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-   -   Post processing: Reporting of fluxes (http://www.cfd-online.com/Forums/fluent/123531-post-processing-reporting-fluxes.html)

dinesh September 15, 2013 14:36

Post processing: Reporting of fluxes
 
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?

macfly September 15, 2013 20:32

Quote:

Originally Posted by dinesh (Post 451836)
water transfers its heat energy to convert phase of material from solid to liquid.

that's why

dinesh September 16, 2013 01:32

Quote:

Originally Posted by macfly (Post 451891)
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.

macfly September 16, 2013 16:57

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.

dinesh September 17, 2013 06:52

Quote:

Originally Posted by macfly (Post 452097)
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.

macfly September 17, 2013 08:21

select
- inlet
- outlet
- either all positive or all negative remaining boundaries

it should balance

dinesh September 17, 2013 13:54

Quote:

Originally Posted by macfly (Post 452205)
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.

macfly September 17, 2013 22:06

1 Attachment(s)
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.

dinesh September 18, 2013 02:34

Quote:

Originally Posted by macfly (Post 452305)
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

macfly September 18, 2013 07:53

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.

dinesh September 18, 2013 09:22

1 Attachment(s)
Quote:

Originally Posted by macfly (Post 452371)
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

macfly September 18, 2013 09:51

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

dinesh September 18, 2013 12:57

Quote:

Originally Posted by macfly (Post 452392)
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

macfly September 18, 2013 13:12

1 Attachment(s)
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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