12 Replies Latest reply on Sep 19, 2014 12:08 PM by Jared Conway

Zero total pressure

Simone Storace Aug 25, 2014 12:05 PM

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

Re: Zero total pressure

Jared Conway Aug 26, 2014 10:39 PM (in response to Simone Storace)

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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- Re: Zero total pressure
  
  Simone Storace Aug 27, 2014 9:00 AM (in response to Jared Conway)
  
  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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  - Re: Zero total pressure
    
    Jared Conway Aug 28, 2014 10:27 PM (in response to Simone Storace)
    
    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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    - Re: Zero total pressure
      
      Simone Storace Sep 11, 2014 4:09 AM (in response to Jared Conway)
      
      Hi Jared,
      sorry for the late.
      Yes surface parameter at the outlet shows zero as the grafic does.
      
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      - Re: Zero total pressure
        
        Jared Conway Sep 11, 2014 7:38 PM (in response to Simone Storace)
        
        can you revisit what issue you are still faced with in the simulation and post your model
        
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        Re: Zero total pressure
        
        Simone Storace Sep 16, 2014 5:43 AM (in response to Jared Conway)
        
        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
        
        condotto.SLDPRT.zip 74.3 KB
        
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        Re: Zero total pressure
        
        Jared Conway Sep 18, 2014 5:11 PM (in response to Simone Storace)
        
        how long do you run this for?
        
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        Re: Zero total pressure
        
        Jared Conway Sep 18, 2014 8:02 PM (in response to Simone Storace)
        
        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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        Re: Zero total pressure
        
        Jared Conway Sep 18, 2014 8:03 PM (in response to Simone Storace)
        
        what i also dont understand is why i get mass fraction of steam but not volume fraction of steam
        
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        Re: Zero total pressure
        
        Jared Conway Sep 18, 2014 9:12 PM (in response to Simone Storace)
        
        soemthign that might be worth trying is moving the outlet BC way far away from that heat transfer BC.
        
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Re: Zero total pressure

Kim KyuHyung Sep 18, 2014 10:03 PM (in response to Simone Storace)

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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- Re: Zero total pressure
  
  Jared Conway Sep 19, 2014 12:08 PM (in response to Kim KyuHyung)
  
  would agree that global density should be a goal
  
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