What do compressor/turbine stages mean, and why we need a different number of stages?

I am new to aviation. I realize that LP (Low Pressure) and HP (High Pressure) compressors/turbines have a various number of stages.

1. What do stages mean?
2. What is the reason to have a different number of stages for LP and HP?
• May 10, 2019 at 19:15

For axial compressors (what you'd find in a typical turbofan), each stage is:

1. A rotating disk of blades

They're called rotors and stators, respectively. So those two rows of blades are one stage.

The rotors increase the air's velocity, the stators turn this velocity into pressure, and repeat (see image below). The following stage would compress the air more, and so on, incrementally increasing the pressure.

Likewise but in reverse for the turbine stages (the fixed blades also come first).

Source: The Jet Engine (book)

• Or rotating blades and stationary vanes. May 10, 2019 at 18:34

A piston engine compresses air by enclosing it in a chamber and then "closing in the walls", so to speak, with the piston so that the air has nowhere to go and it gets mechanically compressed. The piston engine is a movie theater with the doors locked and walls that close in.

On a jet engine, you are compressing air by squeezing it into a smaller space in a constant stream, depending on the velocity and inertia of air mass to do the squeezing. It's more like a movie theater where there is an open exit and you yell "Fire" so people at the exit are being squeezed together and crushed by the mass and inertia of the larger crowd behind them all being forced into the same spot.

For this to work efficiently and get a decent compression ratio, you have to impart energy to the air stream all along its path down the converging passage it's on. It's like having the theater exit being a converging hallway with people stationed along its length, in stages you might say, giving theater goers a big shove so that by the time they are going through the small exit they are running at full speed and everybody is able to make it through.

An axial compressor is just a big fan with multiple blade elements, stages, (with stators in between to manage the flow direction for the next stage) that moves the air along in baby steps all along the convergent path to the inlet of the burner can. How many stages is a design balancing act - more stages, more compression, but more weight and complexity; so somewhere on a given engine there is a sweet spot in the number and design of stages and the designer has to find it.

Engines get broken up into LP and HP compressor/turbine sections when it's desirable to split the compressor into independent segments, each with its own turbine to drive it, that are free to turn at different speeds, which makes the engine able to handle larger variations in inlet flow. The number of stages in each section is a function of the compression ratio the designer wants to achieve with each compressor section. A dual spool configuration like that adds weight and complexity, so there has to be a payoff operationally to be worth it, so you will some engines with two spools and some without.