wall surface temperature
 

I specify an internal heat generation for a solid block surrounded by the air domain, the surface temperature of that block is calculated based on free convection between the block and its surrounding air. the problem is the surface temperature is way too high. is it due to the mesh? Thanks.

I do not think the mesh has got to do with the surface temperature in a natural convection problem, it depends, in your case, to how much is the amount of heat generation and the surface area (try to touch a 100W light bulb few minutes after you turn it on).
Good luck

I am just concerned that the boundary layer may not be modeled properly with the mesh near the surface. Do you think those commercial codes might have issue on this? Thanks.

if you are worried about the quality of your mesh, you can either
repeat your analysis with a denser mesh, or attach a picture of your mesh and details about your analysis so the readers of this forum can give their advice.
Good Luck

Dear friends:
I think the problem is the wrong heat source value in the block region.
recheck it.