Analysis of natural convection and the generation of entropy within an enclosure filled with nanofluid-packed structured pebble beds subjected to an external magnetic field and thermal radiation

The efficient heat transfer and energy storage during periods of excess energy production, like peak solar or wind power generation, is facilitated by the compact design and closely packed pebble bed thermal energy storage system. This study focuses on analyzing natural convection and entropy generation in a closed chamber filled with water/Al2O3-water nanofluid containing eight spherical pebbles arranged in a structured manner, referred to as packed beds while considering the presence of an external magnetic field and surface thermal radiation. The single-phase and two-phase models for nanofluid equations are solved using Ansys Fluent, employing a nondimensional approach for the calculations and presenting the results. Two cases with two-dimensional cavities in the presence of a magnetic field and conductive solid blocks are considered for validating the numerical method. Besides two-dimensional cases, another three-dimensional case is considered to evaluate heat transfer for the air-filled cubic cavity. The active parameters are the Hartmann number, Rayleigh number, and solid-toliquid thermal conductivity ratio. The findings are displayed for different parameters, encompassing the mean Nusselt number, generation of entropy, mean Bejan number, patterns of isotherms, velocity distribution, and localized entropy generation. The averaged Nusselt number decreases by approximately 2 % when applying a magnetic field. However, thermal radiation partially compensates for the negative effect of the strong magnetic field. At a Rayleigh number (Ra) of 106, entropy generation increases by 18 % due to radiation and by 78 % because of the magnetic field. The average Bejan number increases from approximately 0.02 to 0.36 while the Hartmann number increases from 0 to 100 for a single-phase nanofluid without radiation effect.