Transport properties of alumina nanofluids

Abstract

Recent studies have showed that nanofluids have significantly greater thermal conductivitycompared to their base fluids. Large surface area to volume ratio and certain effects ofBrownian motion of nanoparticles are believed to be the main factors for the significantincrease in the thermal conductivity of nanofluids. In this paper all three transportproperties, namely thermal conductivity, electrical conductivity and viscosity, were studiedfor alumina nanofluid (aluminum oxide nanoparticles in water). Experiments wereperformed both as a function of volumetric concentration (3–8%) and temperature(2–50 °C). Alumina nanoparticles with a mean diameter of 36 nm were dispersed in water. Theeffect of particle size was not studied. The transient hot wire method as describedby Nagaska and Nagashima for electrically conducting fluids was used to testthe thermal conductivity. In this work, an insulated platinum wire of 0.003 inchdiameter was used. Initial calibration was performed using de-ionized water andthe resulting data was within 2.5% of standard thermal conductivity values forwater. The thermal conductivity of alumina nanofluid increased with bothincrease in temperature and concentration. A maximum thermal conductivity of0.7351 W m−1 K−1 was recorded for an 8.47% volume concentration of alumina nanoparticles at46.6 °C. The effective thermal conductivity at this concentration and temperature was observed tobe 1.1501, which translates to an increase in thermal conductivity by 22% when comparedto water at room temperature. Alumina being a good conductor of electricity, aluminananofluid displays an increasing trend in electrical conductivity as volumetric concentrationincreases. A microprocessor-based conductivity/TDS meter was used to perform theelectrical conductivity experiments. After carefully calibrating the conductivitymeter’s glass probe with platinum tip, using a standard potassium chloride solution,readings were taken at various volumetric concentrations. A 3457.1% increase in theelectrical conductivity was measured for a small 1.44% volumetric concentrationof alumina nanoparticles in water. The highest value of electrical conductivity,314 µS cm−1, was recorded for a volumetric concentration of 8.47%. In the determination of thekinematic viscosity of alumina nanofluid, a standard kinematic viscometer with constanttemperature bath was used. Calibrated capillary viscometers were used to measure flowunder gravity at precisely controlled temperatures. The capillary viscometers werecalibrated with de-ionized water at different temperatures, and the resulting kinematicviscosity values were found to be within 3% of the standard published values. An increaseof 35.5% in the kinematic viscosity was observed for an 8.47% volumetric concentration ofalumina nanoparticles in water. The maximum kinematic viscosity of alumina nanofluid,2.901 42 mm2 s−1, wasobtained at 0 °C for an 8.47% volumetric concentration of alumina nanoparticles. The experimentalresults of the present work will help researchers arrive at better theoretical models.