Spheres of 40-mm diameter heated to a uniformtemperature

Chapter 5, Problem 5.78

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Spheres of 40-mm diameter heated to a uniformtemperature of 400C are suddenly removed from theoven and placed in a forced-air bath operating at 25Cwith a convection coefficient of 300 W/m2?K on thesphere surfaces. The thermophysical properties of thesphere material are ??3000 kg/m3, c?850 J/kg?K,and k?15 W/m?K.(a) How long must the spheres remain in the air bathfor 80% of the thermal energy to be removed?(b) The spheres are then placed in a packing carton thatprevents further heat transfer to the environment.What uniform temperature will the spheres eventu-ally reach?

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