15.5 Flashcards
What must you ensure about combustion in a combustion chamber?
Combustion must be completed (No unburnt fuel must leave the combustion chamber)
What are the two main components of a combustion chamber?
Combustion chamber casing and flame tube
What can the casing be split into?
Inner and outer casing
What is the function of combustion chamber casing?
To take air pressure loads and protects the internal and external parts from the hot combustion gases.
What gives to the flame tube, and other chamber components such as fuel nozzles and igniter plugs?
The housing (casing)
What does the flame tube do?
Controls and guides the flame
What is the flame tube made up of?
Segments with holes and slots
What sprays fuel into the centre of the airflow? And what happens after its ejected?
Fuel nozzles/Injectors. The fuel vaporises
What creates ignition of the fuel-air mixture?
Electrically by the ignitor plug which switches off as soon as combustion occurs
Why does the ignitor plug turn off after combustion?
Because combustion continues as long as fuel and air are supplied to the engine?
What are the two primary conditions necessary for safe combustion?
The ratio of air to fuel
The air velocity
Why must air velocity be slow enough in the combustion chamber?
To make sure that the combustion occurs in it
What is the ratio of air to fuel?
15:1
What are the two parts of compressor air?
The primary and secondary airflow
What airflow is used for the combustion process?
Primary (20-30%)
What does the rest of the airflow do?
Bypass the combustion come and mixes with the hot gas in the dilution zone
What occurs in the primary zone?
The flame stabilisers
Where can ‘Can-Type’ combustion chambers be found?
On old gas turbines with a centrifugal compressor
Each combustion chamber has its own what?
Air supply duct
Fuel nozzle
Flame tube
Casing
What are the 3 advantages of Can- type combustion chambers?
Simple design
They can be removed and replaced individually
Great structural strength
What are the two types of Can-type combustion chambers?
Single or multiple chambers
What are the three disadvantages of can type combustion chambers?
Heavy, need a lot of space
Complicated air supply ducts
Difficult to ignite different chambers
What is a common feature of a ‘Can-annular’?
All the flame tubes have a common inner and outer casing.
Advantage of ‘Can-annular’?
Smaller and lightweight
Disadvantage of ‘Can-Annular’?
Aerodynamic losses are high and ignition from one flame tube to the other is difficult
What type of combustion chamber is commonly used of modern gas engines?
Annualar
How can you tell that a combustion chamber is annular?
It has one flame tube!
The annular consists of what 3 major parts?
Inner lining
Outer lining
The dome
What is the purpose of the dome?
It has openings for the fuel nozzles which are surrounded by the swirl vanes
What two types of combustion chambers are ideal for use in the APU?
Can type and Annular type
What combustion chamber is known as a reverse combustion chamber?
Annualar
What are advantages of Annular combustion chambers?
Smaller and lighter
Large combustion area for greater efficiency
Flame can spread easily
Combustion is complete and smokeless
What are the disadvantages of annular combustion chambers?
They are expensive and time consuming for maintenance
What are the three sections in an annular combustion chamber?
The flame tube
Combustion case
External components fitted to outer wall
What does the combustion case contain?
An inner wall
An outer wall
Diffuser
What are the 2 integral parts of a combustion case?
Stator vanes also known as exit guide vanes
A diffuser
What Is fed through the outer wall after the last compressor stage?
Bleed air that is fed to the pneumatic system
Other than openings for bleed air, what is also located on the outer wall?
Ignitor bosses
Fuel nozzle pads
How many ignitor plugs are installed on the outer wall of the combustion case?
2 in the lower half of the combustion case
The combustion case/the compressor rear frame is classed as a what?
A frame
What does the combustion case also house?
Bearings for HP rotor
Main components of the flame tube?
Inner and outer cowl
Inner and outer liner
Dome
What does the dome section of the flame tube do?
It connects inner liner with outer liner
Houses swirlers with swirl vanes
What do the inner and outer liners do?
They hold the flame tube to the combustion case
What stays the same across all engines in regards to a flame tube?
The flame tube is fixed at one end and moveable at the other
How many dimensions is the flame tube fixed in at the aft end?
3
What SOMETIMES allows the flame tube to expand at the rear?
The fact its fixed at the front
What is the dome centred in the combustion case by?
Mounting pins
What does primary airflow do?
Enters the flame tube through the swirl vanes
What route does secondary airflow take?
Passes the cowl and enters the flame tube through the liners
What is the purpose of secondary airflow?
Forms a cooling film on inner surface of liners to protect it from heat (film cooling)
What is a more sophisticated method for film cooling, and why?
Transpiration as it requires less air
How does transpiration work?
The inner surface of the flame tube liners has detachable tiles
How does the secondary airflow enter the flame tube?
Through holes in the liner
What is the main cause for pollution?
Unburnt fuel
When do these pollutants occur?
If combustion is incomplete due to an incorrect fuel to air ratio
What is a rich mixture?
Too much fuel for primary air in the combustion chamber
If a rich mixture doesn’t burn all of its fuel, where will it dump it?
Exhaust
Excessive fuel and low combustion also supports the build up of what?
Carbon Monoxide
What is a lean mixure?
If there is insufficient fuel available in combustion chamber which causes less pollution but flame can extinguish
How does increasing the temperature reduce carbon monoxide emissions?
Nitrogen oxide build up
What happens in an annular flame tube?
Fuel nozzle releases fuel, igniter ignites fuel and swirl vanes ensure the primary air is reversing to ensure the flame doesn’t extend through. Secondary airflow is used to cool the inner lining and ensure it creates a vortex that makes the air stay in.
What is the purpose of the diffuser?
Increase static pressure which the swirling vanes
What part of the inner wall is removable to allow for installation of the flame tube?
Rear
