Convection inside of a cube

[jones79]

Hello!

I need to calculate the convection inside of an air-filled cube and don´t know which BCs to use to achive this.

A file showing the system is attached.

The sun is shining on the cube, load curves are used to simulate the sun shining on different surfaces along the calculation.

The cube is made of steel, having a 100mm thick wall and is filled with air.

Applying the convection onto the outside of the cube is quite straight forward, a load curve is used to

simulate a varying ambient temperature.

The heat flux inside through the air-filled space is calculated automatically by setting the thermal conduction of the air.

But how can I set the convection from the inner steel surface into the air? The ambient temperature of this convection, the temperatur inside of the cube, is determined while the calculation takes place.

And having shared border lines between parts, to which part do I apply a BC? Does it matter? In the example with the cube: do i apply a BC to the inner side of the steel surface or the outer surface of the air-filled space? Do the BCs have to be applied to both surfaces?

Thanks in advance!

Kind regards,

Walter

[Joey.X]

According to your description, it appears that the problem is not a standalone thermal problem, but the conjugant heat transfer problem which has coupled fluid flow and thermal with shared or not shared parts between fluid and thermal domains.

Change analysis type to Multiphysics/transient coupled fluid-thermal,  it simulates the nature convection which has buoyancy force involved.

Your current thermal BCs could be applied to this analysis types. Also note that this analysis type is relatively computantional expensive and needs small time steps for solution convergence.

[jones79]

Thanks for the quick response! That's it!

My other question was:

If parts have shared borders does it matter to which a BC is applied? E.g. having two boxes next to each other with a bonded contact and I want to apply a heat flux from the surface in contact, does it matter to which part it is applied?

And if it does not matter, are there other scenarios, where it does matter?

Kind regards,

Walter

[Joey.X]

For applying boundary conditions (BCs), usually BCs are applied on the exterior surface of simulation domain rather the interior surface between parts. I don't the reason to apply thermal flux BC on one of the contact surfaces except you raise some good reasons, simulation modeling always comes from real world physical models.

Btw, fluid flow internal fan BC is an exception as what I know, since it has the pressure jump(or drop) across the part contact surface.

If you want simulate the thermal contact between parts, is this what you want? For Yes, both surfaces of the contact pair needs to be picked and apply contact properties.

No matter bonded or contact surface between parts in thermal analysis, the heat flux from both side is automatically handled by software.

[jones79]

Thank you for the answer.

Kind regads,

Walter