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kola77 September 18, 2010 10:56

Simulating a gap
Hello, I have a thrivial HVAC problem to solve, and my university doesn't provide me with ansys airpak, so I have to use CFX. Since I am only learning how to use it, problems are imminent and I kindly ask you for your help.

Basically, what I am faced with, is a gap between a room with two different temperatures, as seen on the picture below. What I am interested in is the velocity in the gap.
I built a model in design modeler out of three parts, two larger ones for the rooms, and a tiny one representing the gap. Now, what my problem is, I don't know exactly how to define my regions. The general idea I have is to set boundary details of the two larger rooms to "wall" on all faces except the one with the tunnel, aswell as the outer "coating" of the connecting tunnel. However, I'm then left with the remaining two faces, which shouldn't be set to an opening, but i have to do so in order to have some opening at all. How would you suggest I approach the modeling?

The next thing I'm interested in is if there is any way for me to set the temperature of one room to 40C and the other one to 5C, without having to define the temperature of the opening? If no, is it ok for me to set it to 22.5C?

I hope somebody can help me with my problem and thank you in advance.

ghorrocks September 19, 2010 05:54

If you have to define the gap then it is an interface. Hopefully you won't have to do this.

It looks like what you want is to start it up with a given initial condition (40C one side, 5C the other) and see how it evolves over time. This is possible.

kola77 September 19, 2010 07:54

Why do you think an interface would be a bad idea? I've done them before and they seemed just fine.

But is there a simpler way to do it? I've managed to define different initial conditions in each of the rooms, but there is no temperature gradient in the results, so I guess I must be doing something wrong.

ghorrocks September 19, 2010 18:42

There is nothing wrong with an interface. Just don't use them unless you need them. This should be able to be meshed as a single body with no need for interfaces.

What I suggest is about as simple as it gets. What is the problem with it?

If nothing happens in the flow than how are you driving the flow? You will need buoyancy to speed the mixing of the hot and cold gases. Also, are you sure there is no air conditioning flows?

kola77 September 21, 2010 03:36

I now used the interface boundary, and still, I don't get the results I wish to. The problem is that I don't get any velocity in the region of the gap at all. There are no airflows, such as airconditioning or similar. I've only set all the other boundaries to "wall", and the remaining 2 to interface and defined two domains (air @ 40 and 5C), I didn't fiddle with anything else, since I want to make it simple. What I am using to drive the process is the difference of temperatures in the rooms, which should result in at least a slight increase in velocity, but it doesn't.
Any ideas what I did wrong?

ghorrocks September 21, 2010 06:32

Do you want buoyancy to drive the flow? Have you set it up correctly?

If you don't want buoyancy in the flow then the only action would be diffusion of the temperature and that would be very slow.

kola77 September 21, 2010 11:51

Thanks for your help ghorrocks, it works now I've also included buoyancy. Cheers!

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