A Review on Nanofluids: Fabrication, Stability, and Thermophysical Properties

Abstract

Nanofluids have been receiving great attention in recent years due to their potential usage, not only as an enhanced thermophysical heat transfer fluid but also because of their great importance in applications such as drug delivery and oil recovery. Nevertheless, there are some challenges that need to be solved before nanofluids can become commercially acceptable. The main challenges of nanofluids are their stability and operational performance. Nanofluids stability is significantly important in order to maintain their thermophysical properties after fabrication for a long period of time. Therefore, enhancing nanofluids stability and understanding nanofluid behaviour are part of the chain needed to commercialise such type of advanced fluids. In this context, the aim of this article is to summarise the current progress on the study of nanofluids, such as the fabrication procedures, stability evaluation mechanism, stability enhancement procedures, nanofluids thermophysical properties, and current commercialisation challenges. Finally, the article identifies some possible opportunities for future research that can bridge the gap between in-lab research and commercialisation of nanofluids.

Aluminium Nanofluids Stability: A Comparison between the Conventional Two-Step Fabrication Approach and the Controlled Sonication Bath Temperature Method

“…Furthermore, the crystallite sizes (D hkl ) of the highest peaks of Al and α-Al 2 O 3 have shown to be about 50 nm, at (111), and 91 nm, at (042), respectively. The D hkl values were obtained using the Scherrer formula [17][18][19][20], which is demonstrated in Equation (4).…”

“…Each nanofluid sample was prepared by placing the NPs first inside the vial then injecting 20 mL of as-prepared water, using a disposable syringe, on top of the nanopowder after which the vial was tightly sealed using the provided caps. The concentrations of NPs used were 0.1, 0.5, and 1.0 vol.%, for each experimental setup, which was calculated by using the mixing theory (Equations (1) and (2)) that is widely used by many researchers in the field [4,15].…”

“…where filled with water to the recommended operating level by the manufacturer, to agitate the mixture. This kind of particle dispersion method is known as the two-step approach, which is a common procedure used for the production of nanofluids by many researchers [4,16]. The fabrication process then took one of the following two routes: (1) conventional two-step method, where the sonicator bath temperature initially starts at 25°C and ends at 54°C, without external interference ( Figure 2); and (2) controlled temperature two-step approach, where the device bath temperature was controlled for a set of temperatures from 10°C to 60°C, with a margin of ±1°C.…”

“…This is because the nanofluid effective thermal conductivity, which is the net thermal conductivity of the mixture, is seen to have a value within the range of the particles and the hosting fluid thermal conductivities. The highest effective thermal conductivity in any fabricated nanofluid can be achieved by optimizing the colloidal stability, which means that the dispersion of NPs needs to be maintained in a homogeneous manner at all time; otherwise, its thermal, along with the physical, properties will gradually degrade [4].…”

“…Several methods were developed to evaluate the stability of nanofluids, such as (1) zeta potential analysis, (2) centrifugation method, (3) spectral analysis approach, (4) 3ω-method, (5) electron microscopy analysis, and (6) sedimentation photograph capturing method [4]. From all of the aforementioned evaluation approaches, the sedimentation photograph capturing method is considered to be the simplest, cheapest, and is a reliable approach for measuring nanofluid stability [7,8].…”