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Wall conditions

Question asked by Alexey Yuritsyn on Jan 30, 2017
Latest reply on Feb 21, 2017 by Alexey Yuritsyn

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Hello, everybody. I have a problem with, I suppose, easy question. just have limited time and need to get correct answer.

stainless tube L = 2000 mm. Inlet-> Methane 1100 C. 1,5 m3/h. outside air 20 C.

So I need to find out whether a heat exchanger is requited or not.

Everything is clear but I never clearly understood WALL CONDITIONS...what should I choose and why? Heat transfer coefficient or wall temperature? I suppose heat tran coef. but what number? I've done a heat exchanger exmeriment before but they just put 5 W/m2*K without explanation why, where it should be taken from.

And when other parameters must me applied? like surface heat generation rate and wall temperature?

Thank you for explanation in advance and sorry for asking elementary questions.

Holger Parchmann Feb 2, 2017 6:04 AM

Correct Answer

Hello Alexey,

if you define a wall temperature you get a boundary condition that doesnt change. It is all the time the same temperature.

If you want to take into consideration that the temperature of the outer wall can change then you have to define another wall condition. Wall temperature and heat transfer coefficient could be fine if you know the proper value. The coefficient depends on the used materials and on the state of the flow if it is turbulent or not. If you dont know the value it could be better to run an external analysis, then the software calculates the heat transfer itself.

BR Holger

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Holger Parchmann Feb 2, 2017 6:04 AM

Hello Alexey,

if you define a wall temperature you get a boundary condition that doesnt change. It is all the time the same temperature.

BR Holger

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Alexey Yuritsyn @ Holger Parchmann on Feb 2, 2017 7:32 AM

Thank you very much.

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Alexey Yuritsyn @ Holger Parchmann on Feb 8, 2017 8:48 AM

Hello. one more time)

Could you tell me how to set it up?

I mean..should I do external study with the same parameters which I have in internal study ( i mean ..inlets, outlets) and how to combine them afterwards?

Thanks in advance)

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Holger Parchmann @ Alexey Yuritsyn on Feb 8, 2017 9:09 AM

Yes, you can define all the boundary conditions in the same way. In the general settings you get more options, you could specify ambient parameters there, f.e. the ambient tmeperature.

But you should use the option "heat transfer in solids" in the general settings.

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Amit Katz Feb 8, 2017 9:39 AM

Heat transfer coefficient is tricky. Usually when we are in school learning heat transfer basics this number is given to us in a homework assignment, but in reality there is no physical property that it relates to. Heat transfer coefficient is something you define from the data after the experiment is run, you can't numerically derive it from physical properties of materials alone.

In your case what I would do is model a section of pipe and a large bounding container around it, which will have surfaces defined as environmental pressure. Make sure you leave plenty of room for air around the pipe. I would not recommend modeling the entire length of the pipe, because your cell count will be astronomical. You will have to use some judgement here and balance your domain size with your time limits and computational resources.

This model will allow you to simulate heat exchange between the pipe and ambient air. Hopefully it should give you some idea of the heat transfer rates and the amount of heat loss from your working fluid per unit length.

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Alexey Yuritsyn @ Amit Katz on Feb 21, 2017 6:28 AM

Hello! I did an external test and got this above and bottom

I am not sure about results.

Boundary conditions for methane is 0,00083 m3/s = 3m3/h. T=1100 C. this lid in the tube stands in a position where heat zone is over. But I don't know what material should it be...insulator...or high temperature metal..or what? and I need to get it right coz I need to know what material to choose for the flange and other stuff.

Could you comment, please?

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Alexey Yuritsyn @ Amit Katz on Feb 21, 2017 6:28 AM

And I don't understand this two lines going up from the tube??

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Holger Parchmann @ Alexey Yuritsyn on Feb 21, 2017 7:14 AM

Hi,

these two lines show some mesh errors. You have to refine the walls.

And dont use Isolator as solid material. You dont have a realistic heat transfer then.

But I cant help you with the question which material fits your problem best, I'm sorry.

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Alexey Yuritsyn @ Holger Parchmann on Feb 21, 2017 7:58 AM

Thank you for the answer. here I used tungsten if it is a correct translation. but anyway I supposed it is far from real. cooled too fast I suppose.

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9 Replies

Holger Parchmann Feb 2, 2017 6:04 AM
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Correct Answer
Hello Alexey,
if you define a wall temperature you get a boundary condition that doesnt change. It is all the time the same temperature.

If you want to take into consideration that the temperature of the outer wall can change then you have to define another wall condition. Wall temperature and heat transfer coefficient could be fine if you know the proper value. The coefficient depends on the used materials and on the state of the flow if it is turbulent or not. If you dont know the value it could be better to run an external analysis, then the software calculates the heat transfer itself.
BR Holger
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- Alexey Yuritsyn @ Holger Parchmann on Feb 2, 2017 7:32 AM
  Mark Correct
  Correct Answer
  Thank you very much.
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- Alexey Yuritsyn @ Holger Parchmann on Feb 8, 2017 8:48 AM
  Mark Correct
  Correct Answer
  Hello. one more time)
  Could you tell me how to set it up?
  I mean..should I do external study with the same parameters which I have in internal study ( i mean ..inlets, outlets) and how to combine them afterwards?
  Thanks in advance)
  Like • Show 0 Likes0
  Actions
  - Holger Parchmann @ Alexey Yuritsyn on Feb 8, 2017 9:09 AM
    Mark Correct
    Correct Answer
    Yes, you can define all the boundary conditions in the same way. In the general settings you get more options, you could specify ambient parameters there, f.e. the ambient tmeperature.
    But you should use the option "heat transfer in solids" in the general settings.
    BR
    Like • Show 0 Likes0
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Amit Katz Feb 8, 2017 9:39 AM
Mark Correct
Correct Answer
Heat transfer coefficient is tricky. Usually when we are in school learning heat transfer basics this number is given to us in a homework assignment, but in reality there is no physical property that it relates to. Heat transfer coefficient is something you define from the data after the experiment is run, you can't numerically derive it from physical properties of materials alone.

In your case what I would do is model a section of pipe and a large bounding container around it, which will have surfaces defined as environmental pressure. Make sure you leave plenty of room for air around the pipe. I would not recommend modeling the entire length of the pipe, because your cell count will be astronomical. You will have to use some judgement here and balance your domain size with your time limits and computational resources.
This model will allow you to simulate heat exchange between the pipe and ambient air. Hopefully it should give you some idea of the heat transfer rates and the amount of heat loss from your working fluid per unit length.
Like • Show 0 Likes0
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- Alexey Yuritsyn @ Amit Katz on Feb 21, 2017 6:28 AM
  Mark Correct
  Correct Answer
  
  Hello! I did an external test and got this above and bottom
  
  I am not sure about results.
  Boundary conditions for methane is 0,00083 m3/s = 3m3/h. T=1100 C. this lid in the tube stands in a position where heat zone is over. But I don't know what material should it be...insulator...or high temperature metal..or what? and I need to get it right coz I need to know what material to choose for the flange and other stuff.
  Could you comment, please?
  Like • Show 0 Likes0
  Actions
- Alexey Yuritsyn @ Amit Katz on Feb 21, 2017 6:28 AM
  Mark Correct
  Correct Answer
  And I don't understand this two lines going up from the tube??
  Like • Show 0 Likes0
  Actions
  - Holger Parchmann @ Alexey Yuritsyn on Feb 21, 2017 7:14 AM
    Mark Correct
    Correct Answer
    Hi,
    
    these two lines show some mesh errors. You have to refine the walls.
    And dont use Isolator as solid material. You dont have a realistic heat transfer then.
    But I cant help you with the question which material fits your problem best, I'm sorry.
    Like • Show 0 Likes0
    Actions
    - Alexey Yuritsyn @ Holger Parchmann on Feb 21, 2017 7:58 AM
      Mark Correct
      Correct Answer
      Thank you for the answer. here I used tungsten if it is a correct translation. but anyway I supposed it is far from real. cooled too fast I suppose.
      Like • Show 0 Likes0
      Actions

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