Fixed temperature at the wall boundary
 

Hi,

I am asking for your help regarding posing fixed temperature boundary condition in steady state simulation.
I am trying to run free convection simulation.
My domain is simple. A hot steel box in the open air.
A box shaped air fluid domain with boolean subtraction of small box.
Air domain size is 1m x 1m x 1.4m and small void box is 0.1m x 0.1m x 0.055m.
I applied the pressure inlet boundary conditions on the bottom and sides of air domain and pressure outlet on top of air domain.(I applied these BC as open atmosphere. If it is not really suitable, please let me know)
Gravity is in the -z direction.
On the small void box, fixed temperature of 800K is posed on top of box and the others are insulated.
After static simulation, I would like to see 800K on top of box and temperature gradient from 800K to 300K (ambient) as z increases from hot surface.
But static simulation shows almost 300K on top of hot surface, meaning that 800K cooled down to 300K.
Could you let me know how to maintain 800K in the static simulation?
Radiation was turned off just for practice.
Thank you.