CFD and Ground exchanger?

meesrl · Posts: 9

HI all

I am writing this post while the simulation is running, so I actuallly don't know what is going to happen. I would try to simulate an air flow passing through the undergound level of a building. This level a tunnel 24 meter long. My goal is to calculate the temperature of the air at the end of this tunnel in order to evaluate if this system could be an helpfull solution for the air treatment machines energy saving.
I tried to model the situation with walls and floor adiacent to the ground and an adiabatic block over the roof. In the first surface I drawn a cfd boundary with an air flow of 10000 l/s while at the end I drawn a CFD boundary extraction, same debit.

My questions are:
- is my model correct in order to obtain my results?
- is it possible to do that with DB CFD ?
- is it possible to change the cfd boundaries properties after having drawing it?
- how can I modify the in flow air temprature?

thank you for your answer and compliments for the program

G

Steve Potter · Posts: 306 Location: newcastle

Hi G,

It's not clear from your post whether or not you have defined surface temperature boundary conditions for the tunnel. If not, you will need to add temperature patches to the walls and floor to represent the adjacent ground temperature.

meesrl · Posts: 9

Thanky you Steve

well, i have not defined surface temperature boundary conditions, i simply modified the adiacency of the walls setting to Adjacent to Ground. How can I do that?

Actually i find strange results because, even if I set the boundary temperature on the 'Drawing options' to 34°C (in the inlet supply) I obtain, after the simulation, a temperature fixed of 20°C from the beginning to the end of the tunnel.
Tomorrow mornig i will attach my model if you could give it a look it would be great.

Thank you
G

Steve Potter · Posts: 306 Location: newcastle

Hi G,

When you upload your model, can you make sure that you first save it as an SKH file and then just upload the SKH file. Otherwise the DSB file that contains the CFD results will be too large.

Regards,

Steve

meesrl · Posts: 9

Steve

the shk file is 7 mb while the dsb is 2.66 mb.

which one is better??
Tks

Steve Potter · Posts: 306 Location: newcastle

Hi G,

In this case, the smaller 2.66 MB DSB file would be better.

Regards,

Steve

meesrl · Posts: 9

Here you are the file.

Now temperature decrease along the tunnel but i am not sure about the surface temperature of the ground.

How can i change it?

how can i see the numbers of one slice (i.e. table report)?

Thank you steve

Steve Potter · Posts: 306 Location: newcastle

Hi G,

Your model looks okay but you will need to add temperature boundaries to the zone surfaces that are adjacent to the ground in order to define boundary conditions for the ground adjacencies. The beta version has no automatic link with EneryPlus output - this feature will be included in the release version. You can add temperature boundaries in the same way as you added the supply and extracts but just select a temperature boundary from the boundary drop list.

You can't create tabular output at the moment although we have plans to introduce this facility. In the meantime you will need to use temperature contours, however you can define your own contours in order to see the distribution of specific temperatures.

Regards,

Steve

meesrl · Posts: 9

Great Steve

I added temperature boundaries and it works ok. I am not sure how to read results, i am insterested in the medium temperature at the exctraction... i can see colors, i will extimate, ok.
But I am not sure to trust my results: if I set boundary conditions to 14° I mean that the walls are fixed at 14° but in the reality the temperature of the internal walls will be at the same temperature of the air , and I will find 14° only deep in the ground, far enough from my tunnel.
I should evaluate the energy flow from the ground to the air, considering the ground thermal transmittance. How can I?

Thanks G

Steve Potter · Posts: 306 Location: newcastle

Hi G,

Yes, you will need to find some way of estimating the surface temperture on the inside face of the tunnel. Theoretically, it is possible to model the whole system using DB CFD because it does have an in-built conjugate heat transfer mechanism (i.e. it models heat transfer through solids with correct interfacing at solid/fluid boundaries) but currently the thermophysical properties are hard-set to typical construction material properties, however we will offer user-definable solid properties in due course and a mechanism to model this kind of problem.

Regards,

Steve

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