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Handbook of Turbomachinery Second Edition Revised - Ventech!

Handbook of Turbomachinery Second Edition Revised - Ventech!

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Three-Dimensional Effects<br />

The three-dimensional flow characteristic in a turbine passage in a cascade<br />

environment is well understood, and there is good agreement among<br />

investigators regarding the major flow structures (Sieverding and Langston)<br />

[65–66]. Recent experimental measurements suggest that the local efficiency<br />

gains from a pressure-side tip platform extension can be significant. A<br />

number <strong>of</strong> studies revealed that there is considerable secondary flow that is<br />

directed toward the tip region especially between the leading edge and<br />

around 30% axial chord. This is explained by Dring & Joslyn [67] as a<br />

relative eddy, which carries the fluid toward the blade tip. It is a simple<br />

physical phenomenon where the secondary flow vorticity locally dominates<br />

the main-stream flow because in this region <strong>of</strong> the blade the axial Mach<br />

numbers are quite low ð40:3Þ. The secondary flow can result in a slightly<br />

higher heat transfer due to skewing <strong>of</strong> the near-wall flow.<br />

One <strong>of</strong> the significant effects <strong>of</strong> rotor flow is the redistribution <strong>of</strong> inlet<br />

temperature radial pr<strong>of</strong>ile along the blade passage. There is also an unsteady<br />

circumferential effect generated by combustor burners. It is observed that<br />

the secondary flow within the passage can be augmented due to temperature<br />

distortion. As the rotor blade passes through a combustor hot streak, the<br />

flow within the blade passage becomes density-stratified and suffers<br />

incidence changes and additional secondary vorticity. The accompanying<br />

Mach number distributions result in quite high velocities in the tip gap<br />

region, which generate correspondingly high shear stress and heat transfer in<br />

the blade-tip gap. As the hot gas reaches the trailing edge <strong>of</strong> the blade, it<br />

migrates to the suction surface and also penetrates through the tip gap.<br />

Radial Gas Temperature Pr<strong>of</strong>ile<br />

Due to the typical distribution <strong>of</strong> centrifugal forces along the blade height<br />

with its maximum at the root section and minimum at the tip, a decline in<br />

gas temperature toward the blade platform is necessary to achieve uniform<br />

creep-rupture life through the blade height. In addition, blade material<br />

properties usually allow significantly higher metal temperature than the<br />

surrounding end-wall structures: turbine disc under the platform and stator<br />

components that form the radial clearance between rotor and stator. For<br />

these reasons, a blade inlet radial temperature pr<strong>of</strong>ile has to be skewed<br />

toward the end walls with the peak temperatures located usually between<br />

50% and 70% <strong>of</strong> the blade height [68]. This radial temperature pr<strong>of</strong>ile has to<br />

be formed in the combustor and include effects <strong>of</strong> the cooling air that enters<br />

the main gas flow upstream <strong>of</strong> the blade, after cooling the nozzle end walls,<br />

vanes, and disc forward faces. Cooling air discharged from the upstream<br />

Copyright © 2003 Marcel Dekker, Inc.

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