Page 139 - Fluid Mechanics and Thermodynamics of Turbomachinery

120 Fluid Mechanics, Thermodynamics of Turbomachinery
(1) the rotational speed,
(2) the inlet stagnation temperature,
(3) the flow area.

NB. The combination of values for and at R D 0.5 used in this example were
selected from data given by Wilson (1987) and correspond to an optimum total-to-
total efficiency of 91.9%.
Turbine blade cooling

In the gas turbine industry there has been a continuing trend towards higher
turbine inlet temperatures, either to give increased specific thrust (thrust per unit
air mass flow) or to reduce the specific fuel consumption. The highest allowable
gas temperature at entry to a turbine with uncooled blades is 1250 1300 K while,
with blade cooling systems, a range of gas temperatures up to 1800 K or so may be
employed, depending on the nature of the cooling system.
Various types of cooling system for gas turbines have been considered in the past
and a number of these are now in use. Wilde (1977) reviewed the progress in blade
cooling techniques. He also considered the broader issues involving the various
technical and design factors influencing the best choice of turbine inlet temperature
for future turbofan engines. Le Griv` es (1986) reviewed types of cooling system,
outlining their respective advantages and drawbacks, and summarising important
analytical considerations concerning their aerodynamics and heat transfer.
The system of blade cooling most commonly employed in aircraft gas turbines
is where some cooling air is bled off from the exit stage of the high-pressure
compressor and carried by ducts to the guide vanes and rotor of the high-pressure
turbine. It was observed by Le Griv` es that the cooling air leaving the compressor
might be at a temperature of only 400 to 450 K less than the maximum allowable
blade temperature of the turbine. Figure 4.19 illustrates a high-pressure turbine rotor
blade, cut away to show the intricate labyrinth of passages through which the cooling
air passes before it is vented to the blade surface via the rows of tiny holes along
and around the leading edge of the blade. Ideally, the air emerges with little velocity
and form a film of cool air around the blade surface (hence the term “film cooling”),
insulating it from the hot gases. This type of cooling system enables turbine entry
temperatures up to 1800 K to be used.
There is a rising thermodynamic penalty incurred with blade cooling systems as
the turbine entry temperature rises, e.g. energy must be supplied to pressurise the
air bled off from the compressor. Figure 4.21 is taken from Wilde (1977) showing
how the net turbine efficiency decreases with increasing turbine entry temperature.
Several in-service gas turbine engines are included in the graph. Wilde did question
whether turbine entry temperatures greater than 1600 K could really be justified in
turbofan engines because of the effect on the internal aerodynamic efficiency and
specific fuel consumption.

Turbine flow characteristics
An accurate knowledge of the flow characteristics of a turbine is of considerable
practical importance as, for instance, in the matching of flows between a compressor