Derive 3-D non-steady state conduction equation in spherical co-ordinates. Explain the assumption involved.

The hot combustion gases of a furnance are divided from the ambient air and its surroundings, which are at 25 ^oC, by a brick wall 0.15m thick. The brick has a thermal conductivity of 1.2W/mk and a surface emissivity of 0.8 . Under steady state conditions an outer surface temperature of 100 ^oC is calculated. Free convention heat transfer to the air adjoining the surface is divided by a convection coefficient of 20W/m²k. what is the brick inner surface temperature?