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?

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.

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.

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.

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

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

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

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