Enhancement of pool boiling heat transfer using ferromagnetic beads in a variable magnetic field

• Published in: Applied thermal engineering 2020-01, Vol.164, p.114439, Article 114439
• Main Authors: Rahmati, Pouria, Ebrahimi-Dehshali, Massoud, Hakkaki-Fard, Ali
• Format: Article
• Language: English
• Subjects: Beads; Coils; Cooling; Cooling systems; Ferromagnetism; Fluid dynamics; Fluid flow; Heat transfer; Heat transfer coefficients; Magnetic fields; System effectiveness; Working fluids
• ISSN: 1359-4311; 1873-5606
• DOI: 10.1016/j.applthermaleng.2019.114439

•Pool boiling by using ferromagnetic beads in a variable magnetic field is studied.•The heat transfer coefficient is enhanced using the proposed method.•An optimum number of beads is determined by testing different situations.•The proposed pulsating adjustable magnetic field facilitated the movement of beads. The pool boiling is one of the heat transfer mechanisms used in cooling systems. Using an external force to improve the efficiency of thermo-fluid systems is among the most effective approaches, and also highly applicable in enhancing heat transfer. In this study, the pool boiling of water, as the working fluid, in the presence of a variable magnetic field, is studied. To this end, ferromagnetic beads were placed on the boiling surface, and the effect of their movements due to a variable magnetic field was observed and measured. The number of beads was selected such that they would cover 1/3, 1/2 and 2/3 of the surface in different cases and the magnetic field was controlled by altering coils input voltage. The obtained results indicated that in the absence of a magnetic field, the boiling heat transfer coefficient does not vary significantly with the number of beads. However, when the variable magnetic field is applied, the boiling heat transfer coefficient increases due to the mixing induced by the movement of the beads and the consequent turbulence. The present results also revealed that there is an optimum number of beads that maximizes the boiling heat transfer coefficient. Increasing the number of beads disrupts their free motion and delays bubble departure from the boiling surface and as a result the heat transfer coefficient decreases. By applying the optimum number of beads and variable magnetic field, the heat transfer coefficient was enhanced up to 32% on average. Moreover, in this study, the effect of magnetic field frequency on the heat transfer coefficient was also investigated.