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  1. #1
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    Compressible flow



    Iíve been reading a beginners fluid mechanics book, which briefly introduced compressible flow with examples of nozzles, shock waves, Rayleigh and Fanno flow. It runs through the use of equations for basic calculations, but it doesnít go very far with descriptions of what is physically happening with the fluid. Something I canít seem to find with online searches either.

    I think Iíve got my head round the movements and property changes to the fluid in sub/supersonic nozzles with isentropic flow. But when it looks at the effects of heat addition/removal and the effects of friction on sub/supersonic flows in a duct, I donít seem to be able to picture/visualise what is happening that gives the strange changes in the fluid properties.

    Below is a stab at what is happening in Rayleigh flow, maybe someone will be able to see where Iím going wrong.

    Ideal gas with constant mass flow rate, no frictional effects, through constant cross-section duct with heat addition or removal.

    Subsonic - heat added

    I can see a dense gas expanding with the heat addition, the expansion pushing out giving this increase in velocity downstream, and with the relatively slow flow, acting somewhat against the upstream flow. So gives a drop in density and pressure across the heating point.

    Subsonic Ė cooling

    The flow energy is reduced with cooling and it slows (would there be a contraction?), this slower wall of gas has the upstream flow pushing on it increasing the pressure and density.

    Supersonic Ė heat added

    Having a guess here. The increase in energy is causing the molecules to become more excited and that increases the pressure, this wall of pressure is acting against the flow and slows it, but as the flow is moving very fast it doesnít affect the upstream. It just puts a break on it as it passes the heating point. The flow backs-up downstream increasing the density and pressure further.

    Supersonic - cooling

    A guess. The reduction in the energy the molecules have after cooling, gives the reduction in pressure. This low pressure void thatís created allows the upstream to push through into the lower pressure downstream increasing the velocity. The molecules are moving off at a higher velocity giving the reduction in density.

    Thanks



  2. #2
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    Re: Compressible flow

    Quote Originally Posted by seanf View Post
    I’ve been reading a beginners fluid mechanics book, which briefly introduced compressible flow with examples of nozzles, shock waves, Rayleigh and Fanno flow. It runs through the use of equations for basic calculations, but it doesn’t go very far with descriptions of what is physically happening with the fluid. Something I can’t seem to find with online searches either.

    I think I’ve got my head round the movements and property changes to the fluid in sub/supersonic nozzles with isentropic flow. But when it looks at the effects of heat addition/removal and the effects of friction on sub/supersonic flows in a duct, I don’t seem to be able to picture/visualise what is happening that gives the strange changes in the fluid properties.

    Below is a stab at what is happening in Rayleigh flow, maybe someone will be able to see where I’m going wrong.

    Ideal gas with constant mass flow rate, no frictional effects, through constant cross-section duct with heat addition or removal.

    Subsonic - heat added

    I can see a dense gas expanding with the heat addition, the expansion pushing out giving this increase in velocity downstream, and with the relatively slow flow, acting somewhat against the upstream flow. So gives a drop in density and pressure across the heating point.

    Subsonic – cooling

    The flow energy is reduced with cooling and it slows (would there be a contraction?), this slower wall of gas has the upstream flow pushing on it increasing the pressure and density.

    Supersonic – heat added

    Having a guess here. The increase in energy is causing the molecules to become more excited and that increases the pressure, this wall of pressure is acting against the flow and slows it, but as the flow is moving very fast it doesn’t affect the upstream. It just puts a break on it as it passes the heating point. The flow backs-up downstream increasing the density and pressure further.

    Supersonic - cooling

    A guess. The reduction in the energy the molecules have after cooling, gives the reduction in pressure. This low pressure void that’s created allows the upstream to push through into the lower pressure downstream increasing the velocity. The molecules are moving off at a higher velocity giving the reduction in density.

    Thanks
    .

    I'm thinking of the fluid mechanics of the liquid refrigerant. The liquid refrigerant that should be subcooled as a fluid. Are your assumptions based on the same? Because the fluid mechanics of an oil in hydraulics is different, I think?

    As I understand it, as I was taught the expansion and contraction does have an effect on the flow and it does affect the acceleration and deceleration upstream and downstream in a pure liquid but the way I understood it is that liquid is very rarely pure (pure subcooled below saturation with no bubbles). The way I understood it was that even the best quality liquid has micro bubbles inside it caused from a cavitation effect of liquid moving past joints, elbows and such like fittings??

    That then allows the liquid to expand also, Plus I remember working on Subcooled liquid R22 systems in the nineties where the expansion of the subcooled liquid was excessively high and caused no end of issues with mass flow in the system.

    The fluid you are working on needs to be defined.

    Regards
    Rob

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  3. #3
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    Re: Compressible flow

    Hi Rob,

    Just starting with the very basics looking at what happens with an Idea gas, so more like a well superheated vapour. What happens with liquids would be the next question.

    Thanks

  4. #4
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    Re: Compressible flow

    .



    Ah, in that case with an ideal gas, from memory your statements sound correct to me and your theory is based on good assumptions. Now in truth I'm not intelligent enough to argue with anyone who states Rayleigh and Fanno flows in a discussion



    I found reading Rayleigh flow science very interesting if complicated


    Rob
    Last edited by Rob White; 01-10-2018 at 07:03 PM.
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  5. #5
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    Re: Compressible flow

    No doubt my guesses are off, its not something I've ever thought or been taught about. Thanks Rob.


    Sean

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