Free convection occurs between concentric spheres. The inner sphere is of diameter D_i = 50 mm and temperature T_i = 50°C, while the outer sphere is maintained at T_o = 20°C.  Air is in the gap between the spheres. What outer sphere diameter is required so that the convection heat transfer from the inner sphere is the same as if it were placed in a large, quiescent environment with air at T_∞=20°C?