No Thermal Results for natural convection

[Anonymous]

Hey, 

I've been trying to run a natural convection study on a simplified enclosure (as a starting point). I've been through the tutorials. My Air is set as variable. I do not have any extra boundary settings other than Pressure and temperature on the inlet and outlet of the external volume. 

Please help as i've tried everything. 

Regards

Sandeep

[Jon.Wilde]

Hi Sandeep,

Few points to make here:

1. You have internal surface Boundary Conditions - this is not recommended. Suppress the external volume from the mesh
2. Check your model scale, it looks like it is a few km tall. You can change this when you launch a new model by right clicking on 'Geometry' on the top left

Kind regards,

Jon

[Anonymous]

Hi Jon,

In regards to your points:

1) I am unclear what you mean by this. Do you mean to say I have boundary conditions on the enclosure and have not applied them? and therefore I should not be using the external volume?

2) The Scale is actually accurate ~ around 2.7m tall however for some reason autocad when displaying units on the axes displays the exponential 4 instead of 3. (Changing the unit scale to m proved this)

I have changed the model as i found there were geometry errors.

Thanks

Sandeep

[Anonymous]

Forgot to upload new file, apologies.

Regards

Sandeep

[Jon.Wilde]

Hi Sandeep,

Surface Boundary Conditions (BC's) can only go on the external boundary of the model. Some of yours are inside the outer layer of air, which will not work.

Either move the BC to the outer surface or suppress the outer volume from the mesh. Same goes for the ones at the bottom.

[Anonymous]

Hi Jon

Apologies Jon, but i am confused. There are exactly 3 boundary conditions all of which are in their respective areas as demonstrated by the natural convection tutorial. Please correct me if i am wrong, I've put a pressure condition and a temperature condition at the bottom of the external air and a pressure condition at the top of the external air. There are no Surface area boundary conditions present inside the external air volume. 

Please refer to the new design study i've uploaded with this post. 

Regards,

Sandeep

[apolo_vanderberg]

Sandeep,

Just to be sure as you answered it before however I want to be clear.

Is this model's air domain supposed to be ~82m tall and the enclosure roughly 17m tall?

At this size I can't imagine that 200W would heat the block much

That is what is in your latest model you attached. I would recommend checking as you might have a scaling issue (note the 8e4 on the Bounding Box which would correlate with 8e4mm based on your length units)

Also I noticed from some of the logs taht you were stopping the analysis after a 10 or so iterations. The thermal solution does progress a bit slower at the beginning to help with stability, and as such if you run it out thermal results do eventually show although I'm not sure if this is expected given size of the model overall

[ihilali]

Hi,

I have analyzed horizontal tubes array heated by ambient air in free convection. . I used external volume acording to enclosure sizes. Boundary conditions: tubes surface 5 celcius and Pressure on the top and bottom and a Temp (Ambient) on the top. mbient temperature 25 celcius. I attached cad file and my results. I want to see your results  for comparing.

thanks in advance.

[Jon.Wilde]

Hi ihilali,

To be clear:

We should have a P=0 on top and bottom and a T (ambient) on the bottom, as the air will heat up as it rises.

Also, only use T boundary conditions internally if they are applied to suppressed parts, as we cannot use internal surface Boundary Conditions. The alternative is to apply a volumetric heat load to the tubes themselves.

Thanks,

Jon

[apolo_vanderberg]

Also looking purely at your results, you do not have proper mesh on the tubes. They are not showing as round and you are not capturing well the flow between them or as it dissipates away.