simulating the environment seen by the low-pressure turbine at high altitudes by producing Reynolds numbers varying from 25,000 to 400,000. The L1A blade profile is currently being tested. Coefficient of pressure (Cp) plots show a less than 1% difference between surface pressure locations when comparing the most inboard and outboard test blades. This agreement demonstrates the flow uniformity in the tunnel. Cp plots also compared favorably to the literature, validating the BUC operation and providing insight into how Reynolds numbers and free stream turbulence intensity (FSTI) affect flow separation. The literature and this study showed the size and reattachment of the separation bubble was highly dependent on the FSTI for lower Reynolds numbers (25,000 to 200,000). This comparison also showed that the size of the separation bubble and the location was not heavily impacted by FSTI for Reynolds numbers above 200,000. Tests in the future will be conducted to determine the actual FSTI of the BUC. Once completely validated, future studies with the BUC may include use of particle image velocimetry (PIV) to visualize the flow, a gold foil steady state technique using liquid crystals to measure heat transfer, and a series of deposition tests using surface roughness (sandpaper or textured sprays) to measure performance loss under these conditions. The ultimate goal of this research is to improve blade design in the low pressure turbine for all commercial and military aircraft.

Experimental Study of the Unsteady Aerodynamics in a Linear Cascade With Low Reynolds Number Low Pressure Turbine Blades

1997 ◽  
Author(s):  
Christopher G. Murawski ◽  
Rolf Sondergaard ◽  
Richard B. Rivir ◽  
Kambiz Vafai ◽  
Terrence W. Simon ◽  
...  
Keyword(s):  
Reynolds Number ◽  
Turbulence Intensity ◽  
Turbine Blades ◽  
Reynolds Numbers ◽  
Low Pressure ◽  
Freestream Turbulence ◽  
Suction Surface ◽  
Linear Cascade ◽  
Low Pressure Turbine ◽  
Flow Reynolds Number

Low pressure turbines in aircraft experience large changes in flow Reynolds number as the gas turbine engine operates from takeoff to high altitude cruise. Low pressure turbine blades are also subject to regions of strong acceleration and diffusion. These changes in Reynolds number, strong acceleration, as well as elevated levels of turbulence can result in unsteady separation and transition zones on the surface of the blade. An experimental study was conducted in a two-dimensional linear cascade, focusing on the suction surface of a low pressure turbine blade. The intent was to assess the effects of changes in Reynolds number, and freestream turbulence intensity. Flow Reynolds numbers, based on exit velocity and suction surface 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 slightly rearward movement of the onset of separation and shrinkage of the separation zone. Increasing the freestream turbulence intensity, without changing Reynolds number resulted in a shrinkage of the separation region on the suction surface. Increasing both flow Reynolds numbers and freestream turbulence intensity compounded these effects such that at a Reynolds number of 300,000 and a freestream turbulence intensity of 8.1%, the separation zone was almost nonexistent. The influences on the blade’s wake from altering freestream turbulence and Reynolds number are also documented. The width of the wake and velocity defect rise with a decrease in either turbulence level or chord Reynolds number. Numerical simulations were performed in support of experimental results. The numerical results compare well qualitatively with the low freestream turbulence experimental cases.

scholarly journals Aerodynamic Characteristics of Different Airfoils under Varied Turbulence Intensities at Low Reynolds Numbers

2020 ◽  
Vol 10 (5) ◽  
pp. 1706 ◽  
Author(s):  
Yang Zhang ◽  
Zhou Zhou ◽  
Kelei Wang ◽  
Xu Li
Keyword(s):  
Reynolds Number ◽  
Turbulence Intensity ◽  
Flow Separation ◽  
Numerical Study ◽  
Reynolds Numbers ◽  
Jet Flow ◽  
Aerodynamic Characteristics ◽  
Aerodynamic Performance ◽  
Low Reynolds Numbers ◽  
Low Reynolds

A numerical study was conducted on the influence of turbulence intensity and Reynolds number on the mean topology and transition characteristics of flow separation to provide better understanding of the unsteady jet flow of turboelectric distributed propulsion (TeDP) aircraft. By solving unsteady Reynolds averaged Navier-Stokes (URANS) equation based on C-type structural mesh and γ - Re ˜ θ t transition model, the aerodynamic characteristics of the NACA0012 airfoil at different turbulence intensities was calculated and compared with the experimental results, which verifies the reliability of the numerical method. Then, the effects of varied low Reynolds numbers and turbulence intensities on the aerodynamic performance of NACA0012 and SD7037 were investigated. The results show that higher turbulence intensity or Reynolds number leads to more stable airfoil aerodynamic performance, larger stalling angle, and earlier transition with a different mechanism. The generation and evolution of the laminar separation bubble (LSB) are closely related to Reynolds number, and it would change the effective shape of the airfoil, having a big influence on the airfoil’s aerodynamic characteristics. Compared with the symmetrical airfoil, the low-Reynolds-number airfoil can delay the occurrence of flow separation and produce more lift in the same conditions, which provides guidance for further airfoil design under TeDP jet flow.

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.

Sign in / Sign up

Export Citation Format

Share Document