................
How Gas Turbine Engines Work
BY MARSHALL
BRAIN
When you go to an airport and
see the commercial jets there, you can't help but notice the huge engines that
power them.
Most commercial jets are
powered by turbofan engines, and turbofans are one example of a general class
of engines called gas turbine engines.
You may have never heard of
gas turbine engines, but they are used in all kinds of unexpected places.
For example, many of the
helicopters you see, a lot of smaller power plants and even the M-1
Tank use gas turbines.
In this article, we will look
at gas turbine engines to see what makes them tick!
Types
of Turbines
There
are many different kinds of turbines:
· You
have probably heard of a steam turbine.
Most power plants use coal, natural gas, oil or a nuclear reactor to
create steam. The steam runs through a huge and very carefully designed
multi-stage turbine to spin an output shaft that drives the plant's generator.
· Hydroelectric dams use water turbines in
the same way to generate power. The turbines used in a hydroelectric plant look
completely different from a steam turbine because water is so much denser (and
slower moving) than steam, but it is the same principle.
· Wind turbines, also known as wind mills, use
the wind as their motive force. A wind turbine looks nothing like a steam
turbine or a water turbine because wind is slow moving and very light, but
again, the principle is the same.
A gas turbine is an extension
of the same concept. In a gas turbine, a pressurized gas spins the turbine.
In all modern gas turbine
engines, the engine produces its own pressurized gas, and it does this by
burning something like propane, natural gas, kerosene or jet fuel.
The heat that comes from
burning the fuel expands air, and the high-speed rush of this hot air spins the
turbine.
Advantages
and Disadvantages of Jet Engines
So why
does the M-1 tank use a 1,500 horsepower gas turbine engine instead of a
diesel engine?
It turns out that there are
two big advantages of the turbine over the diesel:
· Gas
turbine engines have a great power-to-weight
ratio compared to reciprocating engines. That is, the amount of power
you get out of the engine compared to the weight of the engine itself is very
good.
· Gas
turbine engines are smaller than
their reciprocating counterparts of the same power.
The main disadvantage of gas
turbines is that, compared to a reciprocating engine of the same size, they are expensive.
Because they spin at such
high speeds and because of the high operating temperatures, designing and
manufacturing gas turbines is a tough problem from both the engineering and
materials standpoint.
Gas turbines also tend to use
more fuel when they are idling, and they prefer a constant rather than a
fluctuating load.
That makes gas turbines great
for things like transcontinental jet aircraft and power plants, but explains
why you don't have one under the hood of your car.
The Gas Turbine Process
Gas
turbine engines are, theoretically, extremely simple. They have three parts:
· Compressor - Compresses the incoming
air to high pressure
· Combustion area - Burns the fuel and
produces high-pressure, high-velocity gas
· Turbine - Extracts the energy
from the high-pressure, high-velocity gas flowing from the combustion chamber
The
following figure shows the general layout of an axial-flow gas
turbine -- the sort of engine you would find driving the rotor of a helicopter,
for example:
In
this engine, air is sucked in from the right by the compressor. The compressor
is basically a cone-shaped cylinder with small fan blades attached in rows
(eight rows of blades are represented here).
Assuming the light blue
represents air at normal air pressure, then as the air is forced through the
compression stage its pressure rises significantly.
In some engines, the pressure
of the air can rise by a factor of 30. The high-pressure air produced by the
compressor is shown in dark blue.
Combustion Area
This
high-pressure air then enters the combustion area, where a ring of fuel
injectors injects a steady stream of fuel.
The fuel is generally
kerosene, jet fuel, propane natural gas.
If you think about how easy
it is to blow a candle out, then you can see the design problem in the
combustion area -- entering this area is high-pressure air moving at hundreds
of miles per hour.
You want to keep a flame
burning continuously in that environment. The piece that solves this problem is
called a "flame holder," or sometimes a "can."
The can is
a hollow, perforated piece of heavy metal. Half of the can in cross-section is
shown below:
The injectors are
at the right. Compressed air enters through the perforations. Exhaust gases
exit at the left.
You can see in the previous
figure that a second set of cylinders wraps
around the inside and the outside of this perforated can, guiding the compressed
intake air into the perforations.
The Turbine
At the left of the engine is the turbine section. In this figure there
are two sets of turbines.
The
first set directly drives the compressor. The turbines, the shaft and the
compressor all turn as a single unit:
At the far left is
a final turbine stage, shown here with a single set of vanes. It drives the
output shaft.
This
final turbine stage and the output shaft are a completely stand-alone,
freewheeling unit. They spin freely without any connection to the rest of the
engine.
And
that is the amazing part about a gas turbine engine -- there is enough energy
in the hot gases blowing through the blades of that final output turbine to
generate 1,500 horsepower and drive a 63-ton M-1 Tank!
A
gas turbine engine really is that simple.
In the case of the
turbine used in a tank or a power plant, there really is nothing to do with the
exhaust gases but vent them through an exhaust pipe, as shown.
Sometimes
the exhaust will run through some sort of heat exchanger either to extract the
heat for some other purpose or to preheat air before it enters the combustion
chamber.
The discussion here
is obviously simplified a bit.
For
example, we have not discussed the areas of bearings, oiling systems, internal
support structures of the engine, stator vanes and so on.
All
of these areas become major engineering problems because of the tremendous
temperatures, pressures and spin rates inside the engine.
But
the basic principles described here govern all gas turbine engines and help you
to understand the basic layout and operation of the engine.
Gas Turbine Variations
Large jetliners use what are
known as turbofan engines,
which are nothing more than gas turbines combined with a large fan at the front
of the engine.
Here's the basic (highly
simplified) layout of a turbofan engine:
You can see that the core of
a turbofan is a normal gas turbine engine like the one described in the
previous section.
The difference is that the
final turbine stage drives a shaft that makes its way back to the front of the
engine to power the fan (shown
in red in this picture).
This multiple concentric shaft approach, by the
way, is extremely common in gas turbines. In many larger turbofans, in fact,
there may be two completely separate compression stages driven by separate
turbines, along with the fan turbine as shown above.
All three shafts ride within
one another concentrically.
The purpose of the fan is to
dramatically increase the amount of air moving through the engine, and
therefore increase the engine's thrust.
When you look into the engine
of a commercial jet at the airport, what you see is this fan at the front of
the engine.
It is huge -- on the order of
10 feet (3 m) in diameter on big jets, so it can move a lot of air.
The air that the fan moves is
called "bypass air" (shown in
purple above) because it bypasses the turbine portion of the engine and moves
straight through to the back of the nacelle at high speed to provide thrust.
A turboprop engine
is similar to a turbofan, but instead of a fan there is a conventional propeller at the front of the engine.
The output shaft connects to
a gearbox to reduce the speed,
and the output of the gearbox turns the propeller.
Thrust
Basics
The goal of a turbofan engine is to produce thrust to drive the airplane forward.
Thrust
is generally measured in pounds in the United States (the metric system uses
Newtons, where 4.45 Newtons equals 1 pound of thrust).
A
"pound of thrust" is equal to a force able to accelerate 1 pound of
material 32 feet per second per second (32 feet per second per second happens
to be equivalent to the acceleration provided by gravity).
Therefore,
if you have a jet engine capable of producing 1 pound of thrust, it could hold
1 pound of material suspended in the air if the jet were pointed straight down.
Likewise,
a jet engine producing 5,000 pounds of thrust could hold 5,000 pounds of material
suspended in the air.
And
if a rocket engine produced 5,000 pounds of thrust applied to a 5,000-pound
object floating in space, the 5,000-pound object would accelerate at a rate of
32 feet per second per second.
Thrust is generated
under Newton's principle that "every action has an equal and opposite
reaction."
For
example, imagine that you are floating in space and you weigh 100 pounds on
Earth. In your hand you have a baseball that weighs 1 pound on Earth.
If
you throw the baseball away from you at a speed of 32 feet per second (21 mph /
34 kph), your body will move in the opposite direction (it will react) at a speed of 0.32 feet per second.
If
you were to continuously throw baseballs in that way at a rate of one per
second, your baseballs would be generating 1 pound of continuous thrust.
Keep
in mind that to generate that 1 pound of thrust for an hour you would need to
be holding 3,600 pounds of baseballs at the beginning of the hour.
If
you wanted to do better, the thing to do is to throw the baseballs harder. By
"throwing" them (with of a gun, say) at 3,200 feet per second, you
would generate 100 pounds of thrust.
Jet
Engine Thrust
In a
turbofan engine, the baseballs that the engine is throwing out are air molecules.
The air molecules are already
there, so the airplane does not have to carry them around at least.
An individual air molecule
does not weigh very much, but the engine is throwing a lot of them and it is
throwing them at very high speed.
Thrust is coming from two
components in the turbofan:
· The gas turbine itself - Generally a nozzle
is formed at the exhaust end of the gas turbine (not shown in this figure) to
generate a high-speed jet of exhaust gas. A typical speed for air molecules
exiting the engine is 1,300 mph (2,092 kph).
· The bypass air generated by the fan - This
bypass air moves at a slower speed than the exhaust from the turbine, but the
fan moves a lot of air.
As you can see, gas turbine
engines are quite common. They are also quite complicated, and they stretch the
limits of both fluid dynamics and materials sciences.
If you want to learn more,
one worthwhile place to go would be the library of a university with a good
engineering department.
Books on the subject tend to
be expensive, but two well-known texts include "Aircraft Gas Turbine Engine Technology" and
"Elements of Gas Turbine Propulsion."
There is a surprising amount
of activity in the home-built gas-turbine arena, and you can find other people
interested in the same topic by participating in newsgroups or mailing lists
on the subject.
Marshall Brain,
Founder
Marshall Brain is the founder of HowStuffWorks. He holds a bachelor's degree in electrical engineering from Rensselaer Polytechnic Institute and a master's degree in computer science from North Carolina State University. Before founding HowStuffWorks, Marshall taught in the computer science department at NCSU and ran a software training and consulting company. Learn more at his site.
Marshall Brain is the founder of HowStuffWorks. He holds a bachelor's degree in electrical engineering from Rensselaer Polytechnic Institute and a master's degree in computer science from North Carolina State University. Before founding HowStuffWorks, Marshall taught in the computer science department at NCSU and ran a software training and consulting company. Learn more at his site.
No comments:
Post a Comment