A copper sphere is immersed in an airflow where free stream temperature is 27 C and the heat transfer coefficient between sphere and stream is 35 W/mK. Find the diameter of the copper sphere if sphere temperatures are measured to be 66 C at initial condition and 55 C at 70 seconds later. The density, specific heat and thermal conductivity values for copper can be taken as 8933 kg/m, 389 J/kg and 398 W/mK respectively. Validate the lumped capacitance assumption.