1. JET PROPULSION
Part 2
Combustion

2. Introduction We know that the compressor,
situated at the front of the engine,
is driven by the turbine,
and performs two functions -
it draws air into the engine
and it compresses it
before delivering it into the combustion chamber.

3. Introduction The air from the compression section,
at anything up to 450 lb/sq in,
passes into the combustion chamber.
This chamber is designed to achieve
the most efficient combustion
of the fuel/air mixture,
so that the maximum possible heat energy
is extracted from the fuel
in order to give the greatest rise in temperature
and hence expansion of gas.

4. THE COMBUSTION PROCESS
Combustor Operation
THE COMBUSTION PROCESS
In a Piston Engine -
Fuel is mixed with air before entering the cylinder
The fuel/air mixture is then compressed
It is then ignited by a spark
Once for 2 revs of the engine (in a 4 stroke cycle)
In a Jet Engine -
Air is compressed and forced into the combustor
The fuel is then sprayed in, under pressure
It is then ignited by a spark, but only once to start
Combustion is then continuous

5. Combustor Operation THE COMBUSTION PROCESS We’re going to look
at what happens here
The
Combustion
Chamber
But first we’ll look at the layout
of the combustion system

6. Combustor Operation THE COMBUSTION PROCESS
Combustion Chambers (Cans)
OUTER
CASING
Engine Structure
and Shaft
We will convert this to a basic diagram form

7. THE COMBUSTION PROCESS
Combustor Operation
THE COMBUSTION PROCESS
1st JET ENGINES
Each CAN has
a Fuel Spray
Nozzle
(FSN)
WHITTLE etc.
INNER
CASING
OUTER
CASING
Flame is
contained
within
these tubes
‘FLAME-TUBE’
Engine Structure
and Shaft

8. THE COMBUSTION PROCESS
Combustor Operation
THE COMBUSTION PROCESS
The next stage of design
was having the cans
connected
together
to equalise
the pressures,
and to spread
the flame
during the
start-up
TYPICAL OF
DART SERIES
TURBO PROPS
IGNITERS

9. Combustor Operation THE CAN-ANNULAR LAYOUT TYPICAL OF
A development of the
can type design
where all the
flame tubes
have an
FSN and
connectors,
but they
are all
surrounded
by an
annular casing.
TYPICAL OF
SPEY & TAY
BYPASS
ENGINES
Annular
outer casing
also forms
the engine
structure
IGNITERS
Individual cans with inter-connectors

10. THE CAN-ANNULAR LAYOUT
Combustor Operation
THE CAN-ANNULAR LAYOUT
TYPICAL OF
SPEY & TAY
BYPASS
ENGINES

11. Annular inner casings & flame tube
Combustor Operation
THE ANNULAR LAYOUT
This design features
both an annular
outer casing
as well as an
inner annular
flame tube
and is the
design of
choice for
modern
engines.
TYPICAL OF
MODERN BYPASS
ENGINES
Annular
outer casing
Annular inner casings & flame tube

12. Annular inner casings & flame tube
Combustor Operation
THE ANNULAR LAYOUT
TYPICAL OF
MODERN BYPASS
ENGINES
It allows more
FSN’s
to be fitted.
(up to 20 or more
depending on
engine size)
Annular
outer casing
Gives even
temperature
face to
turbine blades
Annular inner casings & flame tube

13. Combustor Operation THE ANNULAR LAYOUT TYPICAL OF MODERN BYPASS
ENGINES

14. Combustor Operation THE CROSS SECTION OF ALL THESE
Single Can Layout
Can Annular Layout
THE CROSS SECTION OF ALL THESE
LAYOUTS IS VIRTUALLY IDENTICAL
AND THEREFORE SO IS THE AIRFLOW
Annular
Layout

15. The Combustion Process
Inner
Flame Tube
82%
Cooling Flow
Outer
Combustion Casing F S N
Approximately 82% of the air from the compressor passes around
the inner flame tube and then into it via a number of ‘dilution’ holes
it is therefore known as
‘Dilution Air’.

16. ‘Re-Circulating Vortex’.
The Combustion Process
Inner
Flame Tube
82%
Cooling Flow
Outer
Combustion Casing
Recirculating Vortex F S N
18%
Vortex Flow
18% of the air passes into the flame tube where it splits into two main flows:-
Approximately half of the air swirls into the flame tube via the FSN fuel jet.
The other half passes around swirl vanes to produce what is known as a
‘Re-Circulating Vortex’.

17. The Combustion Process
Inner
Flame Tube
82%
Cooling Flow
Outer
Combustion Casing
Recirculating Vortex
Fuel Feed F S N
18%
Vortex Flow
The fuel is sprayed in under high pressure.
The turbulent air of the Recirculating Vortex
ensures good air and fuel mixing
and therefore efficient combustion.

18. The Combustion Process
Inner
Flame Tube
82%
Cooling Flow
Outer
Combustion Casing
Ignition
Fuel Feed F S N
18%
Vortex Flow
The flame temperature can be in excess of 2500ºC which is then reduced
by the dilution air to around 1000ºC before entering the turbine.
An even flow of the dilution air is extremely important
as any slight blockage will unbalance the air around the flame
and could produce a disastrous burn through.

19. Check of Understanding
The air from the compression section
passes into the combustion chamber
at anything up to what lb/sq in?
250 lb/sq in
350 lb/sq in
450 lb/sq in
550 lb/sq in

20. Check of Understanding
In a jet engine combustion is
Once for rev
of the engine
Once for 2 revs
of the engine
Once for 4 revs
of the engine
Continuous

21. Check of Understanding
Which of these statements applies to
the Dart Series of engines?
Each can was individual
Cans connected together
Cans connected together
with annular outer casing
Cans connected together
with annular inner casing

22. Check of Understanding
Which of these statements applies to
the Can-Annular layout?
Allows for more FSN’s
to be fitted
Annular inner casings
and flame tube
Is the design of choice
for modern engines
The outer casing also forms
the engine structure

23. Check of Understanding
Which layout gives an even temperature face
to the turbine blades?
The Whittle Layout
Single Can Layout
Can-Annular Layout
Annular Layout

24. Check of Understanding
In the combustion chamber,
air passes around the inner flame tube
and into it via a number of holes.
What is this air called?
Recirculation Air
Distribution Air
Dilution Air
Redirected Air

25. Check of Understanding
Approximately how much of the air from
the compressor passes into
the inner flame tube via ‘dilution’ holes? 9%
18%
62%
82%

26. Check of Understanding
The dilution air reduces the flame temperature
to around what level?
2500 degrees
2000 degrees
1000 degrees
500 degrees

27. JET PROPULSION
End of Presentation