A circular disk of plastic, with a diameter of 0.2 m and a thickness of 0.02 m, 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 0.2 m/s.

(a) Find the convective heat transfer coefficient. Use Wedekinds correlation listed below, and use the physical properties of air at the average temperature between T0 and Ta. = 0.591 ^ 0.564 *r^0.333

(b) Find the time required to cool the disk surface to Tf = 50oC. For the plastic, use = 1200 kg/m^3, k = 0.34 W/(m*K), and T = 1.3 x 10-7 m^2/s.

A circular disk of plastic, with a diameter of 0.2 m and a thickness of 0.02 m, is initially at a 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 0.2 m/s. uniform temperature of To = (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 TO and Ta. Nup = 0.591 * Rep.564 Pr0.333 1200 kg/m, (b) Find the time required to cool the disk surface to T= 50C. For the plastic, use p k = 0.34 W/(m*K), and ar 1.3 x 10-7 m/s. =