(a) A 12.7 mm diameter copper sphere is suddenly immersed into an air stream. The initial temperature of the sphere is 66°C, and the air temperature is 27°C. A temperature sensor records the sphere surface temperature as 55°C after the sphere has been immersed in the airstream for a period of 69 seconds.
Note: For pure copper: $\rho = 8,933 \text{ kg/m}^3$, $c_p = 389 \text{ J/kgK}$, $k = 398 \text{ W/mK}$.
(i) Determine the convection heat transfer coefficient for air flowing over the sphere, stating all the assumptions made. (5 marks)
(ii) Determine the Biot number for the sphere and comment on its use in this situation. (3 marks)