Jet Engine
A jet engine is a mechanism in which air is scooped from the front of the engine and is then compressed and used in burning of the fuel carried by the engine to produce a jet for propulsion. The usual types of jet engines are turbojet, ramjet and pulsejet.
Fig 11.8 A Turbojet Engine
Fig 11.9 An Appropriate Control Volume Comprising the Stream of
Fluid Flowing through the Engine
Fluid Flowing through the Engine
A turbojet engine consists essentially (Fig. 11.8) of -
- a compressor,
- a combustion chamber,
- a gas turbine and
- a nozzle.
A portion of the thermal energy of the product of combustion is used to run the gas turbine to drive the compressor. The remaining part of thermal energy is converted into kinetic energy of the jet by a nozzle. At high speed fiight, jet engines are advantageous since a propeller has to rotate at high speed to create a large thrust. This will result in excessive blade stress and a decrease in the efficiency for blade tip speeds near and above sonic level. In a jet propelled aircraft, the spent gases are ejected to the surroundings at high velocity usually equal to or greater than the velocity of sound in the fluid at that state.
In many cases, depending upon the range of fight speeds, the jet is discharged with a velocity equal to sonic velocity in the medium and the pressure at discharge does not fall immediately to the ambient pressure. In these cases, the discharge pressure p2 at the nozzle exit becomes higher than the ambient pressure patm. Under the situation of uniform velocity of the aircraft, we have to use Eg. (10.18d) as the momentum theorem for the control volume as shown in Fig. 11.9 and can write
 | |
 |
where, Fx is the force acting on the control volume along the direction of the coordinate axis ”OX” fixed to the control volume, V is the velocity of the aircraft, u is the relative velocity of the exit jet with respect to the aircraft, 
and 
are the mass flow rate of air, and mass burning rate of fuel respectively. Usually is very less compared to 
usually varies from 0.01 to 0.02 in practice).
and are the mass flow rate of air, and mass burning rate of fuel respectively. Usually is very less compared to usually varies from 0.01 to 0.02 in practice).
The propulsive thrust on the aircraft can be written as
 | (11.20) |
 |
The terms in the bracket are always positive. Hence, the negative sign in FT represents that it acts in a direction opposite to ox, i.e. in the direction of the motion of the jet engine. The propulsive power is given by
 | (11.21) |
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