COMBUSTION Flashcards
What is the function of a combustion chamber?
Produce a hot stream of gas for turbines and the jet nozzle.
(Continuous combustion of fuel/air mixture)
Why should the combustion have minimum pressure loss?
Increase engine efficiency.
What are the 2 main components of a combustion chamber?
- Chamber casing (protects engine parts from hot combustion gases
- Flame tube (controls and guides the flame)
What 2 conditions are essential for correct and safe combustion?
Correct air/fuel mix
Air velocity must be slow enough to make sure combustion is completed inside the chamber.
What ratio of air/kerosene is required for complete combustion?
15 parts of air for 1 part of fuel.
What % of airflow from the compressor goes to the combustion chamber?
20-30%
Primary airflow
What does the secondary airflow from the compressor do?
Bypasses the primary combustion zone and mates with the hot gas in the dilution zone.
(Mainly used for cooling)
What happens in the primary zone?
The flame stabilises.
(Some of secondary air is used to keep the flame away from the flame tube walls)
(Centre of flame can reach 2000 Celsius)
Where can can-type combustion chambers be found?
Old gas turbine engines with centrifugal compressors.
Single or multiple can
What do interconnector tubes do in a can-type combustion chamber?
Permit the ignition from one chamber to another.
Equalise pressure in all combustion chambers.
What is an advantage of can-type combustible chamber?
Simple in design, good structural strength and can be individually removed and replaced.
What is an disadvantage of can-type combustion chamber?
Heavy and need a lot of space.
The ducts lead to high aerodynamic losses.
Difficult ignition from one chamber to another.
What is a can-annular combustion chamber?
Set of flame tubes installed around the combustion section.
What are advantages of a can-annular combustion chamber?
- Structural strength
- Simple air supply
- Smaller and lighter
What are disadvantages of can-annular combustion chamber?
High aerodynamic losses
Ignition from one flame tube to another is difficult.
What is the most common combustion chamber used on modern gas engines?
Annular combustion chamber.
What is an annular combustion chamber?
Has only one flame tube supported by an inner and outer casing.
What are the 3 major parts of the flame tube in an Annular combustion chamber?
Dome (openings for fuel nozzles and swirl vanes)
Inner liner
Outer liner
Where would a reverse flow combustion chamber be found?
Annular combustion chambers on small gas turbine engines or APU.
What are advantages of annular combustion chambers?
- Smaller and lighter at same airflow
- Combustion complete and smokeless
- Better thermal load distribution for the turbine
- High service life
What is a disadvantage of annular combustion chambers?
High maintenance costs
High manufacturing costs
What does the combustion case consist of?
Outer case
Inner case
Diffuser
What is the rear flange of the inner wall for?
Attach nozzle guide vanes to the high pressure turbine and support the flame tube.
What is a CDP?
Compressor discharge pressure seal.
Why does the outer wall have some openings?
Supply bleed air for the pneumatic system.
How many igniters are there?
Usually 2.
What is normally used for borescope ports for the combustion chamber and nozzle guide vanes of the first stage of high pressure turbine?
Openings for fuel nozzle and igniter plugs.
What does the combustion case house?
The bearings for the high pressure rotor.
What does each swirler in a flame tube have to hold the nozzle tip in its centre?
A bore.
What is the attachment of a flame tube always?
Fixed at one end and moveable at the other end.
How is the outer line support of the flame tube assembly fixed?
Axially.
What does the mounting assembly of the flame tube ensure?
Long service life from stresses in axial and radial direction during thermal expansion.
What does the dome section of the flame tube assembly do?
Connect the inner lining to the outer lining.
Housing swirlers with swirl vanes.
What is film cooling?
Secondary airflow forming a cooling film on the inner lining protecting it from high temp combustion gases.
What were used to get an optimum cooling film on older aircraft and modern aircraft?
Older- Corrugated strips or splash cooling strips.
Newer- machined cooling rings
Which is a more sophisticated method of film cooling and requires less air for cooling?
Transpiration cooling method.
What material can be used in flame tubes to protect materials against heat?
Thin coating of ceramic material.
