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

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 pi 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 (H-acr) for the transition of heat transfer modes are also analyzed, which is not available in literature. For the case of Ha = 175 and phi = 2.5%, the S-max and Nu(mean )reduce about 48.1% and 14.6%, respectively, as theta 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 Ha(cr) under various magnetic field orientations.

Automated summary

This study conducts numerical analysis on the heat transfer performance of natural convection inside a cavity filled with nanofluids of Al2O3-water under an external magnetic field. It is found that the magnetic field angles have a significant impact on the relationship between nanoparticle concentration and heat transfer. The study classifies heat transfer patterns under different magnetic field angles and analyzes the critical Hartmann number, providing a basis for predicting the critical Hartmann number under magnetic field conditions.