Why do axial compressor stages have to get smaller and smaller to increase fluid pressure?

Question

Why can't we design axial compressor stages of which have the same length? We are adding kinetic energy to fluid and then get it back as potential energy-static pressure. So why do we need that they get smaller?

Is it because to move fluid from stator of previous stage to rotor of next stage, since we know dynamic energy increases as volume decreases in such flows?

Answer 1

Here is an easy way to think of this.

We start with a piston inside a cylinder full of air. We apply a force to the piston and the piston moves into the cylinder, compressing the air. Now the volume inside the cylinder is less, and the pressure is greater.

Then we apply a little more force to the piston. it moves a little further, compresses the air a little more, and thereby reduces its volume and increases its pressure a little more.

So the further we carry on the compression process, the smaller the volume of the air becomes i.e., it occupies less space.

Now we compare this to a multistage turbocompressor, which sucks air in on one end and pushes compressed air out the other. As the air moves from one stage to the next, its volume decreases (it fits into less space) and so the turbine wheels get progressively smaller and smaller as you move from one stage to the next.

In this sense you can imagine the turbocompressor as if it were a magic piston that continuously and progressively compresses the air inside a magic cylinder.