Effect of magnetic fields on thermal conductivity in a ferromagnetic packed bed Shahin Shafiee a,⇑ , Mary Helen McCay b , Sarada Kuravi c a Department of Mechanical and Aerospace Engineering, Florida Institute of Technology, Melbourne, FL 32901, USA b National Center for Hydrogen Research, Florida Institute of Technology, Melbourne, FL 32901, USA c Department of Mechanical and Aerospace Engineering, New Mexico State University, Las Cruces, NM 88003, USA article info Article history: Received 20 September 2016 Received in revised form 6 April 2017 Accepted 10 April 2017 Available online 12 April 2017 Keywords: Heat transfer Thermal conductivity Magnetism Magnetic field Porous bed Particles bed abstract Parameters like solid particle conductivity, shape, size, contraction/expansion and porosity along with saturating fluid conductivity and pressure are important parameters when studying effective thermal conductivity of a porous bed. This paper introduces a novel heat transfer enhancement method for ferro- magnetic material particle beds by exposing them to an external magnetic field. Two materials were experimentally studied: magnetite (Fe 3 O 4 ) and iron filings (random composition of iron oxides). A series of twelve trials were performed using different magnetic field configurations and intensities. The mag- netic fields which were used include non-uniform, semi-homogeneous and non-homogeneous fields which formed by four different magnet configurations with magnetic intensities from 819 Gauss to 4667 Gauss. In all the cases it is shown that applying magnetic fields increases the heat transfer rate in particle beds. The improvement rate is 7–15% for most cases studied. However, the configuration and intensity of the field determined the extent of improvement while non-homogeneous fields pro- duced the greater effect. Ó 2017 Elsevier Inc. All rights reserved. 1. Background Particulate beds have been the subject of study for many years because of their different applications. Some applications of porous media include catalytical and chemical particle beds, mass separa- tor units, thermal insulation, and debris beds [1]. Likewise, the beds which consist of magnetic particles find their application in magnetic fluids, catalysis, magnetic resonance imaging, and data storage. Magnetic particles are produced from different sources like iron oxides (Fe 3 O 4 and c – Fe 2 O 3 ), pure metals (Fe and Co), and alloys (CoPt 3 and FePt). Magnetic particles are chemically very active [2]. Another important characteristics of these particles is that, upon exposure to a magnetic field, they show the properties of supermagnetic materials and each particle acts as a small magnet. Several studies are available in literature that measure the heat transfer and flow in particle beds. Some of the references are dis- cussed here. Schroder et al. [3] measured the local heat transfer in wooden and slate porous beds. They measured both the particles and the filling gas temperatures in vicinity of the particles and used the data to calculate the local heat transfer coefficient. They also studied the effect of radiation by using a one-dimensional statisti- cal model. Xu et al. [4] performed experiments to study the fluid flow and effective thermal parameters in a column of randomly sized particles. Under a constant wall temperature condition, they studied the axial and radial temperature distributions. The param- eters which they considered in their study included particle diam- eter, particle thermal conductivity and fluid velocity. They suggested that a proper selection of good thermal conductive fill- ing material is important for enhanced heat transfer rates in a par- ticles bed. They also concluded that increased flow rate of the filling gas increases the effective thermal conductivity of the bed while smaller particle size has a significantly negative effect on thermal conductivity of the bed. Nsofor and Adebiyi [5] performed experimental measurements of the forced convection gas-particle heat transfer coefficient in a packed bed. They suggested a correlation for convective heat trans- fer in high temperature packed beds while remained concentrated on uncertainty analysis to ensure accuracy of their correlation. Srinivasan and Raghunandan [6] used a packed bed as a heat exchanger while a solid propellent gas generator was used to sup- ply room temperature gas to the bed. They experimentally studied the transient temperature response of the bed with varying inlet gas temperatures under turbulent gas flow regime. They suggested a procedure for calculating unsteady gas temperature at the outlet http://dx.doi.org/10.1016/j.expthermflusci.2017.04.014 0894-1777/Ó 2017 Elsevier Inc. All rights reserved. ⇑ Corresponding author. E-mail address: sshafiee2011@my.fit.edu (S. Shafiee). Experimental Thermal and Fluid Science 86 (2017) 160–167 Contents lists available at ScienceDirect Experimental Thermal and Fluid Science journal homepage: www.elsevier.com/locate/etfs