Turbines

There will often be more than one turbine stage and these extra stages will be used to drive additional compressors, fans, propellers or gearboxes, depending on what type of engine it is being used in. We will talk about the various types of jet engine elsewhere but for the time being we will assume our engine has only one compressor and one turbine.

The turbine section is made up of two elements, the nozzle guide vanes and the turbine rotor. The air leaving the combustion chamber will be swirling and it is the job of the nozzle guide vanes to straighten out the airflow and to direct it on to the turbine blades at the correct angle for maximum efficiency.

The turbine blades are attached to a rotor disk, usually using the "fir-tree" root method. The turbine rotor has a shaft to connect it to the compressor it is driving. The turbine shaft is usually hollow which means that the shafts of any additional turbines can run inside it.

The turbine blade has to live in a very hostile environment. Not only has it got to cope with temperatures exceeding 1,600 degrees, it also has to tolerate rotating at speeds of up to 20,000 rpm. The only way a modern turbine blade can endure such high temperatures is to provide it with some form of cooling system.

Turbine blades can be made with tiny hollow internal passages through which cool air is fed under pressure. Note the term "cool" is a relative one because if "cold" air were to touch the white hot turbine blade then it would shatter with the thermal shock. Instead, "cool" air at a temperature of about 300 degrees is tapped from the latter stages of the air going through the compressor.

As the temperature in the turbine section is often higher than the melting point of the turbine blade material, any loss of cooling air would mean the rapid failure of the turbine.

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