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Basic Overview
The image above shows how a jet engine would be situated in a modern
military aircraft. In the basic jet engine, air enters the front intake and
is compressed (we will see how later). Then the air is forced into
combustion chambers where fuel is sprayed into it, and the mixture of air
and fuel is ignited. Gases that form expand rapidly and are exhausted
through the rear of the combustion chambers. These gases exert equal force
in all directions, providing forward thrust as they escape to the rear. As
the gases leave the engine, they pass through a fan-like set of blades
(turbine), which rotates a shaft called the turbine shaft. This shaft, in
turn, rotates the compressor, thereby bringing in a fresh supply of air
through the intake. Below is an animation of an isolated jet engine, which
illustrates the process of air inflow, compression, combustion, air outflow
and shaft rotation just described.
The
process can be described by the following diagram adopted from the website
of Rolls Royce, a popular manufacturer of jet engines.
This process is the essence of how jet engines work, but how exactly does
something like compression (squeezing) occur? To find out more about each
of the four steps in the creation of thrust by a jet engine, see below.
SUCK
The engine sucks in a large volume of air through the fan and compressor
stages. A typical commercial jet engine takes in 1.2 tons of air per second
during takeoff—in other words, it could empty the air in a squash court in
less than a second. The mechanism
by which a jet engine sucks in the air is largely a part of the compression
stage. In many engines the
compressor is responsible for both sucking in the air and compressing it. Some engines have an additional fan that
is not part of the compressor to draw additional air into the system. The fan is the leftmost component of the
engine illustrated above.
SQUEEZE
Aside from drawing air into the engine, the compressor also pressurizes the
air and delivers it to the combustion chamber. The compressor is shown in the above image just to the left of
the fire in the combustion chamber and to the right of the fan. The compression fans are driven from the
turbine by a shaft (the turbine is in turn driven by the air that is
leaving the engine). Compressors can achieve compression ratios in excess
of 40:1, which means that the pressure of the air at the end of the
compressor is over 40 times that of the air that enters the compressor. At full power the blades of a typical
commercial jet compressor rotate at 1000mph (1600kph) and take in 2600lb
(1200kg) of air per second.
Now
we will discuss how the compressor actually compresses the air.
As can be seen in the image above, the green fans that compose the
compressor gradually get smaller and smaller, as does the cavity through
which the air must travel. The air
must continue moving to the right, toward the combustion chambers of the
engine, since the fans are spinning and pushing the air in that direction. The result is a given amount of air
moving from a larger space to a smaller one, and thus increasing in
pressure.
BANG
In the combustion chamber, fuel is mixed with air to produce the bang, which
is responsible for the expansion that forces the air into the turbine.
Inside the typical commercial jet engine, the fuel burns in the combustion
chamber at up to 2000 degrees Celsius. The temperature at which metals in
this part of the engine start to melt is 1300 degrees Celsius, so advanced
cooling techniques must be used.
The combustion
chamber has the difficult task of burning large quantities of fuel,
supplied through fuel spray nozzles, with extensive volumes of air,
supplied by the compressor, and releasing the resulting heat in such a manner
that the air is expanded and accelerated to give a smooth stream of
uniformly heated gas. This task must be accomplished with the minimum loss
in pressure and with the maximum heat release within the limited space
available.
The amount of fuel
added to the air will depend upon the temperature rise required. However,
the maximum temperature is limited to certain range dictated by the
materials from which the turbine blades and nozzles are made. The air has
already been heated to between 200 and 550 °C by the work done in the
compressor, giving a temperature rise requirement of around 650 to
1150 °C from the combustion process. Since the gas temperature
determines the engine thrust, the combustion chamber must be capable of
maintaining stable and efficient combustion over a wide range of engine
operating conditions.
The air brought in by
the fan that does not go through the core of the engine and is thus not
used for combustion, which amounts to about 60 percent of the total
airflow, is introduced progressively into the flame tube to lower the
temperature inside the combustor and cool the walls of the flame tube.
BLOW
The reaction of the expanded gas—the mixture of fuel and air—being forced
through the turbine, drives the fan and compressor and blows out of the
exhaust nozzle providing the thrust.
Thus, the turbine has the task of providing power to drive
the compressor and accessories. It
does this by extracting energy from the hot gases released from the
combustion system and expanding them to a lower pressure and temperature. The continuous flow of gas to which the
turbine is exposed may enter the turbine at a temperature between 850 and
1700 °C, which is again far above the melting point of current
materials technology.
To produce the
driving torque, the turbine may consist of several stages, each employing
one row of moving blades and one row of stationary guide vanes to direct
the air as desired onto the blades. The number of stages depends on the
relationship between the power required from the gas flow, the rotational
speed at which it must be produced, and the diameter of turbine permitted.
The desire to
produce a high engine efficiency demands a high turbine inlet temperature,
but this causes problems as the turbine blades would be required to perform
and survive long operating periods at temperatures above their melting
point. These blades, while glowing red-hot, must be strong enough to carry
the centrifugal loads due to rotation at high speed.
To operate under these conditions, cool air is forced out of many small
holes in the blade. This air remains close to the blade, preventing it from
melting, but not detracting significantly from the engine's overall
performance. Nickel alloys are used to construct the turbine blades and the
nozzle guide vanes because these materials demonstrate good properties at
high temperatures
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