Free convection occurs between concentric spheres. The inner sphere is of diameter Di = 50 mm and tem- perature Ti = 50°C, while the outer sphere is main- tained at To = 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 environ- ment with air at Too = 20°C?