Aeromechanical Optimization of a Winglet-Squealer Tip for an Axial Turbine

2014 ◽  
Vol 136 (7) ◽  
Author(s):  
Zbigniew Schabowski ◽  
Howard Hodson ◽  
Davide Giacche ◽  
Bronwyn Power ◽  
Mark R. Stokes
Keyword(s):  
Mechanical Performance ◽  
Turbine Blades ◽  
Careful Consideration ◽  
Rate Of Change ◽  
Leakage Loss ◽  
Simple Method ◽  
Axial Turbine ◽  
Low Speed ◽  
Aerodynamic Loss ◽  
Tip Leakage

The possibility of reducing the over tip leakage loss of unshrouded axial turbine rotors has been investigated in an experiment using a linear cascade of turbine blades and by using CFD. A numerical optimization of a winglet-squealer geometry was performed. The optimization involved the structural analysis alongside the CFD. Significant effects of the tip design on the tip gap flow pattern, loss generation and mechanical deformation under centrifugal loads were found. The results of the optimization process were verified by low speed cascade testing. The measurements showed that the optimized winglet-squealer design had a lower loss than the flat tip at all of the tested tip gaps. At the same time, it offered a 37% reduction in the rate of change of the aerodynamic loss with the tip gap size. The optimized tip geometry was used to experimentally assess the effects of the opening of the tip cavity in the leading edge part of the blade and the inclination of the pressure side squealer from the radial direction. The opening of the cavity had a negligible effect on the aerodynamic performance of the cascade. The squealer lean resulted in a small reduction of the aerodynamic loss at all the tested tip gaps. It was shown that a careful consideration of the mechanical aspects of the winglet is required during the design process. Mechanically unconstrained designs could result in unacceptable deformation of the winglet due to centrifugal loads. An example winglet geometry is presented that produced a similar aerodynamic loss to that of the optimized tip but had a much worse mechanical performance. The mechanisms leading to the reduction of the tip leakage loss were identified. Using this knowledge, a simple method for designing the tip geometry of a shroudless turbine rotor is proposed. Numerical calculations indicated that the optimized low-speed winglet-squealer geometry maintained its aerodynamic superiority over the flat tip blade with the exit Mach number increased from 0.1 to 0.8.

Aeromechanical Optimisation of a Winglet-Squealer Tip for an Axial Turbine

2010 ◽  
Author(s):  
Zbigniew Schabowski ◽  
Howard Hodson ◽  
Davide Giacche ◽  
Bronwyn Power ◽  
Mark R. Stokes
Keyword(s):  
Mechanical Performance ◽  
Turbine Blades ◽  
Careful Consideration ◽  
Rate Of Change ◽  
Leakage Loss ◽  
Simple Method ◽  
Axial Turbine ◽  
Low Speed ◽  
Aerodynamic Loss ◽  
Tip Leakage

The possibility of reducing the over tip leakage loss of unshrouded axial turbine rotors has been investigated in an experiment using a linear cascade of turbine blades and by using CFD. A numerical optimisation of a winglet-squealer geometry was performed. The optimisation involved the structural analysis alongside the CFD. Significant effects of the tip design on the tip gap flow pattern, loss generation and mechanical deformation under centrifugal loads were found. The results of the optimisation process were verified by low speed cascade testing. The measurements showed that the optimised winglet-squealer design had a lower loss than the flat tip at all of the tested tip gaps. At the same time, it offered a 37% reduction in the rate of change of the aerodynamic loss with the tip gap size. The optimised tip geometry was used to experimentally assess the effects of the opening of the tip cavity in the leading edge part of the blade and the inclination of the pressure side squealer from the radial direction. The opening of the cavity had a negligible effect on the aerodynamic performance of the cascade. The squealer lean resulted in a small reduction of the aerodynamic loss at all the tested tip gaps. It was shown that a careful consideration of the mechanical aspects of the winglet is required during the design process. Mechanically unconstrained designs could result in unacceptable deformation of the winglet due to centrifugal loads. An example winglet geometry is presented that produced a similar aerodynamic loss to that of the optimised tip but had a much worse mechanical performance. The mechanisms leading to the reduction of the tip leakage loss were identified. Using this knowledge, a simple method for designing the tip geometry of a shroudless turbine rotor is proposed. Numerical calculations indicated that the optimised low-speed winglet-squealer geometry maintained its aerodynamic superiority over the flat tip blade with the exit Mach number increased from 0.1 to 0.8.

scholarly journals On Scaling Method to Investigate High-Speed Over-Tip-Leakage Flow at Low-Speed Condition

2018 ◽  
Vol 140 (6) ◽  
Author(s):  
Hongmei Jiang ◽  
Li He ◽  
Qiang Zhang ◽  
Lipo Wang
Keyword(s):  
High Speed ◽  
Turbine Blades ◽  
Flow Distribution ◽  
Tip Clearance ◽  
Leakage Flow ◽  
Tip Leakage Flow ◽  
Scaling Method ◽  
Low Speed ◽  
Tip Leakage ◽  
Speed Condition

Modern high-pressure turbine blades operate at high-speed conditions. The over-tip-leakage (OTL) flow can be high-subsonic or even transonic. From the consideration of problem simplification and cost reduction, the OTL flow has been studied extensively in low-speed experiments. It has been assumed a redesigned low-speed blade profile with a matched blade loading should be sufficient to scale the high-speed OTL flow down to the low-speed condition. In this paper, the validity of this conventional scaling approach is computationally examined. The computational fluid dynamics (CFD) methodology was first validated by experimental data conducted in both high- and low-speed conditions. Detailed analyses on the OTL flows at high- and low-speed conditions indicate that, only matching the loading distribution with a redesigned blade cannot ensure the match of the aerodynamic performance at the low-speed condition with that at the high-speed condition. Specifically, the discrepancy in the peak tip leakage mass flux can be as high as 22%, and the total pressure loss at the low-speed condition is 6% higher than the high-speed case. An improved scaling method is proposed hereof. As an additional dimension variable, the tip clearance can also be “scaled” down from the high-speed to low-speed case to match the cross-tip pressure gradient between pressure and suction surfaces. The similarity in terms of the overall aerodynamic loss and local leakage flow distribution can be improved by adjusting the tip clearance, either uniformly or locally.

scholarly journals On Scaling Method to Investigate High-Speed Over-Tip-Leakage Flow at Low-Speed Condition

2017 ◽  
Author(s):  
Hongmei Jiang ◽  
Li He ◽  
Qiang Zhang ◽  
Lipo Wang
Keyword(s):  
High Speed ◽  
Flow Distribution ◽  
Tip Clearance ◽  
Leakage Flow ◽  
Tip Leakage Flow ◽  
Scaling Method ◽  
Low Speed ◽  
Aerodynamic Loss ◽  
Tip Leakage ◽  
Speed Condition

Modern High Pressure Turbine (HPT) blades operate at high speed conditions. The Over-Tip-Leakage (OTL) flow, which plays a major role in the overall loss generation for HPT, can be high-subsonic or even transonic. In practice from the consideration of problem simplification and cost reduction, the OTL flow has been studied extensively in low speed experiments. It has been assumed a redesigned low speed blade profile with a matched blade loading should be sufficient to scale the high speed OTL flow down to the low speed condition. In this paper, the validity of this conventional scaling approach is computationally examined. The CFD methodology was firstly validated by experimental data conducted in both high and low speed conditions. Detailed analyses on the OTL flows at high and low speed conditions indicate that, only matching the loading distribution with a redesigned blade cannot ensure the match of the aerodynamic performance at the low speed condition with that at the high-speed condition. Specifically, the discrepancy in the peak tip leakage mass flux can be as high as 22.2%, and the total pressure loss at the low speed condition is 10.7% higher than the high speed case. An improved scaling method is proposed hereof. As an additional dimension variable, the tip clearance can also be “scaled” down from the high speed to low speed case to match the cross-tip pressure gradient between pressure and suction surfaces. The similarity in terms of the overall aerodynamic loss and local leakage flow distribution can be improved by adjusting the tip clearance, either uniformly or locally. The limitations of this proposed method are also addressed in this paper.

The Reduction of Over Tip Leakage Loss in Unshrouded Axial Turbines Using Winglets and Squealers

2013 ◽  
Vol 136 (4) ◽  
Author(s):  
Zbigniew Schabowski ◽  
Howard Hodson
Keyword(s):  
Flow Pattern ◽  
Large Scale ◽  
Turbine Blades ◽  
Suction Side ◽  
Leakage Loss ◽  
Tip Leakage ◽  
Gap Flow ◽  
Computational Fluid Dynamics Cfd ◽  
Axial Turbines ◽  
The Impact

The possibilities of reducing the over tip leakage loss of unshrouded rotors have been investigated using a linear cascade of turbine blades and computational fluid dynamics (CFD). The large-scale blade profile is the same as that of the tip profile of a low-speed high-pressure research turbine facility. The impact of various combinations of squealer and winglet geometries on the turbine performance has been investigated. The influence of the thickness of the squealers has also been assessed. It was found that a 22% reduction in loss slope was possible, when compared to the flat tip blade, using simple tip modifications. The results obtained with the suction side squealer and cavity tip agreed well with the work of other researchers. Three winglet-based tip geometries were tested. One was a plain winglet, the other two had squealers applied. A significant impact of the squealers and their shape on the tip gap flow pattern and loss generation was found. The physical processes occurring within the tip gap region for the tested geometries are explained using both numerical and experimental results. The impact of the flow pattern within the tip gap on the loss generation is described. Good agreement between CFD and the experimental data was found. This shows that CFD can be used with confidence in the design process of shroudless turbines.

The Reduction of Over Tip Leakage Loss in Unshrouded Axial Turbines Using Winglets and Squealers

2007 ◽  
Author(s):  
Zbigniew Schabowski ◽  
Howard Hodson
Keyword(s):  
Flow Pattern ◽  
Large Scale ◽  
Turbine Blades ◽  
Suction Side ◽  
Leakage Loss ◽  
Tip Leakage ◽  
Gap Flow ◽  
Axial Turbines ◽  
The Impact ◽  
Good Agreement

The possibilities of reducing the over tip leakage loss of unshrouded rotors have been investigated using a linear cascade of turbine blades and CFD. The large-scale blade profile is the same as that of the tip profile of a low-speed HP research turbine facility. The impact of various combinations of squealer and winglet geometries on the turbine performance has been investigated. The influence of the thickness of the squealers has also been assessed. It was found that a 22% reduction in loss slope was possible, when compared to the flat tip blade, using simple tip modifications. The results obtained with the suction side squealer and cavity tip agreed well with the work of other researchers. Three winglet-based tip geometries were tested. One was a plain winglet, the other two had squealers applied. A significant impact of the squealers and their shape on the tip gap flow pattern and loss generation was found. The physical processes occurring within the tip gap region for the tested geometries are explained using both numerical and experimental results. The impact of the flow pattern within the tip gap on the loss generation is described. Good agreement between the CFD and the experimental data was found. This shows that the CFD can be used with confidence in the design process of shroudless turbines.

Design Modification of a Passive Tip-Leakage Control Method for Axial Turbines: Linear Cascade Wind Tunnel Results

2013 ◽  
Author(s):  
Albert Benoni ◽  
Reinhard Willinger
Keyword(s):  
Control Method ◽  
Flow Direction ◽  
Turbine Blades ◽  
Chord Length ◽  
Leakage Flow ◽  
Tip Leakage Flow ◽  
Leakage Loss ◽  
Design Modification ◽  
Linear Cascade ◽  
Tip Leakage

Tip-leakage losses can contribute up to one third of the overall losses in unshrouded axial turbine blades. A passive tip-leakage flow control method is used to reduce the tip-leakage loss. Taking into account a modified discharge coefficient model, an inclination of the injection against the tip-leakage flow direction is said to have an even better effect on reducing the tip-leakage loss. To prove the effect, linear cascade measurements have been carried out at three different gap widths from 0.85% to 2.50% chord length. The used geometry is an up-scaled turbine blade tip cross section with weak turning. A single blade is fitted with an injection channel which is inclined by 45° against the tip-leakage flow direction. The flow field of the modified blade was measured 0.31 axial chord length downstream of the cascade using a pneumatic five-hole probe. The tip-leakage loss is reduced by passive tip-injection and further by inclined injection. The reduction can be significant at small gap widths. Detailed results are presented for a gap width of 1.40% chord length.

scholarly journals Blade Loading Effects on Axial Turbine Tip Leakage Vortex Dynamics and Loss

2012 ◽  
Author(s):  
A. C. Huang ◽  
E. M. Greitzer ◽  
C. S. Tan ◽  
E. F. Clemens ◽  
S. G. Gegg ◽  
...  
Keyword(s):  
Pressure Distribution ◽  
Vortex Dynamics ◽  
Vortex Breakdown ◽  
Turbine Blades ◽  
Pressure Rise ◽  
Tip Clearance ◽  
Leakage Loss ◽  
Blade Loading ◽  
Tip Leakage ◽  
Dominant Feature

Numerical simulations have been carried out to define the loss generation mechanisms associated with tip leakage in un-shrouded axial turbines. Tip clearance vortex dynamics are a dominant feature of two mechanisms important in determining this loss: (i) decreased swirl velocity due to vortex line contraction in regions of decreasing axial velocity, i.e., adverse pressure gradient and (ii) vortex breakdown and reverse flow in the vortex core. The mixing losses behave differently from the conventional view of flow exiting a turbine tip clearance. More specifically, it is shown, through both control volume arguments and computations, that as a swirling leakage flow passes through a pressure rise, such as in the aft portion of the suction side of a turbine blade, the mixed-out loss can either decrease or increase. For turbines the latter typically occurs if the deceleration is large enough to initiate vortex breakdown, and it is demonstrated that this is the case in modern turbines. The effect of blade pressure distribution on clearance losses is illustrated through computational examination of two turbine blades, one with forward loading at the tip and one with aft loading. A 15% difference in leakage loss is found between the two, due to lower clearance vortex deceleration (lower core static pressure rise) with forward loading, and hence lower vortex breakdown loss. Additional computational experiments, carried out to define the effects of blade loading, incidence, and solidity, are found to be consistent with the proposed ideas linking blade pressure distribution, vortex breakdown and turbine tip leakage loss.

scholarly journals Vortex Patterns Investigation and Enstrophy Analysis in a Small Scale S-CO2 Axial Turbine

Energies ◽  
2021 ◽  
Vol 14 (19) ◽  
pp. 6112
Author(s):  
Qiyu Ying ◽  
Weilin Zhuge ◽  
Yangjun Zhang ◽  
Can Ma ◽  
Jinlan Gou ◽  
...  
Keyword(s):  
Energy Loss ◽  
Vortex Flow ◽  
Flow Patterns ◽  
Small Scale ◽  
Vortex Interaction ◽  
Tip Leakage Vortex ◽  
Axial Turbine ◽  
Aerodynamic Loss ◽  
Tip Leakage ◽  
Passage Vortex

Supercritical carbon dioxide (S-CO2) Brayton cycle system is a promising closed-loop energy conversion system frequently mentioned in the automotive and power generation field in recent years. To develop a suitable design methodology for S-CO2 turbines with better performance, an understanding of the vortex flow patterns and associated aerodynamic loss inside a S-CO2 turbine is essential. In this paper, a hundred-kilowatt level S-CO2 axial turbine is designed and investigated using a three-dimensional transient viscous flow simulation. The NIST Span and Wagner equation of state model that considers the real gas effects is utilized to estimate the thermodynamic properties of the supercritical fluid. The numerical methods are experimentally validated. The results indicates that the aspect ratio and tip-to-hub ratio are different in the S-CO2 turbine from that in the gas turbine, and the vortex flow patterns are influenced notably by these geometrical parameters. Both the vortex structure and moving tracks of passage vortices are changed as a result of large centrifugal force. An interaction between tip leakage vortex and hub passage vortex is observed in the impeller passage and its formation and development mechanism are revealed. To further explore the aerodynamic loss mechanism caused by vortex interaction, the energy loss in the impeller passage is analyzed with the enstrophy dissipation method, which can not only accurately calculate the energy loss but also estimate how the vortical motions occur. It is found that the enstrophy and energy loss can be effectively reduced by vortex interaction between tip leakage vortex and hub passage vortex. The results in this study would increase the knowledge of vortex flow patterns in S-CO2 turbine and the proposed enstrophy production method can be used intuitively to provide a reference for flow vortical motion study in turbines.

Parametric study of injection from the casing in an axial turbine

2019 ◽  
Vol 234 (5) ◽  
pp. 582-593
Author(s):  
Sarallah Abbasi ◽  
Afshin Gholamalipour
Keyword(s):  
Flow Structure ◽  
Control Method ◽  
Turbine Blades ◽  
Leading Edge ◽  
Tip Clearance ◽  
Leakage Flow ◽  
Tip Leakage Flow ◽  
Axial Turbine ◽  
Tip Leakage ◽  
Turbine Performance

Tip leakage flow reduces both efficiency and performance of axial turbines and damages turbine blades as well. Therefore, it is of great importance to identify and control tip leakage flow. This study investigated the effect of flow injection (from the casing), alongside flow structure, on turbine performance. Additionally, the effect of different injection parameters, including injection mass flow rate, angle, location, and diameter on the turbine performance are evaluated. A numerical analysis of the flow in a two-stage axial turbine was employed by using CFX software. To ensure the accuracy of the results, turbine performance curves were compared with the experimental results, which are in good agreement. Analyses revealed that active control method reduces tip leakage flow, improves turbine performance, and increases the efficiency by 1% to 5% as well. A parametric investigation of the tip injection has sought to identify how various parameters affect the turbine performance. The cross-section diameter and the angle of injection had no significant increase on efficiency. Additionally, results showed that at a point 9 mm further from the leading edge, the injection degree of effectiveness is optimum. Finally, analysis of the flow structure in the tip clearance region supported the tip leakage flow reduction.

The Performance of a Low Speed One and a Half Stage Axial Turbine With Varying Rotor Tip Clearance and Tip Gap Geometry

1994 ◽  
Author(s):  
G. Morphis ◽  
J. P. Bindon
Keyword(s):  
Secondary Flows ◽  
Tip Clearance ◽  
Surprising Result ◽  
Loss Mechanism ◽  
Axial Turbine ◽  
Low Speed ◽  
Tip Leakage ◽  
Second Stage ◽  
Gap Flow ◽  
Flow Loss

The performance of a low speed axial turbine followed by a second stage nozzle is measured with particular reference to the understanding of tip clearance effects in a real machine and to possible benefits of streamlined low loss rotor tips. A radiused pressure edge was found to improve the performance of b single stage and of a one and a half stage turbine at the small tip clearance levels for which the radius was selected. This is in contrast to cascade results where mixing loss reduced the benefits of such tips. Clearance gap flow appears therefore to be just like other turbine flow where the loss mechanism of separation must be avoided. Loss formation within and downstream of a rotor are more complex than previously realized and do not obey the simple rules that have been used to design for minimum tip clearance loss. For example, approximately 48% of the tip leakage mass flow within a rotor appears to be a flat wall jet rather than a wrapped up vortex. The second stage nozzle efficiency was found to be significantly higher than for the first stage and to even increase with tip clearance. This is a surprising result since it means that not only is there a reduction in secondary flow loss but also that rotor leakage and rotor secondary flows do not generate downstream mixing loss.

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