1. Consider the packed bed of aluminum spheres described in Problem 5.12 under conditions for which the bed is charged by hot air with an inlet velocity of V = 1 m/s and temperature of Tg,i = 300°C, but for which the convection coefficient is not prescribed. If the porosity of the bed is s = 0.40 and the initial tem- perature of the spheres is Ti = 25°C, how long does it take a sphere near the inlet of the bed to accumulate 90% of its maximum possible energy?
2. The use of rock pile thermal energy storage systems has been considered for solar energy and industrial process heat applications. A particular system involves a cylindrical container, 2 m long by 1 m in diameter, in which nearly spherical rocks of 0.03-m diameter are packed. The bed has a void space of 0.42, and the den- sity and specific heat of the rock are p = 2300 kg/m3 and cp = 879 J/kg · K, respectively. Consider conditions for which atmospheric air is supplied to the rock pile at a steady flow rate of 1 kg/s and a temperature of 90°C. The air flows in the axial direction through the container. If the rock is at a temperature of 25°C, what is the total rate of heat transfer from the air to the rock pile?