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Turbines

The development of the turbine engine for aircraft began independently in Germany and Great Britain in the 1930s. In Germany, Hans von Ohain developed the engine for the first jet flight in 1939. Germany developed the Messerschmitt 262 towards the end of World War II. 

In Britain, Frank Whittle obtained the patent for the turbine engine in the 1930s and in 1941 the aircraft was developed. The first British jet aircraft, the Gloster Meteor, also flew at the end of World War II.

Principles of operation

The operation principle of the turbine is conceptually simple. It converts the kinetic energy of a moving fluid into mechanical energy by using the motion of the fluid to turn a rotor. The rotor, in turn, is connected to a device that does useful work. The moving fluid could be water, steam, air, or hot gases. Turbines driven by steam are widely used for electrical power generation. 

An aircraft gas turbine engine generates hot, pressurized gas by burning fuel in a confined space.

The gas, in the process of escaping from the engine, drives a turbine that, in turn, drives the compressor that compresses the air entering the engine. The hot compressed gas leaves the engine at high velocity, generating the force or thrust that pushes the aircraft forward.

The aircraft turbine engine is sometimes called a reaction engine because it exemplifies Newton's Third Law, which states:

For every action there is an equal but opposite reaction. In this case, the action is the expulsion of the hot exhaust gas from the rear of the turbine. The reaction is the thrust imparted to the engine – thrust that the engine transmits to the rest of the airframe.

Turbines consist of three sections, each with a different function: compression section, combustion section, and turbine section. 

The compression section draws in and compresses ambient air and delivers the compressed air to the combustion section.

In the combustion section, fuel is continuously injected into the compressed air through a set of nozzles. The fuel evaporates as it mixes with the hot air, and then burns. The hot combustion gases are then forced into the turbine section by the high pressure at the outlet of the compressor.

The turbine section contains a series of pairs of stator vanes and rotor blades. Stator vanes are stationary and accelerate the gas flow to push on the rotor blades.

Finally, the hot mixture of combustion gases and air is accelerated through a nozzle at the rear of the engine. It is this exhaust stream that produces the thrust that drives the aircraft.

The combination of a turbine, the compressor it drives, and the connecting shaft is called a spool.

Types of turbines

Turbojet. The first turbine engines had a single spool and were called turbojet engines. They perform better at high altitudes and high airspeeds.

Turbofan. A turbofan engine uses a turbojet as a core, but has a large fan mounted in front of the compressor section. These fans are visible in commercial aircraft. The fan, which, like the compressors, is driven by a turbine, acts like a propeller; it pushes air to create thrust. It differs from a conventional propeller in that it has lots of wide blades spaced closely together and is surrounded by a tight cowling. 

Some of the air driven by the fan goes directly into the core while the rest goes around the outside of the turbine. This bypass air provides most of the thrust in turbofan engines, approximately 85 percent, while the hot exhaust gases provide only about 15 percent.

Combustion in the engine

Turbines have igniters to initiate combustion. After this, combustion is sustained by the continuous injection of fuel into the flame.

Continuous combustion requires a stable flame, which, in turn, requires matching the fuel and airflow rates to the flame propagation rate.

The flame propagation rate is fairly slow, ranging from about 0.5 m/sec for a static mixture, to around 10 to 30 m/sec under turbulent flow conditions. The flame in the combustion zone is stable only if its propagation velocity is greater than the velocity of the primary air, otherwise the flame would blow out.