How can a plane fly? (II) – The magic of a gas turbine

Today we will talk about one of the most important applications of kerosene, which thanks to its properties, described below,  is the perfect fuel to use in gas turbines.

An aircraft, like a bicycle or a train, needs a force to push. On a plane, unlike other vehicles (where you only need the drive to overcome the drag and inertia), the momentum is important for producing the air circulation in the wings, origin of the stall.

The traction force (exerted by the motor behind) is obtained accelerating a backwards  ambient air mass until  higher speed than the plane; according with the Newton’s 3^rd law, this action causes one reaction of the same intensity but opposite,  which pushes the plane forward.  Acceleration of the air mass is obtained by rotation of a propeller, move, in our case, for a gas turbine.

These machines are made up of four parts: compressor, combustion chamber, turbine and outlet nozzle. Initially, air enters in the compressor area; here, a first rotor compresses the air, after that, a second rotor compresses even more the air, and so on until reaching 10 to 40 times the input air pressure. This air passes through diffusers to the combustion chamber where a constant flow of fuel is burned in an almost isobaric process.

Combustion causes the expansion of the gases at high pressure, temperature (until 1500 degrees) and speed. On the way out, this jet drives a shaft that shares the turbine and the compressor. En general, this shaft rotates at 10000 rpm. Finally, this gas jet is expelled to the atmosphere through the outlet nozzle.

When designing a motor two important parameters are its weight and volume because of the effect they have on the performance of the plane.

In thisP-v diagram of a gas turbine you can see that the work generated by the turbine between 3-3’ is used to rotate the shaft. This is the same axial work the compressor received, then the areas enclosed by each of the curves (w12 and w33´) and the vertical axils have the same numerical value.

Turbofan is a modern version of the turbine. In this device, a big rotor in front of the section of the compressor produces a flow of air at low pressure which does not pass through the fuel chamber, however, is mixed with the jet output by increasing the air mass acceleration. This new system increases the efficiency of the turbine, consuming less fuel and producing lower noise level.