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Old   September 15, 2013, 14:36
Default Post processing: Reporting of fluxes
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Hi
I carried out 2D analysis in which water transfers its heat energy to convert phase of material from solid to liquid. Water flows coaxialy through a container having phase change material.
When i do reporting of fluxes/total heat transfer rate and select inlet and exit of water, there is some finite value which comes in net heat transfer. Incoming is not equal to outgoing which violates energy conservation. can anybody tell whats wrong?
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Old   September 15, 2013, 20:32
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Quote:
Originally Posted by dinesh View Post
water transfers its heat energy to convert phase of material from solid to liquid.
that's why
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Old   September 16, 2013, 01:32
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Quote:
Originally Posted by macfly View Post
that's why
but according to conservation of energy the net heat exchange must be zero. So if i select all the zones my net heat transfer should come to zero but i am getting some finite value.
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Old   September 16, 2013, 16:57
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I guess there is a wall and its shadow counterpart on the boundary of the phase change material? You can see that the heat flux is the same on each side but with opposite sign, just unclick one of the fluxes and your energy balance should be right, unless you have bad energy convergence.

Last edited by macfly; September 16, 2013 at 19:33.
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Old   September 17, 2013, 06:52
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Quote:
Originally Posted by macfly View Post
I guess there is a wall and its shadow counterpart on the boundary of the phase change material? You can see that the heat flux is the same on each side but with opposite sign, just unclick one of the fluxes and your energy balance should be right, unless you have bad energy convergence.
I have checked the case in which i selected inlet and outlet. for the rest of boundary conditons one is positive and other is negative but when i select the inlet and outlet there is difference in the amount of heat exchange.
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Old   September 17, 2013, 08:21
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select
- inlet
- outlet
- either all positive or all negative remaining boundaries

it should balance
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Old   September 17, 2013, 13:54
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Quote:
Originally Posted by macfly View Post
select
- inlet
- outlet
- either all positive or all negative remaining boundaries

it should balance
the difference in the value is coming only because of difference in inlet and exit values. rest all other are cancelling each other.
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Old   September 17, 2013, 22:06
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Look at the picture, it makes no sense if you select both -75 and 75 W fluxes in the energy balance. 75 W remains in the volume and energy balance across boundaries is ok: 100 - 75 - 25 = 0
I don't know what else to say.
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File Type: jpg balance.jpg (16.3 KB, 10 views)
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Old   September 18, 2013, 02:34
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Quote:
Originally Posted by macfly View Post
Look at the picture, it makes no sense if you select both -75 and 75 W fluxes in the energy balance. 75 W remains in the volume and energy balance across boundaries is ok: 100 - 75 - 25 = 0
I don't know what else to say.
please see the attached files for doubts

https://www.dropbox.com/sh/0oxd79vwf6nsly0/CtkIa4vU6R
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Old   September 18, 2013, 07:53
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The net result of 401.2946 W looks like the 401.2 W across interface_1. Is it possible that interface_2 and part-fins are within the PCM? interface_1 is the only boundary between water and PCM? If yes, it looks ok.
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Old   September 18, 2013, 09:22
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Quote:
Originally Posted by macfly View Post
The net result of 401.2946 W looks like the 401.2 W across interface_1. Is it possible that interface_2 and part-fins are within the PCM? interface_1 is the only boundary between water and PCM? If yes, it looks ok.

Interface_1 is not the only boundary between water and PCM, there is another interface 2 the outer region which consists of outer wall of copper and copper fins, and then comes the PCM
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Last edited by dinesh; September 18, 2013 at 09:28. Reason: more explanation
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Old   September 18, 2013, 09:51
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then the energy balance is just fine

draw a control volume around water only, and look what goes in and out : 377094.3 W - 376693.0 W - 401.2 = 0.1 W
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Old   September 18, 2013, 12:57
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Quote:
Originally Posted by macfly View Post
then the energy balance is just fine

draw a control volume around water only, and look what goes in and out : 377094.3 W - 376693.0 W - 401.2 = 0.1 W
The why fluent window(image 3) shows a net heat transfer value=401. As per screenshot of fluent help (image 1)it says net value should be small/or zero
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Old   September 18, 2013, 13:12
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There is no shadow wall in the Fluent user guide, it's not the same situation.

You have to put some thinking when doing energy balance, you cannot just select everything and abracadabra. Your case is really just like the picture I draw in post #8.

here is the control volume I talked about in post #12
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