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A turbine engine works on the principle of accelerating a small mass of air backwards rapidly, thus pushing the engines forward (action reaction law).
Jet Engine Thrust = m x (Vj - Vv)
m = Airmass Flow
Vj = Jet exhaust velocity
Vv = TAS
The working cycle
The 5 stages are identical to those for a piston engine: induction, compression, combustion, power and exhaust. However, for a turbine engine the combustion occurs at a constant pressure and is continuous (constant volume and intermittent for a piston engine).
A turbo-jet is essentially a heat engine meaning that the higher the temperature in the combustion chamber, the greater the efficiency of the engine.
However, there has to be a temperature limit to protect the nozzle guide vanes and the turbine blades behind the combustion chamber.
1. Compression
- Work is done to increase the pressure and decrease the volume of the air
- As a result, temperature also rises in the compressor
2. Combustion
- Temperature is further increased by burning the mixture
- As a result, volume increases (at an almost constant pressure)
3. Expansion
- Some of the energy in the gas stream is converted into mechanical energy by the turbine
- There is a decrease in pressure and temperature, thus resulting in an increased volume
Duct Design
The required changes in pressure and velocity throughout the engine is accomplished by the different shapes of the ducts (passages).
- A Divergent duct
Velocity decreases / Pressure increases / Temperature increases
A divergent duct is placed between the compressor outlet and combustion chamber inlet because we want to decrease the velocity to obtain a steady flame in the chamber, the pressure is also further increased.
- A convergent duct
Velocity increases / Pressure decreases / Temperature decreases
An example of such a duct is in the turbine between the nozzle guide vanes (static blades) and turbine blades (rotating blades).
Spools
When a compressor and turbine are joined together on one shaft, the unit is referred to as a 'spool'. Part of the energy is taken from the gases when passing through the turbine so that it can drive the compressor. The rest is extracted to act as thrust.
A twin spool is also possible. For example in the turbo-prop engine where the high pressure spool (turbine closest to combustion chamber outlet) drives the compressor and the low pressure spool drives the propeller.
A Free Power Turbine is one which is not connected to any of the compressors which gives it a wider operating speed range since it does not have to consider the 'ideal speed' for the compressor anymore.
Low by-pass ratio engine
- An amount of the airflow (depending on the by-pass ratio) flows down the by-pass duct while the rest continues through the high pressure compressor.
- In the mixer unit in the exhaust system, both flows with rejoin again
High by-pass ratio (turbo-fan) engine
- Incorporates a fan (low pressure compressor) in front of the engine where the pressure is raised before the airflow splits in 2
- Thrust is almost completely dependent on the by-pass airflow (high mass, low velocity)
- About 80% of the thrust comes from the cold by-pass air
- About 20% of the thrust comes from the hot core
Formulas (exercises)
By-pass Ratio = By-pass mass flow / HP Compressor mass flow
By-pass mass flow = Inlet mass flow - HP Compressor mass flow
HP Compressor mass flow = Inlet mass flow - By-pass mass flow
Jet Engine Thrust = m x (Vj - Vv)
m = Airmass Flow
Vj = Jet exhaust velocity
Vv = TAS
The working cycle
The 5 stages are identical to those for a piston engine: induction, compression, combustion, power and exhaust. However, for a turbine engine the combustion occurs at a constant pressure and is continuous (constant volume and intermittent for a piston engine).
A turbo-jet is essentially a heat engine meaning that the higher the temperature in the combustion chamber, the greater the efficiency of the engine.
However, there has to be a temperature limit to protect the nozzle guide vanes and the turbine blades behind the combustion chamber.
1. Compression
- Work is done to increase the pressure and decrease the volume of the air
- As a result, temperature also rises in the compressor
2. Combustion
- Temperature is further increased by burning the mixture
- As a result, volume increases (at an almost constant pressure)
3. Expansion
- Some of the energy in the gas stream is converted into mechanical energy by the turbine
- There is a decrease in pressure and temperature, thus resulting in an increased volume
Duct Design
The required changes in pressure and velocity throughout the engine is accomplished by the different shapes of the ducts (passages).
- A Divergent duct
Velocity decreases / Pressure increases / Temperature increases
A divergent duct is placed between the compressor outlet and combustion chamber inlet because we want to decrease the velocity to obtain a steady flame in the chamber, the pressure is also further increased.
- A convergent duct
Velocity increases / Pressure decreases / Temperature decreases
An example of such a duct is in the turbine between the nozzle guide vanes (static blades) and turbine blades (rotating blades).
Spools
When a compressor and turbine are joined together on one shaft, the unit is referred to as a 'spool'. Part of the energy is taken from the gases when passing through the turbine so that it can drive the compressor. The rest is extracted to act as thrust.
A twin spool is also possible. For example in the turbo-prop engine where the high pressure spool (turbine closest to combustion chamber outlet) drives the compressor and the low pressure spool drives the propeller.
A Free Power Turbine is one which is not connected to any of the compressors which gives it a wider operating speed range since it does not have to consider the 'ideal speed' for the compressor anymore.
Low by-pass ratio engine
- An amount of the airflow (depending on the by-pass ratio) flows down the by-pass duct while the rest continues through the high pressure compressor.
- In the mixer unit in the exhaust system, both flows with rejoin again
High by-pass ratio (turbo-fan) engine
- Incorporates a fan (low pressure compressor) in front of the engine where the pressure is raised before the airflow splits in 2
- Thrust is almost completely dependent on the by-pass airflow (high mass, low velocity)
- About 80% of the thrust comes from the cold by-pass air
- About 20% of the thrust comes from the hot core
Formulas (exercises)
By-pass Ratio = By-pass mass flow / HP Compressor mass flow
By-pass mass flow = Inlet mass flow - HP Compressor mass flow
HP Compressor mass flow = Inlet mass flow - By-pass mass flow
