15.6 - Combustion Flashcards
What are the 4 types of combustion sections?
Multiple-can
Can-annular
Annular-through flow
Annular-reverse flow
What is the ratio of fuel and air needed for a jet engine?
15 parts air to 1 part fuel
What is used to cool the flame tube?
Secondary air.
Either flowing between ceramic tiles and the interior walls, or by transpiration cooling where a film of air flows between laminations.
What is usually put on combustion liners?
A whitish coating which helps combat carbon deposits and erosion. Magnesium zirconate
Where would you see can type combustors?
Old gas turbine engines with centrifugal compressors
What are the advantages of can type combustors?
Simple design
Good structural strength
Can be maintained individually
What are disadvantages of can type combustors?
Large and heavy
Need lots of space
Complex compressor ducting (aerodynamic losses)
Ignition difficulties from one chamber to another
Where would you find a can-annular combustion chamber?
Older jet engines
What are advantages to a can-annular combustion chamber?
Smaller and lighter than multiple can design
Doesn’t need complex air ducting
Good strength
What are disadvantages to the can-annular combustion chamber?
High aerodynamic losses
Ignition difficulties from one chamber to another
Where would you find an annular combustion chamber?
Modern jet engines
How much shorter is the annular combustion chamber to its predecessors?
75%
What are the advantages of an annular combustion chamber?
Smaller and lighter High efficiency (larger combustion area) Complete combustion and smokeless Flame can spread easily Better thermal load distribution Less cooling air required Very high service life
What are the disadvantages to the annular combustion chamber?
Expensive
Removal is difficult and time consuming
What is the reverse flow combustion chamber?
A smaller annular combustion chamber that you would find on smaller engines or APU’s
What is a DAC?
Dual annular combustion chamber
What is the purpose of using a DAC?
Reducing pollutants in the fuel;
Hydrocarbons
Carbon monoxide
Nitrogen oxides
How is a DAC more efficient?
Has an inner and outer set of combustion areas, meaning the air-fuel ratio is controlled much better and reduces emissions.
When is a flame-out more likely to occur?
During takeoff- if large amounts of water are ingested
During decent- when throttled back, less fuel but high airflow through engine
What is the purpose of supplying air to the burner shroud and head?
It minimises carbon formation on the burner face
How is the combustion chamber drain valve closed?
By the combustion chamber gas pressure. It will open on engine shut down.
What is the purpose of the swirl vanes in the combustion chamber?
Gas re-circulation. They direct and slow the air as it enters the combustion chamber, and when it hits the secondary airflow, it creates a Toroidal Vortex.
What type of welding is used for the fabricated liner of the flame tube?
Electric resistance welding
What is the overall range of varying air/fuel ratio in a combustion chamber?
45:1 and 130:1
What happens to the flow due to pressures in a duplex burner in terms of it’s main manifold?
It is restricted until the proper pressure is obtained.
What does Carbon forming on fuel spray nozzles do?
Causes distortion of the fuel spray flow and direction. Can cause hot-spots within the combustion chamber
What percentage of air is used in the combustion process?
40%
What temperature is the flame in the combustion chamber?
2000 degrees C
What is “Hot Streaking”?
A partially clogged fuel nozzle
What is the purpose of the shroud placed around the fuel nozzles?
It prevents carbon build up
Where does a vaporising burner inject fuel in comparison to airflow?
Against the airflow
When is water/methanol injection used?
At high altitudes OR low air density
In at atomising burner, where is the fuel atomised? (also known as a vaporiser)
Partly inside the swirl chamber and partly outside the injector
