Turbine cooling :
5. High thermal efficiency is dependent upon high turbine entry temperature, which is limited by the
turbine blade and nozzle guide vane materials. Continuous cooling of these components allows their
|Fig. 9-2 Nozzle guide vane and turbine blade cooling arrangement.|
and vane integrity. Heat conduction from the turbine blades to the turbine disc requires the discs to be
cooled and thus prevent thermal fatigue and uncon- trolled expansion and contraction rates.
6. An air cooled high pressure nozzle guide vane and turbine blade arrangement illustrating the
cooling airflow is shown in fig. 9-2. Turbine vane and turbine blade life depends not only on their form but
also on the method of cooling, therefore the flow design of the internal passages is important. There
have been numerous methods of turbine vane and turbine blade cooling which have been used
throughout the history of gas turbines. Generally, single pass internal (convection) cooling was of great
practical benefit but development has lead to multi-pass internal cooling of blades, impingement cooling
of vanes with external air film cooling of both vanesand blades, these are shown in fig. 9-3. and fig. 9-4.
7. The 'pre-swirl nozzles' (fig. 9-2) reduce the temperature and pressure of the cooling air fed to the
disc for blade cooling. The nozzles also impart a substantial whirl velocity to assist efficient entry of
the air into the rotating cooling passages.
8. Cooling air for the turbine discs enters the annular spaces between the discs and flows
outwards over the disc faces. Flow is controlled by interstage seals and, on completion of the cooling
function, the air is expelled into the main gas stream (fig. 9-5); see para. 23., Hot gas ingestion.