Gas Turbine, Subsystems III
In WW-II and shortly thereafter, piston powered aircraft peaked in power, performance and complexity wise. Power went up to over 4000 bhp for large multi-row radial engines. Only to be defeated by the jet, which was developed by (among others) Germany's Dr. Hans von Ohain and separately in the UK by Sir Frank Whittle. Its principles are based on the "Aeolipile" of the ancient Greek scientist Hero and other great thinkers like Leonardo da Vinci and the laws of Isaac Newton.
Compared to a piston, the gas turbine has less parts and the moving parts rotate in only one direction without stopping and accelerating as the pistons normally do in a engine. Thus, a running gas turbine is basically free of the vibrations normally found in piston models, which translates in much longer service life (TBO) and higher reliability.
Two subjects here: Fire prevention and engine noise. Gas turbine engines have been know to create a lot of noise around airports, and much has been done to get them as quiet as possible with great success.
Jet Engine Noise
With an increasing population around airports it is important that the noise levels coming from aircraft are limited as far as possible. Houses should be sound proved too and people wanting to live near an airport should realize before hand the consequences of living there.
Older aircraft used to be hush kitted, where a noise limiting package was installed afterwards, which is not so effective as designing an engine with noise suppressing in mind on the design table. Research has shown that noise is generated in three major parts: fan or compressor, turbine and the exhaust. Where the noise from the exhaust is the more predominant one.
Originates in the shear zone between the high velocity gas stream and the atmosphere. Small eddies exists near the exhaust nozzle and further away the eddies become larger and each with their own resonating frequency. In the core of the gas stream the velocity exceeds Mach 1.0 and these shock waves also produce a lot of noise.
Compressor / Turbine noise
These noises are generated when air is compressed and passes of stationary and rotating vanes where the speed and density of the airflow, blade construction, type and length will dictate the generated frequency of the noise. Normally this will be heard when the engine is at low thrust because the exhaust noise becomes more predominant when selecting higher thrust levels.
Exhaust noise is reduced by increasing the mixing with the atmosphere thus reducing the relative velocity. In addition engine manufacturers are applying sound absorbing materials inside of the engine near the compressor fan and turbine. The high bypass ratio GTE has two exhaust streams, the cold air surrounds the hot exhaust and this mixing reducing the noise.
Sound absorbing materials are made of thin layers of aluminum, titanium or steel separated by a honeycomb structure and this is bonded inside the engine during construction in the factory.
Principle of a Gas Turbine Engine
Gas turbine aircraft have certain features to lower the risk of fire: prevention, detection and extinguishing systems. Cockpit crew use checklists with actions to take but it is essential that to put out a fire the supply of combustibles or air must be stopped or the temperature must drop below that needed to sustain the fire.
Engines are designed and installed in the aircraft in such a way that the risk is minimized. Fuel and oil components are placed in cool zones and are drained and ventilated and a fire proof bulkhead of steel or titanium sits between the cool zone and the combustors and turbines. The pods are ventilated with enough atmospheric air flow not to cause drag or that the extinguishing agent would not be able to do its work.
This must be rapid without creating any false alarms. The detection system may consist of a series of detector units or continuous elements sensing type. Usual placement is in ventilation outlets providing early warning.
Three types are used for fire detection:
- Thermistor, semiconductor which reacts to temperature
- Thermocouple, creates an EMF in relation to temperature
- Continuous electrical element, capacitance type which reacts to temperature
In the cockpit a distinct ringing bell is used to alert the crew in addition to other visual or tactile warning methods.
The engine must be shutdown (by cutting fuel and ignition) before releasing the extinguishing agent. Halon or sometimes CO2 is used and held in metal bottles and flows through metal tubes and sprayed onto selected areas. Two shots are available on the aircraft (not engine). Do not try to restart the engine, this may result in starting the fire again but now without extinguishing agent on board.