Aluminum balls are heated up to 580 K and then exposed to air at 283 K to cool down with an average heat transfer coefficient of 25 W/m2·K. The balls are 3 cm in diameter. Find the temperature at r =...

Aluminum balls are heated up to 580 K and then exposed to air at 283 K to cool down with an average heat transfer coefficient of 25 W/m2·K. The balls are 3 cm in diameter. Find the temperature at r = 0.5 cm after 75 seconds of exposure. Is the lumped capacitance an acceptable solution here? [Ans.: 329 K].