Basic Gas Turbines Flashcards
Maximum thrust from a gas turbine is limited by the…
maximum turbine temperature
higher temp means greater volume which means more mass flow
EPR is a measure of
Static pressure at the exhaust outlet to the inlet air pressure.
A blocked EPR probe will cause the gauge to over or under-read?
Over-read (high amount of static pressure)
The main difference between a turbojet and a turbofan engine is
The addition of a large fan (N1) to the compressor which produces most of the thrust, compared to a turbojet which gets all the thrust from the exhaust (mass flow of air)
The benefit of a high bypass turbofan in comparison to turbojets is
Better efficiency and quieter
Turboprops have low exhaust velocity because…
Most of the thrust (exhaust) produced is absorbed by the turbine and delivered to the propeller via a reduction gearbox.
The Gas Turbine working cycle is known as the brayton cycle and is characterised by…
constant pressure during combustion, and simultaneous occurence of all events (inductions, compression, ignition, exhaust).
What do convergent ducts do to static pressure and temperature, and velocity
Decrease pressure and temperature, increase velocity
What do divergent ducts do to static pressure and temperature, and velocity
Increase pressure and temperature, decrease velocity
What is different about a centrifugal flow compressor to an axial flow compressor
A centrifugal flow compressor makes use of an impeller to accelerate mass air flow and impart a pressure rise, and a diffuser (divergent duct) to further increase the pressure.
An axial flow compressor makes use of a series of rotor blades (impellers) seperated by stator vanes (diffusers - divergent ducts) which increase pressure at each stage.
Stator vanes are shaped to
direct air from one set of rotors to the other at the correct angle of attack
An advantage of an axial flow compressor to a centrifugal flow compressor is…
an axial flow compressure gives a bigger pressure rise for the same frontal area (pretty much a bunch of centrifugal compressors lined up)
A limitation of stator vanes is that they…
only provide air to the rotors at the correct angle of attack during design RPM (92%-98%)
The purpose of an Variable stator Vane airflow control system is to
maintain the correct angle of the stator vanes to deliver air to the rotors at the correct angle of attack during operation away from design RPM
A stall occurs when…
The compressor blade angle of attack exceeds the critical angle of attack
A stall is LIKELY to occur during….
rapid RPM changes
A surge (flow reversal) is likely to occur when
the pressure between the compressor and combustion chamber begins to neutralise
Symptoms of a surge are
dropping RPM, increased fuel flow, and an immediate rise in EGT
Compared to a centrifugal flow compressor, axial flow compressors…
ahieve greater efficiency and power, and are simpler and cheaper, however centrifugal compressors trade power for reliability (less likely to stall or surge)
What do bleed air valves do?
Used on axial flow compressors, a valve is opened during off-design RPM periods to reduce the compressor outlet static pressure
When are a rich flameout and lean flameout likely to occur
Rich: low speed high power climb
Lean: High speed low power descent
Remember power is fuel flow, too much power with not enough air is too much fuel, and vice versa
Of all the types of combustion chambers (single, multiple, tubo-annular, and annular), which is superior and why?
Annular in terms of combustion efficiency, cost and weight
Less bullshit to get through, less energy to lose
Nozzle Guide Vanes are
Stationary Convergent Ducts located at the front of a TURBINE which increase velocity to help rotate the turbine
What are the two types of rotor blade types?
Impact: Absorbs energy from air flow like a water wheel
Impact/Reaction: Lots of little aerofoils. More efficient than just impact.
The purpose of a shroud is to…
Reduce gas leakage, reduce vibration, and increase strength of the blades.
What is blade creep and what are the 3 types?
Blade creep is a gradual increase in length over time which can cause damage as the blades hit the casing or brake. Primary blade creep occurs during manufacturing, and secondary blade creep occurs during service, and tertiary creep occurs at the moment of failure.