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Hi all,

I'm attempting to model the heat transfer of a certain geometry. Essentially, the geometry has a series of connected "holes". I wouldn't consider it to be microfluidic or a porous media, as the holes are rather large (approximately 1/2 inch diameter holes). The problem is, I can't get the solution to converge, and being new to COMSOL, I have no idea where to start.

For simplification, I'm using a 1"x1"x1" cube with a 3/4" x 3/4" holes through the cube in each direction. Obviously, the goal would be to have a bunch of these geometries connected, but if I can't get a single one to converge (fewer elements, therefore faster computation), then I'll never get a bunch of them to converge. I'm using the non-isothermal flow module. For boundary conditions, I have one side set at 400K, and another at 300K. The rest are insulated. I can get this model to converge, as its a straight forward heat transfer problem. But I'd also like to know the effects of free convection. If I place a volume force on the fluid domain (F = -nitf.rho*g), the model shows a nice flow inside the cube. But now I'd like to open up the boundaries to ambient air . I selected open boundaries on each of the faces of the fluid domain. But once I do this, the model does not converge. It won't even converge if just 2 of the faces have open boundaries. Any ideas? Any help would be greatly appreciated. Thanks!