A flat wall of firestone has thickness δ = 0.325 m and material properties λ = 1.15W/K m, a = 0.613 · 10−6 m2/s. One of its surfaces is insulated, whilst heat is transferred to the surroundings (ϑS =...

A flat wall of firestone has thickness δ = 0.325 m and material properties λ = 1.15W/K m, a = 0.613 · 10−6 m2/s. One of its surfaces is insulated, whilst heat is transferred to the surroundings (ϑS = 20.0 ◦C) at the other. The wall has an initial temperature ϑ0 = 180.0 ◦C. At time t1 the temperature of the non-insulated surface is ϑW1 = 50.0 ◦C; at time t2 = t1 + 8.50 h the surface temperature has fallen to ϑW2 = 41.9 ◦C.

 a) What value has the heat transfer coefficient α decisive for the cooling?

 b) Find the temperatures of the insulated surface at times t1 and t2.