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Basic Gas Turbines, Turbofan, Turboprop, Turbojet, Turboshaft - Coggle…
Basic Gas Turbines
Cold section
Intakes & inlets
Dirt and sand separation
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Vortex-generator
Flicks particles out the side, and delivers clean air from the middle of the tube
Bird-catcher
Tight corner about which air, lighter than heavy dirt/dust particles, can travel
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Pitot intake
Pointed slightly down so that airflow comes in relatively square -- max ram pressure recovery; max efficiency
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Compressors
Why decrease cross-sectional area? Volume decreases, pressure increases.
Compressor stall
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Axial-flow compressor consists of a series of small aerofoil sections. The leading edges of the blades are affected by the passage of air through the previous stage -- cascading.
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- Rotor AoA too small = insufficient compression
- Rotor AoA > critical angle = loss of lift, stall
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Stall always spreads (cascades), reducing compression and rotor RPM.
Due to:
- velocity/rotational speed
- airflow through engine
Compressor surge
Reduce likelihood of surge
- must reduce pressure in combustion chamber by closing throttle and ensuring stable flow at engine intake
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- CLOSE THROTTLE!
- Reduce AoA
- Increase airspeed
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Centrifugal compressor
Advantages:
- shorten engine length
- one big boost of compression
- robust, easier to manufacture
Disadvantages:
- only one compression boost
- only increases pressure a little in comparison
- most efficient way to have a centrifugal compressor is to have two (one feeding the other)
- for similar thrust to axial, larger frontal area
Impeller to accelerate, diffuser to slow & increase pressure
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Axial flow compressor
- Set of rotor blades -- slightly convergent; velocity AND pressure increase
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- Set of stator vanes -- divergent; reduce velocity, increase pressure.
Also direct airflow to next stage of compression at optimum AoA.
Last row usually are air straighteners to remove swirl & deliver to combustion system at a uniform axial velocity.
Advantages:
- can have multiple in stages to achieve a very high compression ratio
- can use to boost the inlet pressure to a centrifugal compressor
- more thrust for same frontal area
- improved efficiency and reduced TSFC
Mid-span shroud attached to each blade, contacts adjacent blades so they all support each other and reduces vibration
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Compressor care:
COMPRESSOR WASH! Salt build-up, etc.
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Combined compressors
Axial compressor feeding boosted inlet pressure to centrifugal compressor, which then gives final boost
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Multi-spool compressors
Advantages:
- Operational flexibility, high CR (more efficient over varying RPM), quick acceleration, easier starting, reduces chance of compressor stall
Two or more rotor/shaft assemblies, each driven by their own turbine. Each spool runs at its ideal tip/rotational speed
Most suitable for bypass-type engine, where front/low-pressure compressor handles a much larger airflow than the high-pressure compressor
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Requirements:
- minimum drag on airframe
- decrease air velocity, increase pressure
- furnish engine with uniform, distortion-free airflow
- Air inlet ducts
- Compressor
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Hot section
Combustion Chambers
Purpose
- Burn fuel/air mixture
- Release heat energy
- Expand gas
- Accelerate gas stream for turbine operation
Fully burn:
- minimum fuel, for the
- maximum heat energy, within the
- smallest combustion chamber, and produce
- minimal emissions
Operation
- Supplies large volumes of air
- Spray nozzles supply fuel
- Igniters only for starting and relighting
- Burns large quantities of fuel
- Continuous flame
- Extreme heat released
- Gas expanded and accelerated
- Should maintain 15:1 air:fuel mixture
Snout
Flare
Swirl vanes & chambers
Fuel spray nozzle
Primary zone
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Secondary air holes
Dilution air holes
- 1 more item...
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Swirl vanes & chambers provide a whirl to the airflow, the an opposite direction to the fuel spray cone.
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Combustor types
Multi-can
Individual cans with individual shrouds, external to the core structure -- interconnector between cans; so only two igniter plugs are required as the flame propagates through the chambers using the interconnectors
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Annular-through flow
- Larger volume of air for the same area
- 75% shorter; weight-saving
Single donut piece - rather than individual flame tubes - so no interconnectors. Still uses two igniters, and combustion chamber drain valve (drains excess fuel that pools at the bottom of the chamber, so that on next start there is no huge fireball)
Reverse-flow annular
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Flame tubes and liners (of the ring-shaped annular chamber) are surrounding the turbine(s) -- if turbine fails, blades must go through several layers of combustion chamber & casing before launching out of the engine
After air enters through the reverse flow, and is combusted, curl assembly turns the gas path back 180° to the turbine.
Materials
Should be resistant to heat, creep, corrosion, fatigue (vibration stress)
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- Combustor
- Turbine
- Exhaust
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Brayton CycleWhere Otto cycle is cyclical and each different stage occurs one after the other in the same location, the Brayton Cycle offers a relatively constant pressure for combustion in comparison.
- Engine sucks in air through air intake and compressor.
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- Compressor also squeezes the air, pressurising it and delivering it to the combustion chamber.
Driven by turbine, connected by shaft; turbine driven by exhausting gases.
Diameter gets smaller and smaller, thus air's volume decreases through the engine.
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- Combustion chamber fuel nozzles mix fuel with the air to produce the bang/burn -- air expands rapidly, and is forced into the turbine.
Burns up to 2000C, while the metal melts around 1300C, so the chamber required advanced cooling techniques.
Must combust uniformly with minimum pressure loss and maximum heat release. Gas temperature determines engine thrust.
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- Exhausting the expanded gas mixture drives the turbine, thus the fan and compressor, and blows it out of the exhaust nozzle, providing the thrust.
Turbine extracts energy from hot gases after combustion chamber by expanding them to a lower pressure and temperature
Turbine may consist of several stages - one stage is a row of rotor blades and a row of stationary guide vanes, which direct airflow onto the rotors at the optimum AoA.
For high efficiency, the air at the turbine inlet must be high above the metal's melting point -- so cool air is forced through small holes in the blades.
All stages occur simultaneously, eliminating 3 idle strokes.
Differences with Otto cycle:
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- Constant pressure
- More fuel burn, shorter time
- More output for size
- Smaller frontal areas (for given engine sizes)
- No reciprocating parts
Newton's 3rd Law: every action has an equal and opposite reaction.
- Gas passing through a jet engine has a reaction as it passes through each stage
- Gas leaving a jet engine reacts
What drives the aircraft through the sky is when the gas stream achieves its greatest acceleration in the combustion chamber of the turbojet.
Boyle's Law: at a constant temperature, pressure of a gas varies inversely with volume changes.
- Large intake area; air goes through, volume decreases, pressure increases
Charles' Law: under constant pressure, volume of a gas varies proportionally with absolute temperature changes
- Feed high-pressure airflow, close-together oxygen molecules into combustion chamber = burns easier
- Expansion of gas under constant pressure when burning/temperature increasing; volume increases, exhaust gas accelerates out
Bernoulli's Theorem: convergent & divergent ducts.Convergence = increased velocity into the turbine.
- Nozzle guide vanes (NGVs) and turbine blades
Divergence = increased pressure during compression
Turbofan
Basic turbofan
Intake/fan
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GEARED TURBOFAN: Gearbox between fan & low pressure shaft to spin each at optimum angular velocities, halving turbine speed. Allows fan diameter to be increased (produces more thrust; F=ma) while keeping blade tips subsonic (maintains fuel efficiency).
While in a turbojet, all of the intake air passes through the combustion chamber and turbines, in a turbofan some of this air is ducted around the outside, bypassing these components.
The fan forces the bypassed air rearwards, converting the mechanical energy from the gas turbine to do so and produce thrust.
The gas turbine drives the ducted fan, while also contributing to thrust itself.
Mass flow bypassing engine core/mass flow passing through the core = bypass ratio
High bypass ratio = more fan thrust than jet thrust.
Low bypass ratio = less fan thrust than jet thrust.
Utilises Newton's 2nd law: F = ma. Burns less fuel to send a large amount of air slowly backwards -- more efficient than burning lots to accelerate a small mass really fast.
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Turboprop
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- Efficient for short runs, medium to low speeds (most efficient around 300kt)
- 10-15,000ft
- Prop becomes inefficient at high speeds
Turbojet
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- Unlimited thrust
- Limited by fuel burn
Turboshaft
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Optimised to produce shaft power, rather than jet thrust