1.2 THEORY OF GAS TRUBNINE ENGINE

 THEORY OF GAS TRUBNINE ENGINE  

Gas Turbine Engines used for aircraft propulsion are broadly speaking jet-producing devices where a working fluid undergoes a series of thermodynamic processes. These processes are, if it is taken ideally, isentropic compression (in air inlet diffuser and compressor), heat addition (in combustion chamber), isentropic expansion (in turbine and propelling nozzle). Thermodynamic cycle for gas turbine engines comprising these processes is the Joule/Brayton Cycle.  Ref: Figure 1.1 (a).

Although the diagram in Figure 1.1 (a) is a closed cycle, aero engines works actually on open cycle as it is clear in Figure 1.1 (b).

Figure 1.1

Objective of this cyclic performance of the working fluid is to produce a net propulsive force that is used by the aircraft for its flight through the atmosphere overcoming the drag force. Different types of engines use this working fluid differently to have the same end result.

When a propeller turbine is used, the net shaft work (W₃₄ + W₁₂) is simply supplied to the airscrew (i.e. propeller). If propulsion is by jet, the turbine is required to supply merely the compressor work and it uses only part of the expansion to atmospheric pressure, from 3 to 5. The remaining expansion, from 5 to 4, occurs in the propulsion nozzle. Ref. Figure 1.1(b).

Cyclic processes consisting the Brayton cycle are executed in different and separate working zones or sections as illustrated in Figure 1.2. These sections are the basis of constructional build up of a turbine engine.

Figure 1.2

The mechanical arrangement of the gas turbine engine is simple, for it consists of only two main rotating parts, a compressor and a turbine, and one or more combustion chambers. To these three basic parts are added intake at the front and an exhaust unit at the rear. See Figure 1.3 illustrating a gas turbine engine (turbojet) for the aircraft.

Figure 1.3

How this arrangement of engine sections, producing propulsive force generates propulsive force is the theory of jet propulsion.