# What is the purpose of blades on a compressor?

What is the point of having blades on a compressor. I heard on NASA that it alters the flow but how would this help. As the flow of a fluid increases, the pressure decreases, and vice versa. The rotors increase the speed of the air and descreases the pressure and the stators decrease the speed of the air and increase the pressure. How would this help create higher pressure. Why not just have the rotors? I know that the ducts this whole thing goes in gets smaller as it goes on, does this play a role? Don't get all complicated here on me, I'm still learning about airplanes and flying. Try to be as simple as possible, thank you.

• By blades, do you mean stator vanes? – Sanchises Nov 3 '17 at 7:55
• @Sanchises I mean ths rotor and the stator – Itzyoboi Nov 3 '17 at 22:51
• There is such a thing as a ramjet, which does not require compressor blades, but you have to be flying fast to make it work at all and it is not practical for most kinds of aircraft. – David K Nov 5 '17 at 6:07

The statement about the rotor blades ("the rotors increase the speed of the air and descrease the pressure") is not correct, because Bernoulli's Equation does not apply in this situation. Hence, the statement "as the flow of a fluid increases, the pressure decreases, and vice versa" isn't appropriate for the rotating compresser blades.

Bernouilli's equation (below) assumes no energy is being added or removed from the fluid. This is true when the fluid is flowing through the stationary stator vanes, but not the rotating blades.

The rotating blades increase the velocity, but do not decrease the static pressure, becuase they are adding mechanical work (energy) to the fluid. This energy (or power) comes from the turbine.

When no energy is being added to the fluid:

$$total\space pressure = static\space pressure + 1/2 \rho v^2$$

where $\rho$ is the density and $v$ is the velocity.

This is the mathematical equivalent of your statement "as the flow of the fluid increase, the static pressure decreases".

But, the rotating blades add total energy. So this equation does not apply, so as the velocity increases, static pressure does not go down. But, total energy has increased, by the mechanical work done by the blades. So the value on the left of the equation goes up as the fluid flows through the rotating blades. Next, the fluid flows through the stationary stator vanes. Because they are static, they add no energy, so Bernoulli's equation now does apply. Hence, as the velocity is decreased, the static pressure goes up (i.e. there is no change in total pressure).

If you just have the rotor blades, and no stator vanes, the compressor would just make the fluid move (i.e an increase in the total pressure) but with no increase in static pressure. The stator vanes slow the fluid down, and convert the velocity into an increase in static pressure.

You need to understand the difference between total and static pressures.

• so the air moves past the rotation blades and the total pressure is increased beucase it's adding mechanical work (energy). Does this mehcinal work also make the air flow? Then when it passes by the stator blades it increases the static pressure, correct? – Itzyoboi Nov 3 '17 at 22:59
• @Itzyoboi. Your comments are correct. Adding mechanical work to the fluid is necessary to make it flow, but is not the only thing necessary. If you replaced the blades with rods for example, energy would still be added to the fluid, but it would just be stirred up. The aerofoil shape of the blades is also crucial. And if you just had aerofoil shaped blades, and they did not move, the air will not flow either. You need both. – Penguin Nov 5 '17 at 11:12
• ok makes perfect sense, thank you. Now my only question is why would you need an increase in static pressure? And does the energy that the air have also heat it, or does the velocity that it has heat it? – Itzyoboi Nov 6 '17 at 2:15
• @Itzyoboi. Combusting the fuel at high pressure improves the combustion efficiency. See figure 1 here:en.m.wikipedia.org/wiki/Brayton_cycle The velocity in the combustor can't be too high, or the flame gets blown out (crudely speaking). Typically the velocity is lower than the aircraft speed, so the flow is decelerated. Compressing the air cause its temperature to rise. An inefficient compressor also causes it to rise a bit more than it theoretically should. – Penguin Nov 7 '17 at 6:57
• @Itzyoboi. Sorry, combustion at high pressure improves the overall efficiency of a gas turbine. I can't edit my comment above. – Penguin Nov 7 '17 at 7:06

You're talking about an engine. Bernouilli is valid for free flow when no energy is added to the airflow, and adding energy to the airflow is exactly what a jet engine does. Inside an engine, it is perfectly possible to increase both pressure and speed. It is not done that way because higher pressure is what is required, but it is possible.

This energy added to the airflow is applied by the rotors, in the same way that a propeller does. In doing so, they also make the airflow swirl around a bit, and the stators twist the flow back. That is all they do, make life easier for the next rotor stage to do their work. Stators don't impart any energy on the flow.

• Do the stators take away energy? Does the energy that is added to air, does it increase the pressure? – Itzyoboi Nov 6 '17 at 5:01
• @Itzyoboi Only friction energy, a tiny bit. The energy added to air means that total pressure rises, which can be distributed to dynamic pressure or static pressure or both. The main function of the compressor is to increase the static pressure, so the increase in total pressure added by the rotor will be channeled into static pressure as much as possible. – Koyovis Nov 6 '17 at 5:13
• thank you, you make sense. And the energy also adds heat correct? Or does the velocity add heat? – Itzyoboi Nov 6 '17 at 18:33
• All energy added is partly transformed into heat. Temperature follows similar lines to pressure: total temperature stays constant if no energy is added, and can be transformed into either internal energy or kinetic energy or both: at a specific total temperature, static temperature rises when velocity lowers and vice versa. – Koyovis Nov 6 '17 at 20:46
• so it gets hotter in the section of the stator blades? And why do you need an increase in static pressure instead of total pressure? Why do compressors want static pressure? – Itzyoboi Nov 6 '17 at 21:25

In simple terms, the compressor blades (both the fixed stator blades and the rotating "fan" blades) in the front of the engine are an air pump: the rotating blades actively draw air into the engine and squeeze it, raising its pressure. the work required to compress the air is large, and comes from the power-extraction turbine at the back end of the engine. Many rotor/stator pairs are present in the compressor side of the engine; each one squeezes the air further and further, raising its pressure further and further. At the end of the process, the air exits the compressor and is sent on to the combustors.