Combustion Flashcards
What is the purpose of the combustor?
- To release heat at the correct air/fuel ratio
- Accelerate airflow into a smooth, uniformly heated gas stream while producing a minimum pressure drop.
What do swirl vanes do?
Slows the axial velocity of the air by imparting a radial motion for the required air/fuel mixing time
What percentage of the combustor airflow are Primary airflow and Secondary airflow?
Primary: 20-25%
Secondary: 75-80%
What is the purpose of primary airflow?
Used in the combustion
- 10-15% enter the snout, 10% enter the primary zone through the combustion walls.
What are the 3 purposes of secondary airflow?
- Centers and shapes the flame
- Provides a cooling air blanket for the liner
- Dilutes the mixture to a suitable temperature for turbines.
In modern combustion chambers, what are the operating efficiencies at sea level and altitude?
Sea Level: 100%
Altitude: 98%
What is combustion stability?
- When the combustor is able to support combustion at a wide range of operating conditions
As the mass airflow is increased, what happens to the range of the rich and lean operating limits?
Reduced
What are ignition limits?
Similar to combustion stability limits but they lie well within the Stable Region.
Why are ignition limits situated well within the Stable region?
Because it is harder to establish combustion when ‘Cold’.
- Operations outside the ignition limits can lead to Flame Out
What is flame out, what are the two types of flame out?
When the flame is extinguished
- Rich Flame Out: Too much fuel is sprayed
- Lean Flame Out: Too little fuel is sprayed
How does Rich Flame Out happen?
- Occurs with very fast acceleration
- Over-rich mixture increases combustor pressure to the point where compressor airflow stagnates and extinguishes the flame.
How does Lean Flame Out happen?
- Occurs at low fuel pressure, low engine speeds or high altitude
- Sets up weak mixture that is easily blown out
What conditions can trigger a Flame Out?
- Turbulent weather
- High Altitude
- Slow Acceleration during maneuvers
- High speed maneuvers
What percentage does non-pollutants make up from engine emmisions?
81%
- Water vapor and inert gases
What percentage does pollutants make up in engine emmisions?
0.04% of emissions
- Smoke
- Unburned Hydrocarbons
- Carbon monoxide
- Oxides of Nitrogen (NOx)
How is the majority of pollutants formed?
Incomplete combustion and inefficiencies of combustion
How is NOx formed as a pollutant?
High flame temperatures converting ozone into nitric acid.
What are the 4 types of combustion chambers?
- Multiple Can
- Annular (Basket type)
- Can-Annular
- Reverse flow annular
What is a Multiple can combustion chamber?
Individual combustion cans arranged radially around the engine (Older)
What are the advantages and disadvantages of a Multiple Can combustion chamber?
Advantages:
- Individual cans can be replaced without disturbing other cans
Disadvantages:
- Uneven turbine temperatures
- Extreme temperatures in the turbine if a burner can fail, leading to turbine failure.
What is a Annular (basket type) combustion chamber?
Consists of a single flame tube which is contained in the inner/outer casing.
What are the advantages and disadvantages of a Annular type combustion chamber?
Advantage:
-For the same output, length is 75% less than a Can-Annular system (Weight and Cost saving)
- Elimination of flame propagation problems found in Can type chambers.
Disadvantage:
- Maintenance requires engine to be removed from the aircraft.
What is a Can-Annular combustion chamber?
Consists of a flame tube fitted inside a common outer casing with INTERCONNECTOR tubes for flame propagation.
What are the advantages of a Can-Annular combustion chamber?
Advantages:
- Allows for on-wing maintenance of the hot section
- Shorter than the Can Type arrangement and has more uniform heat distribution.
What is a Reverse Flow Annular combustion chamber and what are its advantages?
Where the airflow path is an ‘S’ shape
- Allows for longitudinally compact engine
- Provides pre-heating of compressor air.
What is the combustion liner and what is it made of?
The perforated inner section where the fuel burning is controlled. Subject to high temperatures and prone to cracking.
- Made of stainless steel
Is stop drilling acceptable to stop crack propagation on the liner?
No
Is minor cracking acceptable on the combustion liner?
Yes, it relieves stress in the steel and crack propagation ceases.
When marking, what markers can be used?
Approved markers.
- Never use graphite, lead, grease, or wax pencils due to carbon and lead becoming infused into the metal at high temperatures.
Does liquid jet fuel burn?
No, only burns when vaporized or atomized so the fuel can mix with the air and form a combustible mixture.
What are the two different types of fuel nozzles?
- Atomizing (splits liquid into small molecules)
- Vaporizing (turns liquid into vapors)
What are the 4 types of atomizing nozzles?
- Simplex
- Duplex
- Spill type
- Air blast type
How many spray orifices and spray patterns does a Simplex fuel nozzle have?
1 spray pattern and 1 spray orifice.
In a Simplex fuel nozzle, what can be used to reduce the velocity of the fuel and improve atomization?
Flutes that impart a swirling motion on the fuel.
What can be added to a simplex nozzle to help with engine starting?
A second, smaller nozzle can be incorporated called the ‘Primer Nozzle’ or the ‘Starting Nozzle’ to spray a very fine mist when starting, then it switches off.
How many flow rates and spray patterns does a Duplex fuel nozzle have?
2 different rates and flow patterns.
- One for low airflow and one for high airflow.
What are the two different types of Duplex fuel nozzles?
- Single Line Duplex
- Dual Line Duplex
How does a Single Line Duplex nozzle work?
Receives fuel from 1 inlet port and uses a flow divider to distribute fuel through 2 orifices.
- Centre Orifice (Primary/Pilot Fuel) sprays at wide angle for startup.
- Outer Orifice (Secondary/Main Fuel) opens at a preset fuel pressure to spray at a higher volume and pressure, narrowing fuel pattern to prevent fuel from contacting combustion chamber at high power.
How does the Dual Line Duplex nozzle work?
Two Inlet Ports: Primary Fuel Inlet and Secondary Fuel Inlet.
- Flow of fuel to the 2 inlets is controlled by the Pressurization and Dump Valve which contains a single flow divider for all fuel nozzles.
In terms of fuel dividers, how is the Dual Line Duplex fuel nozzle different from the Single Line Duplex nozzle?
- A Dual Line Duplex has a single fuel divider for all fuel nozzles (A.K.A Pressurization and Dump Valve)
- A Single Line Duplex has a flow divider for each nozzle.
What does the Fuel Pressurization and Dump Valve do?
Acts as a central fuel divider for the Dual Line Duplex nozzle.
- The pressurization component controls fuel flow during starting and ground idle.
- The dump valve is used to drain the manifold after combustion and prevents left-over fuel from boiling and leaving contaminants to clog fuel passageways.
What is the Spill Type nozzle?
- A type of Simplex nozzle which has a passage from the swirl chamber for fuel spilling away?
What is the advantage of a Spill Type nozzle?
Allows the fuel supply to the nozzle to remain at a high pressure at all stages of the engine operation as excess fuel is spilled away prior to passing the atomizing nozzle.
What is an Air Blast nozzle and what is the primary use?
Uses a high velocity airflow to enhance atomization
- Most effective during starting operations.
How does the Vaporizing nozzle work?
Premixes the air and fuel using combustor heat to vaporize the fuel before entering combustion.
What is a disadvantage of the Vaporizing nozzle?
Does not provide an effective spray pattern during engine starting.
- A small set of atomizing nozzles are used during starting to overcome this.
