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Compressible or imcompressible?

  1. Sep 1, 2008 #1

    I'm doing the simulation by using the CFD Fluent.
    The reactor is 10 cm diameter and 1 m height.

    There is the water inlet (15mm) tangentially to the wall at the bottom end to produce the upward vortex flow.
    and the air-bubble injection axially at the middle of bottom end.

    The water inlet flowrate is maximum 900 L/hr
    and the air inlet flowrate is about 40 L/hr

    Fluent is the computational fluid dynamic package which base on the Navier-stoke equation.

    with my simulation it will be two phases :water-air
    My question is, Navier-stoke equation is used only for imcompressible liquid.
    Is Air imcompressible?
    If not, is there the assumption to explain that I can use two phases in Fluent?

    all comments are welcome :)
    Thank you very much,
  2. jcsd
  3. Sep 1, 2008 #2


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    Air is not an incompressible fluid. If you can make the argument that your pressure is relatively low, then you can go on that assumption, but you would be introducing some error.

    I am not really sure about the last part of your question when you ask
    Are you wondering if you will have two phase flow?
  4. Sep 1, 2008 #3
    Thank you very much FredGavin,

    my last part question is because,
    I searched in the internet and I got to know about the Mach number.
    There is a case that Air is incompressible.
    but I don't understand very well about Mach number.

    Still, I want to say Air is incompressible in order to simulate in Fluent with two phases.
    But is there any assumption to say that air is incompressible ? (so it can be used with Navier stoke's equation)

    as you said, we can make argument that the pressure is relatively low.
    How low? :)

    Thank you very much again,
  5. Sep 3, 2008 #4
    In referencing the Mach number (M), you will need to know the velocity (v) of the air and the speed of sound at your given conditions (sea level altitude?). Incompressible flow does not occur in nature, but for "slow" enough flows, incompressiblity can be assumed. For basic calculations, you can assume incompressible flow for v < 100 m/s, but using M<0.3 is a much better solution
  6. Sep 4, 2008 #5
    to be clear nothing is incompressible. fluids like water and fuel are not easily compressed and gases like air are very compressible. But you should be aware of the limitations of this assumption even when using "incompressible" fluids, or in your case a mixture which will be much more compressible than water itself.
  7. Sep 5, 2008 #6


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    Even though fluids like water are strictly compressible, if one looks at the real compressibility of water, the incompressible assumption is a very valid one. Water loses only about 1.5% of it's volume at 6000 psi (or there abouts). For 90% of common engineering problems, incompressibility is a good assumption.

    This is the kind of rationale you need to present to back up your modeling technique.
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