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modeling U-tube using 2D extruded mesh

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Old   March 3, 2011, 02:31
Default modeling U-tube using 2D extruded mesh
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Dear friends,

I'm a new user of CFX (since 2 month only ) and I'm trying to model a long vertical U tube with surounding ground. the length is in the order of hundred meter and diameter of the tube is 1 inch.

Regarding disprotionate I'd like to use the '2d extruded meshing ' option but for that i should define two identical pairs (top and bottom) therefore I suppressed the bend of the tube at the bottom which results on two separated tubes (downwind and upwind flows)

now in CFX pre, I've to define 4 boundaries in concern with the fluid. it's ok for the inlet and outlet in the top but how i can define the two remaining boundaries at the bottom to connect them. i.e how i can put on cfx pre that the exit temperature is the same as the inlet temp.

i hope i was clear if not i can send an image

thanks guys
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Old   March 3, 2011, 04:04
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Q) You tub with surrounding ground ?????? What does that mean ???

Figure may help.
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Old   March 3, 2011, 20:29
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Thank you for your reply.
I'm modeling a geothermal heat exchanger, vertical U shaped tube embedded in the soil. so I have also to model the surrouding soil as cylindrical volume. however as the height is about 100meter, i'd like to use the 2D extruded mesh. but for that i have to remove the Tube bend at the bottom which will result on two separeted tubes (up and down) so my question how i can connect the two bottom boundaries on the bottom in CFX pre.

here is a picture i found in the net
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Old   March 3, 2011, 21:17
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OK !!!!!!! So what I understand is that you need to simulate the flow through the U-tube in the soil. Am I right ???

If your simulation involves the entire heat exchanger then am sorry. Am not the one you should be asking.

Anyways, presuming you want to simulate the flow over a pipe in the soil.
It is a very simple problem. No need to make a cylinder outside. You just need to know the BC of the pipe walls. Like the temperature (i.e the average temperature of the soil). Set the outlet to pr. outlet and give velocity inlet at inlet.

Use a Structured mesh for the pipe. You can run a steady state.

Is this what you wanted ?????
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Old   March 4, 2011, 00:50
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Thank you for your fast reply.

I will model only the U shaped Tube in the soil (the underground part)

However i need to take into account the surrounding ground to observe the heat diffusion in the soil and calculating the outlet temperature of the fluid. (in transient mode)

what do you mean by structured mesh ?? I'm afraid to have to deal with huge number of nodes since the height is about 100m and the pipe diameter is only 0.025m. (the diameter of the ground cylinder is 10m) and i will make long simulation in time.

I saw that with 2D extruded mesh the problem can be very easy only the fact to overcome the bend of the tube at bottom by some tricks (i can separate the pipes to get two identical faces top and bottom, however how i can define the boundaries at bottom if i do like this)

thanks again
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Old   March 4, 2011, 02:13
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Your comment "i can separate the pipes to get two identical faces top and bottom, however how i can define the boundaries at bottom if i do like this"

1.) Getting a curve is very simple. There is no trick involved in it.

2.) Look into tutorials to see structured mesh. Just Google the image of it and you will have an idea. You can use other meshes I guess without any problems. If using tri elements, make sure you have a good prism layer.

3.) I did not get the last sentence. What do you mean by define the bottom boundaries. You will have an inlet, outlet and the wall.

Where does bottom comes from ???
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Old   March 4, 2011, 03:25
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thank you now i have a good idea about the structured mesh, and that what i want to do however i don't know how to get such structured mesh i use ansys 12.1 and the mesh is done by AqwaWB (cfx mesh method).
all what i can do is unstructured mesh. there is one option to work with the '2D extruded mesh' to get structured mesh in the height of the whole cylinder. however i need to define two identical surfaces (bottom and top of the cylinder) which is in reality impossible because of the bend of the Utube. one trick is to supress the bend but the only resulting problem that i need to define two other boundaries later in CFX-pre for the fluid at the bottom outlet and inlet with the assumption that are identical Tout=Tin.

I hope u can help me or give more detail on how i can use structured mesh in AqwaWB meshing.
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Old   March 4, 2011, 09:50
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Am sorry. Never heard of AqwaWB meshing. I use ICEM/GAMBIT.

Have a nice day.
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Old   March 4, 2011, 09:56
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Am sorry. Never heard of AqwaWB meshing. I use ICEM/GAMBIT.

Have a nice day.
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Old   April 6, 2011, 11:39
Default ground type u tube heat exchanger-gambit
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Sir,
I 'm new to GAMBIT and FLUENT.Started using it last month .I'm a M.E student and my project focus on creating geometry by Gambit and simulating :conduction heat transfer of borehole of a vertical ground heat exchanger type U-tube. could u give basic idea of how to create geometry and points to take care while simulating?Please help me...
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Old   April 6, 2011, 11:49
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I dont know how I can help you to create a geometry either orally or through mails. There are tonnes of tutorials available online. Find out which one suits your model the best. Try playing with gambit/ICEM for some time. That's how every one does it.
Let me know if I an help you in any other way.

Prof. Chaos
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