STEP BY STEP SOLUTION

A circular disk of plastic, with a diameter of 0.2m and a thickness of 0.02m, is initially at a uniform temperature of T0=120C. It is cooled from both sides by air, at Ta=20C, flowing parallel to the circular faces of the disk. The air speed is 0.2m/s. (a) Find the convective heat transfer coefficient. Use Wedekind's correlation listed below, and use the physical properties of air at the average temperature between T0 and Ta. NuD=0.591ReD0.564Pr0.333 (b) Find the time required to cool the disk to Tf=21C. For the plastic, use =1200kg/m3, k=0.33W/(mK), and T=1.3107m2/s