15.05 Combustion Section Flashcards
What is the purpose of the combustion section?
It produces a hot stream of gas for the turbines and jet nozzle by continuous combustion of a fuel-air mixture
What are the requirements for safe and efficient combustion?
- safe ignition
- stable combustion
- complete combustion
- equal temperature distribution
- combustion with minimum pressure loss
What properties must the combustion chamber have?
- lightweight and small
- have a cooling system
- high operating lifetime
What is the purpose of the combustion chamber casing?
It protects the internal and external engine parts from the hot gases and also takes air pressure loads
When are the igniters turned off?
After the engine has been started and is self-sustaining
What are the two primary conditions for safe combustion?
- the ratio of air to fuel must be correct
- the air velocity must be slow enough for complete combustion
What happens to the airflow from the compressor?
It splits into the primary airflow and secondary airflow
What is the primary airflow from the compressor?
It is approximately 20% to 30% of the air which is used for the combustion process
What ratio of kerosene to air mixture is required for complete combustion?
15 parts air to 1 part fuel
What is the secondary airflow from the compressor?
- bypasses the primary combustion zone and mixes with hot gas in dilution zone
- used for cooling (as temperature 2000°C in centre of flame)
How does the secondary airflow bypass the primary combustion zone?
It enters the dilution zone of the flame tube via dilution air holes
What are the three types of combustion chamber?
- the can-type combustion chamber
- the can-annular combustion chamber
- the annular combustion chamber
Where would you find a single-chamber combustion chamber?
Small gas turbine engines such as APU
What is the purpose of the interconnector tubes in a multiple-can type combustion chamber?
They connect all chambers together and ensure equal pressure throughout and also ignition from the two chambers with the igniter plugs
What are the advantages of a can-type combustion chambers?
- simple design
- very good structural strength
- can be installed and removed individually
What are the disadvantages of a can-type combustion chambers?
- difficult ignition from one chamber to another
- heavy and require a lot of space
- complicated air supply ducts
What type of combustion chamber has flame tubes surrounding it?
The “can-annular” combustion chamber
What are the advantages of can-annular combustion chambers?
- simple air supply ducts
- good structural strength
- smaller and lighter than the simple multiple can-type
What are the disadvantages of using the can-annular combustion chambers?
- Aerodynamic losses are high
- ignition from one flame tube to another is difficult
What type of combustion chamber is the most commonly used one on modern gas tubing engines?
The annular combustion chamber
What is important about the annular combustion chamber’s outer casings?
They are part of the load carrying engine structure
What are annular combustion chambers also known as because of their special design?
Reverse flow combustion chamber
What are the advantages of annular combustion chambers?
- better thermal load distribution
- smaller and lighter
- larger combustion area = better efficiency and smokeless combustion
- flame is easier to spread over the flame tube
What are the disadvantages of annular combustion chambers?
- expensive to make
- removal is difficult and time consuming
Why are annular combustion chambers used if they have disadvantages?
The disadvantages are compensated for by a very high service life
What makes up an annular combustion chamber?
- a combustion case
- a flame tube
- external components fixed to outer wall
On the outer wall of an annular combustion case, what are the mounting flanges for?
They are for bleed air tubes and cooling air tubes
What is the purpose of the flame tube in an annular combustion chamber?
It stabilises the flame during combustion
What are the stator vanes of the last part of the HP compressor also known as?
Exit guide vanes
What is a design feature of the inner wall of an annular combustion chamber?
It is usually removable so that the flame tube can be installed
Where are the igniter bosses installed in comparison to the fuel nozzles?
They are installed slightly behind the fuel nozzles
The fuel nozzle openings and the igniter plug openings can also be used as…?
Borescope inspection ports of the flame tube
What is the purpose of the bore in a swirl vane?
It holds the nozzle tip in the centre
How is the flame tube bolted to the combustion case?
By a fixed point at one end and a movable point at the other (thermal expansion)
What is important about the mounting principle of a combustion chamber?
It allows for long service life for the flame tube
What is film cooling?
The secondary airflow that enters the flame tube through liners instead of the swirl vanes creates a film of cooling air on the inner surface of the liners.
What flame tube cooling methods are used on modern combustion chambers?
- machined cooling rings
What flame tube cooling methods were used in older combustion chambers?
- Corrugated strips
- splash cooling strips
What is the most sophisticated method of film cooling in the combustion chamber?
Transpiration cooling method
What is a thin coating of ceramic material on the inner surface of the combustion chamber used for?
Heat protection
