# Transient CHT Simulation slow variable increase

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 April 22, 2016, 10:57 Transient CHT Simulation slow variable increase #1 Member   Marcel Jay Join Date: Jun 2014 Location: Berlin Posts: 52 Rep Power: 10 Hello Community, sorry for the, I guess, newbie question, but maybe someone can explain the issue in a few words. I'm trying to simulate the heating of a tool (while air is cooling it). Steady State solution gives me an equilibrium of about 530K from 293K (room temperature). In the first timestep it's already 423K. Since there is a heat flow of 16W/mm˛ and from experiments I guess it shouldnt take more than 30secs to reach that temperature. The transient Simulation is supposed to show how hot the tool gets after let's say 2 seconds. The solution though, gives me a heating of about 1,5K after 10s with 0.05s timesteps. I set a minimum of 3 loops per timestep but the residuals are all below 1e-4 after 5 timesteps already. Any idea what I did wrong? Or is transient not the appropriate type for my examination? Cheers and have a nice weekend Marcel

 April 22, 2016, 17:11 #2 Senior Member   Erik Join Date: Feb 2011 Location: Earth (Land portion) Posts: 1,129 Rep Power: 22 Check all your material properties, and the math of your hand calc. Did you put in specific heat in kJ instead of Joules?

 April 23, 2016, 07:19 #3 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,326 Rep Power: 138 For CHT simulations make sure you use imbalances as a convergence parameter. CHT simulations are famous for having the residuals tightly converged but the imbalances are miles off - and this results in the global parameters like average body temperature being inaccurate. So you need to define convergence to include imbalances as well so these global conservation balances are checked before you declare a time step converged.

 April 23, 2016, 13:25 #4 Member   Join Date: Dec 2009 Posts: 44 Rep Power: 15 I would use a tighter residual tolerance than (RMS?) 1e-4. For production runs, I would advise using MAX 1e-4, and possibly even 1e-6.

 April 25, 2016, 10:53 #5 Member   Marcel Jay Join Date: Jun 2014 Location: Berlin Posts: 52 Rep Power: 10 Thank you for your replies! @Erik: The transient CFX simulation is all the same as my steady-state. I just changed it to transient and set the initial conditions. @Glenn: My imbalances are even better than the steady state ones. All below 0.1% except the P-mass imbalance which was fluctuating between 1 and 4%. @cfdgremlin: I can try that but after 150 timesteps the max residuals were below 1e-5. Shouldn't the solution be closer to reality?

April 25, 2016, 13:18
#6
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Erik
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Quote:
 Originally Posted by marcel_jay Thank you for your replies! @Erik: The transient CFX simulation is all the same as my steady-state. I just changed it to transient and set the initial conditions.
Right, but Specific Heat won't matter in your steady state calculation. It will matter in your transient.

April 26, 2016, 07:37
Problem solved
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Marcel Jay
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Quote:
 Originally Posted by evcelica Right, but Specific Heat won't matter in your steady state calculation. It will matter in your transient.
Thank you for your input. I think the problem is solved here.
I had funny values, not for specific heat capacity but for molar mass and density, because I knew these wouldnt matter for my steady state.

Since you said specific heat doesn't matter in a steady state, I was discussing this with my boss and after a thought experiment I was arguing, that once you have multiple solid bodys with differing interface areas and conductivites within a cooling fluid domain, the specific heat would actually matter. I concluded that only when you have one homogenous body being circulated by a fluid the specific heat wouldn't matter.

Cheers

 April 26, 2016, 08:30 #8 Senior Member   Join Date: Jun 2009 Posts: 1,665 Rep Power: 29 For steady state cases without motion (either solid or fluid), the specific heat capacity has not influence on the temperature distribution. Once time comes into play either by transient, or motion of the body the heat capacity has a strong influence.

 May 2, 2016, 17:11 #9 Senior Member   Erik Join Date: Feb 2011 Location: Earth (Land portion) Posts: 1,129 Rep Power: 22 @ Marcel_jay: Right, specific heat of the solid will not matter in steady state. I meant this for your specific case, since it sounds like you are just modeling one solid body, and not modeling the fluid. There is no solid or fluid motion. I don't see where specific heat would matter in (an ideal) steady state problem, if there were more than one solid body with differing interface areas and conductivities as you say? But I'm intrigued in how you came up with this? Opaques answer seems correct in the most general sense to me.