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Will an axial compressor, like the one described starting on page 275 of Mechanics and Thermodyanmics (PDF link), work if the flow was assumed to be ideal (i.e. incompressible and inviscid)?

Specifically, what mechanism will slow the flow down in the axial direction? It's my understanding that slowing the flow down is the job of the stators, but I fail to see how they accomplish this in the axial direction. The author states on page 284 that the axial flow velocity through the compressor is made constant by design but never explains how that design is achieved and I'm curious if that design could still be achieved with an incompressible fluid. -Thanks in advance!

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    $\begingroup$ Are you asking mathematically or physically? $\endgroup$
    – JZYL
    Dec 19, 2019 at 13:58
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    $\begingroup$ I'm wondering what a compressor might do with an incompressible fluid? $\endgroup$
    – Haukinger
    Dec 19, 2019 at 15:50
  • $\begingroup$ @JZYL, I think I'm looking for more of a physical explanation based on the math (i.e. an incompressible fluid is not physically possible so the mathematical assumptions must be considered) $\endgroup$
    – eball
    Dec 19, 2019 at 16:44
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    $\begingroup$ @Haukinger, haha I see the seeming paradox, you cannot compress a fluid by increasing it's density (because it's incompressible), but you can increase it's pressure. I guess my question is trying to determine whether a compressor would work on a fluid like water (essentially incompressible). $\endgroup$
    – eball
    Dec 19, 2019 at 16:48
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    $\begingroup$ Yes, you can axially compress water: en.wikipedia.org/wiki/Axial-flow_pump $\endgroup$
    – JZYL
    Dec 19, 2019 at 16:56

1 Answer 1

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First of all, incompressible flow is subsonic, however subsonic flows are not all incompressible. An ideal gas flow may be "incompressible" until it meets the compressor. Even if the vehicle is travelling at supersonic speed, the flow is reduced to subsonic ("incompressible") flow by the intake system before reaching the compressor. At this point the subsonic flow gets compressed. Presumably this is what is meant here.

The compressor stages each reduce the cross-sectional area, while the fans draw the gas backwards to maintain velocity. Thus the gas, ideal or not, must compress to fit the reduced volume available to it. This compression creates heat which raises the gas temperature, further raising its pressure.

Viscosity is necessary to establish a circulatory flow component around each airfoil-shaped blade (the Kutta condition). It is this circulatory component which creates the velocity differentials needed for the Bernoulli principle to create the dynamic pressure differentials which make the blades do their work. Superfluid helium (close to absolute zero) has no viscosity and has been found to exert neither lift nor drag on an immersed body.

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  • $\begingroup$ Incompressible flow is subsonic. Subsonic flows are not all incompressible... $\endgroup$
    – JZYL
    Feb 6, 2020 at 17:59

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