|
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
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).
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) |
|
