If you have multiple stage
turbines there is a set of guide vanes between each turbine disc to redirect
the flow again before the next turbine blade. They are stationary so are
sometimes called stators.
With the turbine you want a high velocity, so turbine blades are designed to
extract power from the hot expanding gasses to turn the compressor, by using
the high velocity hitting the turbine blade. The guide vanes straighten the
flow before the next set of turbine blades, but in effect it decreases the pressure
and increases the velocity. This also prevents excessive backpressure which
could work its way forward and affect the combustion process.
The compressor on the other hand is designed to increase the pressure through
each of the stages, so you can deliver a good mass flow to the combustion
chambers. However the increasing pressure will try to reverse the flow, so the
convergent duct is designed to keep the velocity constant through the whole
compressor, so that the momentum of the airflow prevents reverse flow. In
effect the velocity is therefore increased in each rotor section.
Velocity increases and
decreases through each set of rotors and stators, and by using a convergent
duct the velocity therefore stays constant, but the pressure is steadily
increasing through each stage.