Enhancement of thermal conductivity using magnetite and silver nanoparticles

Traditional heat transfer fluids such as water, oil and ethylene glycol mixtures were proven to be inherently poor heat transfer fluids by previous studies in this research area. However, in many high-technology industries, the most significant issue is to achieve the miniaturization of heat transfer devices on one hand and the increase in heat flux on the other. The usual enhancement techniques for heat transfer can hardly meet these challenges. Because of this, nanofluids are prepared by dispersing nanometer-sized solid particles in these traditional heat transfer fluids to increase the thermal conductivity and therefore the heat transfer performance. In this study, the mechanism of thermal conductivity enhancement of fluids in the presence of silver and magnetite nanoparticles is investigated. Colloidal silver with large surface area and high electrical and thermal conductivity and magnetite nanoparticles with superparamagnetic properties under applied external magnetic fields have attracted attention to study the thermophysical property enhancements. In order to achieve this, these metallic nanoparticles of known size, shape and concentration are synthesized and solubilized in polar and nonpolar solvents to obtain different nanofluids. Synthesized hydrophilic and hydrophobic silver and magnetite nanoparticles are found to have hydrodynamic diameters in considerable nanometer ranges and to be highly stable using Dynamic Light Scattering (DLS) and Transmission Electron Microscopy (TEM). Crystal structures of both nanoparticles are verified by X-Ray Diffraction (XRD) studies. The next step after characterization of nanoparticles is the measurement of thermophysical properties such as density, viscosity and thermal conductivity for all obtained nanofluids. Overlapping with the most previous studies, viscosity and density of base fluids indicate enhancements upon addition of nanoparticles. Thermal conductivity results vary depending on the type of dispersing medium. In nonpolar medium, thermal conductivity is found to increase with increasing the particle fraction; in contrast to that in polar medium, it is found to decrease for the same type of particle.