Thermophysical properties of water ethylene glycol (WEG) mixture-based Fe₃O₄ nanofluids at low concentration and temperature

Highlights

• Preparation and characterization of Fe₃O₄ water/ethylene glycol (50/50 in vol%) based nanofluids

• Experimental investigation of thermal conductivity, dynamic viscosity and surface tension

• Influence of nanoparticle loading 0.01/0.1 vol% and temperature 253.15–293.15 K

• Enhanced properties relevant for cooling applications

Abstract

In the present work, Fe₃O₄ nanoparticles produced by the ultrasonic precipitation method and characterized by XRD, SEM, and BET methods are used to produce nanofluids using a mixture of water and ethylene glycol (WEG 50:50) as a base fluid and both sodium dodecyl sulfonate and oleic acid as surfactants. The thermal conductivity, dynamic viscosity and surface tension of these Fe₃O₄ nanofluids are experimentally evaluated for temperatures ranging from 253.15 to 293.15 K and different volume concentrations of nanoparticles, 0.01, 0.05 and 0.1% respectively. Experiments indicate that the thermophysical properties of nanofluids are strongly dependant on concentrations of nanoparticles and temperatures, particularly at sub-zero temperatures. Actually, it is shown that the thermal conductivity of nanofluids increases with almost 9.5%, and 14.3%, at 263.15 K and 293.15 K respectively, with 0.1 vol%. The thermal conductivity enhancement of nanofluids with concentration and temperature is compared to some relevant theoretical models. A good agreement is achieved with a comprehensive model taking into consideration effective medium theory, the nanolayer effect of molecules around the solid particle, Brownian motion of nanoparticles encompassing aggregation and nano-convection. It is also found that the dynamic viscosity of nanofluids decreases with nanoparticle content in particular below 273.15 K, up to 40% at 0.1% in volume. Surface tension decreases by adding the surfactant to the base fluid and then increases with Fe₃O₄ concentration with nearly 38% and 33% with 0.1% in nanoparticle volume fraction at 253.15 and 293.15 K, respectively. Finally, these results are promising in view of Fe₃O₄ nanofluids use in cooling applications.

Introduction

Nanofluids, now well-known as suspended nanoparticles in molecular liquids, are promising thermal liquids, because of their thermal transfer properties that are generally significantly better than base fluids [1]. Water, ethylene glycol, propylene glycol, engine oil, and refrigerants are commonly used in literature as base fluids. Nanofluids have special properties that make them conceivably valuable in many applications, such as microelectronics, engine transmission oil, refrigeration, drilling, lubrication, thermal storage, the improved heat transfer efficiency of chillers, engine cooling and hybrid-powered engines, domestic refrigerator-freezers, machining cooling, nuclear reactor, pharmaceutical processes, etc. [2]. Nanofluid thermal conductivities are usually higher than the ones of base fluid and the enhancement is linked to nanoparticle content, size, shape, nature… However, this is also generally coupled to viscosity enhancement that is detrimental in view of practical perspectives. Among all nanofluids, Fe₃O₄ nanofluids have great potential applications, because of unique magnetic properties that can be combined to thermal efficiency. Basically, Fe₃O₄ oil-based nanofluids were introduced by Akoh et al. [3]. Use of magnetic fluids for heat transfer applications has been reported previously in [[3], [4], [5]]. Li et al. [6] investigated the viscosity and the thermal conductivity of Fe₃O₄ nanoparticles dispersed in water-based fluid with a volume fraction from 1% to 5% wt. at 293.15 K. The impact of agglomeration and alignment of nanoparticles on the thermal physical properties of Fe₃O₄ nanofluids in the volume fraction range of 0.5–5% have been studied by Zhu et al. [7]. The outcomes demonstrated that Fe₃O₄ nanofluids from have higher thermal conductivities than various compound aqueous nanofluids at room temperature. They found that even at the volume fraction of 0.005 the thermal conductivity ratios increased by >15.0%. In wider context, the open literature shows that most researches have been performed at intermediate and high temperatures in particular to enhance the rate of heat transfer in heating applications. The use of nanofluids and their marketing in cooling industrial applications is still limited due to the weak development of nanofluid research at low temperatures [8]. Aladag et al. [9] studied Al₂O₃ and CNT water-based nanofluids at small concentrations for a range of temperatures from 271.15 to 283.15 K. Experiments demonstrated that the nanofluids behaved as either Newtonian or non-Newtonian fluids, based on the shear rate. Halelfadl et al. [10] reported the steady-state dynamic viscosity of water-based nanofluids based on multi-walled carbon nanotubes, taking into account the influence of particle volume fraction (0.0055% and 0.55%) and temperature from 273.15 to 313.15 K. Nanofluids behaved as shear-thinning materials for high particle content while the nanofluids are rather Newtonian for lower particle content. Water with ethylene glycol or polyethylene glycol is usually used as a base fluid at low temperature because of the low freezing point of water. Some researchers used water/ethylene glycol and water/propylene glycol mixtures as a base fluid for nanofluid preparation. Namburu et al. [11] studied the viscosity CuO nanofluid for 40:60% of W/EG mixture in the temperature range from 238.15 to 323.15 K. Kulkarni et al. [[11], [12], [13]] studied the effect of low temperature on viscosity of CuO, SiO₂, and Al₂O₃ water/ethylene glycol-based nanofluids in the same temperature range. Their results indicate that for higher temperatures, the nanofluids behave as Newtonian fluids and for lower temperature, a shear-thinning behaviour was obtained. They indicated that with the increase in temperature, the viscosity reduces exponentially. Naik et al. [14] measured the viscosity of CuO nanoparticles into water and propylene glycol (40:60 by weight) with different particle volume concentration of 0.025 to 1.2% at temperatures between 258.15 and 303.15 K. They noted an exponential increase in viscosity as temperature decreases. ϒ-Fe₂O₃ nanoparticles with different volume fractions from 0.005 to 0.02 were considered by Guo et al. [15] to produce nanofluids with a mixture of water and ethylene glycol with volume ratio of 55:45 in the temperature range from 263.15 to 333.15 K. Sundar et al. [16] investigated the viscosity measurement of Fe₃O₄ nanoparticles dispersed in water/ethylene glycol mixture at concentrations ranging from 0 to 1.0 vol%, the temperature range varying from 273.15 to 323.15 K. It was shown that nanofluid viscosity rises with volume concentration and decreases with temperature. Naik and Sundar [17] also prepared CuO nanofluid with a water/propylene glycol mixture (30:70%) as base fluid and noticed thermal conductivity enhancements of 10.9% and 43.37% for 1.2 vol% and at 298.15 and 338.15 K, respectively. Their results also indicated that the thermal conductivity of CuO nanofluids is improved with the increase in the concentration of CuO nanoparticles. Sundar et al. [18] also investigated the thermal conductivity and viscosity of Fe₃O₄ nanoparticles into 20:80% and 40:60% of propylene glycol and water mixture as based fluids in the temperatures of 273.15 and 333.15 K. Results show that the nanofluid thermal conductivity increases with nanoparticle content and temperature. Nanofluid viscosity also increases with concentrations of nanoparticles. The rheological measurements between 263.15 and 313.15 K for TiO₂ nanoparticle dispersions in polyethylene glycol were also reported by Yapici et al. [19].

In summary, this short literature overview shows that the thermophysical properties of nanofluids are still rarely investigated at low and sub-zero temperatures (in °C), in particular using Fe₃O₄ nanoparticles that are considered as emerging and promising candidates for nanofluid applications. Therefore, the aim of this paper is to investigate the thermophysical properties of Fe₃O₄ nanofluids such as the thermal conductivity, viscosity and surface tension. The nanofluids are presently produced with water-ethylene glycol mixture (WEG 50:50 by volume at 20 °C) and with sodium dodecyl sulfonate (SDS) and oleic acid (OA) as surfactants. Several concentrations in nanoparticles were considered, such as 0.01, 0.05 and 0.10 vol% respectively. Also, the nanofluid properties were experimentally evaluated in the temperature range from 253.15 to 293.15 K to verify the performance of such nanofluids in cold condition and demonstrate their potential as coolants.