The Development of Axial Turbine Leakage Loss for Two Profiled Tip Geometries Using Linear Cascade Data

1992 ◽  
Vol 114 (1) ◽  
pp. 198-203 ◽  
Author(s):  
J. P. Bindon ◽  
G. Morphis
Keyword(s):  
Discharge Coefficient ◽  
Separation Bubble ◽  
Tip Clearance ◽  
Loss Reduction ◽  
Analysis Method ◽  
Leakage Loss ◽  
Linear Cascade ◽  
Blade Tip ◽  
Loss Coefficients ◽  
Internal Gap

To assess the possibility of tip clearance loss reduction and to explore the nature and origin of tip clearance loss, blade tip geometries that reduce the roughly 40 percent of total loss occurring within the gap were studied. The shapes investigated aimed at reducing or avoiding the gap separation bubble thought to contribute significantly to both internal gap loss and to the endwall mixing loss. It was found that radiusing and contouring the blade at gap inlet eliminated the separation bubble and reduced the internal gap loss but created a higher mixing loss to give almost unchanged overall loss coefficients when compared with the simple sharp-edged flat-tipped blade. The separation bubble does not therefore appear to influence the mixing loss. Using a method of assessing linear cascade experimental data as though it were a rotor with work transfer, one radiused geometry, contoured to shed radial flow into the gap and reduce the leakage mass flow, was found to have a significantly higher efficiency. This demonstrates the effectiveness of the data analysis method and that cascade loss coefficient alone or gap discharge coefficient cannot be used to evaluate tip clearance performance accurately. Contouring may ultimately lead to better rotor blade performances.

scholarly journals The Development of Axial Turbine Leakage Loss for Two Profiled Tip Geometries Using Linear Cascade Data

1990 ◽  
Author(s):  
Jeffrey P. Bindon ◽  
George Morphis
Keyword(s):  
Discharge Coefficient ◽  
Separation Bubble ◽  
Tip Clearance ◽  
Loss Reduction ◽  
Analysis Method ◽  
Leakage Loss ◽  
Linear Cascade ◽  
Blade Tip ◽  
Loss Coefficients ◽  
Internal Gap

To assess the possibility of tip clearance loss reduction and to explore the nature and origin of tip clearance loss, blade tip geometries which reduce the roughly 40% of total loss occurring within the gap were studied. The shapes investigated aimed at reducing or avoiding the gap separation bubble thought to contribute significantly to both internal gap loss and to the endwall mixing loss. It was found that radiusing and contouring the blade at gap inlet eliminated the separation bubble and reduced the internal gap loss but created a higher mixing loss to give almost unchanged overall loss coefficients when compared with the simple sharp edged flat tipped blade. The separation bubble does not therefore appear to influence the mixing loss. Using a method of assessing linear cascade experimental data as though it were a rotor with work transfer, one radiused geometry, contoured to shed radial flow into the gap and reduce the leakage mass flow, was found to have a significantly higher efficiency. This demonstrates the effectiveness of the data analysis method and that cascade loss coefficient alone or gap discharge coefficient cannot be used to accurately evaluate tip clearance performance. Contouring may ultimately lead to better rotor blade performances.

Blade Tip Leakage Loss Reduction by Means of Passive Tip Injection: Linear Cascade Wind Tunnel Results

2017 ◽  
Author(s):  
Jonas Rejek ◽  
Stefan aus der Wiesche ◽  
Reinhard Willinger
Keyword(s):  
Wind Tunnel ◽  
Turbine Blade ◽  
Working Fluid ◽  
Slight Reduction ◽  
Loss Reduction ◽  
Leakage Loss ◽  
Linear Cascade ◽  
Tip Leakage ◽  
Blade Tip ◽  
Tip Injection

In the open literature, an innovative concept for turbine blade tip leakage loss reduction by means of passive tip injection was recently proposed. The present paper presents experimental results obtained for an unshrouded turbine blade corresponding to a 50 % reaction stage. The experiments were performed in a low-speed linear cascade wind tunnel facility with air as working fluid. The effect of passive tip injection on the resulting loss was investigated by detailed five-hole-probe measurements. Cascades with three different tip gap heights and blades with and without passive injection were considered. Special attention was spent to the actual upstream conditions. The detailed flow field measurements showed that at the blade tip exit the leakage flow merged with the main flow and rolled up to a tip leakage vortex. The linear cascade wind tunnel results indicated a slight reduction of the resulting total pressure loss coefficient due to the passive tip injection. The observed tip leakage loss reduction was well comparable with the predictions of simplified analytical model.

The Measurement and Formation of Tip Clearance Loss

1989 ◽  
Vol 111 (3) ◽  
pp. 257-263 ◽  
Author(s):  
J. P. Bindon
Keyword(s):  
Pressure Gradient ◽  
Secondary Flow ◽  
Separation Bubble ◽  
Tip Clearance ◽  
Turbine Cascade ◽  
High Loss ◽  
Performance Improvements ◽  
Internal Loss ◽  
Internal Gap ◽  
First Time

The detailed development of tip clearance loss from the leading to trailing edge of a linear turbine cascade was measured and the contributions made by mixing, internal gap shear flow, and endwall/ secondary flow were identified, separated, and quantified for the first time. Only 13 percent of the overall loss arises from endwall/secondary flow and of the remaining 87 percent, 48 percent is due to mixing and 39 percent is due to internal gap shear. All loss formation appears to be dominated by phenomena connected with the gap separation bubble. Flow established within the bubble by the pressure gradient separates as the gradient disappears and most of the internal loss is created by the entrainment of this separated fluid. When this high-loss leakage wake enters the mainstream, it separates due to the suction corner pressure gradient to create virtually all the measured mixing loss. It is suggested that the control of tip clearance loss by discharge coefficient reduction actually introduces loss. Performance improvements may result from streamlined tip geometries that optimize the tradeoff between entropy production and flow deflection.

Effect of Endwall Motion on Blade Tip Heat Transfer

2003 ◽  
Vol 125 (2) ◽  
pp. 267-273 ◽  
Author(s):  
V. Srinivasan ◽  
R. J. Goldstein
Keyword(s):  
Mass Transfer ◽  
Suction Side ◽  
Tip Clearance ◽  
Pressure Gradients ◽  
Tip Leakage Flow ◽  
Blade Loading ◽  
Linear Cascade ◽  
Tip Leakage ◽  
Blade Tip ◽  
Wind Tunnel Data

Local mass transfer measurements were conducted on the tip of a turbine blade in a five-blade linear cascade with a blade-centered configuration. The tip clearance levels ranged from 0.6 to 6.9% of blade chord. The effect of relative motion between the casing and the blade tip was simulated using a moving endwall made of neoprene mounted on the top of the wind tunnel. Data were obtained for a single Reynolds number of 2.7×105 based on cascade exit velocity and blade chord. Pressure measurements indicate that the effect of endwall motion on blade loading at a clearance of 0.6% of blade chord is to reduce the pressure gradients driving the tip leakage flow. With the introduction of endwall motion, there is a reduction of about 9% in mass transfer levels at a clearance of 0.6% of chord. This is presumably due to the tip leakage vortex coming closer to the suction side of the blade and ‘blocking the flow,’ leading to reduced tip gap velocities and hence lower mass transfer.

High Coverage Blade Tip Film Cooling

2004 ◽  
Author(s):  
M. Kuwabara ◽  
Keizo Tsukagoshi ◽  
T. Arts
Keyword(s):  
Gas Turbine ◽  
Turbine Blade ◽  
Film Cooling ◽  
Gas Turbines ◽  
Separation Bubble ◽  
Density Ratio ◽  
Tip Clearance ◽  
Experimental Program ◽  
Film Cooling Effectiveness ◽  
Blade Tip

More sophisticated cooling schemes are required for the turbine blade due to the demand of increased turbine temperature for improved performance. Although the tip portion of a turbine blade is one of the most critical portions in a gas turbine, there are few studies on cooling this portion compared to those for airfoil, especially film cooling strategies. Industrial gas turbines have a more uniform gas temperature profile than aero engines. For these applications, it is more important to understand the characteristics of tip film cooling to improve the blade durability and gas turbine performance by reducing cooling air. A numerical and experimental program was initiated to study film cooling effectiveness on a flat blade tip as a function of tip gap and mass flux ratios. Flow visualization tests were conducted with and without film cooling to verify the numerical CFD findings. The predictions and visualization results showed that a separation bubble forms at the pressure side edge that increases with tip gap. Film effectiveness measurements were carried out on a 1.3X scale blade model in a low speed test while simulating the normalized pressure distribution typical of an engine design. The engine density ratio of the coolant to mainstream was replicated in the film cooling tests to provide the best simulation of the engine. Two rows of holes were placed near the tip of the blade to provide high film coverage prior to the flowing over the tip. The data shows that film effectiveness increases with decreasing tip clearance. Blowing ratio provides an improvement due to the added mass flow, which was shown by a non-dimensionalized correlation.

The Effect of the Casing Movement Relative to the Blades on the Tip Leakage Loss in Axial Flow Compressors

2011 ◽  
Author(s):  
Guillaume Pallot ◽  
Dai Kato ◽  
Hidekazu Kodama ◽  
Kazunari Matsuda ◽  
Hideo Taniguchi ◽  
...  
Keyword(s):  
Axial Flow ◽  
Tip Clearance ◽  
Experimental Results ◽  
Test Facility ◽  
Leakage Loss ◽  
Suction Surface ◽  
Linear Cascade ◽  
High Entropy ◽  
Tip Leakage ◽  
Creation Rate

This paper investigates the effect of the casing movement relative to the blades on the tip leakage loss generation mechanisms by using experimental results from a linear cascade test facility, and viscous numerical results. Traverse measurements in the pitch-wise and span-wise directions are made using a five-hole Pitot tube at the inlet and exit planes of a compressor linear cascade comprising seven equally-pitched blades. The blades are two-dimensionally stacked with a cross section representing a typical rear stage rotor of a highly loaded axial-flow compressor. A moving belt, driven by a motor and a pulley system, runs linearly at constant speed under the horizontally suspended cascade to simulate the relative motion of the blade and the casing. Tip clearance can be adjusted by changing the height of the blades. The experimental results, at 2% and 4% tip clearance to blade heights, indicate that the tip leakage loss decreases when the casing is in movement. The Reynolds-averaged Navier-Stokes numerical calculations with Spalart-Almaras turbulence closure model, run with the experimental boundary conditions, agree well with the test data, especially in terms of dependencies of the leakage loss magnitude on the relative movement between the blade and the casing. It is interesting that, contrary to the tendency in the leakage loss to decrease, the computed tip leakage mass flow rate increases with moving endwall. The computations show two distinct regions of high entropy creation rate near the blade tip. The first one is located close to the blade suction surface where the leakage flow leaves the clearance gap. The second one is located further from the suction surface and the entropy creation rate in this region decreases when the casing is in movement. This paper attempts to provide a qualitative analysis of the flow mechanisms involved in the entropy generation in the second regions. Finally Computations of a high loaded rotor show that the second region identified in the static cascade may also be present in the case of rotating cascades.

scholarly journals Validation Studies of Linear Oscillating Compressor Cascade and Use of Influence Coefficient Method

2020 ◽  
Vol 142 (5) ◽  
Author(s):  
H. M. Phan ◽  
L. He
Keyword(s):  
Boundary Layer ◽  
Tip Clearance ◽  
Influence Coefficient ◽  
Far Field ◽  
Compressor Cascade ◽  
Linear Cascade ◽  
Influence Coefficient Method ◽  
Blade Tip ◽  
Coefficient Method ◽  
Blade Tip Clearance

Abstract Advanced predictions of blade flutter have been continually pursued. It is noted however that validation cases of unsteady CFD methods against experimental cases with detailed 3D unsteady pressures are still rather lacking. The main objectives of the present work are two-folds. First, validate and understand the characteristics of blade tip clearance, as well as a bubble-type flow separation for an unsteady CFD solver against a 3D oscillating cascade experiment. And second, examine the applicability of the influence coefficient method (ICM) as widely used in an oscillating linear cascade setup. In the first part, the capability of a widely used commercial solver (CFX) for unsteady flows induced by a 3D oscillating compressor cascade is examined. The present computations have shown consistently a destabilizing effect of increasing blade tip clearance, in agreement with the experiment. More remarkably, the computational analyses reveal a distinctive interplay between the inlet endwall boundary layer and the tip clearance in relation to the aerodynamic damping. Different inlet endwall boundary layer thicknesses are shown to lead to qualitatively different aeroelastic stability characteristics in relation to tip clearance. The aero-damping variation with the tip clearance under the influence of the inlet endwall boundary layer seems to correlate closely to a balancing act between the passage vortex and the tip leakage vortex. The tip clearance aeroelastic behavior seems also in line with a simple quasi-steady analysis. On the other hand, the mid-chord laminar bubble separation on suction surface, though with a clear signature in the local aero-damping, has negligible effects on the overall stability. The second part aims to examine computationally the applicability of the influence coefficient method in a linear cascade setup. The comparison between the cascade-based ICM data and a baseline “tuned cascade” shows that the differences in the sensitivity to the far-field treatment can be significant, depending on inter-blade phase angles. On the other hand, non-linearity effects closely relevant to the basic linear assumption of the ICM are shown to only have a small influence. The present results suggest that extra caution should be exercised when comparing a CFD-based tuned cascade model with a finite cascade-based ICM model, at conditions close to acoustic resonance. The resultant discrepancies may well arise from the inherently different far-field sensitivities between the two models, rather than those typical numerical and physical modeling aspects of interest (e.g., meshing, spatial and temporal discretization errors as well as turbulence modeling).

scholarly journals Validation Studies of Linear Oscillating Compressor Cascade and Use of Influence Coefficient Method

2019 ◽  
Author(s):  
H. M. Phan ◽  
L. He
Keyword(s):  
Boundary Layer ◽  
Tip Clearance ◽  
Influence Coefficient ◽  
Far Field ◽  
Compressor Cascade ◽  
Linear Cascade ◽  
Influence Coefficient Method ◽  
Blade Tip ◽  
Coefficient Method ◽  
Blade Tip Clearance

Abstract Advanced predictions of blade flutter have been continually pursued. It is noted however that validation cases of unsteady CFD methods against experimental cases with detailed 3D unsteady pressures are still rather lacking. The main objectives of the present work are two-folds. Firstly, validate and understand the characteristics of blade tip clearance, as well as a bubble-type flow separation for an unsteady CFD solver against a 3D oscillating cascade experiment. And secondly, examine the applicability of the Influence Coefficient Method (ICM) as widely used in an oscillating linear cascade setup. In the first part, the capability of a widely used commercial solver (CFX) for unsteady flows induced by a 3D oscillating compressor cascade is examined. The present computations have shown consistently a destabilizing effect of increasing blade tip clearance, in agreement with the experiment. More remarkably, the computational analyses reveal a distinctive interplay between the inlet endwall boundary layer and the tip clearance in relation to the aerodynamic damping. Different inlet endwall boundary layer thicknesses are shown to lead to qualitatively different aeroelastic stability characteristics in relation to tip clearance. The aero-damping variation with the tip-clearance under the influence of the inlet endwall boundary layer seems to correlate closely to a balancing act between the passage vortex and the tip-leakage vortex. The tip clearance aeroelastic behaviour seems also in line with a simple quasi-steady analysis. On the other hand, the mid-chord laminar bubble separation on suction surface, though with a clear signature in the local aero-damping, has negligible effects on the overall stability. The second part aims to examine computationally the applicability of the influence coefficient method in a linear cascade setup. The comparison between the cascade based ICM data and a baseline ‘tuned cascade’ shows that the differences in the sensitivity to the far-field treatment can be significant, depending on interblade phase angles. On the other hand, non-linearity effects closely relevant to the basic linear assumption of the ICM are shown to only have a small influence. The present results suggest that extra caution should be exercised when comparing a CFD-based tuned cascade model with a finite cascade-based ICM model, at conditions close to acoustic resonance. The resultant discrepancies may well arise from the inherently different far-field sensitivities between the two models, rather than those typical numerical and physical modelling aspects of interest (e.g. meshing, spatial and temporal discretization errors as well as turbulence modelling).

Turbine Blade Tip Leakage Flow Control: Thick/Thin Blade Effects and Separation Line Studies

2008 ◽  
Author(s):  
Julia E. Stephens ◽  
Thomas Corke ◽  
Scott C. Morris
Keyword(s):  
Turbine Blade ◽  
Pressure Loss ◽  
Flow Behavior ◽  
Surface Flow ◽  
Blade Loading ◽  
Linear Cascade ◽  
Gap Flow ◽  
Blade Tip ◽  
Loss Coefficients ◽  
Thin Blade

An experiment was conducted in a linear cascade of Pratt and Whitney Pak-B turbine blades for an exit Mach number of 0.3 to simulate the flow in the tip-gap region of a low pressure turbine blade row. The experiment focused on the independent effects of thickness-to-gap (t/g) and gap-to-chord (g/c) ratios on the tip-gap flow behavior. Two extreme g/c ratios of 5% and 8% were chosen, for which four tip t/g ratios were simulated using pressure-side winglets. The flow was documented through blade-tip and end-wall static pressure measurements, and downstream total pressure loss coefficients. Additionally, surface flow visualization was performed on the blade tip end for a greater understanding of the gap-flow behavior. The response of the flow to passive flow control using a partial suction-side squealer tip at each of the t/g and g/c cases was documented. The intention was to examine any sensitivity of the flow to the g/c ratio that might be attributed to the t/g ratio in a manner that can be categorized as “thick” or “thin” blade behavior. For this, the focus was on possible changes in the size and location of separation and reattachment lines on the blade tip end. The results presented in this paper indicate that the behavior of the flow in the tip-gap region of a linear cascade turbine blade depend both on t/g and the g/c ratios. Downstream loss coefficients generally decreased with increasing t/g for the 5% g/c case, while they were relatively steady with a slight increase with increasing t/g for the 8% g/c case. The squealer reduced pressure loss coefficient for both g/c cases. It was seen to have a peak effectiveness at t/g of about 3.7 for the 5% g/c case, and at t/g of 3.3 for the 8% g/c case, with diminishing effectiveness at larger t/g ratios. Blade loading showed similar dependence on both g/c and t/g. For the baseline flat-tip case, the tip loading initially decreased with increasing t/g up to about 3.5. For values greater than 3.5, the tip loading increased slightly. The addition of the squealer tip increased the tip loading for all of the t/g ratios at the 5% g/c ratio. The trend with t/g generally followed that of the baseline flat-tip case. However, for the 8% g/c case, the blade loading increased almost linearly with increasing t/g, and the squealer increased loading at the smallest t/g but decreased loading for all other t/g cases investigated. In all cases regardless of the t/g, the surface flow visualization revealed a well defined separation and reattachment region. However, the chordwise location of the start of the separation moved toward the trailing edge with increasing t/g and, independently, with increasing g/c. Therefore, no “thick” or “thin” blade regimes were found, and flow characteristics were determined to depend on both g/c and t/g ratios.

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