Compressor/turbine ducting
 

G'day.

I have been pondering this problem for the last half an hour and can't seem to think of a reason.

In the compressor of a turbine engine the physical duct in which the compressor is set is a converging duct. This arrangement decreases pressure and increases velocity of gas flow. However, inside this converging duct the compressor/stator relationship is to increase pressure through divergence in the blades.

The same is true however in reverse in the turbine section. The engine now encorporates a turbine in a diverging duct which contains converging nozzles. The ducting is increasing pressure and reducing velocity however the turbine/nozzle guide vanes are configured as to decrease pressure and increase velocity.

My question is as to why the ducts are designed to achieve seemingly opposite gas results as opposed to the turbine/compressor set up.

Actually, in the process of compression, more molecules are squeezed into a smaller volume. This DECREASES velocity, internally within the rotor/stator arrangements. To compensate for this, the external ducts (compressor cases) are converging to reverse this effect and keep velocity up!

And exactly the reverse happens in the turbine stages.

If you can find a rotor/stator airflow velocity vector diagram, showing both high-power and low-power ops, you can begin to appreciate the complexity.

You can change the volumn by maintaining the outer physical duct constant in diameter while changing the rotor inner blade root diameter up or down.

This is more obvious in looking at an engine cross-section

It's to maintain almost constant axial velocity.