An axial turbine
is composed of a series of fan-blade assemblies mounted together inside a cone-shaped housing, with each assembly getting successively narrower in diameter
the deeper into the housing it is. The purpose of an axial turbine is usually the compression
of large volumes of air; each fan assembly compresses the air a little more than the previous (larger) one, until at the end of the line the air is at a much greater pressure than it was when it entered. The air is also quite a bit hotter due to the compression. Each fan blade compresses air by hurling it against the side of the housing via centrifugal force
; the blades are usually angled in order to better scoop the air and direct it towards the sides.
Axial turbines are most commonly found in larger turbojet engines, since they are very proficient at compressing large quantities of air. (Smaller turbojet engines usually employ a radial turbine instead.) Some jet engines employ intercoolers to cool down the air after it exits the turbine; this allows more air to flow through the engine. These turbines are usually spooled up using a "starter bullet", which is powered by an APU. Once the turbine is rotating fast enough to supply the combustion chamber with sufficient air to maintain combustion with the jet fuel, the starter bullet disengages and the turbine is driven by another turbine on the other end of the combustion chamber. That turbine is driven by the exhaust gas from the combustion chamber, and drives the air intake turbine by means of a common shaft.
As with radial turbines, axial turbines can be designed to work either way - they can either be driven by external force and used to compress a fluid, or be used to convert the kinetic energy of a compressed fluid into rotation to drive a shaft. In a turbojet engine, the turbine that compresses the air coming into the engine is driven by a common shaft shared with another turbine on the other side of the engine which is driven by exhaust from the combustion chamber.