Temperature Dependent Convective BC for Conduction

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Coffer_001
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Temperature Dependent Convective BC for Conduction

Post by Coffer_001 »

Hi:

I'm trying to apply convective BCs to a transient conduction model. I want to use a gas temperature that is a function of time. The Elmer Model Manual seems to say that can be done. See below.

Heat flux depending on heat transfer coefficient α and external temperature Text may be written as
− k ∂T/∂n = α(T − Text). (1.14)
Both variables α and Text can be constant or functions of time, position or other variables.

I've modified my sif file many, many times in attempt to get it to work. The closest I can get is what appears to be time dependent temperature boundary conditions but not hc (Ts-Tinf(t)) as the results are unchanged regardless h=1 or h=0.001. I believe my problem is in the below section from the sif.

Boundary Condition 1 ! time in Hours
Name = "AC Gas Temperature + Hc BC"
Target Boundaries (2) = 1 12
Heat Transfer Coefficient =1.0
! Heat Transfer Coefficient =0.001
Temp = Variable Time
Real
0.0 90.7 ! Time in Degrees F & Hrs - 2" Panel 1st step - 2/1/23
0.1 94.2
0.2 95.2
0.3 93.1

My .msh and sif files are attached and welcome any help that anyone will provide.

thanks

gsw
case.sif
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case.sif
(3.71 KiB) Downloaded 39 times
Coffer_001
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Re: Temperature Dependent Convective BC for Conduction

Post by Coffer_001 »

Attached is the msh file.
2InPanel.msh
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kevinarden
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Re: Temperature Dependent Convective BC for Conduction

Post by kevinarden »

I believe ther may be an issue with using # for comments instead of !
I think LUA interprets the # as a variable. You also have three solvers, 2 defined, and only need to make 1 active.
ep is an obsolete post file, most users are using vtu
mcase.sif
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Coffer_001
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Re: Temperature Dependent Convective BC for Conduction

Post by Coffer_001 »

I removed the R comments and replaced with !. The # was not a good idea.

I changed from 3 to 2 solvers. I need the 2nd solver for Save Scalars.

Obsolete Elmerpost is ok for this simple problem.

Unfortunately all those excellent catches did not resolve the problem. I get the same result if I use h=0.001 or h=1.0.

Any more thoughts on what I might be doing wrong? I appreciate that you took the time to read my post and share your experience with me! The quick response is greatly appreciated!!
kevinarden
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Re: Temperature Dependent Convective BC for Conduction

Post by kevinarden »

The second solver will run without calling it.

If you are calling h the Heat Transfer Coefficient in the boundary condition then it only woks if you have a heat flux, external temperature, or radiation on. As it stands you have set temperature as a function of time. I would expect the answer to be the same.
Coffer_001
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Re: Temperature Dependent Convective BC for Conduction

Post by Coffer_001 »

Good to know. Thank you for the lesson

Yes h is the Heat Transfer Coefficient.

Is there another way to accomplish what I am trying to do? I've tried a Convective Heat Transfer Coefficient with a time dependent External Temperature and not had success

The Elmer Manual says: Both variables α and Text can be constant or functions of time, position or other variables.

Thanks!
kevinarden
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Re: Temperature Dependent Convective BC for Conduction

Post by kevinarden »

I am not quite sure what you are trying to do, but you may have to specify a radiation boundary to transfer heat in either direction.

something like

Heat Transfer Coefficient = 1.0
External Temperature = Variable Time
Real
...
End
Radiation = Idealized
kevinarden
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Re: Temperature Dependent Convective BC for Conduction

Post by kevinarden »

I ran some test and the previous comment did not matter. You do not need convection nor radiation. What did work was time. In heat transient it takes a long time for heat to flow. In my test case;
a square aluminum plate
Initial body temperature = 70
External temperature = 120
h = 1.0

it took 20 seconds for the body to start rising in temperature, then it started increased linearly to 120.

with h = 0.001
after 2000 seconds the body only reached 70.5

I would say your results do not significantly differ because they really do not significantly differ after only 1.8 seconds. Heat change is not fast unless you make it fast with lots of heat input.
Coffer_001
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Re: Temperature Dependent Convective BC for Conduction

Post by Coffer_001 »

I'm trying to do a conduction solution with convective BCs that does not involve CFD - not a conjugate HT problem.

I tried External Temperature = Variable Time before with no success, but repeated today in case I had done something wrong. See pertinent section listed below:

Initial Condition 1
Name = " Initial Temperature"
Temp = Real 70.0 ! Change to match experimental data
End

Boundary Condition 1 ! time in Hours
Name = "AC Gas Temperature + Hc BC"
Target Boundaries (2) = 1 12
Heat Transfer Coefficient =1.0
! Heat Transfer Coefficient =0.001
External Temp = Variable Time
Real
0.0 90.7 ! Time in Degrees F & Hrs - 2" Panel 1st step - 2/1/23
0.1 94.2
0.2 95.2
0.3 93.1
0.4 90.9
.
.
.
1.7 105.3
1.8 109.7
End
End


That seems like the right thing to do when reading the Manual instructions. Before as now it produces 70 F which is the same Temp as the initial condition - which is odd. I conclude that External Temperature must be a CONSTANT OR I'm doing something wrong. External Temp works well if you enter a constant.

There's no radiation involved in my problem so I don't think it should be included.

Thanks for sticking with me. I appreciate you sharing your expertise and time!!
kevinarden
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Re: Temperature Dependent Convective BC for Conduction

Post by kevinarden »

I will experiment with External Temperature = Time on my simple case.
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