Most modern passenger and military aircraft are powered by gas turbine engines, which are also called jet engines. The first and simplest type of gas turbine is the turbojet. How does a turbojet work?
On this slide we show a computer animation of a turbojet engine. The parts of the engine are described on other slides. Here, we are concerned with what happens to the air that passes through the engine. Large amounts of surrounding air are continuously brought into the engine inlet. (In England, they call this part the intake, which is probably a more accurate description, since the compressor pulls air into the engine.) We have shown here a tube-shaped inlet, like one you would see on an airliner. But inlets come in many shapes and sizes depending on the aircraft's mission. At the rear of the inlet, the air enters the compressor. The compressor acts like many rows of airfoils, with each row producing a small jump in pressure. A compressor is like an electric fan. We have to supply energy to turn the compressor. At the exit of the compressor, the air is at a much higher pressure than free stream. In the burner a small amount of fuel is combined with the air and ignited. (In a typical jet engine, 100 pounds of air/sec is combined with only 2 pounds of fuel/sec. Most of the hot exhaust has come from the surrounding air.) Leaving the burner, the hot exhaust is passed through the turbine. The turbine works like a windmill. Instead of needing energy to turn the blades to make the air flow, the turbine extracts energy from a flow of gas by making the blades spin in the flow. In a jet engine we use the energy extracted by the turbine to turn the compressor by linking the compressor and the turbine by the central shaft. The turbine takes some energy out of the hot exhaust, but there is enough energy left over to provide thrust to the jet engine by increasing the velocity through the nozzle. Because the exit velocity is greater than the free stream velocity, thrust is created as described by the thrust equation. For a jet engine, the exit mass flow is nearly equal to the free stream mass flow, since very little fuel is added to the stream.
The thrust equation for a turbojet is given on a separate slide.
Last Updated Thu, Jun 12 04:47:01 PM EDT 2014
by Tom Benson