Posted 9-29-2004 at 05:04 PM

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

http://science.howstuffworks.com/turbine3.htm

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 or 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.

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.

REFERENCE:
http://science.howstuffworks.com/turbine.htm