Nanofluids are suspensions of nanoparticles in fluids that exhibit a noticeable enhancement of their thermal and physical properties even at moderate nanoparticle concentrations.
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Mixing nanoparticles with the base liquid alters the viscosity and density of the nanofluid, thus affecting its rheology. In most cases, the use of nanofluids requires stable flow conditions. Hence, understanding the rheological properties of the nanofluids is of critical importance for the further development of their practical applications.
Nanofluids are a relatively new class of colloidal suspensions that consist of nano-sized particles (with at least one of their principal dimensions in the range of 1-100 nm) mixed in a base fluid.
The nanoparticles can be metallic or non-metallic, such as oxides, carbides, ceramics, and carbon-based. Alternatively, a mixture of different nanoparticles or even nanoscale liquid droplets can be used to create a nanofluid. The base fluid may be a low-viscosity liquid like water, refrigerant, high-viscosity liquids, such as ethylene glycol, mineral oils, or a mixture of different types of liquids.
Surpassing the Thermophysical Properties of Conventional Fluids
The research on nanofluids initially started at the Argonne National Laboratory in the USA, where the term 'nanofluid' was first introduced in 1995.
At the early stages of their work, the researchers observed that adding nanoparticles to conventional fluids remarkably enhanced the thermal conductivity, thermal diffusivity, viscosity, and convective heat transfer coefficients compared to the base fluids.
Most of the early research focused on the nanofluids' thermal conductivity and was conducted under macroscopically static conditions (without flow).
However, various experiments revealed that the enhanced thermal conductivity might be coming at the cost of increased viscosity and density of the nanofluid. In addition, it became clear that the classical models often were unable to explain adequately the observed enhanced thermal conductivity and viscosity of the nanofluids.
Extensive research in the past decade revealed that the thermal behavior of these complex systems differed from solid-solid composites or standard solid-liquid suspensions.
Variables, such as the size, shape, and surface properties of the nanoparticles, affect the viscosity and thermal properties of the nanofluids, resulting in a surprisingly efficient heat transport compared to standard solid-liquid suspensions.
Owing to their enhanced thermophysical properties, nanofluids are regarded as promising candidates for advanced heat transfer applications, such as airconditioning, refrigeration, and power generation.
In such flow-related practical applications, the dependence of the nanofluid's viscosity 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 fluids, defined as the ratio between shear stress and shear strain (or the velocity gradient of the flow). It is associated with the fluid's resistance to deformation.
Heat Transfer and Rheological Behavior of Nanofluids
During the last decade, several studies of a wide range of nanofluids with a diverse composition, including copper oxide, and alumina nanoparticle suspended in water, have found that nanofluids containing less than 13% in volume of nanoparticles behave as Newtonian fluids.
In other words, the shear stress depends linearly on the shear strain, and the viscosity is independent of the flow conditions such as flow rate and pressure drop.
Under such conditions, increasing the nanoparticle concentration resulted in increased viscosity of the nanofluid. For example, nanofluids containing around 5% in volume of nanoparticles, exhibited a viscosity increase of more than 80%. Such an increase in viscosity is usually associated with increased pumping losses and pipe clogging when using Newtonian nanofluids in heat transfer applications.
Non-Newtonian Rheology Facilitates Flow
Further research revealed that higher concentrations of nanoparticles, particularly high aspect ratios objects like nanotubes and nanofibers, can dramatically alter the rheological properties of the nanofluids. Such nanofluids can exhibit non-Newtonia behavior where the viscosity decreases with the increase of the shear rate (so-called shear-thinning behavior).
In nanofluids, the non-Newtonian shear-thinning behavior is caused by several factors, all of them related to the structural reorganization of the fluid phases due to flow. The alignment of the highly anisotropic nanoparticles and the segregation of the different phases in the fluid results in a decreased viscosity.
Future Applications of Carbon-Based Hybrid Nanofluids
Recent developments have involved the investigation of carbon-based non-Newtonian hybrid nanofluids that contain a mixture of metal oxide nanoparticles and high aspect ratio graphene nanosheets or carbon nanotubes suspended in water, ethylene glycol, or propylene glycol.
Such complex nanofluids exhibited a combination of non-Newtonian shear-thinning and Newtonian behavior at high and low shear rates, respectively. By tuning the rheology of the hybrid nanofluids, the researchers were able to minimize the pressure drop under high flow conditions while still benefiting from the enhanced thermal conductivity of the nanofluid.
These non-Newtonian nanofluids are of particular interest for future heat transfer applications like solar heat collector systems and nuclear reactor cooling. Here, the heat transfer nanofluid can percolate through a porous medium under natural convection without hindrance due to excessively high viscosity.
Continue reading: Improving the Rheology of Drilling Fluid with Nanoparticles.
References and Further Reading
Ali, N., et al. (2021) Carbon-Based Nanofluids and Their Advances towards Heat Transfer Applications-A Review. Nanomaterials (Basel, Switzerland), 11(6), 1628. Available at: https://doi.org/10.3390/nano11061628
Sharma A.K., et al. (2020) Rheological Behaviour of Hybrid Nanofluids: A Review. In: Katiyar J., Ramkumar P., Rao T., Davim J. (eds) Tribology in Materials and Applications. Materials Forming, Machining and Tribology. Springer, Cham. Available at: 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 at: https://doi.org/10.1007/978-981-13-9833-9_10
Sohel Murshed, S. M., and Estellé, P. (2017) A state of the art review on viscosity of nanofluids. Renewable and Sustainable Energy Reviews, 76, 1134-1152. Available at: https://doi.org/10.1016/j.rser.2017.03.113
Sharma A.K., et al. (2016) Rheological behaviour of nanofluids: A review. Renewable and Sustainable Energy Reviews, 53, 779-791. Available at: https://doi.org/10.1016/j.rser.2015.09.033