So I understand that the rotors add dynamic pressure to the airflow and this dynamic pressure airflow is transformed into static pressure in the stator vanes. But now does the airflow still have dynamic pressure once it leaves the stator vanes, or does it get completely transferred into static pressure?

And the static pressure stays in the airflow and keeps increasing as it goes on, it doesn't get transformed into dynamic pressure because the movement of the rotor vanes create the dynamic pressure, is that correct?

  • $\begingroup$ Axial compressor: "the rotor increases the absolute velocity of the fluid and the stator converts this into pressure rise" $\endgroup$ – mins Nov 27 '17 at 7:38
  • $\begingroup$ @mins That seems odd to me. I was under the impression that the stators were there to redirect the flow, to cause the slightly orthogonal flow from the rotors into straight axial flow. The compression is from the converging shape of the chamber. Rotors accelerate the air actually into a converging chamber $\endgroup$ – TomMcW Nov 27 '17 at 21:29
  • $\begingroup$ If the chamber didn't converge pressure wouldn't rise, just velocity. But I'm terrible with physics $\endgroup$ – TomMcW Nov 27 '17 at 21:30
  • $\begingroup$ @TomMcW: Not quite. The stators pre-swirl the flow so it hits the rotors at the right angle. The rotors introduce swirl in the opposite direction, hence a new line of stators before the next rotor disc. Now the converging compressor - without the converging shape the pressure would still rise but the flow would slow down. Slower flow means less dynamic pressure, and the later rotors would have lower efficiency. Convergence is needed to keep flow speed constant and each compressor disc at top efficiency. $\endgroup$ – Peter Kämpf Nov 28 '17 at 0:11

..the rotors add dynamic pressure to the airflow..

..this dynamic pressure airflow is transformed into static pressure in the stator vanes.

are both incorrect partially correct statements, but can be open to confusion. Dynamic pressure is transformed into static pressure continuously, by narrowing the passage through the compressor. The rotors add total pressure to the airflow, some of which is extra dynamic pressure and some extra static pressure.

  • The rotor vanes increase the energy contents of the gas stream. Another name for the energy contents of a gas stream is enthalpy.
  • Energy contents can be transformed into gas speed when allowed to expand (dynamic pressure) or into static pressure when you keep the volume confined.

For ease of reference, just take a car tyre as an example. Some little rotating implements in an air pump at the gas station provide a pressurised air stream into the tyre. Stop pumping and measure the tyre pressure: what you're measuring is static pressure. The weather news talks about high pressure and low pressure systems, that is static pressure as well. It's energy contents that has not been converted into a velocity yet, like increasing potential energy of a rock by lifting it up further.

Now get a match stick and press on the tyre valve. The air streams out of the tyre, this air stream can be called dynamic pressure. You can use the outflowing air to inflate another flat tyre, you're then converting static pressure in tyre 1 into dynamic pressure into static pressure in tyre 2. End pressure in tyre 1 is lower than when you opened the valve: total energy contents of tyre 1 is now divided over 2 tyres. No energy was added, only pumps can do that, by converting electrical energy into air pressure energy.

For the axial compressor in a jet engine: * The rotor vanes act as the air pump, and add energy into the air stream: total pressure is increased. * The stator vanes do nothing with the energy content of the air stream: total pressure stays the same. The swirl is removed, and some dynamic pressure is converted into static pressure.

From The Jet Engine issue 5 by Rolls Royce: notice how the total pressure rises in the rotors, and the static pressure rises as a function of the passage narrowing.

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  • $\begingroup$ @Penguin thanks for your comments, they were helpful. $\endgroup$ – Koyovis Nov 29 '17 at 10:09
  • $\begingroup$ So how does the airflow have static pressure if the airflow is moving? Otherwise a pretty good explanation! $\endgroup$ – Itzyoboi Dec 4 '17 at 21:04
  • $\begingroup$ @Itzyboi Moving airflow has a static pressure as well as a dynamic pressure, and ome can comvert into the other. A stationary airflow has only static pressure. Air pressure is the average impact of air molecules, and they only stop moving at 0 deg Kelvin. $\endgroup$ – Koyovis Dec 4 '17 at 21:18

The rotors are what imparts mechanical energy to the working fluid, increasing its enthalpy. Stators are there in order to redirect the momentum of the gas flow toward the rotor of the next compressor stage. The gas upon exiting the stator of the last stage is going to have at least some stagnation pressure associated with its current momentum, but its more convenient to describe its energy state in terms of enthalpy.


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