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Two different HTC values in a single anular pipe

Question asked by Simone Storace on Mar 26, 2015
Latest reply on Apr 23, 2015 by Jared Conway

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Good morning,

I have a problem / doubt.

I want to post-process some HTC values (heat transfert coefficient) in a CFD model.

The CAD is very simple, two concentric pipes in which steam evolves (an image is appended).

The boundary conditions are:

- Mass flow inlet = 30 kg/s (steam)

- Inlet temperature = 735 K

- Inlet Pressure = 102 absolute bar

- Outlet Pressure = 101 absolute bar

Results:

[FACE1] External pipe External face = 100 w/(m2 k) imposed in General Settings

[FACE2] External pipe Internal face = 5000 w/(m2 k)

[FACE3] Internal pipe External face = 400 w/(m2 k)

[FACE4] Internal pipe Internal face = 320 w/(m2 k)

How is it possbile that Face 2 and Face 3 have so different HTC values? They are 2 faces seeing "each other" and the steam-flowing on them is the same (same velocity, same temperature, same density, etc...).

Obviously there is a bit temperature difference on the two pipes (4/5 kelvin), but I do not think that it could be the cause of theese unusual HTC values.

Thank you

Greetings,

Simone

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Anular pipe.PNG363.2 KB

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Jared Conway Mar 31, 2015 12:26 AM
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just to be clear, you want 2 and 3 to be exactly the same?

is the surface temperature on them exactly the same?

can you post some screenshots of the results or better yet the file?
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- Simone Storace @ Jared Conway on Apr 1, 2015 10:01 AM
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  Hi Jared,
  the temperature values are respectively:
  Face 2 = 724 K
  Face 3 = 734.9 K
  Face 4 = 735 K
  I expected that the two HTC values were very close to each other, not exactly the same.
  In my opinion:
  - Nusselt number should be constant because Prandtl and Reynolds have no significant variations (dynamic viscosity in fact is between 2.68E-05 and 2.74E-05 Pa*sec on the opposite surfaces [face 2 and face 3])
  - HTC = (Nu * Fluid conducibility) / (Equivalent Diameter) ---> Nusselt almost constant, Kfluid almost costant, Equivalent Diameter little variations.
  I attach an image with HTC values on th faces 2-3-4.
  The whole model is too heavy to upload, I upload an alternative test with different boundary conditions and different diameter values, but the concept is the same.
  Thanks a lot
  Greetings
  Simone
  Attachments
  - tubo.SLDPRT.zip132.7 KB
  - Pipe.PNG209.6 KB
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  - Jared Conway @ Simone Storace on Apr 1, 2015 1:50 PM
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    something maybe to note is that HTC is a post-processed value based on the reference temperature
    but if your solid and fluid temperatures are the same, your reference temp is the same, your HTC should be the same
    
    i haven't had a chance to check your sample but your sample shows the exact same issue?
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  - Jared Conway @ Simone Storace on Apr 1, 2015 1:58 PM
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    looking at the file now
    can you describe what conditions you want to test in?
    i see steam and 10000kPA, is that correct? i don't think that is in the range of the steam model
    does the problem go away if you use air and steel and 101325Pa and a room temp?
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    - Jared Conway @ Jared Conway on Apr 1, 2015 1:59 PM
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      and how long does it take to solve and does it converge/complete?
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      - Simone Storace @ Jared Conway on Apr 2, 2015 7:22 AM
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        Hi Jared,
        I know HTC is a post-processed value based on a reference temperature, but I don't know which temperature is the reference one in SW Simulation.
        The boundary condition are the real conditions within pipe, 100 abs bar and 735 K (I've listed them in the first post).
        At this pressure and temperature steam is a supeheated steam, I've understood SW gives a range of values only for the phase transition (as described in the Technical Reference).
        I don't know if using air or water the problem could be the same or not, but unfortunately I have to use steam.
        
        The convergence is quite fast, 1 or 2 hours. Did you run the model yesterday?
        Can you tell me which is the procedure to set the reference temperature? Is it the "Initial temperature" in "Initial Conditions" (for fluid and solid)??
        
        Thanks
        Greetings
        Simone
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        Jared Conway @ Simone Storace on Apr 2, 2015 11:23 AM
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        hi, i didn't run the problem because the inputs didn't make sense to me.
        
        regarding your questions, here are the limits for steam:
        
        200.15 K < T < 583.15 K, 10^3 Pa < P < 10^7 Pa.
        
        I think you may be reading too deep into the phase transition comment, I've always been told by the developers this is the range.
        
        so you are outside the range for steam. you may need to come up with your own properties.
        
        regarding reference pressure, if you look in the help, you'll see where it is found, i believe it is under flow simulation tools > results > parameters. by default it is set to your initial temperature.
        
        and regarding using air and steel, I'm only suggesting that as a troubleshooting method.
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        Simone Storace @ Jared Conway on Apr 2, 2015 2:29 PM
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        Hi Jared,
        first of all thank you for the quick reply and your suggestions.
        Oh, it's a bad news for me, I have to run some cases with supeheated steam (above 583K) and so I must re-check some old calculation now that are out of this range.
        Tomorrow I'll try to run a simulation with air and other simulation with steam in a correct range.
        If you want, You could run my model with some correct steam conditions and check if the HTC values on faces 2 and 3 are different or not.
        I'll do the same thing.
        Thanks
        Greetings
        Simone
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        Jared Conway @ Simone Storace on Apr 3, 2015 11:41 AM
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        i'm also not clear on what the expected outlet conditions are. with your components not merged together the software could get very confused. is there supposed to be an outlet on the inner pipe and the outer pipe?
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        Simone Storace @ Jared Conway on Apr 17, 2015 6:16 AM
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        Hi Jared,
        as suggested, I made some test with steam in correct range conditions and another test with air.
        Nothing is changed, the HTC values are different within the model.
        Example:
        OLD calculation with STEAM 100bara and 735K
        - [FACE2] External pipe Internal face = 5000 w/(m2 k)
        - [FACE3] Internal pipe External face = 400 w/(m2 k)
        - [FACE4] Internal pipe Internal face = 320 w/(m2 k)
        
        New calculation with STEAM 4.75bara and 500K
        - [FACE2] External pipe Internal face = 444 w/(m2 k)
        - [FACE3] Internal pipe External face = 7.3 w/(m2 k)
        - [FACE4] Internal pipe Internal face = 8.5 w/(m2 k)
        
        New calculation with AIR 4.75bara and 500K
        - [FACE2] External pipe Internal face = 503 w/(m2 k)
        - [FACE3] Internal pipe External face = 7.5 w/(m2 k)
        - [FACE4] Internal pipe Internal face = 9 w/(m2 k)
        
        I've use, for each project, an inlet velocity boundary conditions of 35m/s.
        
        Greetings
        Simone
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        Jared Conway @ Simone Storace on Apr 23, 2015 2:13 PM
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        hi, did you fix the modeling issues that i mentioned? please post the latest model and a description of what the setup should look like
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Two different HTC values in a single anular pipe

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