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Zero total pressure

Question asked by Simone Storace on Aug 25, 2014
Latest reply on Sep 19, 2014 by Jared Conway

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Hello,

I'm trying to condensate steam along a duct.

I've read that FlowSimultiation allows only mass fraction of condensate not greater than 5%, so i've imposed boundary condition to not overcome this limit.

My BC are:

Inlet mass flow rate = 1.676 * 10^(-3) kg/s
Static pressure Outlet = 5 bar
T inlet = 425 K
Duct Lenght = 0.8 meters
Duct Diameter = 0.02 meters
Thermal flux = - 5 watt (surface surce)

The analysis have no problem to converge, but there is something i can't understand:

The values of total temperature and total pressure on outlet surface are zero.

The internal surface of the duct is equally divided in two part and the heat exchange takes place only in the second part; the first one is adiabatic.

The velocity and static pressure profile seem correct compared to the standard profile in a tube.

The next picture shows the results of a cutplot plotting total pressure.

Is there something that I'm wrong?

Is there something in fluid-dynamic theory that i've forgot?

Thanks for the answers

Greetings

Simone

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Jared Conway Aug 26, 2014 10:39 PM

can you double check that your min and max are set to the min and max for the plot.

also what are the surface parameters reading at the inlet and outlet. assume you've checked that mass flow in and out match?

if you turn off all the fancy stuff like humidity..etc does it work any better?

does your plot of static pressure look better than total pressure? (is there a reason why you are plotting total pressure?)

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Simone Storace @ Jared Conway on Aug 27, 2014 9:00 AM

Hi Jared,

yes, min and max are automatic in the legend.

The calculation is very simple. It consists in a tube with two circolar faces, one inlet and one outlet. I've used "surface parameter" on these faces to investigate total pressure value and the other values like mass flow rate, static pressure, density, etc....

There aren't parameters like Humidity, gravity, and other "fancy stuff" but only over-heated steam in inlet.

The contour of static pressure and velocity look better and there isn't outlet vortex or recirculation in outlet surface.

I needn't the values of total pressure along the duct for my analysis, but I would understand why Flow Simulation does this.

Total pressure is the sum of static pressure and dynamic pressure. Both are obviously positive and non-zero (either in reality or in simulation), am I wrong?

Thanks

Simone

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Jared Conway @ Simone Storace on Aug 28, 2014 10:27 PM

what does the surface parameter at the outlet show for total pressure? does it match what is shown in the graphic?

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Simone Storace @ Jared Conway on Sep 11, 2014 4:09 AM

Hi Jared,

sorry for the late.

Yes surface parameter at the outlet shows zero as the grafic does.

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Jared Conway @ Simone Storace on Sep 11, 2014 7:38 PM

can you revisit what issue you are still faced with in the simulation and post your model

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Simone Storace @ Jared Conway on Sep 16, 2014 5:43 AM

Hi Jared,

uploaded the model described above.

The results file is too big to be uploaded.

It shows that a few condensation occurs (mass fraction of steam on the outlet surface is 0.9999) but the total pressure value is still zero on the oulet surface.

Thanks

Greatings

Simone

Attachments

condotto.SLDPRT.zip74.3 KB

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Jared Conway @ Simone Storace on Sep 18, 2014 5:11 PM

how long do you run this for?

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Jared Conway @ Simone Storace on Sep 18, 2014 8:02 PM

i can confirm the issue on my end. if i remove the -5W heat transfer the problem goes away. same if i'm using air instead of steam. will have to think about that a bit though.

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Jared Conway @ Simone Storace on Sep 18, 2014 8:03 PM

what i also dont understand is why i get mass fraction of steam but not volume fraction of steam

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Jared Conway @ Simone Storace on Sep 18, 2014 9:12 PM

soemthign that might be worth trying is moving the outlet BC way far away from that heat transfer BC.

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Kim KyuHyung Sep 18, 2014 10:03 PM

Flow simulation is using Goal convergence not a residual.

So you should consider parameter related your problem.

As I know, if you consider condensation, YOU MUST SET Density Goal.

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Jared Conway @ Kim KyuHyung on Sep 19, 2014 12:08 PM

would agree that global density should be a goal

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12 Replies

Jared Conway Aug 26, 2014 10:39 PM
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Correct Answer
can you double check that your min and max are set to the min and max for the plot.

also what are the surface parameters reading at the inlet and outlet. assume you've checked that mass flow in and out match?

if you turn off all the fancy stuff like humidity..etc does it work any better?

does your plot of static pressure look better than total pressure? (is there a reason why you are plotting total pressure?)
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- Simone Storace @ Jared Conway on Aug 27, 2014 9:00 AM
  Mark Correct
  Correct Answer
  Hi Jared,
  yes, min and max are automatic in the legend.
  The calculation is very simple. It consists in a tube with two circolar faces, one inlet and one outlet. I've used "surface parameter" on these faces to investigate total pressure value and the other values like mass flow rate, static pressure, density, etc....
  There aren't parameters like Humidity, gravity, and other "fancy stuff" but only over-heated steam in inlet.
  The contour of static pressure and velocity look better and there isn't outlet vortex or recirculation in outlet surface.
  I needn't the values of total pressure along the duct for my analysis, but I would understand why Flow Simulation does this.
  Total pressure is the sum of static pressure and dynamic pressure. Both are obviously positive and non-zero (either in reality or in simulation), am I wrong?
  Thanks
  Simone
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  - Jared Conway @ Simone Storace on Aug 28, 2014 10:27 PM
    Mark Correct
    Correct Answer
    what does the surface parameter at the outlet show for total pressure? does it match what is shown in the graphic?
    Like • Show 0 Likes0
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    - Simone Storace @ Jared Conway on Sep 11, 2014 4:09 AM
      Mark Correct
      Correct Answer
      Hi Jared,
      sorry for the late.
      Yes surface parameter at the outlet shows zero as the grafic does.
      Like • Show 0 Likes0
      Actions
      - Jared Conway @ Simone Storace on Sep 11, 2014 7:38 PM
        Mark Correct
        Correct Answer
        can you revisit what issue you are still faced with in the simulation and post your model
        Like • Show 0 Likes0
        Actions
        Simone Storace @ Jared Conway on Sep 16, 2014 5:43 AM
        Mark Correct
        Correct Answer
        Hi Jared,
        uploaded the model described above.
        The results file is too big to be uploaded.
        It shows that a few condensation occurs (mass fraction of steam on the outlet surface is 0.9999) but the total pressure value is still zero on the oulet surface.
        Thanks
        Greatings
        Simone
        Attachments
        condotto.SLDPRT.zip74.3 KB
        Like • Show 0 Likes0
        Actions
        Jared Conway @ Simone Storace on Sep 18, 2014 5:11 PM
        Mark Correct
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        how long do you run this for?
        Like • Show 0 Likes0
        Actions
        Jared Conway @ Simone Storace on Sep 18, 2014 8:02 PM
        Mark Correct
        Correct Answer
        i can confirm the issue on my end. if i remove the -5W heat transfer the problem goes away. same if i'm using air instead of steam. will have to think about that a bit though.
        Like • Show 0 Likes0
        Actions
        Jared Conway @ Simone Storace on Sep 18, 2014 8:03 PM
        Mark Correct
        Correct Answer
        what i also dont understand is why i get mass fraction of steam but not volume fraction of steam
        Like • Show 0 Likes0
        Actions
        Jared Conway @ Simone Storace on Sep 18, 2014 9:12 PM
        Mark Correct
        Correct Answer
        soemthign that might be worth trying is moving the outlet BC way far away from that heat transfer BC.
        Like • Show 0 Likes0
        Actions
Kim KyuHyung Sep 18, 2014 10:03 PM
Mark Correct
Correct Answer
Flow simulation is using Goal convergence not a residual.
So you should consider parameter related your problem.

As I know, if you consider condensation, YOU MUST SET Density Goal.
Like • Show 1 Like1
Actions
- Jared Conway @ Kim KyuHyung on Sep 19, 2014 12:08 PM
  Mark Correct
  Correct Answer
  would agree that global density should be a goal
  Like • Show 0 Likes0
  Actions

Zero total pressure

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