Effect of different magnetic field angles on the relationship between nanofluid concentration and heat transfer

Abstract

The present study performs a numerical analysis for heat transfer performance of natural convection inside a cavity filled with nanofluids of Al2O3-water under an external magnetic field. Adding nanoparticles to a fluid will increase both thermal conductivity and fluid viscosity. However, the influence of magnetic field angles from a complete period of 0 to π on the relationship between the nanoparticle concentration and heat transfer remains unclear. The governing equations are solved by finite-volume methods and a two-phase mixture model is adopted. From detailed simulations, heat transfer patterns of nanofluids can be divided into three groups according to the magnetic strength: Type-A (Peak mode), Type-B (Critical zone), and Type-C (Increasing mode). The influences of magnetic field angles on the critical Hartmann number (Hacr) for the transition of heat transfer modes are also analyzed, which is not available in literature. For the case of Ha = 175 and φ = 2.5%, the Smax and Numean reduce about 48.1% and 14.6%, respectively, as θ changes from 45° to 135°, and the larger Ha number is needed for the transition of heat transfer pattern. Finally, one empirical formula with a determination coefficient 0.96 is proposed to predict Hacr under various magnetic field orientations.