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Convection/Heat Transfer problems

18 REPLIES 18
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Message 1 of 19
kevinjbecker
815 Views, 18 Replies

Convection/Heat Transfer problems

Greetings!

 

I have successfully created a few flow models, but I have now been tasked with showing how heat from equipment within a building.

 

I have read through the documentation and watched some of the videos, but none have helped.  I have tried to create the model with Total Heat Generation (in W) and with surfaces set to a Temperature (F), but I am not getting what I expected.

 

This building houses several generators and engines.  I created a pretty simplified model in Revit,  I setup two options in this file, one using Total Heat and the other with surface Temperature.

 

Ambient is 97F

Engines operate at 214f (large polygonal)

Exhaust and Turbo operates a732f (60"d Duct and boxes)

Generators (Boxes opposite end from Exhaust) operate at 241f

all openings in walls are outside air for cooling (18m3/s)

vents in roof

 

18 REPLIES 18
Message 2 of 19
Jon.Wilde
in reply to: kevinjbecker

Hi,

 

Please can you share a CFZ file with us? We cannot open a CFDST without the associated files, which are all created when you double click a CFZ.

 

There will be one (_support) in the folder above or you can save one from the interface 'S' -> Save Share File.

 

Thanks,

Jon

Message 3 of 19
kevinjbecker
in reply to: kevinjbecker

Here is my base model with only materials and (air) boundary conditions assigned.  This model was working earlier but now refuses to solve.

 

Message 4 of 19
kevinjbecker
in reply to: kevinjbecker

here is the cfz file

Message 5 of 19
Jon.Wilde
in reply to: kevinjbecker

Hi Kevin,

 

This should be a pretty easy and quick fix, there are 2 rules to note and making these changes should see this running well:

 

  • We cannot have pressure and flow Boundary Conditions (BC) on the same surface. The inlets look good, v and T makes sense. The outlets should just be P=0 and nothing else
  • You cannot have internal surface BC's. You have a pressure and Temperature on the floor. All BC's must be on the external surfaces of what we are meshing. You could use a film coefficient on the underside of the entire model to allow heat to escape if that is the goal? 5W/m2/K at 97F is sensible. 

 

Otherwise, this looks great, nice work.

 

Kind regards,

Jon

Message 6 of 19
kevinjbecker
in reply to: Jon.Wilde

Thank Jon.

 

What I am really trying to show is how the heat for the engines, exhaust, and generators build up within the envelope of the building so we can see where we can and can not run cable tray (derating due to excessive heat).  Once we have a base line done, I will change the location(s) and size(s) of the inlets to see how the changes things.  Lastly, we may design some kind of additional HVAC to cool the areas where the cable trays are.

 

I have taken everything except inlet volume and outlet press out of the model.  it looks to be running.

 

Would you recommend using total heat generation or temperature to represent the heat gain (from the equipment) into the space?

Message 7 of 19
Jon.Wilde
in reply to: kevinjbecker

No problem.

 

Volumetric heat load is probably best. You could use a fixed temp if you suppressed the engines from the mesh, I would have thought this would be less realistic?

 

Kind regards,

Jon

Message 8 of 19
kevinjbecker
in reply to: kevinjbecker

Hello Jon,

 

(Atleast) one more question....

 

the generator actually have an internal fan that blows the heated air up into the ceiling space.  if I can't have a BC within the model space, how should I show that air/heat flow?

Message 9 of 19
Jon.Wilde
in reply to: kevinjbecker

Hi Kevin,

 

There are 2 approaches here.

 

  1. If you have a known temp, supress the generator and have a recessed cylinder in the top - the end of which you will have a temp and flow rate Boundary Condition assigned.
  2. If the temp is not known and you are just pulling local air from one side of the fan to the other, we have an axial/internal fan material for this exact purpose. You would need to model a cylinder to represent the blades, a shroud around the outside (suppressed from the mesh if you choose).

 

There is a decent reference in the Wiki on Axial Fans here. The inlet to that model is less than ideal though, very short.

 

Kind regards,

Jon

Message 10 of 19
kevinjbecker
in reply to: kevinjbecker

Jon,

Does it need to be a cylinder?  Could I recess a rectangular void in Revit?

 

 

Message 11 of 19
Jon.Wilde
in reply to: kevinjbecker

Yup 🙂

Message 12 of 19
kevinjbecker
in reply to: Jon.Wilde

Can I still apply a total heat factor to the supress volume?  We want to show the heat convection off the equipment and the exhaust (temp/cfm) too.   I'm thinking that I can not.

 

Does the supressed volume still affect air flow?

Message 13 of 19
kevinjbecker
in reply to: Jon.Wilde

Jon,

 

Can you please tell me what I have done wrong.  I am getting flow, but no convection from the equipment.

 

I keep getting this error when I try to upload the cfz file "The contents of the attachment doesn't match its file type"

Message 14 of 19
Jon.Wilde
in reply to: kevinjbecker

Hi Kevin,

 

You should be able to apply a Volumetric Heat Load to each solid, provided it is not suppressed from the mesh.

Be sure to suppress the volume where the temp and flow outlet is though, so we avoid an internal Boundary Condition - you may need to split the volume into different parts in CAD to enable you to do this.

 

If you can share your CFZ also it would help. The max file size is 2MB on here I think, Dropbox or Autodesk360 ?

Message 15 of 19
kevinjbecker
in reply to: Jon.Wilde

Jon,

 

I'm completely stumped.  I'm sure it's an obvious answer like BC internal to the model.

 

I couldn't upload it to dropbox (blocked by IT), see if you can get it from my google drive folder

 

https://drive.google.com/file/d/0B-YDZPVOWv4Remh2ZG1tT1lxRjg/edit?usp=sharing

Message 16 of 19
Jon.Wilde
in reply to: kevinjbecker

Hi Kevin,

 

It will likely be something really simple as you say, helps for me to look at it though.

 

I'll pm you, I have an idea how we can share the model.

Message 17 of 19
kevinjbecker
in reply to: Jon.Wilde

Message 18 of 19
Jon.Wilde
in reply to: kevinjbecker

Hi Kevin,

 

Thank you for sharing this file.

 

Your setup looks really good to me, I cannot fault it.

 

The questions you raise regarding applying a temperature vs a heat load are ones that you alone can answer. I suggest you assign what is most realistic. If the part surface temperatures are constant, known and uniform then we can use this (you will need to suppress the solid parts). If not then I would suggest a heat load. You can always double check the surface temps of the converged model. 

 

I suggest 3 things:

 

  1. You either refine the mesh or utilise mesh adaption here, just to be sure you are properly capturing what is occurring
  2. You investigate the results in more detail (if you have not already). Use cut-planes with vectors to visualise the flow. This will hopefully help explain why some regions are hot when this was not expected
  3. You may also wish to consider turning on Radiation here, we have a large overall temperature differential so it will have a significant effect, just be sure that your material settings (emissivity and reflectivity (which is 1 -(emissivity+transmissivity) are accurate)

Kind regards,

Jon

 

By way of demonstrating how we can view results, check this out. It shows velocities on a cut-plane, flow from each of the exhausts and where it travels to and also the temperatures of each of the generators. This is your model with no changes applied, we should still do the above.

 

I have attached a view settings file for you to import to show it like this (View -> Apply View).

 

results.png

 

Message 19 of 19
kevinjbecker
in reply to: Jon.Wilde

Thanks again Jon.

 

I had made several section, and views to go over with the design team and the owner.  I'm still working with the building and the equipment to get it more accurate.

 

 

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