Handbook of Turbomachinery Second Edition Revised - Ventech!

vortex structure. Their studies shows that significant total-to-total efficiency
gain is possible by the use of tip platform extensions.
For more detailed information dealing with turbine blade-tip heat
transfer, the reader is referred to a recently published review by Bunker [98].
COOLING OF ROTOR AND STATOR STRUCTURES
Cooling Supply and Secondary Air Systems
The role of a secondary air system, which is often also called internal cooling
system, is to deliver cooling air to the cooled gas-path components, cool
rotor, and stator structures and prevent hot gas ingress into internal cavities
and seal leakages between turbine stages. Traditionally, the combustor
secondary air flows were treated independently from the turbine section
using pressure losses through a combustor that were specified by engine
designers. However, the secondary air circuits in the combustor and turbine
sections are closely coupled being supplied from the same pressure source—
the compressor discharge. For this reason, more recently the entire (often
very complicated) flow network of the secondary air system is being
analyzed to provide accurate prediction of local pressure and flow
distributions. Also advanced analytical tools, which must include heattransfer
effects in the flow calculations, started to use pseudo-3D or true 3D
flow modeling for the secondary cooling systems.
Delivering cooling air to the cooled component has to be performed at
lowest pressure losses and with minimal heat pick-up on the way to the
component. Lowest pressure losses in the delivery system are particularly
critical for a shower-cooled leading edge of the stage 1 nozzle, especially for
modern industrial gas turbines designed with a low combustor pressure
drop. A combustor pressure drop in order of 3% of the total compressor
discharge pressure that is often found in these engines leads to a very small
pressure margin at the shower head film discharge holes in the leading edges.
Secondary flow and heat-transfer issues in the disc cavities and bladecooling
delivery systems play a crucial role in the life and structural integrity
of these components. Blade- and rotor-cooling delivery system deserves
special attention—its significant impact on engine performance losses
associated with the cooling air discharge into the main stream must also
be considered. Some issues related to flows and heat transfer in the disc
cavities are discussed in the following section. Minimal heat pick-up is very
important for the cooled stage 1 disc rim and blade structure. There are four
main ways to deliver air from a turbine stator to the blades:
Copyright © 2003 Marcel Dekker, Inc.