Free convection of nanofluids in a porous sensible heat storage unit: Combined effect of time periodic heating and external magnetic field

Abstract

Nanofluids have gained much interest in recent years in heat transfer applications due to their improved thermal properties compared to conventional pure fluids. In many applications, the nanofluid may be subjected to unsteady heating and to the effect of external magnetic field, which can have a considerable effect on the behavior of the nanofluid, especially if the nanoparticles are metallic. In the present study, the flow and heat transfer characteristics of Cu/water nanofluid in a semi-circular cavity are analyzed. The fluid is heated by a heat source located at the bottom wall and having a time-periodic temperature profile. The other walls are adiabatic. Two magnetic sources are located on the top wall. The equations governing the flow and thermal behavior of the nanofluid are presented and converted into the dimensionless form. The equations are then solved by means of the finite volume method. The results are presented in the form of flow and thermal contours, as well as in the variation of the sensible heat, thermal stratification, and Nusselt numbers as functions of time. The impacts of various parameters, namely the temperature frequency, the magnetic number, the porosity, Darcy and Rayleigh numbers, on the results are discussed. In particular, it has been found that the porosity possessed the most important effect on the flow and thermal behaviors of the nanofluid. A 60% increase in Nusselt number is notably observed when the porosity is changed from 0.9 to 0.1.