A Critical Analysis of the Hot-wire Technique for Thermal Conductivity Measurements of Thermal Nanofluids

Abstract

Nanofluids (liquid mixtures with a small concentration of nanoparticles in suspension) have unique properties that make them potentially useful for heat transfer applications. Ceramic and carbon-based nanoparticles as well as nano-sized free metals and metal oxides exhibit good thermal conductivities and heat capacities. When suspended in a carrier fluid (usually with the aid of a dispersant), these materials may potentially improve the heat transfer characteristics of the carrier. The hot-wire technique is a well-developed and widely accepted method for measuring the thermal conductivity of a fluid. The aims of this work were to validate the accuracy of the hot-wire technique for thermal conductivity measurements of metal oxide nanofluids and to determine if the magnetic field that is induced by the current running through the wire influences the measured thermal conductivity. A 3D printed hot-wire cell was used for the experiments. The metal oxide nanoparticles tested were copper oxide and magnetite. The nanofluids were tested at weight loadings of 0.5 %, 1.0 % and 1.5 % and temperatures of 25 °C, 30 °C and 35 °C. To determine the effect of the magnetic field on the measured thermal conductivity additional control experiments were conducted using a double pipe apparatus, using distilled water as a validation medium. The results for the metal oxide nanofluids showed an expected increase in thermal conductivity with temperature and weight percentage of nanoparticles. There was a consistent positive bias in the measurement of the thermal conductivity using the hot-wire cell. Since this consistent bias was present for both the copper oxide (which was diamagnetic) and the magnetite (which was ferromagnetic) it was concluded that the magnetic field induced by the hot-wire was not of a high enough intensity to affect the measured thermal conductivity of the magnetite nanofluids.