Energy imbalance
 

Hi,

I would like to ask a question regarding the engery imbalance.
I have a model which has three domains consisting of water, mould and heat source domains.
In the heat source domain, a high temperature is set as an initial condition of this domain. The heat for the heat source domain is conducted to the mould and the mould is cooled down by the water which passes through the mould inside.

I checked these three domains' energy imbalance (i.e., T-Enegry imbalance for heat source and mould and H-Energy imbance for water) during the transient simulation.

By the way, it shows a negative value in the beginning of simulation. After that, it shows a high positive pitch and then converges to zero.

By the definition of energy imbalance, i.e., (Total in-Total out)/Total in,
the energy imbalance of the heat source domain should be always negative because the heat source domain has only initial high temp.(200C) without any additional heat supply (i.e., it means it does not have any total energy in) and thus it is cooled down continuously by air convection and heat conduction with the mould (temp. 30C) where the cooling water (15C) flows (i.e., there is only total energy out in the heat source domain). However, the above result is different from my expectation.

Could anyone explain or intepret the above result of the energy imbalance of the heat source domain?

Thank you so much,

Jaho

Hi,

If you are referring to the imbalances on the solver manager then your definition of heat balance is wrong. In CFX heat balance is defined as the sum of all the heats and all the heat gains and losses - meaning it should always equal zero. If you are getting large imbalances then you have not converged sufficiently.

Glenn Horrocks

Hi, Glenn

Thank you for your answer.
Then, the energy imblance graph of the heat source domain should show a positive value in the begining of transient simulation and then gradually converge to zero if this domain is just initialized as certain high temperature without any addtional heat supply. (i.e., it becomes to only lose the energy without any gain). However, in my case, it shows a negative value in the beginning of simulation and then a high positive value and finally convergence to zero from a high positive value.
I still wonder why there is this negative value in the energy imbalance of the heat source domain during the simluaiton.
Could you please explain about that if you don't mind?
Thanks again and have a good day.

Best regards,
Jaho

Hi,

No, you did not understand what I said. The energy imbalance on the solver manager should ALWAYS tend towards zero. Any time it deviates from zero represents a convergence error. It does not represent anything heating up or cooling down.

If you are getting significant imbalance errors you need to converge tighter or use smaller timesteps (for a transient simulation).

Glenn Horrocks

Thank you, Glenn. It helps a lot to me.
Have a great day,

Jaho

Weirdly, I have in all my steady state simulations a high Enthalpy Imbalance, (H Energy in Flow), which is mostly around 100% while other variables seem to converge very nicely. Also my P-Mass is always at 100% or -100% (I discussed this in another thread).

So I think in some cases a high Energy imbalance in the fluid can be related to that issue. I think in my case it's the opening entrainment BC which only has a pressure specification.

No, the problem is that when there is no net flow in the simulation any microscopic flow through and inlet or an outlet becomes a 100% imbalance. For example if the domain only has one inlet or outlet then tiny numerical noise creating microscopic flows in and out the only boundary will be shown as 100% imbalance.

This is why imbalances are not the default convergence criterion. They can be very misleading in some cases.