Researches on the nanofluids have been increased very rapidly over the past decade. In spite of some inconsistency in the reported results and insufficient understanding of the mechanism of the heat transfer in nanofluids, it has been emerged as a promising heat transfer fluid. In the continuation of nanofluids research, the researchers have also tried to use hybrid nanofluid recently, which is engineered by suspending dissimilar nanoparticles either in mixture or composite form. The idea of using hybrid nanofluids is to further improvement of heat transfer and pressure drop characteristics by trade-off between advantages and disadvantages of individual suspension, attributed to good aspect ratio, better thermal network and synergistic effect of nanomaterials. This review summarizes recent researches on synthesis, thermophysical properties, heat transfer and pressure drop characteristics, possible applications and challenges of hybrid nanofluids. Review showed that proper hybridization may make the hybrid nanofluids very promising for heat transfer enhancement, however, lot of research works is still needed in the fields of preparation and stability, characterization and applications to overcome the challenges.

A review on hybrid nanofluids: Recent research, development and applications

Nanofluids—a simple product of the emerging world of nanotechnology—are suspensions of nanoparticles (nominally 1–100 nm in size) in conventional base fluids such as water, oils, or glycols. Nanofluids have seen enormous growth in popularity since they were proposed by Choi in 1995. In the year 2011 alone, there were nearly 700 research articles where the term nanofluid was used in the title, showing rapid growth from 2006 (175) and 2001 (10). The first decade of nanofluid research was primarily focused on measuring and modeling fundamental thermophysical properties of nanofluids (thermal conductivity, density, viscosity, heat transfer coefficient). Recent research, however, explores the performance of nanofluids in a wide variety of other applications. Analyzing the available body of research to date, this article presents recent trends and future possibilities for nanofluids research and suggests which applications will see the most significant improvement from employing nanofluids.

Small particles, big impacts: A review of the diverse applications of nanofluids

Up to date review on the synthesis and thermophysical properties of hybrid nanofluids

Recently, the study of transformer oil-based nanofluids became of great interest, due to their prospective properties as a dielectric and cooling medium. However, agglomeration of nanoparticles limits the beneficial properties that can be obtained from nanofluids. So, the work presented in this paper aims to get enhanced dispersion behavior of nanoparticles within transformer oil-based nanofluids. Then, breakdown strength with the enhanced dispersion behavior is evaluated. In order to get enhanced dispersion behavior, nanofluids were prepared using nanoparticles with a surfactant. The surfactant plays a role in the stabilization of nanoparticles and maintaining suspension stability. The considered type of nanoparticles is TiO2 nanoparticles. Two series of nanofluid samples were prepared. Through the first series, the effect of surfactant concentration on dispersion behavior and agglomerate size was studied. The dispersion of nanoparticles was characterized using three different techniques. These techniques are optical microscope analysis, transmission electron microscope (TEM) analysis and zeta potential measurements. Based on these techniques, the suitable concentration of surfactant was identified. The second series of nanofluid samples was prepared with different weight percents of nanoparticles for assessing the breakdown strength. Weibull distribution was used to calculate the breakdown probability for the base oil as well as nanofluid samples. The results showed an enhancement in the breakdown strength by about 27% in comparison to the base oil. Based on the obtained results, mechanisms behind the dispersion behavior and breakdown strength were proposed and discussed.

Dispersion behavior and breakdown strength of transformer oil filled with TiO 2 nanoparticles

The mechanisms by which conductive and dielectric nanoparticles (NPs) trap electrons are explained by the potential well distribution caused by induced or polarized charges on NPs. Thus, the distributions of surface and saturation charges on conductive and dielectric NPs are determined. Given conductive Fe3O4, semiconductive TiO2, and dielectric Al2O3 NPs, insulation performance tests are conducted and ionization models of nanofluids (NFs) based on transformer oil are developed. These models are compared with those of NFs based on pure oil. The NP whose conductivity or permittivity does not match that of the dielectric liquid has a potential well and an increased amount of saturation charges on its interface. This NP influences streamer development strongly and enhances the breakdown of oilbased NF.

Effects of conductivity and permittivity of nanoparticle on transformer oil insulation performance: experiment and theory

In this paper, TiO2 semiconductive nanoparticles with a large relaxation time constant is added into transformer oil to form semiconductive nanofluids (SNFs), with the aim of enhancing insulating characteristics. ac, dc and lightning impulse breakdown voltage and partial discharge (PD) characteristics of oil samples before and after modification were measured according to ASTM standard methods. It was found that SNFs have ac, dc and lightning impulse breakdown voltage up to 1.2 times compared with pure oil. Meanwhile, the partial discharge resistance of SNFs was also dramatically improved. Charge trap and transportation characteristics of both samples have been measured by thermally stimulated current method (TSC) and pulse electroacoustic technique (PEA). It was found that electron shallow trap density and charge decay rate are greatly increased in semiconductive nanoparticles modified transformer oil. It is proposed that electron trapping and de-trapping processes in the shallow traps could be one of the main charge transport processes in dielectric liquids.

Effect of semiconductive nanoparticles on insulating performances of transformer oil

Transformer oil-based nanofluids with TiO2 semiconductive nanoparticles exhibit substantially higher AC and positive impulse breakdown voltage levels than that of pure transformer oils. Thermally stimulated current method (TSC) and pulse electroacoustic technique (PEA) have been used to measure charge trap and transportation characteristics of pure oils and nanofluids. It is found that electron shallow trap density and charge decay rate are greatly increased in nanofluids, i.e., fast electrons may be converted to slow electrons by electron trapping and de-trapping in shallow traps of nanofluids, resulting in improved breakdown performance compared to that of pure oil.

Effect of electron shallow trap on breakdown performance of transformer oil-based nanofluids

A new class of colloidal dielectric fluids is formulated by mixing TiO 2 nanoparticle with transformer oil in order to enhance the dielectric performance of mineral oil. To investigate the impact of this change, both AC and lightning breakdown voltage tests were carried out according to IEC standard respectively. The results of AC tests indicate that mean value and 1% probability breakdown voltage of nanofluid achieves an increase of 1.15 and 1.43 times that of transformer oil respectively. For lightning breakdown voltage, nanofluid was 13.3KV higher than that of transformer oil. It is considered to be mainly attributed to polarization of TiO 2 nanoparticles, which may act as electron traps in the process of electron transfer in electrically stressed nanofluids. Furthermore, the high specific surface area of nanoparticles also increases the probability of electron scattering in the nanofluids, which reduces the impact energy of electrons and prevents the oil from ionizing. All these results confirmed that transformer oil modified with TiO 2 nanoparticles hold a promise to improve its insulating properties.

Breakdown properties of transformer oil-based TiO 2 nanofluid

A new class of colloidal dielectric fluids is formulated with the objective of enhancing the dielectric performance of mineral oil for application in power system. TiO 2 nanoparticles modified by different organic reagents are suspended by means of sonication. Dielectric strength measurements are carried out under AC for a number of different colloidal systems with varying particle mass fraction. It was found that breakdown voltages under power frequency condition have been improved at the optimum composition. The enhancement of breakdown strength (50Hz) of transformer oil-based nanofluid is mainly attributed to dielectric property of TiO 2 nanoparticles, which may act as electron traps in the process of electron transfer in electrically stressed nanofluids. Furthermore, the high specific surface area of nanoparticles also increases the probability of electron scattering in the nanofluids, which reduces the impact energy of electrons and prevents the oil from ionizing. All these results confirmed that transformer oil modified with TiO 2 nanoparticles hold a promise to improve its insulating properties.

Effect of TiO 2 nanoparticles on the breakdown strength of transformer oil

Transmission and distribution transformers are a critical part of the electricity generation and distribution network. The potential consequence of transformer failure can cause operation disruption and financial losses. In recent years, magnetic nanoparticles have been used to enhance the dielectric breakdown strength of transformer oil. Considering the inherent magnetic property of the magnetite nanoparticles, their distribution and stability in the fluid under magnetic field is still a challenge. In this paper, a new class of colloidal dielectric fluids was explored by dispersing TiO 2 nanoparticles into transformer oil through ultrasonic treating. Breakdown voltage measurements under AC stresses were carried out for a number of different colloidal systems with varying nanoparticle content and surface modification. The observed results indicated that the addition of TiO 2 nanoparticles to transformer oil can enhance its dielectric strength and the surface compositions of the TiO 2 nanoparticles play an important role in its dielectric performance. The effect of nanoparticles' surface modification on the breakdown strength of colloidal systems has been investigated.

Xiao-xin Li

Papers

Effect of semiconductive nanoparticles on insulating performances of transformer oil

Effect of electron shallow trap on breakdown performance of transformer oil-based nanofluids

Breakdown properties of transformer oil-based TiO 2 nanofluid

Effect of TiO 2 nanoparticles on the breakdown strength of transformer oil

Preparation and breakdown strength of TiO 2 fluids based on transformer oil