Nowadays in the turbofan Variable Inlet Guide Vanes are more and more used. My question is: which is their function in terms of pressure change?
The question is very specific: I know that they allow to give the most proper direction to the fluid, but do they act also to increase the pressure? So: what happen between the inlet and the outlet of each blade, talking in term of pressure?
I need some reference, if possible.
You need to be careful in understanding the significance of the variable guide vanes (VGVs) being variable. You could think of them as a bit like venetian blinds - when they they are most open they present the least obstruction to the air flow, much as you get the most light through a set of blinds when they are at the right angle to the incoming light. As you close them, you get less flow (light) through them, as they present a greater obstruction to the flow.
The VGVs are only variable to maintain stable flow in the compressor at conditions away from the design condition. The design condition for jet engines is always high power during cruise (or at the top of the initial climb to be pedantic), so the VGVs only start to close at reduced powers.
At high power the vanes are at their most open to maximise flow through the engine, minimising pressure losses across them and maximising thrust. As they are gradually closed when power is reduced, they present more of an obstruction to the flow (acting as effectively a variable area throttle valve in the path of the flow being closed down), so presenting an increased pressure ratio loss (or if you prefer reduced pressure recovery).
The increased use of VGVs really shows designers pushing at the limits of airflow stability in pursuit of better efficiency by maximising pressure ratio per stage at the design condition, at the cost of the increased weight of the mechanisms to drive the VGVs.
Not sure I expressed that very well, but first post on stack exchange.
Variable Nozzle Guide Vanes do the same job as non-variable ones, they are just optimised for a greater range of conditions so their characteristics will be the same.
They direct the airflow onto the turbine blades while at the same time converting pressure energy into kinetic energy.
They will accelerate the flow and therefore reduce the pressure at the vane outlet.
There is also this blog, which is a very good and informative read about gas flow in a turbine.
VIGVs are similiar to IGVs with one major exception:
VIGV's move either mechanically or hydro-mechanically to increase or decrease angle of small blades (located at front of compressor) to maximize airflow into the compressor. Doing this allows airflow to not shock the 1st stage compressor blades which can minimize things such as compressor stall due to speed of aircraft. The faster the air enters the compressor the more chance there will be to stall the compressor/engine.
Google Pegasus turbo fan engine (F402-RR-408) to see how the VIGV's help redirect the air on the dual shaft, counter-rotating compressor/fan. This is an amazing engine that counter rotates to help with torque while the Harrier is in a picture perfect hover.
If the motor did not have this feature the entire fuselage would want to rotate in the direction of the spinning blades inside of the motor. So the first section of the motor is the first of 3 stages of stators followed by a rotating stage of blades alternating by stator/fan/stator/fan blade which are clockwise spinning blades.
VIGV's redirect airflow counter-clockwise to enter the first of 8 stages of clockwise rotating blades.
To my knowledge, this is the only counter rotating motor.
