Anomalous Size Dependent Rheological Behavior of Alumina Based Nanofluids

Rheological characteristics of alumina (Al2O3) nanofluids (NFs) were found to exhibit an unexpected behavior. Two base-fluids viz, water and ethylene glycols (EG) with particles of average diameter of 11, 45 and 150 nm were examined. An anomalous reduction in viscosity compared to that of the base fluid was seen for EG based NFs. However, viscosity reduction was absent in water based NFs. The inter-related effects of particle size, concentration and mode of dispersion (mono or poly-dispersed) were investigated. Particle migration under shear is attributed to the reduction of viscosity. The increase in bulk viscosity with particle size reduction is attributed to the surface forces acting between the particles and the medium in a suspension and the increase of effective volume with size. INTRODUCTION Improving the heat transfer characteristics using suspensions have been a recent excitement especially after the advent of nanofluids (NFs), which are essentially dilute suspensions of nanoparticles dispersed in a base fluid as defined by Das et al.(2008). The works done by Lee et al. (199), Das et al.(2003), Patel et al.(2003) and Prasher et. al.(2005) and many other researchers have demonstrated the anomalous heat transfer and thermal conductivity enhancement of these fluids in the recent past. However, the usage of such fluids demands a comprehensive picture of their flow properties and the physical basis of the effective viscosity of the suspension is one among the dominant properties as it governs the ease of flow and pressure drop during flow. In the viscosity study of Alumina-water NFs (particle size, 13 nm) done by Pak and Cho (1998), they showed higher viscosity ratio (ratio of the viscosity of the NF to that of the base fluid) compared to titanium oxide-water NF (particle size, 27 nm). The viscosity values were significantly larger than the values predicted by the analytical theory of Einstein (1911) for dilute solutions. Putra et al.(2003) demonstrated the Newtonian nature and the effect of temperature on viscosity of alumina water NFs. To see the effect of base fluid other than water, rheological data on alumina based NFs with varying diameters (27, 40 and 50 nm) suspended in propyleneglycol were reported by Prasher et al.(2006). The viscosity ratio was largely independent of particle sizes and the aggregation of particles in NFs was indicated to be the cause of under-prediction by Einstein’s equation. Similar studies were conducted with alumina-water NFs (36 and 47 nm) by Nuguen et al.(2008) and they observed a hysteresis phenomenon on viscosity measurement which sent alarms on the use of NFs in heat transfer applications. Lee et. al(2008) studied about the effective viscosity of Alumina-water NFs at low concentrations of 0.01 % − 0.3 %. They observed a non linier behaviour of viscosity with concentration. Based on the available literature it may be perceived that studies on viscosity of NFs are still sparse as only discrete combinations of particle size, particle-fluid combination and concentrations have been reported without any systematic observation. Many investigators have not taken it to consideration that their particles may have a large distribution of sizes which influences the underlying physics. The present letter reports some surprising observations on the effective viscosity of NFs while trying to investigate the effect of various parameters such as particle size, volume fraction, mode of dispersion and nature of base-fluids. 179 Volume 1 · Number 2 · 2010 EXPERIMENTAL METHODS Nanoparticles used in all the experiments were alumina while the two base-fluids used were water (W) and ethylene glycol (EG). Commercially purchased nano-powders (Nanophase Technologies Corporation and Inframat Advanced Materials) produced by physical methods were used to prepare the NFs. Three particle sizes (11, 45 and 150 nm), varying over an order of magnitude, were used to analyze the effect of particle size. The particles were dispersed in the base-fluids using several hours of ultrasonication and no stabilizers were used as they may change the rheological properties. Figure 1(a)(c) show the TEM images of typical Al2O3 particles in the ‘as received’ state. The average particle dimensions were measured by gas absorption method and were 11, 45 and 150 nm. Rheological properties of EG based NFs were measured using a rheometer (Anton Paar, Physica MCR 301) with a cone and plate geometry (50 mm diameter 2 ̊ angle with 47 μm gap) with the plate temperature controlled by a Peltier unit. The measurement was done at the rate of 50 readings in 10 second interval and the shear rate was varied from 50 s−1 to 250 s−1. The relative viscosities of water based NFs were measured using an Ubbelohde viscometer as the viscosity values were smaller for applying the rheometer with sufficient accuracy. The above comparative viscometer had a capillary bore of 0.5 mm. The ratio of efflux time of NF, tnano to that of base-fluid tbf was taken as the experimental relative viscosity ratio.