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AXIAL TURBINES

Differences between the flow field characteristics of turbines and compressors

Compressor

Turbine

Adverse pressure gradient

Favorable pressure gradient

Low stage pressure ratio (1.2 to 2)

High stage pressure ratio (> 4)

Limited by stall

Limited by choking and blade stress

Moderate temperature

High temperature, requiring cooling

The engine shown above has 10 fan and compressor stages, but only four turbine stages (2 on each spool) to provide enough work to run them. A turbine stage consists of a "nozzle", which is static with respect to the engine walls, and a "rotor". The nozzle is in fact a series of passages between aerodynamically-shaped surfaces (essentially airfoils). The rotor blades may be highly cambered, since flow separation is not as big a concern as in compressors.


Using the same nomenclature as for the axial compressor stage velocity diagram, Work Output per unit mass flow rate

Also,

thus,

Note that here, T01 = T02 (no work in the nozzle).
Again, the work done per unit mass flow rate is proportional to the blade speed achieved, and the turning of the flow.

Impulse Stage

In a impulse turbine stage, the entire pressure drop occurs in the nozzle. The velocity diagram is shown below:

Note that for a turbine stage to operate, the C vector must be considerably longer than the U vector. In the case of an impulse stage, the flow gets turned by the impulse rotor, so that the static termperatures and the relative flow angles are:

50% Reaction Stage

In a 50% Reaction Stage, the static pressure drop is split between the rotor and the nozzle. Here, the velocity diagram is as shown below:

The velocity triangles become symmetric.

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