To move an airplane through the air,
thrust is generated by some kind of
propulsion system. Most modern airliners use
turbofan engines
because of their high thrust and good
fuel efficiency. On this page, we will discuss
some of the fundamentals of turbofan engines.
A turbofan engine is the most modern variation of the basic
gas turbine engine. As with other gas
turbines, there is a core engine, whose
parts and operation are discussed on a
separate page. In the turbofan engine, the core engine is surrounded
by a fan in the front and an additional turbine at the rear. The fan
and fan turbine are composed of many blades, like the core compressor
and core
turbine,
and are connected by an additional shaft. All of
this additional turbomachinery is colored green on the
schematic. As with the core compressor
and turbine, some of the fan blades turn with the shaft and some
blades remain stationary. The fan shaft passes through the core shaft
for mechanical reasons. This type of arrangement is called a two
spool engine; one "spool" for the fan, one "spool" for the core.
Some advanced engines have additional spools for sections of the
compressor which provides for even higher compressor
efficiency.
How does a turbofan engine work? The incoming air is captured by
the engine
inlet.
Some of the incoming air, colored blue on the figure, passes
through the fan and continues on into the core compressor and then
into the
burner,
where it is mixed with fuel and combustion
occurs. The hot exhaust passes through the core and fan turbines and
then out the
nozzle,
as in a basic turbojet.
This airflow is called the core airflow and is denoted by (m dot)c.
The rest of the incoming air, colored light blue on the figure, passes through the fan
and bypasses, or goes around the engine, just like the air
through a propeller. The air that goes
through the fan has a velocity that is slightly increased from free
stream. This airflow is called the fan flow, or bypass flow, and
is denoted by (m dot)f. The ratio of (m dot)f to (m dot)c is called the
bypass ratio  bpr.
bpr = (m dot)f / (m dot)c
The total mass flow rate through the inlet is the
sum of the core and fan flows
(m dot)0 = (m dot)f + (m dot)c
A turbofan gets some of its thrust from the core and some
of its thrust from the fan. If we denote the exit of the core as
station "e", the exit of the fan as station "f", and the free stream
as station "0", we can use the basic
thrust equation for each stream to obtain the
total thrust:
F = (m dot * V)f  (m dot)f * V0 + (m dot * V)e  (m dot)c * V0
We can combine the terms multiplying V0 and use the definition of the
bypass ratio bpr to obtain the final thrust equation:
F = (m dot * V)e + bpr * (m dot)c * Vf  (m dot * V)0
Because the fuel flow rate for the core is changed only a small
amount by the addition of the fan, a turbofan generates more thrust
for nearly the same amount of fuel used by the core. This means that
a turbofan is very fuel efficient. In fact, high bypass ratio
turbofans are nearly as fuel efficient as turboprops.
Because the fan is enclosed by the inlet and is composed of many
blades, it can operate efficiently at higher speeds than a simple
propeller. That is why turbofans are found on
high speed
transports
and propellers are used on
low speed
transports. Low bypass ratio
turbofans are still more fuel efficient than basic turbojets. Many
modern fighter planes actually use low bypass ratio turbofans
equipped with afterburners. They can then
cruise efficiently but still have high thrust when dogfighting. Even
though the fighter plane can fly much
faster
than the speed of sound,
the air going into the engine must travel less than the speed of
sound for high efficiency. Therefore, the airplane inlet slows the
air down from supersonic speeds.
You can explore the design and operation of an afterburning turbojet
engine by using the interactive
EngineSim
Java applet. Set the Engine
Type to "Turbofan" and you can vary any of the parameters which
affect thrust and fuel flow.
Activities:
Guided Tours

Turbofans:
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