0018-9464 (c) 2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TMAG.2017.2705804, IEEE Transactions on Magnetics 0018-9464 © 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. (Inserted by IEEE.) 1 Power Generation Properties of Flow Nanogenerator with Mixture of Magnetic Nanofluid and Bubbles in Circulating System Su-Hun Kim, Jong-Hoo Park, Hong-Soon Choi, and Se-Hee Lee Department of Electrical Engineering, Kyungpook National University, Daegu 41566, Korea A method has been developed for demonstrating a flow nanogenerator by using a mixture of magnetic nanofluid (MNF) and bubbles in a fluid circulating system, and notable phenomena related to the power generation properties of the nanogenerator have been explored. MNF is widely used in various areas because of its interesting magnetic properties under an external magnetic field. The objective of the proposed technique is to obtain the induced electromotive force (EMF) based on Faraday’s law due to the flow of MNF in a closed circulating system. To maximize the induced EMF, magnetic nanoparticles (MNPs) should pass through the induction coil with a perpendicular magnetization direction in accordance with Faraday’s law. To control the magnetization direction of the MNPs, a permanent magnet was employed to produce an external magnetic field that considers the Brownian and Néel motions. To obtain a continuously induced voltage, a circulation system was implemented ensuring the flow of the MNF in the closed cycle. Further, power generation properties were investigated considering electric, magnetic, and fluidic effects. To analyze this complicated physics, a multiphysics analysis was used to calculate the flow pattern of the MNF according to its magnetic properties, and the acquired results were compared with those obtained from the experiment. From these experiments, we investigated the generation properties of the nanogenerator considering the flowrate of the MNF as well as the presence or absence of bubbles within the MNF. Our experimental tests demonstrated that the continuous power generation mode was successfully achieved with a mixture of MNF and bubbles. Index Terms—Magnetic nanofluid, Magnetic nanoparticle, Two-phase flow, Flow nanogenerator, Closed circulating system. I. INTRODUCTION agnetic nanofluids (MNFs) are composed of well- dispersed superparamagnetic nanoparticles of 10-nm size in a base fluid. When an external magnetic field is applied, MNFs exhibit magnetic properties [1]. These properties of MNFs have been employed in various applications such as heat transfer, sensors, micro total analysis systems (μ-TAS), nanofluidics, optical modulators, magneto–optical wavelength filters, drug delivery, and MRI contrast agents [2]–[4]. Recently, energy-related research has been expanding by using MNFs [5]–[9]. To create a more realistic power generation system, we have proposed a nanogenerator with a mixture of MNFs and bubbles as nonmagnetic materials in a closed-circulating system. The proposed nanogenerator system utilizes Faraday’s law to obtain induced electromotive force (EMF) with the flow of MNF [8]–[10]. To increase the power generation efficiency, the magnetization directions of the nanoparticles should be controlled by using an external magnetic field [1], [11]–[13]. For this purpose, we adopted a cylindrical permanent magnet. In addition, for the continuously generating mode, the bubbles as the nonmagnetic material were injected into the cycle to continuously induce change in the magnetic field with time. When an external magnetic field is applied to an MNF, the flow of the MNF, along with the bubbles floating inside the MNF, are affected by the magnetic force. Therefore, it is necessary to thoroughly understand the phenomenon occurring inside the MNF owing to the magnetic force. However, the opaque property of the MNF makes it challenging to observe its interior. Hence, a numerical approach was applied to observe the phenomenon occurring inside the MNF, which could predict the flow pattern of the MNF and the bubbles under the magnetic field. The numerical analysis results were verified by comparing the experimental results with the generated voltage. II. CONTINUOUS POWER-GENERATION MODE WITH A MIXTURE OF MAGNETIC NANOFLUID AND BUBBLES To generate electric power with the nanogenerator technique using an MNF, the continuous power generating mode may prove worthwhile for real-world applications. Therefore, we adopted a generating cycle in which a mixture of an MNF and bubbles is circulated with externally applied energy sources such as wasted heat or vibration. We essentially focused on the continuous power generation mode and explored the characteristics of the power generated in a completely closed cycle. When the flow channel is fully filled with the MNF, the MNPs will be continuously aligned and will pass through the coil loop as shown in Fig. 1(a). Thus, there is no field variation with time, and the induced EMF will be zero. (a) (b) Fig. 1. Schematic representation of the continuous power generation mode in the presence or absence of bubbles. M Corresponding author: Se-Hee Lee (shlees@knu.ac.kr). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier (inserted by IEEE)