CFD of HVAC/Natural ventilation

[brane1012]

Hi!

I am new user using CFD simulation software. My first task is to model the natural ventilation of hotel atrium. Because of the very warm climate the natural ventilation will be modeled in combination with FanCoil (FC) units. I have few basic questions about modeling specific items:

1. Is it possible that simulation CFD software recognizes the names of elements imported from REVIT?! For example - in Revit I have basic wall and when imported into simulation CFD it becomes CAD Surface XXX (like everything else).

2. How should I model supply/return diffusers of FanCoil unit? Let's say that supply and return diffuser of FC unit have each flow 500 m3/h. Should I define 500 m3/h only on supply diffuser and then on return diffuser define pressure p=0 or should I define flow on supply and on return diffuser?! One more thing to note - I have multiple FC units all around atrium so I have multiple supply and return diffusers.

3. How should I define floor FC units? Should I only define it as supply diffuser or there is somekind trick how to also define intake of air?

4. How should I define the entrance of fresh air used for natural ventilation? Should I define that (how?) on the surface on which I want that air to enter into the building or should I simply just delete that surface?

Best regards, Branimir!

[Jon.Wilde]

Hi Branimir,

I saw your PM but would rather post here so that others can benefit from what we discuss. I hope that is OK.

Could I first suggest you run through a couple of the tutorials?

The HVAC includes a fan, which will move flow as I think you are trying to do. 

I would also check out the Thermal Comfort one, just as it is also an atrium. It will answer how to set up the inlet and outlets I think. Although you may wish to just have a P=0 rather than drive the flow.

http://help.autodesk.com/view/SCDSE/2015/ENU/

Then look here to learn about setting up a natural convection study: CFD Help, Natural Convection

Once you have covered these I think you will be in a better place to set up your study, which you could then post here for us to verify your approach.

Kind regards,

Jon

[brane1012]

And the question nr 1.? I think that question is more general...

1. Is it possible that simulation CFD software recognizes the names of elements imported from REVIT?! For example - in Revit I have basic wall and when imported into simulation CFD it becomes CAD Surface XXX (like everything else).

[Jon.Wilde]

I think some we can and some we cannot. Familes and instances of a family should work.

AEC Best Practices - Geometry

It is strange though that it says 'surface' this makes me wonder if the CAD is OK. Is it all 100% 3D volumes?

Thanks,

Jon

[brane1012]

Yes, all are volumes. I attached a picture (Picture 1.jpg) of simple Revit model (basic wall - generic - 200 mm; floor - generic - 300 mm) which I transfered to Simulation CFD via button on Revit taskbar. As you can see all elements are defined as CAD volume (not surface - sorry my mistake...but the principle is same - it didn't recognize Revit walls, floors... :S )

And now I would like to explain something regarding your note - The HVAC includes a fan, which will move flow as I think you are trying to do. That is true but I don't think I need set up that HVAC element as Internal Fan Material Device as explained in tutorial you recommended (http://help.autodesk.com/view/SCDSE/2015/ENU/?guid=GUID-A54215F4-1281-40CE-A1DB-C49B611D6671). On picture 2 you can see the basic scheme of two HVAC units that I am using in my project.

My idea for type 1 FanCoil unit was only to define supply and return diffuser with flow, temperature and surface free ratio (so that I can get speed). Idea for type 2 FanCoil unit was to split one surface (in floor) on half and then one half define as supply diffuser and other half as return diffuser (both ends define with temperature and flow). What do you think?

On picture 3 you can see the basic (draft) scheme of my object with basic principle of HVAC equipment and natural ventilation. HVAC equipment acts as help in very warm weather to cool the fresh external air. My idea was to define all supply and return diffuser with flow and temperature (FanCoil Type 1 and 2), define the external air intake (don't know at this very moment how 🙂 ) and then at the end define exhaust air on top of atrium as p = 0. Any comments?

Thanks for all advices and comments.

Best regards!

[Jon.Wilde]

Honestly I am not sure on the naming (I do not use Revit all that much). Whenever I am working with models like this I would always just select everything in CFD and make it all solid, concrete for example. Then I would assign the air to the one or two volumes that needed to be.

It looks like you might be better off just suppressing the fans. Have some sort of void in the model (created by suppressing a volume from the mesh) where you can have a P=0 on one end to represent the inlet (flow would enter here from the domain and disappear).

The other end would have a flow rate and temp and would enter back into the room. 

For the external intake, extend the air outwards a little bit (length of extrusion = 5x diameter generally). Then you would assign the ambient temp and a P=0 or flow rate, depending on the circumstance.

Kind regards,

Jon

[brane1012]

The other end would have a flow rate and temp and would enter back into the room.

And do I need to define free area ratio (is that possible) so that CFD knows the speed of the flow exiting the diffuser?

My next problem/question is - Is it possible to define how the flow exists the diffuser?. Let's say that the diffusers have somekind of element that directs the flow in specific angle (see Section 1.jpg).

BTW - I am defining my diffusers as described in tutorial on Autodesk site (see Picture 1.jpg).

[Jon.Wilde]

Wouldn't the speed exiting be the same as the speed entering? We can use resistance regions (check out the help) to represent a filter, although we tend to use them in-line and not to change the flow direction. They also need to be surrounded by air and not on any external mdoel surface. Not quite sure how you are approaching it.

I would be inclined to use surfaces to direct the flow. Perhaps get the basics running OK first? 

I wish you had a subscription with us so we could have a screenshare to discuss, it would be so simple to walk through all of this with you 😄

Hope that helps,

Jon

[brane1012]

Yes, inlet and outlet speed is same but I need to calculate exact speed. Because if I have a supply/return diffuser (area=1 m2) then the effective area of that diffuser is about 60 % of 1 m2. So I think this thing with resistance is OK. But I don't undestand what did you mean by this: ...although we tend to use them in-line and not to change the flow direction. They also need to be surrounded by air and not on any external mdoel surface.

I would be inclined to use surfaces to direct the flow.      I will try this and get back to you.

Best regards!

[Jon.Wilde]

I guess what I can do is list a few options and you naybe can choose what works best.

1. If you have a black box with flow in and flow out, we could do this with a suppressed volume within the model. Ideally within a duct. You could have whatever flow speeds you wanted in and out, controlled by boundary conditions
2. You could use an internal fan to move flow. This works in the same way as a real fan would. It moves flow from one side to the other, the flow never disappears into a black box so the flow in and out would be the same
3. I am not entirely sure if these are applicable but you can use a resistance within the air. This is representing a filter and would cause a pressure drop within the air. Like a fan is isn't a black box, the inlet and outlet are connected

I would still use some sort of surfaces/vanes to direct the air but maybe get something running first, you can always push an updated CAD model into the CFD scenario once you have something working.

Does that help?

Jon

[brane1012]

Yeah, I listened to your advice and simulated something simple. I got two test files. The difference between them is that in the first case supply diffuser has flow 1000 m3/h and in the second 10000 m3/h.

The strange thing is that the speed is higher in the first then in the second case :S

Did I do something wrong or what?

And why is temperature in both simulations the same and hasn't changed?

How can I know is the simulation really finished because the results are constantly changing?

p.s. Is there any way that I can get more stable/direct flow so that it reaches floor? (let's say something like this http://machinedesign.com/site-files/machinedesign.com/files/uploads/2014/01/Operating-Room-Better.jp... )

I hope I am not bothering you too much :S

Best regards!

[Jon.Wilde]

Hi,

Could you share a CFZ (support) file? Also, are you aware you can clone a scenario within a single project and change the settings, so you shouldn't need two projects 🙂

Thanks,

Jon

[brane1012]

**** it, I didn't know that I have to share the files like this (cfz type). Again I have learnt something new, tnx! Here are the files.

https://www.wetransfer.com/downloads/d350cea2b440cea815b6e8426f7c257120150109160452/a49932372073e0d2...

Yes, I am aware about scenarios but this are just test projects so it is much easier for me to do that like this.

[Jon.Wilde]

🙂

Yup, they are super useful. Pre-zipped and ready to share!

We actually cannot open a cfdst, the CFZ will create a cfdst for us and all other necessary files/

The issue here is that you have assigned internal boundary conditions (BCs), which is a no-no and will cause issues. BC's must always go on the external boundary of a model. Does that help?

Kind regards,

Jon

[brane1012]

Not sure if I get it. How should I simulate then flow inside room? And what if I have a big atrium with lots of floors and HVAC unit on every floor (then they are all basically inside and I can not define them with external boundary conditions) ? Not quite sure if I completely understand you :S

[Royce_adsk]

Hi Brane,

I caught the highlights in the previous posts, but if you need to move air inside of the model then you'll want to either use the Internal Fan, Heat Exchanger, or blower material. The other option is to suppress the volume the would represent the fan in the HVAC system and apply flow rate condition on the inlet/outlet face such that those faces are now 'external' to the model.

Make sense?

[brane1012]

The issue here is that you have assigned internal boundary conditions (BCs), which is a no-no and will cause issues. BC's must always go on the external boundary of a model. Does that help?

Yeah, I think it actually helped. I am sending you the new model where I moved supply and return diffuser to the top.

https://www.wetransfer.com/downloads/6dd9d1d2fbd0102d43db6b8933a53a3220150109180426/863334b085c5a161...

The only problem is that if I transfer a model from Revit with openings (representing diffusers - see Picture 1.jpg) then simulation CFD doesn't create (recognize) inside air. I think I solved that problem but not quite sure if it is OK, please comment. I filled openings with geometry tool - fill void - build surfaces. After both surfaces were built I clicked fill void (see Picture 2.jpg how it looks). That newly created volume I defined as Fluid - Air. After that (as you can see from file) I defined boundary conditions and started the simulation.

In the model you can see that the results with flow 10000 m3/h are pretty realistic but with 1000 m3/h is something strange :S How can I know is simulation finished? When it's not changing too much? Maybe scenario 2 needs further simulating?!

Best regards, Branimir.

p.s. If you can, please have a look to the part of my next post (reply to Royce) But, honestly I am bit confused. If you look at the handout I attached from the AU university (topic - Supply diffusers) you can see that he is assigning boundary conditions to the nozzles inside of the building, please comment?!

[brane1012]

Hi Royce!

Yes, now it makes sense after Wildej told me that I need to define boundary conditions only on outside surfaces 😄 But, honestly I am bit confused. If you look at the handout I attached from the AU university (topic - Supply diffusers) you can see that he is assigning boundary conditions to the nozzles inside of the building, please comment?!

but if you need to move air inside of the model then you'll want to either use the Internal Fan, Heat Exchanger, or blower material. The other option is to suppress the volume the would represent the fan in the HVAC system and apply flow rate condition on the inlet/outlet face such that those faces are now 'external' to the model.

Can you please explain a little bit more how to do that what you are suggesting or point me to some tutorials?!

Because, my work mostly consists of simulating HVAC equipment - mostly air flow which is always inside building  (let's say large atrium with many floors) and mostly in false ceiling.

And sorry I think accidentally pushed accept as solution on your post :S I hope that doesn't mean that we have to end discussion in this topic :S

Best regards, Branimir!

[Royce_adsk]

Keep in mind that you should be using extensions at the inlets:   

This would be done in Revit with simple masses as one option.  The boundary conditions would then be at the far face.

The supply diffiusers in that handout are along the wall and not 'in' the model. They would be considered to be external surface. If you had a supply in the model from a vent that you didn't want to mesh then you would have to have a block that you would suppress the mesh such that it would then be an external surface.  This block should have a recess in it such that there is a small inlet extension.  Never a good idea to have flush boundary conditions with a larger domain.  Similar to the above picture...

Make sense?

[brane1012]

Hm, strange...because in all AU University tutorials I saw that they are making diffuser as holes in the wall and then filling them with the fill void function :S

I still don't get it what would happen if you had "boxes=rooms" all around your "box=room" on the picture?! How would you then simulate the flow? Or more easily how to simulate flow like in my situation (see attachment) when I have false ceiling?! Simulate only the part of the room from the floor to the false ceiling?

Never a good idea to have flush boundary conditions with a larger domain - What does it mean flush?