# Why do we need stators in compressors or turbines?

Summary:
The need of stators in compressors or turbines
Hey Everybody!

I was wondering why do we need Stators in compressors and turbine, for instance the goal of compressor is to increase the outlet pressure, so that the combustion is effective, but why we don't put just Rotors and no stators in the compressor (or Turbine).

Baluncore
2021 Award
Because there is energy in the rotation of the fluid.
The stator converts much of that into axial flow and greater pressure.

Hidd, berkeman and anorlunda
DaveE
Gold Member
You'll probably enjoy watching AgentJayZ's YouTube videos about jet engines. He explains stuff like this in a correct but "low tech" way.

Some of those "fixed" blades can rotate to redirect the air to optimize the angle of attack of the turbine blades. This allows more power from modern engines by preventing stalling.

Hidd and sysprog
Lnewqban
Gold Member

Hidd and Astronuc
hutchphd
Homework Helper
Summary:: The need of stators in compressors or turbines

why we don't put just Rotors and no stators in the compressor (or Turbine).

Do you understand the difference between axial flow compressors and radial flow compressors? The first Whittle jet engines were radial flow and had no stators. They are less complicated to build but less capable.

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sophiecentaur
Gold Member
One way to look at what happens in a turbine: At the first rotor, the blade can move due to pressure of the moving gas and gets energy transferred from the moving gas and momentum is transferred. Power transferred will be 'pressure times blade speed'. At the first stator, because the stator is attached to the 'Earth' it will not move so Pressure times speed is zero. Conservation of Momentum means there is a finite motion but the moving gas has the same speed ('bounces' with equal opposite velocity). It retains all (most ) of the Kinetic energy and is moving in the right direction for the next rotor.

At the second rotor, the moving gas can give additional KE to the blades (pressure times speed). Same for second stator; no energy transferred and for the second rotor; more KE transferred. Each layer of rotating blades can acquire more energy but none goes to the stators. The shapes and pressures are chosen to maximise this because the pressures / gas speed decreases along the turbine.

Without the stators, the angle of gas flow would need to change from rotor to rotor. The gas would end up 'chasing' the subsequent blades and the leading edges would have to be angled more and more. A pretty smart piece of non-intuitive design imo.

DaveE, Lnewqban and Spinnor
Airfoils and curved surfaces such as the rotors seen in compressors are only efficient with precise incoming flows. If only rotors were utilized, then the flow would not be "primed" or "prepared" for the next rotor. Attached flow is critical when deal with turbomachinery, and if you only have rotors, flow begins to detach. In lamen's terms, air begins to leave the surface of the rotor.

This causes pressure distribution to vary, and while that might not sound too scary, that basically begins to tear apart your engine and cause compression to be inefficient, on top of it. Stators allow the compression process to be controlled- a strictly rotor compressor with no stators would simply break, or be incredibly inefficient.

It's akin to over-exercising your body without rest days. If you over-work your body (air flow) without proper rest (stators) to allow your body (air flow) to be prepared for the next exercise activity (compression, rotor), then your body will soon tear itself apart.

Hidd and sophiecentaur