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

An experimental study of a turbine tip desensitization method based on tip coolant injection was conducted in a largescale rotating turbine rig. One of twenty-nine rotor blades was modified and instrumented to have a tip trench with discrete injection holes directed towards the pressure side. Time accurate absolute total pressure was measured 0.3 chord lengths downstream of the rotor exit plane using a fast response dynamic pressure sensor in a phase-locked manner. The test cases presented include results for tip gap heights of 1.40% and 0.72% of the blade height, and coolant injection rates of 0.41%, 0.52%, 0.63%, and 0.72% core mass flow rate. At a gap height of 1.40% the leakage vortex is large, occupying about 15% blade span. A reduction in gap height causes the leakage vortex to reduce in size and move towards the blade suction side. The minimum total pressure measured, for the reduced gap, in the leakage vortex is about 4% greater. Coolant injection from the tip trench is successful in filling in the total pressure defect originally resulting from the leakage vortex without injection. At the higher tip injection rates the leakage vortex is also seen to have moved towards the blade tip. The high momentum associated with the tip jets affects the total pressure distributions in the neighboring passages.

Axial Turbine Tip Desensitization by Injection From a Tip Trench: Part 1 — Effect of Injection Mass Flow Rate

A numerical study has been conducted to explore the effect of a pressure-side winglet on the flow and heat transfer over a blade tip. Calculations are performed for both a flat tip and a squealer tip. The winglet is in the form of a flat extension, and is shaped in the axial chord direction to have the maximum thickness at the chord location where the pressure difference is the largest between the pressure and suction sides. For the flat tip, the pressure side winglet exhibits a significant reduction in the leakage flow strength and an associated reduction in the aerodynamic loss. The low heat transfer coefficient “sweet-spot” region is larger with the pressure-side winglet, and lower heat transfer coefficients are also observed along the pressure side of the blade. The winglet reduces the average heat transfer coefficient by about 7%. In the presence of a squealer, the role of the winglet decreases significantly, and only a 0.5% reduction in the pressure ratio is achieved with the winglet with virtually no reduction in the average heat transfer coefficient.Copyright © 2003 by ASME

Blade Tip Leakage Flow and Heat Transfer With Pressure-Side Winglet

Numerical investigation of a HPT with different rotor tip configurations in terms of pressure ratio and efficiency

An evaluation of the tip clearance effects on turbine efficiency for space propulsion applications considering liquid rocket engine using turbopumps

This study is an attempt to reduce the effect of the leakage vortex in axial flow turbines. A 3D Navier-Stokes CFD solver with k-e turbulence modeling was used compute the flow through an axial flow turbine with modified blade tip designs. A baseline flat tip case and three modified tip cases were simulated and the leakage flow and vortex for each was analyzed in detail. The three modified blade tip designs each involved adding a chamfer to the tip of the blade, in an attempt to diffuse the leakage flow through the gap and obstruct the leakage flow with the outer casing’s shear layer. Chamfering of the blade tip near the leading edge of the gap and across the entire gap region both failed to reduce the size and strength of the leakage vortex. By chamfering the blade tip near the trailing edge of the gap, the leakage flow inside the gap was turned toward the direction of the blade’s camber. This turning resulted in a decrease in the size and the strength of the leakage vortex and its subsequent losses, while at the same time, did not reduce the blade loading by an appreciable amount.This paper is available in color on the World Wide Web at http://navier.aero.psu.edu/∼jat/research.htmlCopyright © 2001 by ASME

J. Tallman

Papers

Methods for Desensitizing Tip Clearance Effects in Turbines