# Boundary conditions for heat transfer between a hot plate and the ambient air

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July 14, 2015, 19:28
Boundary conditions for heat transfer between a hot plate and the ambient air
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Join Date: Jan 2011
Posts: 45
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I have seen this thread which is about natural convection between a hot plate and the ambient as well, but I can't seem to find the problem in my case. It is attached (execute blockMesh -region air first).

My goal is to simulate a hot plate in a very large room. The plate is square, so I defined a mesh which models only 1/4 of the plate (at the bottom of the room) and the surrounding air, with two symmetry planes.

The plate and the floor are walls, so I use
U: no-slip
p_rgh: fixedFluxPressure
p: calculated
T: on the plate is fixed to 400 K, on the floor fixed to 300 K.

The ambient air has these BCs:
p_rgh: fixedFluxPressure
p: calculated
T: fixed to 300 K.

Are these reasonable?

I use chtMultiRegion(Simple)Foam because I want to refine the hot plate model later. Currently the case just contains one fluid region "air". Both chtMultiRegionSimpleFoam and chtMultiRegionFoam abort after a few time steps, with the temperature diverging:
Code:
```Solving for fluid region air
diagonal:  Solving for rho, Initial residual = 0, Final residual = 0, No Iterations 0
DILUPBiCG:  Solving for Ux, Initial residual = 0.010049355, Final residual = 4.3291669e-08, No Iterations 2
DILUPBiCG:  Solving for Uy, Initial residual = 0.0058214611, Final residual = 8.0462977e-10, No Iterations 3
DILUPBiCG:  Solving for Uz, Initial residual = 0.0069691383, Final residual = 1.3927723e-09, No Iterations 3
DILUPBiCG:  Solving for h, Initial residual = 0.99254951, Final residual = 1.8866287e-08, No Iterations 3
Min/max T:274.66362 400
GAMG:  Solving for p_rgh, Initial residual = 0.99800747, Final residual = 3.9553685e-05, No Iterations 1
diagonal:  Solving for rho, Initial residual = 0, Final residual = 0, No Iterations 0
time step continuity errors (air): sum local = 1.1262753e-05, global = 7.3271104e-07, cumulative = -0.00035973887
GAMG:  Solving for p_rgh, Initial residual = 0.052670331, Final residual = 3.0747754e-08, No Iterations 4
diagonal:  Solving for rho, Initial residual = 0, Final residual = 0, No Iterations 0
time step continuity errors (air): sum local = 3.1563787e-11, global = 5.514505e-12, cumulative = -0.00035973886
ExecutionTime = 28.25 s  ClockTime = 28 s```
In the following time steps Tmin goes further down until it reaches zero, and the solver crashes. Tmax seems to be correct.

This also happens when I set the plate's temperature to 300 K, i.e. no temperature difference between the plate and the ambient air.

What is wrong here? I am using OF2.4.0

Best regards

Christoph
Attached Files
 02_symmetric.zip (10.3 KB, 22 views)

 July 15, 2015, 05:59 #2 Member   Join Date: Jan 2011 Posts: 45 Rep Power: 14 I enabled turbulence (k-epsilon, as in the tutorials) and now it's running fine. Is that a common problem with these setups (natural convection, chtMultiRegion solvers)?

 August 28, 2015, 08:05 chtMultiRegionFoam #3 New Member   dafe Join Date: Jul 2015 Posts: 3 Rep Power: 9 hi foamers, i am modelling flows through pipes (hot and cold), with a heatexchanger coupling the hot to the cold (chtmultiregionfoam). I do not know which files to specify in my heatchanger folder, in the constant folder. example: constant: polymesh coldwater - g, radiation pro, ras pro, thermophysical pro, turbulence pro heatexchanger - ???? I am new to openfoam and have checked out various tutorials but couldn't find an example to give me an idea. thanks...