CHAPTER 7
CHAPTER 7 – INTERNAL COOLING AND SEALING
CONTENTS
PAGE
System Overview
02
Blade/NGV Cooling and Sealing
04
INTERNAL COOLING AND SEALING SYSTEM – System Overview
System Overview
In addition, the bearing chambers are sealed with this airflow;
see Chapter 4 Jet Engine Oil System for details.
Jet engines, like piston engines are ‘Heat’ engines, that is,
they generate their power by using heat to rapidly expand the
air, which passes through it. The temperatures generated are
extremely high, in some cases higher than the metal
components can actually withstand.
System Description
Air is taken from the compressor via internal passageways to
various sections of the engine.
The stage of compressor used as a source is always a higher
pressure than the gas flow passing through the section of the
engine requiring cooling and sealing. For instance the high
pressure front stages of the turbine, would need air from the
rear of the compressor (see Chapter 2 Cycle), as this is the
highest pressure within the engine. Therefore the higher
pressure compressor air can be fed to the front turbine section
and then it will flow into the gas stream, details of typical
airflows can be seen on the following pages.
The reason the engines don’t just melt and fall apart, is
because of the internal Cooling and Sealing flows.
Cooling
The cooling flows are provided, as the name suggests, to
protect the metal components from these damaging
temperatures.
One cooling flow is described in Chapter 2 Cycles, which is
the dilution air into the combustion chambers. Without this
flow the combustion chambers would simply melt and
disintegrate.
With a few exceptions, cooling and sealing air flows are
uncontrolled, that is when the engine is running, the air flows
through the internal passageways.
Similarly, the turbine blade and nozzle guide vane aerofoils
need to be cooled or they too would simply disappear.
Additionally, the cooling airflows ensure the hot gasses do not
impinge on the turbine disc rims, preventing potential disc
failure
In addition to cooling air flows, there is also a requirement for
heating airflows. The front of the engine can be subject to ice
build up in certain conditions, therefore a heating flow to get
rid of the ice is required to keep the engine running efficiently.
Sealing
Icing conditions are not constant, so the anti-icing airflows are
pilot selectable as the need arises.
Sealing is required to ensure the engine runs as efficiently as
possible, i.e. ensuring that all the high energy hot gas flow
passes between the turbine aerofoil sections and not over the
tip or through the root area.
2
Inlet Vane
and Bullet
Anti Icing Air
Disc Cooling,
Blade Film Cooling
and Tip Sealing
Anti Icing Air
Control Valve
Stage 6 Internal Cooling Air
Notional Internal Cooling and
Sealing System
Stage 14 Anti Icing Air
Stage 17 Internal Cooling and Sealing Air
JET ENGINE INTERNAL COOLING AND SEALING
3
INTERNAL COOLING AND SEALING SYSTEM – Blades and NGV’s
History – No Cooling
There are so many holes that the air creates a film or barrier
between the aerofoils and the hot gasses preventing the hot
exhaust gasses from coming into contact with the aerofoil
material.
In early designs, turbine blades (blades and NGV’s) were solid
and had no cooling airflows. By design and progress, Turbine
Entry Temperatures (TET) rose to values that the turbine
blades could not withstand, they would simply melt and be
blasted out the rear of the engine with the gas flow.
Tip Sealing
A small proportion of the same cooling air supply is fed to the
blade tips, between two or three ‘lands’ or projections. When
all the turbine blades are assembled into the turbine disc,
these projections form a continuous channel around the blade
tips.
Therefore a method of being able to protect the blade material
from these high TET’s was needed. In initial designs, aerofoils
were simply made hollow and cooling air passed through from
the root to the tip. It wasn’t long before progress raised TET’s
even higher and ever more sophisticated designs were
required.
The channel is pressurized by this air supply and prevents any
hot gasses from travelling over the blade tip. The more energy
passing between the blades means more energy can be
extracted, i.e. making the engine more efficient.
Aerofoil Internal Cooling
High pressure compressor air is fed, via internal passageways
through the engine to the aerofoil (blade and NGV’s). the air is
then fed into the inside of the aerofoil.
General Points
The rotating turbine blades are generally fed with cooling and
sealing air via the root, the none-rotating NGV’s can be fed
from either the root or through the engine outer casings.
Modern aerofoil internal air passageways are complex, with
single, double or triple pass flows; that is, the air can pass
through the aerofoil up to three times before escaping into the
hot gas flow. Some designs had more than one source of
cooling air.
It is important that the cooling and sealing air is clean, any dirt
(such as sand from desert area operations) can clog the film
cooling holes resulting in blade failure; blade cooling design
has to take this into consideration to ensure engine life.
Internal air offtakes are usually in towards the centre of the
engine, where any dirt would be centrifuged out to the tip area
of compressor blades, reducing the contaminates in the
cooling and sealing air.
Aerofoil ‘Film’ Cooling
Film cooling is a name given to a ‘blanket’ of cooling air
around the outside of the aerofoil material.
The film cooling air fed to the aerofoil surface via hundreds of
small holes allowing the air to escape into the hot gas flow.
4
A
INTERNAL
COOLING FLOW
OUTER SEAL SEGMENT
COMBUSTION
GAS FLOW
OUTER
PLATFORM
SEAL
BLADE AEROFOIL
AND FILM
COOLING FLOW
COMBUSTION
GAS FLOW
FILM COOLING
BARRIER
UP THROUGH
THE ROOT
JET ENGINE INTERNAL COOLING AND SEALING
5