scholarly journals Reynolds, Mach and free-stream turbulence effects on the flow in a low-pressure turbine

2021 ◽  
pp. 1-17
Author(s):  
Maxime Fiore ◽  
Nicolas Gourdain
Keyword(s):  
Reynolds Number ◽  
Mach Number ◽  
Free Stream ◽  
Guide Vane ◽  
Flow Behavior ◽  
Suction Side ◽  
Boundary Layer Separation ◽  
Low Pressure ◽  
Free Stream Turbulence ◽  
Low Pressure Turbine

Abstract This paper presents the Large Eddy Simulation of a Low-Pressure Turbine Nozzle Guide Vane for different Reynolds (Re) and Mach numbers (Ma) with or without inlet turbulence prescribed. The analysis is based on a slice of a LPT blading representative of a midspan flow. The characteristic Re of the LPT can vary by a factor of four between take-off and cruise conditions. In addition, the LPT operates at different Ma and the incident flow can have significant levels of turbulence due to upstream blade wakes. The paper investigates numerically using LES the flow around a LPT blading with three different Reynolds number Re = 175'000 (cruise), 280'000 (mid-level altitude) and 500'000 (take-off) keeping the same characteristic Mach number Ma = 0.2 and three different Mach number Ma = 0.2, 0.5 and 0.8 keeping the same Reynolds number Re= 280'000. These different simulations are performed with 0% Free Stream Turbulence (FST) followed by inlet turbulence (6% FST). The study focuses on different flow characteristics: pressure distribution around the blade, near-wall flow behavior, loss generation and Turbulent Kinetic Energy budget. The results show an earlier boundary layer separation on the aft of the blade suction side when the Re is increased while the free-stream turbulence delays separation. The TKE budget shows the predominant effect of the turbulent production and diffusion in the wake, the axial evolution of these different terms being relatively insensitive to Re and Ma.

Reynolds and Mach Number Effects on the Flow in a Low-Pressure Turbine

2020 ◽  
Author(s):  
Maxime Fiore
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Mach Number ◽  
Kinetic Energy ◽  
Gas Turbine ◽  
Guide Vane ◽  
Flow Behavior ◽  
Low Pressure ◽  
Secondary Flows ◽  
Low Pressure Turbine

Abstract This paper presents the Large Eddy Simulation (LES) of a Low-Pressure Turbine (LPT) Nozzle Guide Vane (NGV) for different Reynolds (Re) and Mach number (Ma). The analysis is based on a slice of the blade that may be representative of midspan flow where secondary flows, hub and shroud contributions are lower. In LPT, the variation of the Re during the mission of the gas turbine is a well-known effect since its value can vary of a factor four between take-off and cruise. This can induce performance variations due to various phenomena with among them suction side boundary layer separation on the aft portion of the blade due to an adverse pressure gradient and laminar boundary layer that can be maintained due to the relatively low Re in LPT. Similarly, the Ma in the LPT may vary depending on the thrust required from the gas turbine at the considered mission phase. The current paper investigates through numerical simulation the flow representative of a medium-sized LPT with three different Reynolds number Re = 175’000 (cruise), 280’000 (mid-level altitude) and 500’000 (take-off) keeping the same characteristic Mach number Ma = 0.2 and three different Mach number Ma = 0.2, 0.5 and 0.8 keeping the same Reynolds number Re = 280’000. The study focuses on different flow characteristics: pressure distribution around the blade, near-wall flow behavior and wake analysis. This includes the related generation of losses and the effect of Re and Ma on these different phenomena. A special emphasis is given to the generation of loss based on an entropy approach and the redistribution of mean kinetic energy towards turbulent kinetic energy. The results show that the increase of the Re has a destabilizing effect on potential separation while the increase of the Ma has a stabilizing effect. The peak in the TKE downstream of the blade is also moved upstream closer to the trailing edge when the Ma is increased.

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.

Separated Flow Transition in a Low Pressure Turbine Cascade: Mean Flow and Turbulence Spectra

2003 ◽  
Author(s):  
Ralph J. Volino ◽  
Christopher G. Murawski
Keyword(s):  
Boundary Layer ◽  
Free Stream ◽  
Separation Bubble ◽  
Suction Side ◽  
Low Pressure ◽  
Turbine Cascade ◽  
Mean Flow ◽  
Turbulence Spectra ◽  
Free Stream Turbulence ◽  
Low Pressure Turbine

Boundary layer separation, transition and reattachment have been studied experimentally in a low-pressure turbine cascade. Cases with Reynolds numbers (Re) ranging from 50,000 to 200,000 (based on suction surface length and exit velocity) have been considered under low free-stream turbulence conditions. Mean and fluctuating velocity profiles and turbulence spectra are presented for streamwise locations along the suction side of one airfoil and in the wake downstream of the airfoils. Hot film gages on the suction side surface of the airfoil are used to measure the fluctuation level and the spectra of the fluctuations on the surface. Higher Re moves transition upstream. Transition is initiated in the shear layer over the separation bubble and leads to boundary layer reattachment. Peak frequencies in the boundary layer spectra match those found in similar cases in the literature, indicating that the important frequencies may be predictable. Spectra in the wake downstream of the airfoils were similar to the spectra in the boundary layer near the trailing edge of the airfoil. Comparisons to the literature indicate that small but measurable differences in the spectra of the low free-stream turbulence can have a significant effect on boundary layer reattachment.

Separated Flow Transition Under Simulated Low-Pressure Turbine Airfoil Conditions—Part 2: Turbulence Spectra

2002 ◽  
Vol 124 (4) ◽  
pp. 656-664 ◽  
Author(s):  
Ralph J. Volino
Keyword(s):  
Shear Stress ◽  
Free Stream ◽  
Boundary Layer Separation ◽  
Streamwise Velocity ◽  
Low Pressure ◽  
Turbulent Shear ◽  
Suction Surface ◽  
Free Stream Turbulence ◽  
Low Pressure Turbine ◽  
Turbulent Shear Stress

Spectral analysis was used to investigate boundary layer separation, transition and reattachment under low-pressure turbine airfoil conditions. Cases with Reynolds numbers ranging from 25,000 to 300,000 (based on suction surface length and exit velocity) have been considered at low (0.5%) and high (9% inlet) free-stream turbulence levels. Spectra of the fluctuating streamwise velocity and the turbulent shear stress are presented. The spectra for the low free-stream turbulence cases are characterized by sharp peaks. The high free-stream turbulence case spectra exhibit more broadband peaks, but these peaks are centered at the same frequencies observed in the corresponding low turbulence cases. The frequencies of the peaks suggest that a Tollmien-Schlichting instability mechanism drives transition, even in the high turbulence cases. The turbulent shear stress spectra proved particularly valuable for detection of the early growth of the instability. The predictable nature of the instability may prove useful for future flow control work.

scholarly journals Measurements in Separated and Transitional Boundary Layers Under Low-Pressure Turbine Airfoil Conditions

2000 ◽  
Vol 123 (2) ◽  
pp. 189-197 ◽  
Author(s):  
Ralph J. Volino ◽  
Lennart S. Hultgren
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Pressure Gradient ◽  
Boundary Layers ◽  
Free Stream ◽  
Separation Bubble ◽  
Low Pressure ◽  
Mean Velocity ◽  
Free Stream Turbulence ◽  
Low Pressure Turbine

Detailed velocity measurements were made along a flat plate subject to the same dimensionless pressure gradient as the suction side of a modern low-pressure turbine airfoil. Reynolds numbers based on wetted plate length and nominal exit velocity were varied from 50,000 to 300,000, covering cruise to takeoff conditions. Low and high inlet free-stream turbulence intensities (0.2 and 7 percent) were set using passive grids. The location of boundary-layer separation does not depend strongly on the free-stream turbulence level or Reynolds number, as long as the boundary layer remains nonturbulent prior to separation. Strong acceleration prevents transition on the upstream part of the plate in all cases. Both free-stream turbulence and Reynolds number have strong effects on transition in the adverse pressure gradient region. Under low free-stream turbulence conditions, transition is induced by instability waves in the shear layer of the separation bubble. Reattachment generally occurs at the transition start. At Re=50,000 the separation bubble does not close before the trailing edge of the modeled airfoil. At higher Re, transition moves upstream, and the boundary layer reattaches. With high free-stream turbulence levels, transition appears to occur in a bypass mode, similar to that in attached boundary layers. Transition moves upstream, resulting in shorter separation regions. At Re above 200,000, transition begins before separation. Mean velocity, turbulence, and intermittency profiles are presented.

scholarly journals Measurements in Separated and Transitional Boundary Layers Under Low-Pressure Turbine Airfoil Conditions

2000 ◽  
Author(s):  
Ralph J. Volino ◽  
Lennart S. Hultgren
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Pressure Gradient ◽  
Boundary Layers ◽  
Free Stream ◽  
Separation Bubble ◽  
Low Pressure ◽  
Mean Velocity ◽  
Free Stream Turbulence ◽  
Low Pressure Turbine

Detailed velocity measurements were made along a flat plate subject to the same dimensionless pressure gradient as the suction side of a modern low-pressure turbine airfoil. Reynolds numbers based on wetted plate length and nominal exit velocity were varied from 50, 000 to 300, 000, covering cruise to takeoff conditions. Low and high inlet free-stream turbulence intensities (0.2% and 7%) were set using passive grids. The location of boundary-layer separation does not depend strongly on the free-stream turbulence level or Reynolds number, as long as the boundary layer remains non-turbulent prior to separation. Strong acceleration prevents transition on the upstream part of the plate in all cases. Both free-stream turbulence and Reynolds number have strong effects on transition in the adverse pressure gradient region. Under low free-stream turbulence conditions transition is induced by instability waves in the shear layer of the separation bubble. Reattachment generally occurs at the transition start. At Re = 50, 000 the separation bubble does not close before the trailing edge of the modeled airfoil. At higher Re, transition moves upstream, and the boundary layer reattaches. With high free-stream turbulence levels, transition appears to occur in a bypass mode, similar to that in attached boundary layers. Transition moves upstream, resulting in shorter separation regions. At Re above 200,000, transition begins before separation. Mean velocity, turbulence and intermittency profiles are presented.

Improvement of Laminar-Turbulent Transition Modeling Within a Low-Pressure Turbine

2016 ◽  
Author(s):  
A. Minot ◽  
I. Salah El-Din ◽  
R. Barrier ◽  
J.-C. Boniface ◽  
J. Marty
Keyword(s):  
Reynolds Number ◽  
Mach Number ◽  
Separation Bubble ◽  
Suction Side ◽  
Low Pressure ◽  
Boundary Layer Transition ◽  
Flow Angle ◽  
Low Pressure Turbine ◽  
Turbulent Transition ◽  
Laminar Turbulent Transition

The flow within turbomachines is intrinsically complex and involves boundary layer transition, separation and vortices such as the tip leakage vortex and wakes. In a low-pressure turbine, as the Reynolds number can be small, the flow over the suction side is likely to separate leading to the formation of a laminar (or transitional) separation bubble. This flow mechanism can be predicted using Large-Eddy Simulation. However the computation is still very expensive in a design framework. Thus, Reynolds-Averaged Navier-Stokes (RANS) method is used in the present investigation to simulate the flow over the low-pressure turbine airfoil T106C. The laminar-turbulent transition is modeled with the γ-Rθt~ model of Menter and Langtry. Following the work of Minot et al. in which the CFD setup was deeply investigated, the present study aims at evaluating the sensitivity to uncertainties relative to experimental values (freestream turbulence, Reynolds number, incidence flow angle and exit isentropic Mach number) and at improving this model regarding the calibration of several functions using optimization process. The uncertainty study highlights the parameters which mainly influence the isentropic Mach number and loss distributions. The new calibration of the Menter-Langtry model improves significantly the flow prediction over the suction side, except for the open bubble configuration.

Separated Flow Transition Under Simulated Low-Pressure Turbine Airfoil Conditions: Part 2 — Turbulence Spectra

2002 ◽  
Author(s):  
Ralph J. Volino
Keyword(s):  
Shear Stress ◽  
Free Stream ◽  
Boundary Layer Separation ◽  
Streamwise Velocity ◽  
Low Pressure ◽  
Turbulent Shear ◽  
Suction Surface ◽  
Free Stream Turbulence ◽  
Low Pressure Turbine ◽  
Turbulent Shear Stress

Spectral analysis was used to investigate boundary layer separation, transition and reattachment under low-pressure turbine airfoil conditions. Cases with Reynolds numbers ranging from 25,000 to 300,000 (based on suction surface length and exit velocity) have been considered at low (0.5%) and high (9% inlet) free-stream turbulence levels. Spectra of the fluctuating streamwise velocity and the turbulent shear stress are presented. The spectra for the low free-stream turbulence cases are characterized by sharp peaks. The high free-stream turbulence case spectra exhibit more broadband peaks, but these peaks are centered at the same frequencies observed in the corresponding low turbulence cases. The frequencies of the peaks suggest that a Tollmien-Schlichting instability mechanism drives transition, even in the high turbulence cases. The turbulent shear stress spectra proved particularly valuable for detection of the early growth of the instability. The predictable nature of the instability may prove useful for future flow control work.

Transition Mechanisms in Laminar Separated Flow Under Simulated Low Pressure Turbine Aerofoil Conditions

2012 ◽  
Vol 135 (1) ◽  
Author(s):  
Jerrit Dähnert ◽  
Christoph Lyko ◽  
Dieter Peitsch
Keyword(s):  
Reynolds Number ◽  
Separation Bubble ◽  
Separated Flow ◽  
Low Pressure ◽  
Main Flow ◽  
Test Facility ◽  
Flow Conditions ◽  
Laminar Separation ◽  
Free Stream Turbulence ◽  
Low Pressure Turbine

Based on detailed experimental work conducted at a low speed test facility, this paper describes the transition process in the presence of a separation bubble with low Reynolds number, low free-stream turbulence, and steady main flow conditions. A pressure distribution has been created on a long flat plate by means of a contoured wall opposite of the plate, matching the suction side of a modern low-pressure turbine aerofoil. The main flow conditions for four Reynolds numbers, based on suction surface length and nominal exit velocity, were varied from 80,000 to 300,000, which covers the typical range of flight conditions. Velocity profiles and the overall flow field were acquired in the boundary layer at several streamwise locations using hot-wire anemometry. The data given is in the form of contours for velocity, turbulence intensity, and turbulent intermittency. The results highlight the effects of Reynolds number, the mechanisms of separation, transition, and reattachment, which feature laminar separation-long bubble and laminar separation-short bubble modes. For each Reynolds number, the onset of transition, the transition length, and the general characteristics of separated flow are determined. These findings are compared to the measurement results found in the literature. Furthermore, the experimental data is compared with two categories of correlation functions also given in the literature: (1) correlations predicting the onset of transition and (2) correlations predicting the mode of separated flow transition. Moreover, it is shown that the type of instability involved corresponds to the inviscid Kelvin-Helmholtz instability mode at a dominant frequency that is in agreement with the typical ranges occurring in published studies of separated and free-shear layers.

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