The Effects of Relative Motion, Blade Edge Radius and Gap Size on the Blade Tip Pressure Distribution in an Annular Turbine Cascade With Clearance

1988 ◽  
Author(s):  
G. Morphis ◽  
J. P. Bindon
Keyword(s):  
Relative Motion ◽  
Separation Bubble ◽  
Tip Clearance ◽  
Flow Visualisation ◽  
Turbine Cascade ◽  
Pressure Surface ◽  
Pressure Distributions ◽  
Pressure Peak ◽  
Blade Edge ◽  
Blade Tip

Flow visualisation and microscopic static pressure measurements were done in the tip clearance region of an annular turbine cascade with a rotating outer casing to simulate the relative motion at the tip of an axial rotor. The effect of relative motion did not have a significant effect on the blade gap pressure distributions. As in previous studies the narrow deep pressure depression on a sharp pressure edge was seen. It was confirmed that the width of the gap separation bubble depends on clearance and a correlation with flow visualisation showed that at the reattachment line there is the expected slight pressure peak. The separation bubble, which is thought to contribute a major part of the leakage loss, was shown to disappear when the pressure surface tip is give a radius of 2.5 gap widths.

scholarly journals Numerical Calculation of Flow for Cascade With Tip Clearance

1994 ◽  
Author(s):  
Shimpei Mizuki ◽  
Hoshio Tsujita
Keyword(s):  
Three Dimensional ◽  
Tip Clearance ◽  
Turbine Cascade ◽  
Governing Equations ◽  
Tensor Form ◽  
Pressure Surface ◽  
Rear Part ◽  
Blade Tip ◽  
Flow Through ◽  
Blade Tip Geometry

Three-dimensional incompressible turbulent flow within a linear turbine cascade with tip clearance is analyzed numerically. The governing equations involving the standard k-ε model are solved in the physical component tensor form with a boundary-fitted coordinate system. In the analysis, the blade tip geometry is treated accurately in order to predict the flow through the tip clearance in detail when the blades have large thicknesses. Although the number of grids employed in the present study is not enough because of the limitation of computer storage memory, the computed results show good agreements with the experimental results. Moreover, the results clearly exhibit the locus of minimum pressure on the rear part of the pressure surface at the blade tip.

Endwall Flow/Loss Mechanisms in a Linear Turbine Cascade With Blade Tip Clearance

1989 ◽  
Vol 111 (3) ◽  
pp. 264-275 ◽  
Author(s):  
A. Yamamoto
Keyword(s):  
Three Dimensional ◽  
Tip Clearance ◽  
Leakage Flow ◽  
Turbine Cascade ◽  
Flow Angle ◽  
Pressure Distributions ◽  
Flow Loss ◽  
Blade Tip ◽  
Flow Vectors ◽  
Blade Tip Clearance

This paper discusses the mechanisms of three-dimensional flows and of the associated losses occurring near the tip endwall region of a linear turbine cascade with tip clearance. The clearance gap sizes and the cascade incidences were chosen as the most important variables affecting the mechanisms. Flows close to the endwall and inside the clearance were surveyed in great detail using a micro five-hole pitot tube of 0.6 mm head size. The results gave very detailed information on the mechanisms, such as leakage flow vectors and pressure distributions throughout the clearance. Interaction of leakage flow with the endwall flow and their associated separation lines, effects of gap size and inlet flow angle on loss generation, and skewness of the three-dimensional endwall flows are also discussed.

Pressure Distributions in the Tip Clearance Region of an Unshrouded Axial Turbine as Affecting the Problem of Tip Burnout

1987 ◽  
Author(s):  
Jeffery P. Bindon
Keyword(s):  
Separation Bubble ◽  
Low Pressure ◽  
High Heat ◽  
Tip Clearance ◽  
Small Scale ◽  
Narrow Strip ◽  
Pressure Surface ◽  
High Heat Transfer ◽  
Axial Turbines ◽  
Secondary Vortices

The pressure distribution in the tip clearance region of a 2D turbine cascade was examined with reference to unknown factors which cause high heat transfer rates and burnout along the edge of the pressure surface of unshrouded cooled axial turbines. Using a special micro-tapping technique, the pressure along a very narrow strip of the blade edge was found to be 2.8 times lower than the cascade outlet pressure. This low pressure, coupled with a thin boundary layer due to the intense acceleration at gap entry, are believed to cause blade burnout. The flow phenomena causing the low pressure are of very small scale and do not appear to have been previously reported. The ultra low pressure is primarily caused by the sharp flow curvature demanded of the leakage flow at gap entry. The curvature is made more severe by the apparent attachement of the flow around the corner instead of immediately separating to increase the radius demanded of the flow. The low pressures are intensified by a depression in the suction corner and by the formation of a separation bubble in the clearance gap. The bubble creates a venturi action. The suction corner depression is due to the mainstream flow moving round the leakage and secondary vortices.

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.

scholarly journals Influences of Tip Cooling Injection on Tip Clearance Control at Design and Off-Design Incidences

2009 ◽  
Vol 2009 ◽  
pp. 1-12 ◽  
Author(s):  
Maosheng Niu ◽  
Shusheng Zang
Keyword(s):  
Heat Transfer ◽  
Tip Clearance ◽  
Air Injection ◽  
Turbine Cascade ◽  
Tip Clearance Flow ◽  
Clearance Flow ◽  
Blade Tip ◽  
Design Value ◽  
Tip Injection ◽  
Clearance Control

A numerical investigation has been performed to study the influences of cooling injection from the blade tip surface on controlling tip clearance flow in an unshrouded, high-turning axial turbine cascade. Emphasis is put on the analysis of the effectiveness of tip injection when the approaching flow is at design and off-design incidences. A total of three incidence angles are investigated, 7.4°, 0°, 0°, 0°, and 7.6°, 0° relative to the design value. The results indicate that even at the off-design incidences, tip injection can also act as an obstruction to the tip clearance flow and weaken the interaction between the passage flow and the tip clearance flow. It is also found that tip injection causes the tip clearance loss to be less sensitive to the incidences. Moreover, with injection, at all these incidences the heat transfer conditions are improved significantly on the blade tip surface in the middle and aft parts of blade. Thus, tip injection is proved to be an effective method of controlling tip clearance flow, even at off-design conditions. Beside that, an indirect empirical correlation is observed to be able to perform well in predicting the losses induced by tip clearance flow at design and off-design conditions, no matter whether air injection is active or not.

The Effect of Rotor Tip Clearance Size Onto the Separated Flow Through a Super-Aggressive S-Shaped Intermediate Turbine Duct Downstream of a Transonic Turbine Stage

2009 ◽  
Author(s):  
A. Marn ◽  
E. Go¨ttlich ◽  
F. Malzacher ◽  
H. P. Pirker
Keyword(s):  
Low Pressure ◽  
Tip Clearance ◽  
Flow Visualisation ◽  
Turbine Stage ◽  
Intermediate Turbine Duct ◽  
Low Pressure Turbine ◽  
Oil Flow ◽  
Blade Tip ◽  
Flow Through ◽  
Aero Engines

The demand of further increased bypass ratio of aero engines will lead to low pressure turbines with larger diameters, which rotate at lower speed. Therefore, it is necessary to guide the flow leaving the high pressure turbine to the low pressure turbine at a larger diameter without any loss generating separation or flow disturbances. Due to costs and weight this intermediate turbine duct (ITD) has to be as short as possible. This leads to an aggressive (high diffusion) and further to a super-aggressive s-shaped duct geometry. In order to investigate the influence of the blade tip gap size on such a high diffusion duct flow a detailed test arrangement under engine representative conditions is necessary. Therefore, the continuously operating Transonic Test Turbine Facility (TTTF) at Graz University of Technology has been adapted: An super-aggressive intermediate duct is arranged downstream of a transonic HP-turbine stage providing an exit Mach number of about 0.6 and a swirl angle of −15 degrees. A second LP-vane row is located at the end of the duct and represents the counter rotating low pressure turbine at a larger diameter. A following deswirler and a diffuser are the connection to the exhaust casing of the facility. In order to determine the influence of the blade tip gap size on the flow through such a super-aggressive s-shaped turbine duct measurements were conducted with two different tip gap sizes, 1.5% span (0.8 mm) and 2.4% span (1.3 mm). The aerodynamic design of the HP-turbine stage, ITD, LP-vane and the de-swirler was done by MTU Aero engines. In 2007 at ASME Turbo Expo the influence of the rotor clearance size onto the flow through an aggressive ITD was presented. For the present investigation this aggressive duct has been further shortened by 20% (super-aggressive ITD) that the flow at the outer duct wall is fully separated. This paper shows the influence of the rotor tip clearance size onto this separation. The flow through this intermediate turbine duct was investigated by means of five-hole-probes, static pressure taps, boundary layer rakes and oil flow visualisation. The oil flow visualisation showed the existence of vortical structures within the separation where they seem to be imposed by the upstream HP-vanes. This work is part of the EU-project AIDA (Aggressive Intermediate Duct Aerodynamics, Contract: AST3-CT-2003-502836).

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.

Numerical Study of the Flow Past a Turbine Blade Tip: Effect of Relative Motion Between Blade and Shroud

2013 ◽  
Vol 136 (3) ◽  
Author(s):  
Sumanta Acharya ◽  
Louis Moreaux
Keyword(s):  
Turbine Blade ◽  
Relative Motion ◽  
High Speed ◽  
Numerical Study ◽  
Tip Clearance ◽  
Leakage Flow ◽  
Static Case ◽  
Blade Tip ◽  
Geometric Effects ◽  
Blade Tips

Turbine blade tips are often the most susceptible to material failure due to the high-speed leakage flow and associated large thermal loadings. In this paper, the effect of the blade rotation and relative motion between the blade tip and shroud is studied numerically. Three different simulations have been undertaken: (1) a static case where the blade and the shroud are stationary (used as the reference case) (2) a linearly moving blade (or shroud) and (3) a rotating blade. Comparisons between cases 1 and 2 identify the effects of relative motion, while comparison between cases 2 and 3 delineate the effects of rotational Coriolis and centrifugal forces. Geometric effects were also studied through different combinations of tip gaps and squealer depths with the relative motion and rotational effects included. The calculations were done using a commercial flow solver, Fluent, using a block body-fitted mesh, Reynolds-averaged transport equations and a turbulence model. Results confirm the significant effects of the relative motion between the blade tip and shroud, and indicate that the assumption of pressure-driven leakage flows for blade tips is inappropriate. While rotational forces also play a role, the magnitude of their effects are relatively small compared to the relative motion effects. Geometric effects are also important with the lower tip clearance reducing leakage flow and allowing the tip coolant to migrate towards the SS with relative motion.

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.

Effects of Upstream Platform Geometry on the Endwall Flows of a Turbine Cascade

2005 ◽  
Author(s):  
Elena de la Rosa Blanco ◽  
H. P. Hodson ◽  
R. Va´zquez
Keyword(s):  
Boundary Layer ◽  
Separation Bubble ◽  
Turbine Cascade ◽  
Blade Profile ◽  
Linear Cascade ◽  
Surface Separation ◽  
Pressure Surface ◽  
Low Pressure Turbine ◽  
Large Pressure ◽  
Blade Row

This paper describes the effect that the endwall geometry has on the endwall flows in the vicinity of the blade platform in a low-pressure turbine. The aim of this work is to assess the effect on blade performance of a step in hub diameter just ahead of the blade row. The blade profile under consideration is of high aspect ratio and characterized by a large pressure surface separation bubble. The tests are conducted on a linear cascade and the experimental results are supported by numerical simulations. Two different steps are employed, i.e., forward facing and backward facing steps. Furthermore, the size of the step and the thickness of the inlet endwall boundary layer are also varied. It was found that the presence of the step ahead of the blade row can significantly alter the structure and the strength of the endwall flows. A backward facing step gives rise to lower losses when compared with a flat endwall. However, the effect is found to be dependent on the step height and the thickness of the approaching boundary layer. A forward facing step, on the other hand, produces higher losses than a flat endwall.

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