Conjugate heat transfer problem

Numerical methods and mathematical models of Elmer
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mark smith
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Location: Peterborough, England

Conjugate heat transfer problem

Post by mark smith » 01 Apr 2019, 20:50

Hi,
I'm doing a coupled heat fluid flow simulation with a fluid channel flowing through a solid with a heat load on it's surface (liquid cooled heat-sink), the only path for the heat is into the fluid.
My solution conserves fluid flow, the heat flux into the surface agrees with my Heat Flux boundary condition but the heat energy isn't conserved, i.e. the difference between the integrals of V.rho.c.T over the inlet and outlet is significantly higher (up to 20%) than the heat load?
Does this suggest that my fluid mesh at the no-slip boundary is not good enough? The error does decrease if I make it finer but I only have the option of tetrahedral or prism elements (I only have GMSH for meshing). With a "fine" mesh using 8 cores my solution time with linear elements is >8hrs and increasing the order to quadratic would be prohibitive and would exceed my available memory ;-(

My tolerance settings in both the Navier-Stokes and the Heat equation are already very tight.
The Reynolds number for the flow is ~1000

Are there any settings that would reduce the energy imbalance error?

Any suggestions?

Thanks in advance, Regards
Mark

mark smith
Posts: 208
Joined: 26 Aug 2009, 18:20
Location: Peterborough, England

Re: Conjugate heat transfer problem

Post by mark smith » 03 Apr 2019, 15:55

Hi,
Just a quick update.
I have run an all linear tet mesh problem, GMSH mesh file size 170Mb (650000 fluid tets 2330000 solid tets), this runs to completion on 8 processors in 20 mins.
The mesh is converted to 2nd order 10 node tets in GMSH (incomplete elements) producing a GMSH file of size 565Mb, this runs (again 8 processors) but refuses to converge, I eventually aborted run after 34 hours ;-(

Comments/observations encouraged.
Regards
Mark

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Re: Conjugate heat transfer problem

Post by raback » 03 Apr 2019, 17:52

Hi Mark

Are you sure that your heat is not convected away at the outlet, or is not eaten up to warm the system in a transient case?

Peter

mark smith
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Joined: 26 Aug 2009, 18:20
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Re: Conjugate heat transfer problem

Post by mark smith » 03 Apr 2019, 18:29

Hi Peter, you asked
Q: "Are you sure that your heat is not convected away at the outlet"?

Indeed it is, in the model the heat cannot leave by any other means. The problem is that the warm fluid leaving the system has more energy than that supplied via the heat flux BC, by 20% more in my linear tet model.

In my 2nd post with the second order tets that wouldn't converge, it was the the fluid flow that would not converge,the heat equation was never started.

Regards
Mark

ark
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Re: Conjugate heat transfer problem

Post by ark » 03 Apr 2019, 20:54

Hi All,

I have similar problems. The case is the air flow around heater (500degC). The walls of channel are modelled by solid bodies.

c.png
c.png (16.38 KiB) Viewed 651 times


The convective flux is calculated by 'convective flux' operator, the fluxes on the solid faces by 'boundary sum' on 'temperature load'.

The flux on internal faces of the channel walls is 4974 W, on external faces 4769 W, so difference is 205 W.
Net convective flow is 1438 W. The outflow from heater 3821 W, perfect radiation outflow 26W, so inside the channel are an extra 2617 W.

Best regards, ark.
Attachments
results.dat
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case.sif
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ark
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Re: Conjugate heat transfer problem

Post by ark » 08 Apr 2019, 20:03

Hi,

I have used heat source (body force) instead of the fixed temperature on heater and the difference decried to about 1100 W.

What is the formula implemented in Elmer for calculating the heat exchange between a fluid and solid?

Here is my mesh (not very 'fluidic'):
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m2.7z
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m1.7z
(491.88 KiB) Downloaded 23 times

ark
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Re: Conjugate heat transfer problem

Post by ark » 13 Apr 2019, 15:47

Hi,
by switching off the radiation I have found that radiation is not included in boundary sum on heater in my original case.

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