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Old   August 14, 2007, 01:04
Default Is this correct?
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Hi all, I am a beginner in using CFD codes. In my curent project, I have a large hollow cylinder immersed in deep sea. The cylinder is hot and is being cooled by the sea water around it. I modelled the cylinder and its components, along with the sea water domain around it. The sea water domain is about 5 times bigger than the cylinder. I gave the bottom face as Pressure inlet and the top face as pressure outlet for the sea water domain and the rest four faces as symmetry. The sea water is at a pressure of 100 bar and 4deg C.

In the cylinder, I have components that are dissipating heat. These components are sorounded by nitrogen in side the cylinder. For nitrogen I gave Ideal gas instead of Bousinessq, because, delta T is very high. I have activated gravity and gave the value as -9.81m/s2.

Is my approach correct? I am observing that the mass residuals are comming down, but it started at an order of 1e4 and it is comming down. I stopped the iterations and checked the velocities and they are 400m/s where as they should be much much less. I cant lay my finger on the place(s) that I am making mistakes. Your help would certainly be helpful. Thank you. regards Siva.
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Old   August 14, 2007, 07:08
Default Re: Is this correct?
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Hello guys, Kindly take some time out of your schedules and look into this please. Your inputs are most valued to me in this situation at this moment. Thank you. regards Siva.
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Old   August 14, 2007, 07:22
Default Re: Is this correct?
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What is missing here is aim of your project? The whole modeling effort will depend upon your objective.

First thing to look at is the mesh. If the components in the cylinder are not too complex, try getting a structured mesh. If not you may want to get the mesh fine in regions where there is conjugate heat transfer. Try getting a boundary layer surrounding the cylinder (for un-structured mesh)

The pressure-pressure boundary condition is making things difficult for the solver. Try specifying a velocity inlet for the sea water.

If I understand you correctly, you have a cylinder placed inside sea water, then comes nitrogen and then components generating heat. how did u specify the heat generation? You may not need to model all these components depending upon your objective.

Where are you measuring the velocity?

Start with in-compressible flow for nitrogen. first calculate the Rayleigh no. and see if the natural convection in N2 flow is laminar or turbulent.

Give more details and you could get more help

Best luck
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Old   August 14, 2007, 08:35
Default Re: Is this correct?
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In my anxiety, I forgot to mention the purpose of the analysis. I am sorry and thank you.

A cylinder is filled with nitrogen and there are some components in the cylinder which are disipating heat. Because of this we observe natural convection of nitrogen in the cylinder. The cylinder walls get heated up. Also, this cylinder is placed very deep in the sea. So the sea water cools the cylinder. The purpose is to model the natural convection in nitrogen and also to see the temperature feild around the cylinder in sea water.

I have given heat source to a few components and for the rest temperature. I have not calculated the Rayleigh number(I dont think I have enough input to calculate that). The sea water is not moving in this case, beacuse the whole thing is placed very deep in the sea. But thank you for your suggestions. I will make necessary changes to accomodate them and see the results.

So, vel inlet(with very very small value), pressure outlet and the rest of the faces as symmetry for the sea water domain is ok?

Or Should I give a heat transfer coefficient value to the cylinder wall and not model the sea water at all. If so, what value and will that be correct. I am confused here. Looking forward eagerly to your comments. Thank you.


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