In-situ high temperature study of the long-term stability and dielectric properties of nanofluids based on TiO2 and SiC dispersions in natural ester oil at various concentrations

Exploring the impact of particle stability, size, and morphology on nanofluid thermal conductivity: A comprehensive review for energy applications

Recent advancements enhance the efficiency of energy conversion processes and leverage nanofluids-novel thermal fluids with nanoparticles (under 100 nm) suspended in conventional fluids. These nanofluids significantly alter thermophysical properties, notably thermal conductivity, which is crucial for evaluating their thermal performance. Despite three decades of intensive research, disagreements persist due to a lack of comprehensive data on how particle size, shape, stability, and others influence thermal conductivity. This review tries to fill this literature gap by critically reviewing how the characteristics that distinguish nanofluids from their micrometer-sized counterparts affect the stability and convective heat transfer. The study compares experimental results in a systemic way that addresses the reported inconsistencies and provides a general summary of the thermal behavior of nanofluids in energy systems. It has also pointed out the lack of reliable hybrid models considering all parameters affecting thermal conductivity. The current study assembles data from different analyses showing that a particle size within the 10-50 nm range may enhance thermal conductivity, depending on the base-fluid used. Likewise, the morphological options available, namely, spherical, ellipsoid, platelet, and blade-like, all have given promise for enhancing thermal conductivity, hence considering morphological issues. Finally, stability, defined by the zeta potential analyses, forms a vital criterion for the long-term sustainability of these enhancements. By consolidating experimental results across different research groups, this review highlights the variability and sometimes contradictory findings in thermal conductivity enhancements, ranging from negligible increases to over 50% improvement in specific nanofluids systems. The absence of reliable hybrid models encapsulating all influencing parameters for predicting thermal conductivity is critically addressed. It is concluded by identifying the main challenges in the field and offering recommendations for standardizing measurement techniques, which include the need for a unified model capable of predicting thermal conductivity enhancements with an accuracy of ±5%.

Unveiling the Micro-Mechanism of Functional Group Regulation for Enhanced Dielectric Properties in Novel Natural Ester Insulating Oil TME-C10

The functional groups in the molecular structure of natural ester insulating oil have a significant impact on its physicochemical and electrical properties. This article takes the novel synthetic ester TME-C10 and traditional natural ester GT molecules as research objects, and based on density functional theory (DFT) calculations, systematically explores the micro-mechanism of the effects of C=C double bonds, ester groups (-COOC), and β-H groups on the performance of insulating oils. The results show that the chemical stability and anti-aging ability of the TME-C10 molecule are significantly improved by eliminating the C=C double bond and β-H group. The electronic behavior of the TME-C10 molecule is mainly controlled by the ester group (-COOC), while the GT molecule is significantly affected by the unsaturated C=C double bond, resulting in a significant difference in the mode of electronic transition between the two molecules: the TME-C10 molecule shows the n→σ∗ transition, while the GT molecule is the π→π∗ transition. In addition, the HOMO orbital energy level, electron transition energy, and ionization energy of the GT molecules are lower than those of the TME-C10 molecules. Under the action of an external electric field, the TME-C10 molecules exhibit excellent dielectric properties. In summary, the TME-C10 molecules not only overcome the aging defects of traditional natural ester insulating oils, but also possess excellent insulation properties, making it a new type of insulating oil material with broad application prospects.

Analytical Solutions for Electroosmotic Flow and Heat Transfer Characteristics of Nanofluids in Circular Cylindrical Microchannels with Slip-Dependent Zeta Potential Considering Thermal Radiative Effects

This study analyzes the impact of slip-dependent zeta potential on the heat transfer characteristics of nanofluids in cylindrical microchannels with consideration of thermal radiation effects. An analytical model is developed, accounting for the coupling between surface potential and interfacial slip. The linearized Poisson-Boltzmann equation, along with the momentum and energy conservation equations, is solved analytically to obtain the electrical potential field, velocity field, temperature distribution, and Nusselt number for both slip-dependent (SD) and slip-independent (SI) zeta potentials. Subsequently, the effects of key parameters, including electric double-layer (EDL) thickness, slip length, nanoparticle volume fraction, thermal radiation parameters, and Brinkman number, on the velocity field, temperature field, and Nusselt number are discussed. The results show that the velocity is consistently higher for the SD zeta potential compared to the SI zeta potential. Meanwhile, the temperature for the SD case is higher than that for the SI case at lower Brinkman numbers, particularly for a thinner EDL. However, an inverse trend is observed at higher Brinkman numbers. Similar trends are observed for the Nusselt number under both SD and SI zeta potential conditions at different Brinkman numbers. Furthermore, for a thinner EDL, the differences in flow velocity, temperature, and Nusselt number between the SD and SI conditions are more pronounced.

Statistical analysis of the impact of FeO3 and ZnO nanoparticles on the physicochemical and dielectric performance of monoester-based nanofluids

This article deals with a comparative study of the physicochemical and electrical properties of monoesters of castor oil compared with their counterparts based on FeO₃ and Z_nO nanoparticles. The results are also compared with those in the literature on triesters, and also with the recommendations of the IEEE C 57.14 standard. The data is analysed statistically using a goodness-of-fit test. The analysis of the viscosity data at 40 °C shows an increase in viscosity. For concentrations of 0.10 wt%, 0.15 wt% and 0.20 wt% these are respectively 5.4%, 9.69%, 12.9% for F_eO₃ NFs and 7.6%, 9.91% and 12.7% for Z_nO NFs. For the same concentrations, the increase in acid number is respectively 3.2%, 2.9%, 2.5% for FeO3 samples and 3.18%, 2.0%, 1.2% for Z_nO samples. For the same concentrations, the fire point shows an increment of 4%, 3% and 2% for F_eO₃ samples and a regression of 8.75%, 6.88% and 5.63% for Z_nO samples. As for the breakdown voltage, for the same concentrations we observe respectively an increment of 43%, 27%, 34% for the F_eO₃. The results show an improvement on partial discharge inception voltage with FeO3 of 24%, 8.13% and 15.21% respectively for the concentrations 0.10 wt%, 0.15 wt% and 0.20 wt%.