Abstract:
The specific power (or specific thrust) of modern gas
turbines is much influenced by the gas temperature at turbine
inlet. Even with the use of the best superalloy available and the
most advanced cooling configurations, there are competitive
pressures to operate engines at even higher gas temperatures.
Ceramic coatings operate as thermal barriers and can allow
the gas temperature to be increased by 50 to 220 K over the
operating gas temperature for an uncoated turbine .
It is important that the surface temperature of the blade
be determined as accurately as possible. Large uncertainties as
to the surface temperature require significant margins for safe
operation .
Blade surface temperatures can be determined with an
accuracy of 10 K using radiation pyrometry and about"30 to 40 K
by calculating the blade temperature based on---gas temperature
measurement of the exhaust gas plane. This'- makes pyrometry an
attractive option for advanced high temperature gas turbines .
However, there is little experience in measuring surface
temperatures of blades coated with ceramic coatings. There is
evidence that the. radiation signal picked up by the pyrometer
will not only depend on the surface temperature but also on a
number of optical properties of the coating. Important among
these are the emissivity of the coating and whether the coating
is translucent. Parameters affecting this are the coating
material, coating surface finish, coating thickness and whether
or not a bond coat is used .
This work explores these variables in a rig that simulates
the conditions within a turbine stage of a gas turbine engine. In
which six thermal barrier coating systems were tested. These
systems are of current interest to gas turbine manufacturers and
users. They include the latest advances in coating technology.
Four stabilized zirconia systems and two alumina based systems
were tested.
It was found experimentally that the surface emissivity of
these coating systems was invariant over the range 873 to 1023 K
surface temperature. It was found that the use of different
stabilizers did not affect the surface spectral emissivity.
In further experiments six turbine wheels were coated with
these systems and tested at turbine entry temperatures of 973,
1073, and 1173 K. It was found that the blade surface temperature
was function of the coating material, coating thickness and
turbine entry temperature. The blade surface temperature was also
function of the blade height being maximum at the blade tip and
minimum at the blade root .
It was found that the C-YPSZ was better insulator than the
rest of the systems. Whilst the blades coated with zirconia
based systems suffered minor loss near the edges, the two alumina
based systems were lost from more than a blade during the test.
This coating loss was picked up by. the pyrometer .
Analysis shows that the measured blade surface temperature
was within 10 K of that calculated. The use of 0.3 mm of C-YPSZ
on air cooled turbine blades caused 250 K surface temperature
increase and 270 K metal temperature decrease for turbine entry
temperature of 1673 K. The metal temperature reduction was as
high as 310 K for coating thickness of 0.5 mm.