Aerodynamic losses due to the formation of a leakage vortex near the tip section of rotor blades form a significant part of viscous losses in axial flow turbines. The leakage flow, mainly induced by the pressure differential between the pressure side and suction side of a rotor tip section, usually rolls into a streamwise vortical structure near the suction side part of the blade tip. The current study uses the concept of a tip platform extension that is a very short “winglet” obtained by slightly extending the tip platform in the tangential direction. The use of a pressure side tip extension can significantly affect the local aerodynamic field by weakening the leakage vortex structure. Phase averaged, time accurate total pressure measurements downstream of a single stage turbine facility are provided from a total pressure probe that has a time response of 150 kHz. Complete total pressure maps in all of the 29 rotor exit planes are measured accurately. Various pressure and suction side extension configurations are compared against a baseline case. The current investigation performed in the Axial Flow Turbine Research Facility (AFTRF) of the Pennsylvania State University shows that significant total to total efficiency gain is possible by the use of tip platform extensions.Copyright © 2001 by ASME

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Aerodynamic Tip Desensitization of an Axial Turbine Rotor Using Tip Platform Extensions

The over tip leakage (OTL) flow that exists between the stationary casing and the rotor tip of a shroudless HP turbine remains a major source of loss of performance for modern aero gas turbines. To date the principal approaches to reducing OTL loss have been to minimize the clearance gap and/or apply a rotating shroud to the rotor. Tip clearance control systems continue to improve, but a practical limit on tip gap remains. A rotating shroud is highly effective but increases the rotor weight, forcing it to run more slowly and thus increasing other aerodynamic losses. Additional means of reducing OTL loss are still needed. Partial shrouds (winglets) have been tried but none have entered commercial service to date. This paper presents a novel design of partial shroud derived from a review of past research. The (arbitrary) objectives were to halve the OTL loss of a shroudless rotor, at less than half the size of a full shroud. This design has been analyzed using a steady flow RANS CFD code to qualitatively determine its benefits. Attention has been paid to its validation and a realistic determination of its capabilities. The winglet is predicted to significantly improve the efficiency of a highly loaded HP turbine, by 1.2 percent-1.8 percent at 2 percent tip gap/span. A detailed understanding of the flow field shows this to be credible.

A Computational Study of a Novel Turbine Rotor Partial Shroud

A comprehensive experimental investigation was undertaken to explore the flow field in the tip clearance region of a turbine rotor to understand the physics of tip leakage flow. Specifically the paper looks at its origin, nature, development, interaction with the secondary flow, and its effects on performance. The experimental study was based on data obtained using a rotating five-hole probe, Laser Doppler Velocimeter, high-response pressure probes on the casing, and static pressure taps on the rotor blade surfaces. The first part of the paper deals with the pressure field and losses. Part II presents and interprets the vorticity, velocity, and turbulence fields at several axial locations. The data provided here indicates that the tip leakage vortex originates in the last half chord. The leakage vortex is confined close to the suction surface corner near the blade tip by the relative motion of the blade and the casing, and by the secondary flow in the tip region. The tip leakage flow clings to the blade suction surface until midchord then lifts off of the suction surface to form a vortex in the last 20 percent of the blade chord. The relative motion between blades and casing leads to the development of a scraping vortex that, along with the secondary flow, reduces the propagation of the tip leakage flow into the mainflow. The rotational effects and coriolis forces modify the turbulence structure in the tip leakage flow and secondary flow as compared to cascades.

Tip Clearance Effects in a Turbine Rotor: Part II — Velocity Field and Flow Physics

Tip clearance losses represent a major efficiency penalty of turbine blades. This paper describes the effect of tip clearance on the aerodynamic characteristics of an unshrouded axial-flow turbine cascade under very low Reynolds number conditions. The Reynolds number based on the true chord length and exit velocity of the turbine cascade was varied from 4.4 × 104 to 26.6 × 104 by changing the velocity of fluid flow. The free-stream turbulence intensity was varied between 0.5% and 4.1% by modifying turbulence generation sheet settings. Three-dimensional flow fields at the exit of the turbine cascade were measured both with and without tip clearance using a five-hole pressure probe. Tip leakage flow generated a large high total pressure loss region. Variations in the Reynolds number and free-stream turbulence intensity changed the distributions of three-dimensional flow, but had no effect on the mass-averaged tip clearance loss of the turbine cascade.Copyright © 2005 by ASME

Effects of Reynolds Number and Free-Stream Turbulence on Turbine Tip Clearance Flow

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Aerodynamic tip desensitization in axial flow turbines

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.Copyright © 1994 by ASME

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G. Morphis

Papers

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