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.
