Compressors & Combustion Systems
Compressors & Combustion
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Compressors & Combustion
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Learning goal
I can explain how compressors raise air pressure and how combustion systems burn fuel while controlling airflow and temperature in a gas turbine engine.
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What words come to mind when you hear compressor and combustor?
Use this to surface prior ideas such as pressure, air, fuel, heat, turbine, blades and burning. Pick out misconceptions too, especially if students suggest that fuel could simply be burned in incoming air.
Why compress the air?
Air must be at higher pressure before fuel is burned.
Combustion needs controlled conditions, not just fuel added to incoming air.
Compressed air supports the engine's thermodynamic cycle.
The compressor prepares the flow for the combustor.
Anchor the opening idea: fuel is not simply burned in free incoming air because the engine needs pressure rise first. Link this to efficient energy conversion in the gas turbine cycle.
What the compressor does
A compressor takes incoming air and increases its pressure, temperature and energy level. It does this by using mechanical power from the turbine through the engine shaft. The compressor does not create energy; it converts shaft work into a pressure rise in the airflow.
Stress the common misconception that pressure rise appears by itself. The energy source is the turbine, transmitted by the shaft.
Energy path through the compressor
Turbine → Shaft power → Compressor → Pressure rise
The turbine supplies the shaft work.
The compressor transfers that work to the air.
The result is higher-pressure air for combustion.
Keep this as the key energy chain. It will connect later to the full engine cycle and the glossary terms shaft power and pressure rise.
Where does the compressor get the energy to raise air pressure?
Fuel injector
Engine shaft
Combustor flame
Air inlet
Checks the energy chain from slides 3 to 4: turbine to shaft power to compressor to pressure rise. Distractors reflect common confusion between energy source, airflow source and fuel source.
Complete the energy path
The compressor does not create energy. It uses shaft power from the turbine to increase air pressure. This gives the combustor higher-energy air under controlled conditions.
All three blanks come directly from the opening compression block and reinforce the idea that compression prepares air for combustion.
Two main compressor types
Centrifugal compressor
Air moves mainly parallel to the engine axis.
Common in modern turbofans, turbojets and many larger gas turbines.
Air is accelerated radially outward by a rotating impeller.
Used where compactness, simplicity and smaller engine size are important.
Axial compressor
The key contrast is airflow direction. Avoid overcomplicating with performance maps at this stage.
Axial compressor basics
Air passes through repeated rows of rotor and stator blades.
A simple pattern is Rotor → Stator → Rotor → Stator.
Each stage adds a small part of the total pressure rise.
Make clear that one stage is a rotor row working with a stator row. Students often see only the moving blades and miss the paired action.
What the rotor does
Rotor blades rotate with the shaft.
They add energy to the airflow.
They increase the air's velocity and energy.
They transfer mechanical shaft power into the moving air.
Use the chain 'shaft power → rotor → air energy' exactly, because it supports later comparison with the stator.
What the stator does
Stator blades are stationary.
They diffuse and slow the airflow appropriately.
They convert some velocity into static pressure.
They redirect the air for the next rotor stage.
Address the misconception that stationary means unimportant. The stator is essential for pressure recovery and flow direction.
Why use multiple stages?
Stage 1 → small pressure increase
Stage 2 → further pressure increase
Stage 3 → further pressure increase
More stages → large overall pressure ratio
A single stage cannot usually produce the required overall pressure rise efficiently, so many small increases are combined.
Emphasise 'small increases repeated many times'. This is the core reason axial compressors are built in stages.
Centrifugal compressor basics
Air enters near the centre of the impeller.
The impeller accelerates the air outward.
The air then enters the diffuser.
The diffuser reduces velocity and converts kinetic energy into pressure.
Point out that the diffuser is the key stationary part here, matching the stator's pressure-recovery role in the axial compressor.
Axial vs centrifugal
Airflow mainly axial
Main rotating part: rotor blades
Stationary part: stators
Multiple stages are common
Often used in larger gas turbines
Centrifugal
Axial
Airflow moves radially outward
Main rotating part: impeller
Stationary part: diffuser
Fewer stages are often possible
Common in smaller or compact engines
If time allows, explain that engineers may choose axial designs for high mass flow and integration into large engine cores.
Why choose one type over another?
Engine designers choose between compressor types by balancing:
Airflow requirement
Overall pressure ratio
Engine size and shape
Simplicity of design
Integration with the rest of the gas turbine
This slide turns the comparison into an engineering decision rather than a memorised table.
Which statement best distinguishes axial from centrifugal compression?
Axial uses fuel
Centrifugal uses stators
Axial flows parallel
Centrifugal has no diffuser
The correct distinction is airflow direction. The distractors target nearby misconceptions: mixing compressor and combustor functions, confusing diffuser/stator roles, and the false idea that centrifugal compressors lack stationary parts.
Axial and centrifugal features
In an axial compressor, the rotor adds energy to the airflow and the stator redirects it for the next stage. In a centrifugal compressor, the diffuser slows the air and helps convert velocity into pressure.
This anchors the load-bearing component roles from slides 6 to 12 and helps separate rotating from stationary parts in both compressor types.
Why are many compressor stages used?
Expected elements: one stage gives only a small pressure rise; repeated stages build a large overall pressure ratio more efficiently; rotor and stator rows work together stage by stage.
The combustor
Receives high-pressure air from the compressor
Receives fuel from the fuel system
Burns the fuel in a controlled manner
Produces a high-energy gas flow for the turbine
Transition clearly from compression to combustion: compressed air is the condition that makes controlled burning possible inside the engine.
What the combustor must achieve
Maintain stable combustion
Release chemical energy from the fuel
Minimise pressure loss
Control temperature
Protect engine components
Students may assume the goal is just 'burn fuel'. Keep the broader system goals visible, especially pressure loss and temperature control.
Why not all the air burns
A gas turbine combustor receives a large mass of compressed air, but only part of it is used directly in the main combustion zone. The rest is managed for secondary combustion tasks, temperature control and cooling. Airflow through the combustor must therefore be carefully divided and directed.
This directly counters the misconception that all compressor air is simply mixed with fuel and burned.
Primary and secondary airflow
Secondary airflow
Primary air
Directed into the main combustion zone
Mixes with fuel
Supports ignition and stable combustion
Helps complete combustion
Controls gas temperature
Includes dilution and cooling functions
Shapes conditions before turbine entry
Keep terminology consistent: primary air, secondary airflow, dilution air and cooling air are related but not interchangeable.
The primary combustion zone
Fuel is injected into a controlled proportion of compressed air in the primary combustion zone. The fuel and air mix, ignition occurs, and combustion is then sustained. The mixture must stay within suitable conditions, because too much or too little fuel relative to air can disrupt stable burning.
You do not need to introduce detailed fuel-air ratios; keep the point at the level of controlled conditions for sustained combustion.
Cooling air protects the engine
Combustion creates extremely high temperatures.
Engine materials have temperature limits.
Cooling air protects combustor liners, turbine parts and other hot-section structures.
Carefully designed passages and holes form protective cooling layers.
Stress that gas temperature can exceed what the material could directly withstand. Cooling is a design necessity, not an optional extra.
Dilution air shapes turbine entry
Additional air is introduced downstream of the primary zone to:
Reduce gas temperature
Create a suitable temperature profile
Protect the turbine
Prepare the gas flow for turbine entry
Differentiate this from cooling air: dilution air mainly manages the gas stream delivered to the turbine, though both support temperature control.
Combustor airflow as a system
Compressor → high-pressure air → primary zone
Fuel + air → controlled combustion
Secondary and dilution airflow → temperature management
Turbine → receives high-energy gas at an appropriate temperature profile
The combustor must balance energy release with temperature control.
This is the whole combustor airflow chain in one view. Use it to connect pressure, fuel burning and turbine protection.
Why is only part of the compressed air used in the primary zone?
To lower inlet drag
To cool the shaft
To manage temperature
To spin the impeller
Correct answer ties together slides 15 to 20: remaining air supports secondary combustion tasks, cooling and dilution to protect components and shape turbine-entry conditions.
Airflow through the combustor
In the combustor, primary air mixes with fuel in the main combustion zone. Further downstream, dilution air reduces gas temperature before turbine entry, while cooling air protects hot engine parts.
Reinforces the distinction between primary, dilution and cooling air using the exact roles taught in slides 16 to 20.
Fuel delivery and stable combustion
Fuel must arrive at the correct flow rate.
It must be supplied at the correct pressure.
Good atomisation supports mixing.
Delivery must be controlled to support ignition, stability and temperature control.
Avoid going into injector hardware detail unless learners already know it. The key idea is that fuel delivery and airflow management must work together.
Why combustor airflow is carefully managed
Combustion requires the right velocity, pressure and air distribution. If the flow moves too quickly through the combustor, stable burning becomes harder to maintain. If temperature is not controlled, downstream turbine components can be damaged.
This is the main purpose of the earlier subtopics on primary air, cooling air and dilution air: they are all forms of airflow management.
Divergent combustor ducts
A divergent section increases flow area.
Larger area → lower flow velocity → more suitable conditions for stable combustion
The exact geometry varies between engines, but the design aim is to support the required pressure and velocity conditions in the combustor.
Keep this conceptual rather than mathematical. The lesson plan introduces the effect, not a full fluid-dynamics derivation.
Energy through compressor and combustor
Turbine → shaft power → compressor rotor → air energy and pressure increase → combustor → fuel + compressed air → thermal energy → high-energy gas → turbine
The compressor prepares the air, and the combustor releases fuel energy into that compressed flow.
This slide resolves the central thread of the lesson by linking the shaft-driven compressor to the fuel-burning combustor and back to the turbine.
Common misconceptions
The stator does nothing because it does not rotate — incorrect; it diffuses, redirects and recovers pressure.
All compressor air is burned — incorrect; some air is used for cooling and dilution.
Cooling air makes the whole engine cold — incorrect; it is used strategically.
A centrifugal compressor has no stationary parts — incorrect; the diffuser is essential.
Use this slide as a corrective summary rather than a quiz. It is a good checkpoint before the glossary.
What is the main purpose of a divergent combustor section?
Raise fuel pressure
Lower flow velocity
Increase blade speed
Burn all the air
Draws on slides 22 to 23: larger flow area reduces velocity and supports stable combustion. Distractors mirror misconceptions from earlier in the lesson.
How does fuel delivery support stable combustion?
Expected elements: correct fuel flow rate, correct pressure, good atomisation and controlled delivery; these support mixing, ignition, stable combustion and temperature control.
What idea from this lesson will you remember best?
Reflection only. Invite students to name one concept, comparison or energy chain that now feels clear.
What helped you understand this topic most today?
Reflection only. Encourage students to mention a specific slide type, explanation, diagram or comparison that helped them understand.
Which part still feels hardest to explain?
Reflection only. Useful for identifying whether students found rotor/stator roles, airflow types or the energy chain most difficult.
Glossary
compressor - The engine section that raises the pressure of incoming air.
pressure ratio - The overall increase in air pressure produced by the compressor.
axial compressor - A compressor in which air flows mainly parallel to the engine axis.
centrifugal compressor - A compressor that accelerates air radially outward from an impeller.
rotor - The rotating blade row that adds energy to the airflow.
stator - The stationary blade row that redirects the airflow and helps recover pressure.
stage - A rotor and stator working together to produce part of the total pressure rise.
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Glossary
impeller - The rotating part of a centrifugal compressor.
diffuser - The stationary passage that slows airflow and converts kinetic energy into pressure.
combustor - The chamber where fuel is burned in compressed air.
primary air - The portion of compressed air directed into the main combustion zone.
secondary airflow - Air used beyond the primary zone for combustion completion, temperature control and flow management.
cooling air - Air directed to protect hot engine components from excessive temperature.
dilution air - Additional air mixed downstream to reduce and shape gas temperature before the turbine.
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Glossary
atomisation - Breaking fuel into fine droplets to improve mixing and combustion.
shaft power - Mechanical power transmitted along the engine shaft.
static pressure - The pressure of the air itself, distinct from pressure due to motion.
temperature profile - How temperature is distributed across the gas flow entering the turbine.
hot section - The engine region exposed to very high temperatures, including the combustor and turbine.
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