Numerical optimization of the air film cooling
One of the major problems in enhancing the specific work output
and efficiency in gas turbines is the maximum possible value of
the turbine inlet temperature due to blade material properties. Up
to about 1300 K uncooled blades can be used. To increase this
maximum, turbine blades need to be cooled (internal or external),
which is usually done by compressor air. Based on its high cooling
efficiency, film cooling is one of the major cooling techniques
for the hottest blades. In film cooling compressed air is injected along the blade surface,
forming a cold boundary layer, thus separating the hot gas from
the blades. The use of cooling is limited, while it leads to an
increased use of compressed air, which in its turn decreases the
systems efficiency. Besides the decrease of system efficiency
through the use of compressed air, film cooling also leads to a
change in the turbine flow conditions; a change in in- and outlet
angles of the flow around the blade.
In film cooling cold air is injected into the boundary layer
through small nozzles in the blade surface. The flow through these
nozzles is laminar with a Reynolds number Re of typically 100-
1000. The speed in the nozzles is of the same order of magnitude
as the free stream velocity.
Impingement of the jets into the (laminar) boundary layer flow
(with a Re typically 1000) is essentially three-dimensional. The
collision of the laminar jet with the boundary layer flow produces
a loca1 turbulent shear layer and changes the local heat transfer
to the blade (when poorly constructed it may even increase the
local heat transfer). It has been well established that the
geometry of the nozzles is an important parameter in film cooling.
The cooling effectively is influenced by local flow phenomena like
separation of the flow in the cooling hole, produced turbulence,
relaminarization and reattachment.
Figure 1. Example of the turbine rotor with
Figure 2. More detailed view of the blade