Learning Forum

[ricardo]

Dear all, 

I wanted to test a capability of a facade system to transfer heat from the outside to the inner side.

Like this:

 

However, when I try to use convection as the only boundary condition as the outer part and leave the other side empty without boundary condition, the simulation won't work (the system says error in the boundary condition). 

The issue is, when we put the boundary condition (i.e. convection), we have to fill the ambient temperature of that particular boundary condition. So we have to 'know' the temperature on the outer and inner side of the facade system.

However, I do not want to fill the ambient temperature on the other side because the temperature on the other side of the facade is unknown. That is why I wanted to do this simulation, to see the temperature of the other side of the system. 

Since I am new to this software, I was wondering if it is possible to do that?

 

Thank you

 

Regards,

Ricardo

[dlooman]

In a steady state thermal analysis with only one convection boundary condition, the solution will be a uniform temperature equal to the ambient temperature.  Why would it be anything else?  

[wrbulat]

If you were to use the MAPDL solver directly (rather than via Mechanical) you would be permitted to solve with this one convection BC. In a steady state thermal analysis, you would find, as Dave noted, that the calculated temperature of the entire structure would be the bulk temperature of the convection BC. 

Is this facade attached to the exterior surface of a wall? Maybe you can include a section of the wall in your model, with the facade attached to it in whatever way it is phyically attached. On the interior surface of the wall you could specify another convection boundary with the bulk temperature being whatever the air temperature of the room is (22 C?). You would then see a resulting temperature ditribution between the interior surface of the wall and the exterior surface of the facade.

Cheers,

--Bill

[ricardo]

 

Hi Bill and Dave, 

 

Thank you for your reply. The whole system is not attached to another wall, so it is a lightweight steel cladding. 

So, what I wanted to do is as follows:

1. Outdoor temperature is known (got from the bureau of meteorology for a certain time)

2. Outdoor facade surface heated up

3. Inner surface will be heated up

4. For transient thermal transfer, the indoor space will be heated up due to radiation from the inner surface of the wall. But for now, I wanted to try a simple steady state first (since most tutorials do steady state)

 

Therefore, my questions are:

 

1. Do we have to assume the indoor ambient temperature at first? I saw a tutorial (not specifically for building, but more for a mechanical device) that just assumes an ambient temperature of 22 degrees celsius on the other side of the heat source boundary condition. What happens if in my case, I only know the outdoor temperature? If I do not put the interior boundary condition (in this case convection with ambient temperature), the simulation is not working.
2. I wanted to know the performance of the facade system to transfer the heat, for example, if the outdoor temperature is 35 degree, this set of facade systems (with a certain thermal transmittance value) will be able to provide certain degree celsius of reduction on the inner surface. But if I already assume the indoor temperature, doesn’t it mean I am ‘forcing’ the simulation to produce an inner surface temperature close to the indoor temperature that I am assuming?

 

Thanks!

 

[ricardo]

Hi Bill, 

Sorry, not familiar with how reply works here and could not tag you before.

Kindly see my reply for your feedback :)

 

Regards

[Rahul Kumbhar]

Hi Ricardo,

You can apply convection with higher ambient temperature on outer boundary. On the inner faces, you can use surface to surface perfect enclosure radiation.

[ricardo]

Hi Rahul,

 

Thank you for your suggestion!

I initially put surface-to-surface radiation inside the assembly like this

If I also put surface-to-surface radiation on the inner side, does it mean the room does not have a fluid movement? 

I tried changing the inner surface boundary into surface-to-surface radiation perfect enclosure instead of convection but it got an error in the boundary condition.

But if I also include convection at the same surface, the simulation runs

I was wondering why

Thanks 

 

Regards,

Ricardo

[Rahul Kumbhar]

Hi Ricardo,

Yes, to model both radiation and fluid movement you need to define both radiation and convection boundary condition.

[ricardo]

Hi Rahul, 

Thank you for your prompt reply!

I wanted to clarify something:

1. If I put convection on the outer side, do I have to also put convection as a boundary condition inside? (Otherwise the simulation won't run?). You previously suggested that I use perfect enclosure radiation to the inside surface as a boundary. But if I only put perfect enclosure radiation on the inside surface instead of convection, the simulation won't run.
2. If I put a convection boundary on the inside, I must put in an "ambient temperature". What do you suggest I do if the ambient temperature inside is unknown? I know the default one from ANSYS is 22 degrees celsius and lots of other tutorials just leave it as the default. But does it mean we have to assume it?

Thank you, really appreciate your help

 

Regards,

Ricardo

[Rahul Kumbhar]

Hi Ricardo,

1. do I have to also put convection as a boundary condition inside? It depends whether you want to model it or not. But the model should be able to solve without convection boundary condition on inside faces.
2. For this problem you should define a convection and radiation which communicate with a unique space node for ambient temperature. The temperature of this space node will change as the heat is added through radiation and convection. But there is no direct GUI way to define this. One nned to use APDL commands to achieve this.

 

[ricardo]

Hi Rahul,

Thanks

It is weird though, the model won't solve without convection boundary conditions on the inside face.

As you can see in this screenshot, I only put convection on the outside, the rest is radiation, but there is a boundary condition error

Would you mind suggesting troubleshooting steps?

 

Thank you again

 

Regards,

Ricardo

[Rahul Kumbhar]

Remove the charts and results with question mark on it. Then try to solve

[ricardo]

Hi Rahul,

 

Tried your suggestions, and the simulations run but encountered an error

Tried looking for a solution to this similar problem:

/forum/forums/topic/problem-with-ansys-surface-to-surface-enclosure-type/

Turned out that the perfect enclosure can only be applied when the sum of the view factor is one. In this case, the inner surface does not have the "pair". Hence, the error happens. 

I guess in the end I need to define the ambient temperature to make this simulation works

 

Thanks for your help

 

Regards,

Ricardo

[Rahul Kumbhar]

Looking at the geometry it appears the three radiation bc wont be able to view each other. Try defining different enclosure number for the radiation conditions which do not see other radiating surfaces. For radiating surfaces which see each other should have same number.

[ricardo]

Hi Rahul,

 

Are you referring to this number?

I tried to simplify the boundary condition to test. I change the number so that the one facing the other radiating surface has the same number and the one that doesn't face anything have a different enclosure number. As you can see, the enclosure number of radiation B is "1", and radiation C is "2"

This is radiation B, where it faces each other. If I only simulate this without the other radiation, the simulation runs perfectly

However, once I include this radiation C, where it does not face any other radiating surface, the simulation won't run, even though I put a different enclosure number. 

Am I understanding the enclosure number correctly?

Thanks

[Rahul Kumbhar]

 

The radiation C will need more faces. What I said applies to your full model where you have innermost faces (six walls) which can see each other. 

 

[ricardo]

Hi Rahul, 

Ah, I see, so it needs a full model with the enclosed space instead of just a section of it.

I'll try 

Thanks