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| Glenn |
Most modern passenger and military aircraft are powered bygas turbineengines, which are also calledjet engines. The first and simplesttypeof gas turbine is the turbojet. How does a turbojet work?
On this slide we show a computer animation of a turbojet engine.Thepartsof the engine are described onother slides. Here, we are concerned with what happens to the airthat passes through the engine. Large amounts of surrounding air arecontinuously brought into the engineinlet.(In England, they call this part the intake, which is probably a moreaccurate description, since the compressor pulls air into theengine.) We have shown here a tube-shaped inlet, like one you wouldsee on an airliner. But inlets come in many shapes and sizesdepending on the aircraft's mission. At the rear of the inlet, theair enters thecompressor.The compressoracts like many rows of airfoils,with each row producinga small jump in pressure. Acompressor is like an electric fan. We have to supply energy to turnthe compressor. At the exit of the compressor, the air is at a muchhigher pressure than free stream. In theburnera small amount of fuel is combined with the air and ignited. (In atypical jet engine, 100 pounds of air/sec is combined with only 2pounds of fuel/sec. Most of the hot exhaust has come from thesurrounding air.) Leaving the burner, the hot exhaust is passedthrough theturbine.The turbine workslike a windmill. Instead of needing energy to turn the blades to makethe air flow, the turbine extracts energy from a flow of gas bymaking the blades spin in the flow. In a jet engine we use the energyextracted by the turbine to turn the compressor bylinkingthe compressor and the turbine by the central shaft. Theturbine takes some energy out of the hot exhaust, but there is enoughenergy left over to provide thrust to the jet engine by increasingthe velocity through thenozzle.Becausethe exit velocity is greater than the free stream velocity, thrust iscreated as described by thethrust equation.For a jet engine, the exit mass flow is nearly equal tothe free stream mass flow, since very little fuel is added to thestream.
Thethrust equationfor a turbojet isgiven on a separate slide.
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