Handbook of Turbomachinery Second Edition Revised - Ventech!

also performs a very important convective cooling function within the film
discharge holes. The most effective system combines internal convective and
external film cooling. In an ideal situation where film cooling approaches
transpiration cooling, the film cooling source air temperature might be close
to the targeted metal temperature. The cooling effectiveness in this case can
be close to 1. This, however, requires large cooling flows and multiple rows
of film holes to achieve full film coverage of the component without
degradation of the film cooling effectiveness between the holes.
Cooling Penalties
For a modern industrial turbine, the cooling airflow requirements are in the
order of 20–25% of the total compressor flow. This large quantity of air is a
source of a significant loss to the engine cycle performance as a whole and
has three effects that tend to partially offset the performance improvement
of high turbine inlet temperature operation. The first is that the air used for
cooling enters the turbine at a lower temperature, reducing temperature of
the gas downstream of combustor. Thus, to deliver a specified power, the
engine has to run at a higher turbine inlet temperature than would an
uncooled engine. The second is that the cooling air bypasses the combustor,
subsequently leading to greater challenges to control emissions and provide
desirable temperature distribution at the combustor exit. The third is related
to aerodynamic losses when the cooling air is reintroduced and mixes with
the higher-velocity main gas stream. The aerodynamic loss, often called the
mixing loss, is caused by the injection of coolant into a turbine airfoil
passage, and its subsequent mixing with the main-stream flow. This is
generally reported in terms of a decrease, or loss, in the total pressure of the
mainstream. Vigorous steps must therefore be taken to minimize both the
quantity of cooling air used and the losses associated with its use, in order to
achieve maximum benefits of the high cycle temperature. This can be a
severe restraint on the degree of freedom with which the cooling design is
performed.
Regardless of what type of open cooling technique is used—internal or
film cooling—the spent air has to be discharged through the wall along the
surface of the airfoil or through its trailing edge. For this reason, when the
applied cooling technique is based on internal cooling, an effort should be
made to utilize the spent air for some film protection. And, for the same
reason, when film cooling is applied an effort has to be made to utilize the
cooler air for some convective cooling along the channels of the film
discharge holes.
Two main objectives related to the penalty reduction have to be
considered during airfoil internal cooling design:
Copyright © 2003 Marcel Dekker, Inc.