Nanofluids are suspensions of nanoparticles in liquids that show a noticeable improvement in their thermal and physical properties even at moderate nanoparticle concentrations.
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Mixing nanoparticles with the base fluid changes the viscosity and density of the nanofluid and thus influences its rheology. In most cases, the use of nanofluids requires stable flow conditions. Therefore, understanding the rheological properties of nanofluids is of crucial importance for the further development of their practical applications.
Nanofluids are a relatively new class of colloidal suspensions made up of nanoparticles (with at least one of their major dimensions in the 1-100 nm range) mixed into a base liquid.
The nanoparticles can be metallic or non-metallic, such as oxides, carbides, ceramics and based on carbon. Alternatively, a mixture of different nanoparticles or even nanoscale liquid droplets can be used to create a nanofluid. The base fluid can be a low-viscosity liquid such as water, refrigerant, high-viscosity liquids such as ethylene glycol, mineral oils or a mixture of different types of liquids.
Outperforms the thermophysical properties of conventional liquids
Research on nanofluids began at the Argonne National Laboratory in the USA, where the term “nanofluid” was first introduced in 1995.
In the early stages of their work, the researchers observed that the addition of nanoparticles to conventional fluids remarkably improved thermal conductivity, thermal conductivity, viscosity, and convective heat transfer coefficients compared to the base fluids.
Most of the early research focused on the thermal conductivity of nanofluids and was carried out under macroscopically static (no flow) conditions.
However, various experiments showed that the increased thermal conductivity could come at the expense of increased viscosity and density of the nanofluid. It also became clear that the classic models were often unable to adequately explain the observed increased thermal conductivity and viscosity of the nanofluids.
Extensive research over the past decade has shown that the thermal behavior of these complex systems differs from solid-solid composites or standard solid-liquid suspensions.
Variables such as size, shape and surface properties of the nanoparticles influence the viscosity and the thermal properties of the nanofluids, which leads to a surprisingly efficient heat transport compared to standard solid-liquid suspensions.
Because of their improved thermophysical properties, nanofluids are seen as promising candidates for advanced heat transfer applications such as air conditioning, cooling and power generation.
In such practical fluidic applications, the dependence of the viscosity of the nanofluid on the shear rate, which results from the fluid circulation under pressure, becomes an essential factor for the overall performance and efficiency of the heat transfer process.
Dynamic viscosity is an important transport property of liquids, defined as the ratio between shear stress and shear strain (or the velocity gradient of the flow). It is related to the fluid’s resistance to deformation.
Heat transfer and rheological behavior of nanofluids
Over the past decade, several studies with a wide range of nanofluids of varying composition, including copper oxide and alumina nanoparticles suspended in water, have shown that nanofluids with a volume of less than 13% nanoparticles behave like Newtonian liquids.
In other words, the shear stress depends linearly on the shear stress and the viscosity is independent of the flow conditions such as flow and pressure drop.
Under such conditions, an increase in the nanoparticle concentration led to an increased viscosity of the nanofluid. For example, nanofluids with a volume of about 5% of nanoparticles showed a viscosity increase of more than 80%. Such an increase in viscosity is usually associated with increased pumping losses and pipe clogging when Newtonian nanofluids are used in heat transfer applications.
Non-Newtonian rheology facilitates the flow
Further research showed that higher concentrations of nanoparticles, especially objects with a high aspect ratio such as nanotubes and nanofibers, can dramatically change the rheological properties of the nanofluids. Such nanofluids can exhibit a non-Newton behavior in which the viscosity decreases with increasing shear rate (so-called shear thinning behavior).
In nanofluids, the non-Newtonian shear thinning behavior is caused by several factors, all of which are related to the structural reorganization of the fluid phases by flow. The alignment of the strongly anisotropic nanoparticles and the segregation of the various phases in the fluid lead to a reduced viscosity.
Future applications of carbon-based hybrid nanofluids
Recent developments have included the study of non-Newtonian hybrid carbon-based nanofluids that contain a mixture of metal oxide nanoparticles and graphene nanosheets or high aspect ratio carbon nanotubes suspended in water, ethylene glycol, or propylene glycol.
Such complex nanofluids showed a combination of non-Newtonian shear thinning and Newtonian behavior at high or low shear rates. By coordinating the rheology of the hybrid nanofluids, the researchers were able to minimize the pressure drop under high flow conditions and at the same time benefit from the improved thermal conductivity of the nanofluid.
These non-Newtonian nanofluids are of particular interest for future heat transfer applications such as solar thermal collector systems and the cooling of nuclear reactors. The heat carrier nanofluid can percolate unhindered through a porous medium under natural convection due to excessively high viscosity.
Read on: Improving the rheology of drilling fluids with nanoparticles.
References and further reading
Ali, N., et al. (2021) Carbon-Based Nanofluids and Their Advances Towards Heat Transfer Applications – An Overview. Nanomaterialien (Basel, Switzerland), 11 (6), 1628.Available from: https://doi.org/10.3390/nano11061628
Sharma AK et al. (2020) Rheological Behavior of Hybrid Nanofluids: A Review. In: Katiyar J., Ramkumar P., Rao T., Davim J. (Eds.) Tribology in materials and applications. Material forming, machining and tribology. Springer, Cham. Available from: https://doi.org/10.1007/978-3-030-47451-5_4
Khan I., et al. (2019) Overview of Nanofluids to Ionanofluids: Applications and Challenges. In: Bhat A., Khan I., Jawaid M., Suliman F., Al-Lawati H., Al-Kindy S. (Eds.) Nanomaterials for Healthcare, Energy and Environment. Advanced Structured Materials, Vol. 118. Springer, Singapore. Available from: https://doi.org/10.1007/978-981-13-9833-9_10
Sohel Murshed, SM, and Estellé, P. (2017) An overview of the state of the art on the viscosity of nanofluids. Renewable and Sustainable Energy Reviews, 76, 1134-1152. Available from: https://doi.org/10.1016/j.rser.2017.03.113
Sharma AK et al. (2016) Rheological Behavior of Nanofluids: An Overview. Renewable and Sustainable Energy Reports, 53, 779-791. Available from: https://doi.org/10.1016/j.rser.2015.09.033
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