Solar Heating Analysis - No Heat Flux and No Temperature Results

[c3110765]

Dear Autodesk,

I am currently conducting a research study using Simulation CFD to perform a thermal heating analysis. The context of this simulation is the construction of a simple 6m square house in Autodesk Revit, using generic models, which has then been loaded into Simulation CFD for a solar heating thermal analysis. I hope to understand the heating effects on the internal brick partition walls, with the sun heating the building through a north facing window and a thermally insulated ceiling preventing downward heating through the roof.

At present, the model solves, and I can visualise the solar heat flux, however I have no temperature results, and upon inspection, very little change occurs within the model regarding temperature. I suspect that my dome environment might be acting in a similar fashion to an ozone layer (in reality) and preventing ratiation from reaching the building in the simulation.

Additionally, I have had to change the glass window in the northern (y axis) of the model to be air, in the hope of allowing the radiation to enter the building through the northern window (the only place the radiation is to be entering the building).

Could someone please help? Im not sure if the emmissivity is set correctly, or if I need initial or boundary conditions adjusted? it should be noted that I am implementing data recorded from previous research studies as the transient external air temperature (as a boundary condition on the external environment (dome))..

For reference to the .ctz file, please see my dropbox link https://www.dropbox.com/s/g7jw3xmoipahwyn/Internal%20Walls%20Version%206_redo%20window.cfz

Regards,

Ron

--

University of Newcastle

Callaghan NSW, Australia 2308

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[Jon.Wilde]

Hi Ron,

A few observations here:

1. You have an internal Boundary Condition (BC) which is not permitted. Let ChFD calculate the heat flux here. You can apply volumetric BC's if you happen to have an internal heat source
2. You are running a conduction only analysis as you have air set to fixed - select the air volumes and edit the Environment to switch from Fixed to Variable - now then air density can change. I would also suggest you run with the default air material. You do however have flow turned off - is this intentional?
3. Ensure you have 2 elements through any material thicknesses - use a uniform mesh to guarantee this is applied to the entire volume. It is important we do this to allow for an accurate temperature gradient
4. I suggest you run with a Steady State anaysis initially, at least until you are happy this works, then switch to the more complex transient setup. 

Kind regards,

Jon

[c3110765]

Hi Jon, thanks for your assistance, It is very much appreciated.

Could i please ask you to clarify what you mean by "2 elements through any material thicknesses", and what this means in terms of implementation?

Kind Regards,

Ron

[c3110765]

Hi,

I have followed the above suggestions, however I am still unable to visualise any results for temperature analysis. I get the feeling that the 'air' filled dome is blocking the solar radiation, even though I've tried both standard 'air' and 'solid air' with transmissivity changed to 1 and emissivity changed to 0.

Please help!!!

[Jon.Wilde]

Good morning, 

I suggest you use air with the default 0.3 emissivity, no need to change this. The emissivity value will switch to the solid material that the air touches, the only time the emisivity of the air is used will be the air-dome.

Regarding the mesh, take a look at a cut-plane and show the mesh on it, you will see that there is sometimes only 1 element across the material thickness - ideally we need 2 (meaning a central node) which allows the heat to transfer more accurately.

I think perhaps you might be expecting to see something that we cannot show. Try looking at the global heat flux as a result, can you see the shadow from the building on the floor?

As radiation passes through a window it would be diffused so we would not show this internally.

Kind regards,

Jon