In this lecture we will understand working principles behind a gas turbine engine. Gas turbines are used mainly for two purposes. First, for power production. Second, for generating thrust force in an aircraft. Even though functions are different working principle behind each case is the same. Here we will understand how gas turbine engine is used for generating thrust force in an aircraft.
Detailed description above lecture is given below.
Gas Turbine Engine for Jet Propulsion
Following figure shows of gas turbine engine of an aircraft. In order to make the flight move forward this engine should produce a force in forward direction.
|Fig.1 Jet force required by aircraft is produced by a gas turbine engine|
This force is produced by jet effect of this exit fluid. When a high velocity fluid is ejected from aircraft engine it will produce a reaction force which will power the aircraft flight. This force is known as jet force. By applying Newton’s 2nd law of motion to jet engine control volume, one can easily deduce magnitude this jet force as follows.
It is momentum out minus momentum in. So if jet velocity is high it means high thrust force! This is why exit portion of a jet engine has got decreasing area, assuming the flow is subsonic. Or the exit portion acts like nozzle which increase jet velocity.
Continuous Production of High Velocity Jet
If we can produce high velocity jet continuously, the engine will be continuous jet force. We will produce this by a combustion process, by injecting fuel into air. This will produce flames with very high velocity.
Fig.2 Combustion produces flames at high energy, which is then transformed to high velocity flames
But for a sustainable combustion process we need the inlet air to the combustion chamber to be at high temperature and pressure.
Use of Compressor
Surrounding air is brought to high temperature and pressure state with help of diffuser plus compressor arrangement. Air gets into the engine by forward motion of engine and sucking effect of compressor. Diffuser increases pressure and temperature of the fluid to some extent by converting some part of kinetic energy. After that compressor comes where both pressure and temperature of the air is raised by by energy supply from compressor.
Fig.3 Diffuser and compressor together raise pressure and temperature of the incoming air
So at outlet of the compressor we will have air at high pressure and temperature. But compressor requires some power input to do this compression process.
Turbine – A Source of Power to Compressor
Power required compressor is given by a turbine which is situated right after the combustion chamber. The turbine absorbs some amount of energy from the high energy fluid and transmits it to the compressor.
Fig.4 The complete gas turbine, which is self sustainable in operation
Nozzle – Production of High Velocity Jet
Now the fluid with high energy can be expanded in a nozzle section to produce a high velocity jet.In nozzle air will expand to surrounding pressure. Thus the process of producing high velocity jet at outlet has become a self sustainable. We will get continuous supply of high velocity jet and thrust force to this aircraft, thanks to synchronized working of all these components.
Thermal Cycle of Gas Turbine – Brayton Cycle
Variation of state of fluid from inlet to exit of gas turbine engine is shown in Fig.5 in a T-s diagram. Point 1 is the inlet condition of a gas turbine engine, which is same as state of surrounding air. Due to diffuser effect pressure and temperature of the fluid increases slightly, entropy remains same assuming this is an adiabatic reversible process (1-2). Next in compressor stage also same process continues, temperature and pressure rise to a level where combustion process is sustainable (2-3). Now fuel injection and heat addition to the fluid, this process happens almost at constant pressure, here pressure raises to very high level (3-4). Right after that, turbine will absorb some amount energy which is required by the compressor. So here temperature and pressure of the fluid comes down (4-5). Now the last section, which produces high velocity jet. This is again a constant entropy process, where internal energy of the fluid gets converted into kinetic energy. Here pressure expands to the surrounding pressure (5-6).
Fig.5 Variation of state of fluid as it executes Brayton cycle
It should be noted here that the exit stream never go back to the inlet condition, at inlet it sucks fresh steam of air in. So this is an open cycle process, but since both this points are having same pressure we can assume pseudo constant pressure process (6-1) in between in order to complete the cycle.