Winglets for Improved Aerothermal Performance of High Pressure Turbines

Coull, John D.; Atkins, Nicholas R.; Hodson, H. P.

doi:10.1115/gt2013-94425

Cited by 14 publications

(27 citation statements)

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“…The two-dimensional streamline at the mid-gap plane is used to describe the flow behaviour near the endwall. The methods of analysis are based on the previous research by Coull et al 8 The analysis process of this section, by contrast, is simplified to validate a basic changing trend of losses under different cases. The cut plane A and cut plane B (two black lines shown in Figure 10 33.5 at the cut plane A and then gradually turns to the Z-aixs direction.…”

Section: Discussion Of Tip Leakagementioning

confidence: 99%

“…An investigation into a novel winglet gained a reduction of 45% in exchange rate at a relatively large gap. 7 Coull et al 8 investigated the effects of winglet tip on the aerodynamic and thermal performance in an unshrouded high-pressure rotor by RANS methods. The results showed that the overhang around the full perimeter of the blade tip greatly improved efficiency, and notably increased the overall heat load, and remarkable efficiency improvements could be obtained without increasing the overall heat transfer to the blade by adopting winglet tip on only the early suction surface.…”

Section: Introductionmentioning

confidence: 99%

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Aerodynamic optimisation of a winglet-cavity tip in a high-pressure axial turbine cascade

Zhou

Chen

Wang

2016

Proceedings of the Institution of Mechanical Engineers, Part G:

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Tip clearance flow between rotating blades and the stationary casing in high-pressure turbines is very complex and is one of the most important factors influencing turbine performance. The rotor with a winglet-cavity tip is often used as an effective method to improve the loss resulting from the tip clearance flow. In this study, an aerodynamic geometric optimisation of a winglet-cavity tip was carried out in a linear unshrouded high-pressure axial turbine cascade. For the purpose of shaping the efficient winglet geometry of the rotor tip, a novel parameterisation method has been introduced in the optimisation procedure based on the computational fluid dynamics simulation and analysis. The reliability of a commercial computational fluid dynamics code with different turbulence models was first validated by contrasting with the experimental results, and the numerical total pressure loss and flow angle using the Baseline k-omega Model (BSL k-o model) shows a better agreement with the test data. Geometric parameterisation of blade tips along the pressure side and suction side was adopted to optimise the tip clearance flow, and an optimal winglet-cavity tip was proven to achieve lower tip leakage mass flow rate and total pressure loss than the flat tip and cavity tip. Compared to the numerical results of flat tip and cavity tip, the optimised winglet-cavity design, with the winglet along the pressure side and suction side, had lower tip leakage mass flow rate and total pressure loss. It offered a 35.7% reduction in the change ratio Á " C Pt,t =Á. In addition, the optimised winglet along pressure side and suction side, respectively, by using the parameterisation method was studied for investigating the individual effect of the pressure-side winglet and suction-side winglet on the tip clearance flow. It was found that the suction-side extension of the optimal winglet resulted in a greater reduction of aerodynamic loss and leakage mass flow than the pressure-side extension of the optimal winglet. Moreover, with the analysis based on the tip flow pattern, the numerical results show that the pressure-side winglet reduced the contraction coefficient, and the suction-side winglet reduced the aerodynamic loss effectively by decreasing the driving pressure difference near the blade tips, the leakage flow velocity, and the interaction between the leakage flow and the main flow. Overall, a better aerodynamic performance can be obtained by adopting the pressure-side and suction-side winglet-cavity simultaneously.

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Section: Discussion Of Tip Leakagementioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Aerodynamic optimisation of a winglet-cavity tip in a high-pressure axial turbine cascade

Zhou

Chen

Wang

2016

Proceedings of the Institution of Mechanical Engineers, Part G:

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“…where the leakage flow quantities (ṁ l , v l ) are extracted from the TGP cells and then radially integrated. In order to determine M m and v m , Coull [7] proposes to use the isentropic values derived from the suction surface pressures gauged at 90% span. A similar approach was adopted here with the difference that the static pressure distributions were sampled at 80% of the span.…”

Section: Cfd Methods and Data Processingmentioning

confidence: 99%

“…Efforts have therefore been made to develop advanced tip geometries for unshrouded blades, aiming to seal the tip gap and reduce the blade heat transfer [6]. A mid-way between the two approaches are the so-called winglets [7]. The design most commonly employed in the industry is a grooved tip (or squealer tip) shape, featuring a partial or full rim which acts as a labyrinth seal [8][9][10].…”

Section: Introductionmentioning

confidence: 99%

Experimental and Numerical Investigation of Optimized Blade Tip Shapes: Part II — Tip Flow Analysis and Loss Mechanisms

Pátý

Cernat

Maesschalck

et al. 2018

Volume 5B: Heat Transfer

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The leakage flows within the gap between the tips of unshrouded rotor blades and the stationary casing of high-speed turbines are the source of significant aerodynamic losses and thermal stresses. In the pursuit for higher component performance and reliability, shaping the tip geometry offers a considerable potential to modulate the rotor tip flows and to weaken the heat transfer onto the blade and casing. Nevertheless, a critical shortage of combined experimental and numerical studies addressing the flow and loss generation mechanisms of advanced tip profiles persists in the open literature. A comprehensive study is presented in this two-part paper that investigates the influence of blade tip geometry on the aerother-modynamics of a high-speed turbine. An experimental and numerical campaign has been performed on a high-pressure turbine stage adopting three different blade tip profiles. The aerothermal performance of two optimized tip geometries (one with a full three-dimensional contoured shape and the other featuring a multi-cavity squealer-like tip) is compared against that of a regular squealer geometry. In the second part of this paper, we report a detailed analysis on the aerodynamics of the turbine as a function of the blade tip geometry. Reynolds-averaged Navier-Stokes simulations, adopting the Spalart-Allmaras turbulence model and experimental boundary conditions, were run on high-density unstructured meshes using the Numeca FINE/Open solver. The simulations were validated against time-averaged and time-resolved experimental data collected in an instrumented turbine stage specifically set up for the simultaneous testing of multiple blade tips at scaled engine-representative conditions. The tip flow physics is explored to explain variations in turbine performance as a function of the tip geometry. Denton’s mixing loss model is applied to the predicted tip gap aerodynamic field to identify and quantify the loss reduction mechanisms of the alternative tip designs. An advanced method based on the local triple decomposition of relative motion is used to track the location, size and intensity of the vortical flow structures arising from the interaction between the tip leakage flow and the main gas path. Ultimately, the comparison between the unconventional tip profiles and the baseline squealer tip highlights distinct aerodynamic features in the associated gap flow field. The flow analysis provides guidelines for the designer to assess the impact of specific tip design strategies on the turbine aerodynamics and rotor heat transfer.

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“…Shroudless high pressure (HP) turbine blades can have peak Mach numbers within the tip gap of 1.8 and, thus, supersonic effects cannot generally be ignored Zhang et al 2011;Moore et al 1989;Moore & Elward 1993). Supersonic effects tend to occur over the aft region of the tip where the blade loading is highest, although the extent of supersonic flow over the tip is strongly dependent on the tip geometry (Coull et al 2014;Zhang et al 2014). This area of the tip is important because the blade thickness in this region is small which means complex tip geometries are often not feasible and this zone is typically difficult to cool.…”

Section: Introductionmentioning

confidence: 99%

Numerical investigation of the flow over a model transonic turbine blade tip

Wheeler

Sandberg

2016

J. Fluid Mech.

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Direct numerical simulations are used to investigate the unsteady flow over a model turbine blade tip at engine scale Reynolds and Mach numbers. The DNS are performed with an in-house multi-block structured compressible Navier-Stokes solver. The particular case of a transonic tip flow is studied since previous work has suggested compressibility has an important effect on the turbulent nature of the separation bubble at the inlet to the tip-casing gap and subsequent flow reattachment. The flow is simulated over an idealized tip geometry where the tip gap is represented by a constant area channel with a sharp inlet corner to represent the pressure side edge of the turbine blade. The effects of free-stream disturbances, cross-flow and pressure-side boundary-layer on the tip flow aerodynamics and heat transfer are studied. For 'clean' in-flow cases we find that even at engine scale Reynolds numbers the tip flow is intermittent in nature, i.e. neither laminar nor fully turbulent. The breakdown to turbulence occurs through the development of spanwise streaks with wavelengths around 15 − 20% of the gap height. Multi-dimensional linear stability analysis confirms the two-dimensional base state to be most unstable with respect to spanwise wavelengths of 25% of the gap height. The linear stability analysis also shows that addition of cross-flows with 25% of the streamwise gap exit velocity increase the stability of the tip flow. This is confirmed by the DNS which also show that the turbulence production is significantly reduced in the separation bubble. For the case when free-stream disturbances are added to the inlet flow, viscous dissipation and the rapid acceleration of the flow at the inlet to the tip-casing gap causes significant distortion of the vorticity field and reductions of turbulence intensity as the flow enters the tip gap. The DNS results also suggest that the assumption of the Reynolds analogy and a constant recovery factor are not accurate, in particular in regions where the skin friction approaches zero while significant temperature gradients remain, such as in the vicinity of flow reattachment.

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Winglets for Improved Aerothermal Performance of High Pressure Turbines

Cited by 14 publications

References 0 publications

Aerodynamic optimisation of a winglet-cavity tip in a high-pressure axial turbine cascade

Aerodynamic optimisation of a winglet-cavity tip in a high-pressure axial turbine cascade

Experimental and Numerical Investigation of Optimized Blade Tip Shapes: Part II — Tip Flow Analysis and Loss Mechanisms

Numerical investigation of the flow over a model transonic turbine blade tip

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