Basic Gas Turbines Flashcards
What are the four types of reaction engines?
Rocket,
Ram Jet,
Pulse Jet,
Gas Turbine
Describe a rocket engine
Self contained air and fuel in pressurised canisters
Describe a Ram Jet engine
No moving parts, Divergent entry duct and convergent exhaust duct. Must have a large forward airspeed to operate (600kt)
Describe a pulse jet
No moving parts, similar to a Ram Jet, Divergent entry duct and convergent exhaust duct. Works on an intermittent cycle.
Describe a Gas Turbine
A Gas Turbine has a turbine driven compressor. It does not require forward airspeed to compress the air entering the engine.
What factor limits the maximum thrust from a gas turbine?
Maximum Turbine Temperature determines maximum thrust.
A large increase in gas temperature from the air intake to the turbine means a large increase in the volume of gas, a large acceleration of the gas, and as a result thrust is produced.
How does density affect the amount of thrust produced?
Cold intake air allows the engine to produce more thrust because cold air has high density. High mass air flow through the engine produces more thrust.
As Density reduces, mass airflow and thrust decreases.
Thrust output is a product of Mass Airflow and acceleration
Thrust output is a product of:
Mass Airflow and Acceleration
THRUST = Mass Airflow x Vj - V.
Vj is the jet exhaust velocity and V is forward speed.
A Spool refers to
A compressor at the front of an engine and it’s accompanying set of turbine wheels at the rear.
EPR - Engine Pressure Ratio refers to?
The ratio of exhaust gas pressure compared to inlet air pressure. Indicates thrust output of a turbojet.
What would a blocked EPR Probe cause?
An over-read of the EPR gauge.
In a High Bypass (Turbofan) engine, describe the N1 Fan
A Large Fan added at the front of the compressor, driven by an extra turbine at the rear of the engine. Produces thrust by giving a relatively small acceleration to a large mass of air.
This bypasses the combustion chamber via a duct around the outside of the engine core.
What does by-pass ratio mean?
The ratio of air which bypasses the engine core compared to the air which passes through the core.
It’s usually around 5:1 or higher. Approximately 75% of thrust is produced by by-pass fan air.
What is the equation for by-pass ratio?
By-pass mass flow
—————————
Engine core mass flow
How are spools numbered?
Front to back: N1 N2 N3 Etc.
Describe a free power turbine turboprop
The most common design of turboprop
Extra turbine stage called the “Free power turbine”.
It is on a separate shaft (Spool) not connected to the compressor or any other shaft or spool. The power from the turbine is fed via the shaft to a reduction gearbox and from there to an output shaft connected to the propeller.
Lower starting torque
Variable prop speed
High torque at low RPM
Describe a direct drive turboprop
The output shaft to the reduction gearbox comes off the main compressor/turbine spool instead of having it’s own independent turbine.
Describe a compound turboprop
Variation on direct drive design.
Two spools, propeller gear box driven from the low pressure compressor spool.
Why is the reduction drive necessary on a turboprop?
Needs to reduce the very high RPM of the turbine to a manageable speed to use by the propeller. Small turboprop engines can rotate at 50000rpm, whereas propellers rotate closer to 2000rpm.
Why do turboprops have low exhaust velocity?
The additional turbines absorb extra energy from the gas stream. A small amount (10%) of thrust is produced by the exhaust.
Describe the Brayton cycle compared to the Otto Cycle?
Continuous cycle:
- Intake
- Compression
- Combustion
- Exhaust
One constant cycle - as opposed to single stages as found in the Otto Cycle
Constant pressure during combustion
What is the role of a convergent duct?
Increases velocity
Decreases pressure and temperature
What is the role of a divergent duct?
Decreases velocity
Increases pressure and temperature.
Describe a Centrifugal Flow Compressor.
Consists of an impeller to accelerate the mass airflow and impart a pressure rise. Has a diffuser (Divergent duct) to further increase the pressure.
Air flows into the centre of the impeller and centrifugal force causes it to accelerate and flow outwards, a pressure rise also occurs through the impeller due to the addition of kinetic energy by the rotation of the impeller.
