Hot combustion gases of a furnace are flowing througha concrete chimney (k =1.4 w/m · °c) of rectangular cross section. the flow section of the chimney is 20 cm ×40 cm, and the thickness of the wall is 10 cm. the average temperatureof the hot gases in the chimney is ti=280°c, and the averageconvection heat transfer coefficient inside the chimney is hi=75 w/m2· °c. the chimney is losing heat from its outer surfaceto the ambient air at to=15°c by convection with a