Enhancement of non-uniform magnetic field on saturated film boiling of magnetic nanofluid (MNF)

Previous studies pointed to the possibility of controlling magnetic nanofluid (MNF) boiling heat transfer by a magnetic field. In this study, the numerical model for the film boiling of the MNF was developed to explore the heat transfer and dynamics characteristics of MNF film boiling under the non-uniform magnetic field. Due to the existence of magnetic field intensity gradient, the dynamics of the separated bubbles in the non-uniform magnetic field was different from that in the uniform magnetic field. First, the single-mode film boiling was performed to study the dynamics of the separated bubble in detail. As the magnetic field intensity increased, the magnetic field force in the vertical direction increased, causing the bubble to escape and rise faster. Due to the non-uniformity of the magnetic field in the vertical and horizontal directions, the results of multi-mode film boiling influenced by the non-uniform magnetic field were different from those of single-mode film boiling. As a result of the horizontal component of the magnetic field force, the instability wavelength of film boiling became shorter and the bubble formation sites on the heated wall gradually deviated towards the central axis during the bubble generation stage. As the magnetic field intensity increased, the deviation of the bubble increased due to the increasing horizontal component of the magnetic field force. More importantly, the heat transfer performance of a MNF with a non-uniform magnetic field applied in this paper was enhanced with a higher magnetic field intensity. The enhancement effect is more pronounced in the film boiling for water near the critical pressure. (C) 2019 Elsevier Ltd. All rights reserved.