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-   -   UDF for cell zone condition (http://www.cfd-online.com/Forums/fluent-udf/118060-udf-cell-zone-condition.html)

rachana May 20, 2013 13:13

UDF for cell zone condition
 
Hi, I am modeling a system in which the temperature of a large block needs to change with time and with depth (inside the block). A thin pipe (carrying hot liquid) is passing through the block and the temperature of the block around the pipe is affected by that.

So, how do I provide a time-dependent cell zone temperature condition? I cannot use "Fixed value" because the temperature profile inside the block needs to be affected by the pipe. Can I use "Source term" somehow for this?

I think I will be able to make the UDF but how (and where) do I use it? :confused: Please help me out.

mvee May 22, 2013 09:04

Can you put some light on problem?
Yes, You can model temperature as a source term which varies with space and time. If you have mathematical model then you can easily put down this into UDF.
Best wishes

rachana May 22, 2013 13:28

I am modeling underground heat pump and i need to study the seasonal variations. So, I have a solid block, which represents the earth, and a horizontal pipe passing through it. Now the temperature of the underground soil will vary with depth and with season. I have an expression for that, where it varies exponentially with depth and sinusoidally with time (in the same equation). Now the temperature of the soil in the pipe vicinity will be affected by the heat pump and the heat pump performance will in turn be affected by the soil temperature.

And this is where I get stuck. If I patch the earth block for a temperature profile that varies with depth, it doesn't include the time dependence. Please help me. Is there any more information I need to provide?

mvee May 23, 2013 03:05

Is it possible to upload schematic diagram along with BC and the expression for temperature?

rachana May 23, 2013 19:55

1 Attachment(s)
I am attaching the schematic of the problem. This is the temperature profile of undisturbed soil:
T_soil = T_avg - A exp[-z (π/365a)^(1⁄2) ]∙ cos{2π/365 [t-t0 - z/2 (365/πa)^(1/2) ]}

Tavg, A, t0 are constants. As you see the "exp" term signifies the change with depth (z). The "cosine" term depends on time (t) and has a phase term that depends on depth.

As the air passes through the pipe at a temperature of T_air, the soil temperature around the pipe will get affected. But the total size of the earth block is very large (decided from literature and personal experience). So at the top, side and bottom surfaces of the earth block, heat flux is zero.

mvee May 24, 2013 01:33

where are you going to define the temperature i.e on which face? When you represent temp. as a function of 'z', it will have 1D variation.
Now if you are saying that this temperature will be for same throughout the 'x', then this condition can be defined by profile condition. This profile can be incorporated as a function of time also. or else you have add the second coordinate into expression. and then you can have UDF.

rachana May 24, 2013 12:35

1 Attachment(s)
This profile applies to the complete earth block, where z=0 at the top of the surface. The variation occurs vertically. The resulting profile (undisturbed soil) at a fixed time should be something like that shown in the figure. It doesn't change horizontally. I obtained this by defining a custom field function and patching the earth block with that. This profile doesn't include time variation. How do I include that? Please let me know.

avi@lpsc August 23, 2013 16:10

Temp.varying with space and Time
 
Did you write udf for mentioning temperature on block surfaces?
I am also trying to model this problem..would you please share the model udf for temp.varying with depth and time.
Shall be thankful:)

JouleBrayton October 15, 2013 06:08

How did you solve this problem?? Is it possible to define time dependent cell condition? Thanks.

reza67 December 3, 2014 16:12

could anyone write this udf ?

i have the same problem


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