Gas Turbine - Combustion Flashcards
Requirements for combustion section
High combustion efficiency
Stability
Reliable
Exit temp does not exceed blades temp limit
Low emissions/low fuel consumption
Where is the highest pressure found in turbine engine
The diffuser after the last compressor before combustion
1st stage of combustion
Fuel injector sprays fuel into recirculating air into chamber
2nd stage of combustion
Electric igniter plug starts combustion then switches off
3rd Stage of combustion
Mixture starts burning and flame spreads around combustion chamber and flame becomes self sustaining
15:1 - 2100 degrees
4th Stage of combustion
Self sustaining speed all engine spools have reached a self sustaining speed where assistance from air starter not needed.
How is the air starter disengaged
Coupling between engine N2 spool and starter by use of centrifugal forces
Distribution of air into combustion phase from diffuser as percentages
20% direct into core
80% secondary air flows around the tube and introduced after combustion
Swirl Vanes do what
Create recirculation to slow air for continuous combustion
What temperature do gases leave combustor
1000 degree to 1500 degrees
Types of combustion layouts
Multiple (old design tubes all around)
Annular (circles around each other)
Turbo annular (canned tubes and circles together)
Advantage of annular combustion layout
Shorter/stiffer and lighter engine
Even distribution
Less wall area required so less cooling so more air for combustion chamber emissions
No flame propagation problems during engine start
Types of fuel injectors for combustion
Air spray injector (simplex):
HP delivery through a swirl chamber then out of nozzle - inefficient at low RPM
Duplex fuel nozzle:
Has a small orifice for low RPM
Has a large orifice for high power
Powered via FADEC
Fuel Burner Drain purpose
Prevent unburned fuel in combustion section
Drains away at lowest points in each section
High Energy Ignition Unit
Delivers 60-100 sparks per minute
25,000 volts
High power output - 12 joules for short periods of high altitude starts/ground start
Lower power - 3-6 joule for continuous ignition
Basically a magneto that pumps up a capacitor until it cannot hold anymore charge
When would we use the high energy ignition system
Engine start
Using contaminated runways
Flying through conditions that could cause flameout
What does the HPSOV do
Delivers fuel
Purpose of swirl vanes
Angle airflow sideways using swirl vanes
Increase radial velocity but reduce axial velocity
What happens to pressure in the combustion chamber
Pressure does not go up as we do not allow gases to expand
Secondary air is used for
Cooling of the flame tube
What reduces axial flow in chamber
Due to swirl vanes/perforated flare and airflow from the sides
What types of combustion chamber has a flame tube
They all do
Percentages of cooling and dilution air in the dilution zone
40% cooling
20% dilution
Highest max temp in the engine is located at
Just before entry to turbine stator
Primary Airflow
20% of the hot stream gassed used to actual combustion
Secondary Airflow
80% of the hot gas stream
Used for cooling (60%)
Stabilise the flame (20%) (primary zone)
Multiple Can - Combustion Chamber
Used in radial compressors
One fuel nozzle in each can
Ignition only present in two cans
Turbo Annular
Used in axial flow compressors
In line with compressor and diffuser so no deflection losses
Can surrounded by single outer casing
Annular Chamber
Main type in axial flow compressors
Single flame tube
2 igniters and lots of fuel nozzles
Less weight and less cooling required
Types of Fuel Nozzles in combustion and where they are found
Simplex - single delivery poor atomisation at low fuel flows
Duplex - primary lower pressure /main orifice for high pressure
2 simplex nozzles in annular chamber near igniter and rest of the duplex type
Fuel drains
Located at lowest point within chamber
Held shut by combustion pressure
Takes 30 secs to drain fuel
Gas Behaviour in combustion
Highest temperature occurs during combustion
Constant pressure - constant stream of gas
Increase in pressure = stall
Another advantage of an annular combustion chamber is
Reduced thermal stress on the turbine
Diffuser is a very
Divergent duct increase pressure and reduce velocity
Continuous Ignition - What does FADEC automatically switch on in response to
Engine anti ice
Maximum flex thrust selected
Idle thrust selected
Thrust control malfunction detected
Stall warning/stick shaker
