The Viscosity of Nanofluids: A Review of the Theoretical, Empirical, and Numerical Models

Abstract

The enhanced thermal characteristics of nanofluids have made it one of the fastest-growing research areas in the last decade. Numerous researches have shown the merits of nanofluids in heat transfer equipment. However, one of the problems is the increase in viscosity due to the suspension of nanoparticles. This viscosity increase is not desirable in the industry, especially when it involves flow, such as in heat exchanger or micro-channel applications INTRODUCTIONColloidal suspension dates back to Maxwell's study in 1873 [1]. Though the idea behind his study was vivid, the imposed problems were too enormous for profitable engineering solutions [2]. In 1995, Choi [3] came up with a pioneering idea based on Maxwell's study and suspended ultrafine particles (nanoparticles) in conventional heat transfer fluids. His invention has opened up a myriad of opportunities in research and development. Nanofluids descriptively are colloidal suspensions containing metallic (Ag, Au, Al, Cu, Ni, etc.), non-metallic (single-and multi-wall carbon nanotube (SWCNT and MWCNT), Si, Graphene, etc.), metallic oxide (Al2O3, CuO, NiO2,TiO2, etc.) and oxides of non-metals (SiO2, SiC, MgO, CaCO3, etc.) nanoparticles suspended in conventional heat transfer fluids such as water, engine oil, ethylene glycol, transformer oil, gear oil or mixture of two or more heat transfer fluids [2,3]. When compared with previous microparticle suspensions in conventional heat transfer fluids, it is a special type of fluid with numerous applications potentials because of its enhanced thermal conductivity, stability and homogeneity [3,4]. Microscale particles in suspensions lead to abrasion, clogging of flow paths, pressure drop and high pumping power requirements, therefore, its sustainability was impossible. Besides, nanofluids can reduce the pumping power in engineering equipment significantly and do not pose the problem of clogging and abrasion of equipment flow paths [5][6][7][8]. Therefore, the design and engineering of physical systems are now being tailored towards using nanofluid as working fluid.The impact of colloidal suspension cuts across the fields of science, biological science, medical, pharmaceutical and engineering. In the context of sustainable energy development and thermal management, nanofluids are becoming more and more significant as the need for efficient thermal management is of paramount importance. Moreover, the level of miniaturisation of devices today as technology advances is overwhelming. Devices such as microprocessors, microelectromechanical systems (MEMS), nanoelectromechanical systems (NEMS), microchannels and lab-on-chips come with high-density heat flux that needs quick heat removal for 3 efficient performance, stability and durability, which, from all indications, could be provided by nanofluids for now [9][10][11][12]. The following are also emerging areas of applications of nanoparticles and nanofluids: (i) they could be useful in medicine in the targeted treatment of malignant cells without damaging healthy tis...

Intriguingly high thermal conductivity increment for CuO nanowires contained nanofluids with low viscosity

“…It means that the nanofluids system has a good liquidity, which may be the result of the combined action of the base fluids and nanoparticles 55 . The low viscosity is of great significance for the flow of fluid and the process of heat transfer and mass transfer 56 .…”

Experimental investigation and model development for effective viscosity of MgO–ethylene glycol nanofluids by using dimensional analysis, FCM-ANFIS and GA-PNN techniques

“…The recent review of Meyer et al [40] gave a comprehensive coverage of the different viscosity models available for nanofluids and other suspensions alike. …”

“…Recently, Hemmat Esfe et al [21] investigated the influence of different MgO nanoparticles sizes (20,40,50 and 60 nm), temperatures (25-55 o C) and volume fractions (0.25-5%) on the thermal conductivity of MgO-EG nanofluids and proposed a correlation for predicting the thermal conductivity of the nanofluids. To the best knowledge of the authors there is no study on the viscosity of ethylene glycol based nanofluids containing MgO, which proposes new correlations including particle size, temperature and volume fraction as parameters.…”

Nanofluids revisited

“…It must be noted at this point that many more such correlations can be found in literature (see e.g. [12,68]). …”