EFFECTS OF DARCY AND REYNOLDS NUMBERS ON THE THERMAL BEHAVIOR OF A THERMOCLINE THERMAL STORAGE SYSTEM

Resumo

This work explores the transient behavior of different flow velocities and porosities on the
charging cycle of a thermocline thermal storage system for a concentrated solar power plant filled with
solid porous material. In order to accomplish this goal, a transient model describing turbulent flow in a
hybrid medium (porous/clear) with mixed convection was used. Macroscopic flow equations are
obtained using the concept of double-decomposition. Discretization of the governing equations was
performed with the SIMPLE method and the SIP procedure was used to relax the algebraic system of
equations. Turbulent flow was modelled using the k-ε turbulence. A two-energy equation model was
employed to assess heat transfer between solid and fluid phases. The analyzed geometry consists of an
axisymmetrical tank with convection in the exterior, hot air flow entering the system from the top and
clear regions at the top and bottom of the tank. Transient temperatures and storage and charge
efficiencies were evaluated for different Da and Re number values. Results indicated that increasing Re
values improve efficiency up to Re= 3.104

and further increases in Re slightly decreases thermal
efficiencies. Finally, it was found that decreasing the permeability of the system by either lowering
porosity or lowering particle diameter was beneficial for the thermal charge efficiency.