Consider a packed bed of 75-mm-diameter aluminum spheres ( ϭ 2700 kg/m 3 , c ϭ 950 J/kg ⅐ K, k ϭ 240 W/m ⅐ K) and a charging process for which gas enters the storage unit at a temperature of T g, i ϭ 300 Њ C. If the initial temperature of the spheres is T i ϭ 25 Њ C and the convection coefficient is h ϭ 75 W/m 2 ⅐ K, how long does it take a sphere near the inlet of the system to accumulate 90% of the maximum possible thermal energy? What is the corresponding temperature at the center of the sphere? Is there any advantage to using copper instead of aluminum?