Flow in thermomagnetic energy conversion loops

Abstract

Ferrofluids are colloidal suspensions of single domain superparamagnetic nanoparticles, typically of the order of 10 nm in diameter, in nonmagnetic liquids. In absence of an external magnetic field the fluid does not show any magnetic behavior due to random thermal Brownian motion of the magnetic nanoparticles. However, the fluid bulk exhibits magnetic polarization when exposed to a magnetic field. Temperature-sensitive magnetic fluid experiences a change in magnetic susceptibility as its temperature changes. Under combined influence of external magnetic and temperature field gradients, a Kelvin body force is established in a ferrofluid bulk such that the high temperature ferrofluid is repelled away and colder ferrofluid is attracted towards the region of stronger magnetic field. This thermomagnetic advection causes enhanced convective heat transfer in ferrofluids. Herein a numerical study of thermomagnetic convection in a rectangular annulus (formed by placing a solid block inside a larger enclosure) is presented. Magnetic field is created by a series of small permanent magnets in a regular or a Halbach array. Heat transfer characteristics under different thermal and magnetic arrangements are presented. The configuration is suitable for a host of engineering applications, e.g., electronics cooling or ferrofluid-based solar thermal or a thermomagnetic energy conversion loop.