Boundary conditions for S2S radiation model in naturally convected square cavity

Mahendra_Paipuri · October 13, 2014, 15:32

Hello All

I am working on natural convection and radiation model in a square cavity having finite wall thickness. The geometry of the problem can be seen in attachment.

The outer part is the solid part and inner part is the fluid which contains air. I see from the tutorials and forums, that I can give the thickness of the wall rather than explicitly meshing it. But I want to mesh the solid part as I have to compare the temperature in the middle of the both flanges with experimental data.

I am using ANSYS meshing module to generate mesh. If I make named sections of the top, bottom, left and right sides of outer edges, I cannot use S2S radiation model and it is giving me error saying no boundary is found adjacent to fluid region. If I make a named section of all internal walls in the meshing module, I am not able to specify a coupled boundary condition on the internal walls and they are acting as adiabatic walls.

Can someone please suggest me how to model this particular problem in FLUENT?

Thanks

Mahendra_Paipuri · October 17, 2014, 06:46

Hello everyone

I managed to make it work by tweaking the geometry and mesh a bit. Now, I got some questions about the physics of the natural convection in a cavity.

As you can see, my problem is with square tubular section with air inside the cavity. The twos side faces are adiabatic while bottom face is heated up and top staying at room temperature. The boundary condition for the bottom face is time dependent temperature, precisely following ISO curve. I am using pressure based transient analysis (PISO) with PRESTO for Pressure spacial discretization and slightly higher than default UDFs for pressure and momentum (0.5 and 0.9 resp).

The velocity contours are losing symmetry and going unstable at Rayleigh number around 4e6. From theory, I understand that turbulence regime starts at Ra 1e8-1e9. Does anyone think it can happen or Am I doing something wrong? I am using Boussinesq approximation and does it give acceptable results for higher temperature differences? If anyone have any suggestions, kindly let me know.

Regards
Mahendra

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October 17, 2014, 06:46		#2
Mahendra_Paipuri New Member Mahendra Paipuri Join Date: Oct 2014 Posts: 3 Rep Power: 12	Hello everyone I managed to make it work by tweaking the geometry and mesh a bit. Now, I got some questions about the physics of the natural convection in a cavity. As you can see, my problem is with square tubular section with air inside the cavity. The twos side faces are adiabatic while bottom face is heated up and top staying at room temperature. The boundary condition for the bottom face is time dependent temperature, precisely following ISO curve. I am using pressure based transient analysis (PISO) with PRESTO for Pressure spacial discretization and slightly higher than default UDFs for pressure and momentum (0.5 and 0.9 resp). The velocity contours are losing symmetry and going unstable at Rayleigh number around 4e6. From theory, I understand that turbulence regime starts at Ra 1e8-1e9. Does anyone think it can happen or Am I doing something wrong? I am using Boussinesq approximation and does it give acceptable results for higher temperature differences? If anyone have any suggestions, kindly let me know. Regards Mahendra