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pressure drop for fan design

Question asked by Lukas Müller on Mar 4, 2017
Latest reply on Apr 13, 2017 by Lukas Müller

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Hi there,

I have a model which is a quit complex building of pipes, which are in parallel and in serial and in additional with some flow resistance (ellbows and so on). At the end I have a outlet-lit, which represents a fan.

Now I like to get the pressure drop at this lit, so I can choose the proper fan in reality.

On the otherside of the model, I have some other lits (which represents several inlets to the parallel pipes, and opened to the ambient surrounding).

By now I tried to use following setup:

- internal analysis

- fluid: air; laminar and turbulent

- adiabatic wall; roughness 10 µm

- standard thermodynamic parameters; no velocity

this boundary condition:

- Inlet lit: ambient pressure

- outlet lit: constant flow rate

and this goals:

- total pressure at inlet and outlet

(here I am not sure, if I choosed the proper pressure (static?), and do I need to set a velocity goal as well, since this is a significant value for the pressure?)

in the next step, i vary the flowrate to create a pressure - flowrate - chart (the characteristic curve of that model).

I never expected to get a value which corresponds to 100 % to the real measurement.

But as I now measured, it doesn`t correspond at all.

So what kind of mistake did i make?

Where is my logical failure?

Thanks in advance.

Amit Katz Mar 6, 2017 8:37 AM

Correct Answer

Lukas, I think you are on the right track but your methodology is flawed. If you set a flow rate at the outlet, and ambient conditions at the inlet, you will not see the correct pressure distribution. This is because the "outlet flow" condition has a set thermodynamic state, which means your pressure and temperature are constant. By default they will be at the ambient conditions defined in your project options.

What you should do is define an outlet static pressure (below ambient of course.) Then as your goal you should measure the flow rate. This way you should be able to build a resistance relationship for your piping.

By the way, I must say that Flow Sim is not the best tool for pipe systems. If you can justify the cost I would recommend looking at 1D flow network analysis software such as Macroflow: http://inresllc.com/products/macroflow/overview.html

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Mark Keown Mar 6, 2017 1:02 AM

You have outlet a constant flow rate. This means no matter what the flow resistance is the flow rate will be the specified value.

Use the Fan command and key-in the fan curve from the fan manufacture. Insure that you provide proper piping before and aft of the fan.

I think what you are looking for is your system resistance (correct me if not correct). If the system resistance is determined then you can read any fan curve and determine the best fan for your application. OR if the fan is fixed the piping can be changed to reduce flow-resistance and in turn move more heat...

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Amit Katz @ Mark Keown on Mar 6, 2017 8:37 AM

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Lukas Müller @ Amit Katz on Mar 26, 2017 12:40 PM

hi,

thank you both for your reply. And I`m sorry for my late answer...

@mark, you are right, I like to have the system resistance. But as the model is quite complex, a specified fan curve leads to some simulation errors (probably wrong start conditions...)

@Amit, thanks for your explanations.

If I define a static pressure at the outlet, i must have a defined flow rate at the inlet (or haven`t I?). Since my model has several inlet, where I don`t know the specified flow rate, I cannot define a flow rate at the inlet...

If I remember properly, I cannot have a pressure in- and outlet?

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Alex Clarke @ Lukas Müller on Apr 11, 2017 7:32 AM

You can apply inlet flow rate to multiple faces as a single boundary condition, I haven't done it before, so would recommend setting up some surface goals to check that the distribution is correct.

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Lukas Müller @ Alex Clarke on Apr 13, 2017 1:36 PM

Thank you for your response, but actually I don`t like to specify the flowrate-distribution, because I don`t know it.

By now I tried to set a static pressure outlet and an ambient pressure inlet, and measure the outlet flowrate.

By that I get a pressure - flowrate - relationship. But I am afraid, that`s a wrong one, since I need to have the sum of the dynamic pressure loss over the entire flow path.

That leads me to my next question:

Is there a way to determine the dynamic pressure loss or discharge coefficient over the entire flowpath?

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5 Replies

Mark Keown Mar 6, 2017 1:02 AM
Mark Correct
Correct Answer
You have outlet a constant flow rate. This means no matter what the flow resistance is the flow rate will be the specified value.
Use the Fan command and key-in the fan curve from the fan manufacture. Insure that you provide proper piping before and aft of the fan.
I think what you are looking for is your system resistance (correct me if not correct). If the system resistance is determined then you can read any fan curve and determine the best fan for your application. OR if the fan is fixed the piping can be changed to reduce flow-resistance and in turn move more heat...
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- Amit Katz @ Mark Keown on Mar 6, 2017 8:37 AM
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  Correct Answer
  Lukas, I think you are on the right track but your methodology is flawed. If you set a flow rate at the outlet, and ambient conditions at the inlet, you will not see the correct pressure distribution. This is because the "outlet flow" condition has a set thermodynamic state, which means your pressure and temperature are constant. By default they will be at the ambient conditions defined in your project options.
  
  What you should do is define an outlet static pressure (below ambient of course.) Then as your goal you should measure the flow rate. This way you should be able to build a resistance relationship for your piping.
  
  By the way, I must say that Flow Sim is not the best tool for pipe systems. If you can justify the cost I would recommend looking at 1D flow network analysis software such as Macroflow: http://inresllc.com/products/macroflow/overview.html
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  - Lukas Müller @ Amit Katz on Mar 26, 2017 12:40 PM
    Mark Correct
    Correct Answer
    hi,
    
    thank you both for your reply. And I`m sorry for my late answer...
    @mark, you are right, I like to have the system resistance. But as the model is quite complex, a specified fan curve leads to some simulation errors (probably wrong start conditions...)
    
    @Amit, thanks for your explanations.
    If I define a static pressure at the outlet, i must have a defined flow rate at the inlet (or haven`t I?). Since my model has several inlet, where I don`t know the specified flow rate, I cannot define a flow rate at the inlet...
    If I remember properly, I cannot have a pressure in- and outlet?
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    - Alex Clarke @ Lukas Müller on Apr 11, 2017 7:32 AM
      Mark Correct
      Correct Answer
      You can apply inlet flow rate to multiple faces as a single boundary condition, I haven't done it before, so would recommend setting up some surface goals to check that the distribution is correct.
      Like • Show 0 Likes0
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      - Lukas Müller @ Alex Clarke on Apr 13, 2017 1:36 PM
        Mark Correct
        Correct Answer
        Thank you for your response, but actually I don`t like to specify the flowrate-distribution, because I don`t know it.
        By now I tried to set a static pressure outlet and an ambient pressure inlet, and measure the outlet flowrate.
        By that I get a pressure - flowrate - relationship. But I am afraid, that`s a wrong one, since I need to have the sum of the dynamic pressure loss over the entire flow path.
        
        That leads me to my next question:
        Is there a way to determine the dynamic pressure loss or discharge coefficient over the entire flowpath?
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pressure drop for fan design

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