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AIRCRAFT PROPULSION : TURBOJET ENGINE

Gas turbines are particularly suited for aircraft propulsion because they are light and compact and have a high power-to-weight ratio. Aircraft gas turbines operate on an open cycle called "Jet propulsion Cycle", as explained below . It can be of turbojet, turbofan or turboprop type.

TURBOJET ENGINE : 

 In a turbojet engine, high velocity air first flows through a diffuser where it is decelerated increasing its pressure. Air is then compressed in the compressor. It is mixed with fuel in the combustion chamber, where the mixture is burned at constant pressure. The high pressure, high temperature combustion gases partially expand in the turbine, producing enough power to drive the compressor and any auxiliary equipment. Finally, the gases expand in the nozzle to the ambient pressure and leave the aircraft at high velocity. In the ideal case, the turbine work is assumed to be equal to the compressor work. The processes in the diffuser , the compressor , the turbine., and the nozzle are assumed to be reversible and adiabatic.

The thrust developed in a turbojet engine in the unbalanced force caused by the difference in the momentum of the air entering the engine and the exhaust gases leaving the engine, so that 

F = (m*V*)exit - (m*V*)inlet = m* (V* exit - V* inlet )


Turbojet Engine

T-s diagram of an ideal cycle
 

The pressure at inlet and exit of the engine are the ambient pressure, For an aircraft cruising in still air, V* inlet is the aircraft velocity. The mass flow rates of the gases at the engine exit and the inlet are different, the difference being equal to the combustion rate of the fuel. But the air -fuel ratio used in jet propulsion engines is usually very high, making the difference very small. Thus m* is taken as the mass flow rate of the air through the engine. For an aircraft cruising at a steady speed, the thrust is used to overcome the fluid drag, and the net force acting on the body of the aircraft is zero. Commercial airplanes save fuel by flying at higher altitude during long strips since the air at higher altitudes is of less density and exerts a smaller drag force on the aircraft.

The power developed from the thrust of the engine is called the propulsive power, Wp , is given by, 

W*p = FV*aircraft = m* (V*exit - V*inlet) V*aircraft

The propulsive efficiency, Np is defined by 

Np = Propulsive power /  Energy Input rate = W*p / Q*in

It is a measure of how efficiently the energy released during combustion is converted to propulsive power. 

Space and weight limitations prohibit the use of  regenerators and intercoolers on a aircraft engines. The counter part of reheating is after burning. The air-fuel ratio in jet engine is so high that the turbine exhaust gases are sufficiently rich in oxygen to support the combustion of more fuel in an afterburner. Such burning of fuel raises the temperature pf gas before it expands in the nozzle, increasing the KE change in the nozzle and consequently increasing the thrust. In the air- standard case, the combustion is replaced by constant pressure heat addition.






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