15.5 Flashcards
For safe and efficient operation of engine, combustion chamber must fulfil what requirements?
Safe ignition of fuel air mixture
Give stable combustion
Combustion must also be complete
Characteristics of combustion chamber
Must be small and light to save weight
Give stable combustion
Combustion must also be complete
Two main components of combustion chamber
Combustion chamber casing
The flame tube- Found inside the casing
-Split into inner and outer casing
Explain a brief combustion process?
Compressed air enters combustion chamber
Fuel nozzles spray fuel into airflow
Fuel and air mixes and is ignited
After start ignition is turned off
Two primary conditions for safe and efficient combustion?
-Ratio of fuel and air must be correct
-Air velocity must be slow enough to make sure that the combustion is completed inside combustion chamber
What is the air to fuel ratio?
15 parts of air
1 part of fuel
Two parts of the airflow from compressor?
Primary airflow- used for combustion
Secondary airflow- used to keep flame away from flame tube walls (mainly used for cooling)
What are the three types of combustion chambers?
Can type
Can annular
Annular
What type of engine is a can type combustion chamber put on?
Old gas turbine engines with centrifugal compressor
How are multiple can type combustion chambers connected around engine?
Inter-connector tubes
What do inter-connector tubes do?
Permit ignition from one chamber to another
They equalise the pressure in all combustion chambers
How can pressure be equalised in can type combustion chambers?
Inter-connector tubes
Advantages of can type combustion chamber?
Simple design
Good structural strength
Removed and replaced individually
Disadvantage of can type combustion chamber?
Heavy
Require lots of space
Complicated air ducts
Difficult ignition from one chamber to another
What are can annular combustion chambers?
Set of flame tubes installed around combustion section
Have common inner and outer casing
3 major parts of annular combustion chambers?
The dome
The inner liner
The outer liner
What does the annular combustion chamber dome have?
Has openings for fuel nozzles which are surrounded by swirl vanes
What does the combustion section consist of?
Combustion case
Flame tube
External components-fitted to outer wall
What does the combustion assembly consist of?
An outer wall
Inner wall
Diffuser
Why is the rear portion of the inner wall usually removable?
So flame tube can be installed
Why are there openings in the outer wall of combustion case?
Through these openings, bleed air from the last compressor stage is fed to the aircraft pneumatic system
Where are the fuel nozzle pads and igniter bosses found?
Outer wall of case assembly
How many igniter plugs are installed in the combustion case of an engine?
2 (found on lower half of combustion case)
Openings for fuel nozzles and igniter plugs can also be used for what?
Borescope inspections of flame tube
Combustion case can also be known as what?
Compressor rear frame
What does the compressor rear frame house?
Bearings for high pressure rotor
Where is the bearing case found in the combustion case?
Inner annulus
Flame tube assembly consists of what?
Outer and inner liner
Outer and inner cowl
Dome
In the flame tube assembly, what does the dome connect?
Connects inner liner with outer liner
What houses swirlers with swirl vanes?
Flame tube assembly Dome
Each swirler has a what to hold the nozzle tip in its centre?
A bore
The flame tube is bolted to combustion case, how is it held in place?
By outer and inner liner supports
In the flame tube assembly, how many pieces make up the outer liner and dome?
One
In the flame tube assembly, how is the dome centred in combustion case?
By mounting pins
Explain flame tube cooling?
There is primary and secondary airflow
Primary enters flame tube through swirl vanes
Secondary passes the cowl and enters flame tube through liners
-this forms a cooling film and helps protect liner from intense heat
Known as film cooling
Explain transpiration cooling method
Requires less air for cooling the flame tube liners
Inner surfaces are covered in detachable tiles called liner segments
Air gets in the through the gap in the liner segments and cools the outer liner segments
As air leaves it creates a cooler film
What coating is used to protect against heat?
Ceramic coating
What type of combustion chamber is used on modern turbofan engines?
Annular
What is the purpose of the combustion chamber flame tube?
To control and guide the flames
What is the one purpose of the combustion casing?
Support fuel nozzles
Airflow into the combustion chamber is split into what?
Primary and secondary airflow
What temperature does the centre of the flame in the combustion chamber reach?
Approx 2000 degrees Celsius
What percentage of airflow from the compressors is used for combustion?
20-30%
Where is the flame stabilised in the combustion chamber?
Primary zone
Two conditions to ensure correct and safe combustion?
Air/fuel ratio
Velocity
