Convective cooling
 

I have 2 cubes 70x70x70 placed with a gap of 1cm. one cube is at 500 deg Celsius other at 30. I want to study the heat transfer between them.

What boundary conditions can I give for this case? The temp of both bodys are changing and heat flux is unknown.

And do do you have in this gap? A fluid? Or are these two blocks immersed in fluid (say, like in a room)? On both cases what you have is a transient simulation where you just set the initial temperature on each block to your known value.

The actual boundary conditions will depend on where these block are, but most likely all other surfaces (the ones that don't belong to the blocks) will be set as adiabatic walls.

I have these blocks in a room. They have air in between. I modeled the room and placed two cubes in them. I patched one cube to be at 500 deg other at 30. But I cannot set the interface between the two, ie between the solid and fluid.