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In an axial flow compressor as used in modern jet engines, the goal of a compressor rotor is to increase the angular momentum of the air by imparting an increase in swirl velocity. (Note: the goal of an entire compressor stage, including rotor and stator, is to raise the pressure of air, but I'm focused purely on the rotor right now).

However, I'm a bit confused as to what imparts this increase in swirl velocity. Is it: a) the rotational motion of the rotors themselves? or b) the lift generating characteristics of the rotor airfoil shape.

Basically, I'm not sure why the blades of the rotor needs to be lift producing entities. It seems that if you had a flat plate it would also increase the rotational velocity to the flow because it itself is rotating. For example, in typical pictures of velocity triangles, nowhere is lift and or drag mentioned.

Hopefully that question makes sense!

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  • $\begingroup$ "the goal of a compressor rotor is to increase the angular momentum of the air" -- is that the goal in & of itself, or something that happens on the way (i.e. a means) to achieving another goal? If you had lots of angular momentum but very slight axial velocity, would that accomplish any useful purpose? $\endgroup$
    – Ralph J
    Commented Apr 29, 2023 at 16:47

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The goal of the rotor is to add energy to the air stream. A rotating propeller/fan/turbine will always add some of this energy in the form of kinetic energy of swirling motion—to add energy the blades must do work, which means apply force on the air in the direction of their motion, and this will inevitably accelerate the air in that direction.

The blades do actually push the air aft, not just in the direction of rotation, but the swirling can't be avoided. So the stator—which can't add energy because, not moving, it can't do work—then redirects the air to convert the kinetic energy of the swirling motion to pressure (or kinetic energy of straight flow in case of a ducted fan; what form it will be depends on the pressure downstream).

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