Question

. Consider a packed bed of 75 mm diameter aluminium spheres (ρ = 2700 kg/m3, c = 950

J/kg K, k = 240 W/m K) and a charging process for which gas enters the storage unit at a

temperature of 300 °C. If the initial temperature of the spheres is 25 °C and the convection

coefficient is h = 75 W/m2 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

aluminium? [Copper properties are: ρ = 8900 kg/m3, c = 400 J/kg K, k = 400 W/m K]​

Answer 1

Answer:

Thermal energy storage systems commonly involve a packed bed of solid spheres, through which a hot gas flows if the system is being charged, or a cold gas if it is being discharged. In a charging process, heat transfer from the hot gas increases thermal energy stored within the colder spheres; during discharge, the stored energy decreases as heat is transferred from the warmer spheres to the cooler gas. Consider a packed bed of 75-mm-diameter aluminum spheres (p = 2,700 kg/m^3; c = 950 J/kg*K; k = 240 W/m*K) and a charging process for which gas enters the storage unit at a temperature of 300 degrees C. The initial temperature of the spheres is Ti = 25 degrees C and the convection heat transfer coefficient is h = 75 W/m^2*K.