Enhanced thermal conductivity of nanofluids: a state-of-the-art review

Adding small particles into a fluid in cooling and heating processes is one of the methods to increase the rate of heat transfer by convection between the fluid and the surface. In the past decade, a new class of fluids called nanofluids, in which particles of size 1–100 nm with high thermal conductivity are suspended in a conventional heat transfer base fluid, have been developed. It has been shown that nanofluids containing a small amount of metallic or nonmetallic particles, such as Al₂O₃, CuO, Cu, SiO₂, TiO₂, have increased thermal conductivity compared with the thermal conductivity of the base fluid. In this work, effective thermal conductivity models of nanofluids are reviewed and comparisons between experimental findings and theoretical predictions are made. The results show that there exist significant discrepancies among the experimental data available and between the experimental findings and the theoretical model predictions.     

Entropy generation analysis of different nanofluid flows in the space between two concentric horizontal pipes in the presence of magnetic field: Single-phase and two-phase approaches

In this paper, entropy generation analysis of different nanofluid flows in the space between two concentric horizontal pipes in the presence of magnetic field by using of single-phase and two-phase approaches was carried out. Single-phase model and two-phase model (mixture) are utilized to model the flow and heat transfer for Newtonian nanofluids in the space between two concentric horizontal tubes subjected to the magnetic field. The Reynolds and Hartman numbers ranges are 500 $\le Re\le$ 1500 and 0 $\le Ha\le$ 20, respectively. In this study, heat transfer of various nanofluids (Al₂O₃, TiO₂, ZnO and SiO₂) and their entropy generation have been investigated. The effect of diameter of particles (water-Al₂O₃ nanofluid) on heat transfer and entropy generation has also been studied. Average Nusselt number in terms of Hartman number and Reynolds number for different nanofluids for single-phase and two-phase models in various volume fractions, entropy generation due to friction, magnet and heat transfer in terms of radial direction for different Hartman numbers, Reynolds number and different nanofluids with different diameter of particles were obtained. We found that in all states, the Nusselt number is higher in two-phase model than in single-phase model. The maximum pressure difference for single- and two-phase models occurs at maximum volume fractions and Hartman number. Also, as the diameter of the nanoparticle increases, the result will be an increase in the temperature of the walls, leading to an increase in entropy generation. Also, as the Hartman number increases, the amount of entropy generation increases.     

Nanofluids thermal conductivity prediction applying a novel hybrid data-driven model validated using Monte Carlo-based sensitivity analysis

Accurate estimation of the thermal conductivity of nanofluids plays a key role in industrial heat transfer applications. Currently available experimental and empirical relationships can be used to estimate thermal conductivity. However, since the environmental conditions and properties of the nanofluids constituents are not considered these models cannot provide the expected accuracy and reliability for researchers. In this research, a robust hybrid artificial intelligence model was developed to accurately predict wide variety of relative thermal conductivity of nanofluids. In the new approach, the improved simulated annealing (ISA) was used to optimize the parameters of the least-squares support vector machine (LSSVM-ISA). The predictive model was developed using a data bank, consist of 1800 experimental data points for nanofluids from 32 references. The volume fraction, average size and thermal conductivity of nanoparticles, temperature and thermal conductivity of base fluid were selected as influent parameters and relative thermal conductivity was chosen as the output variable. In addition, the obtained results from the LSSVM-ISA were compared with the results of the radial basis function neural network (RBF-NN), K-nearest neighbors (KNN), and various existing experimental correlations models. The statistical analysis shows that the performance of the proposed hybrid predictor model for testing stage (R = 0.993, RMSE = 0.0207) is more reliable and efficient than those of the RBF-NN (R = 0.970, RMSE = 0.0416 W/m K), KNN (R = 0.931, RMSE = 0.068 W/m K) and all of the existing empirical correlations for estimating thermal conductivity of wide variety types of nanofluids. Finally, robustness and convergence analysis were conducted to evaluate the model reliability. A comprehensive sensitivity analysis using Monte Carlo simulation was carried out to identify the most significant variables of the developed models affecting the thermal conductivity predictions of nanofluids.

     

Erosion–corrosion synergism in an alumina/sea water nanofluid

Since nanofluids increase the thermal conductivity of a fluid mixture compared with the base fluid, it is important to investigate any damaging effects caused by the presence of the solid particles. Thus, this paper explores the nanofluid synergistic effects produced by the addition of 1 g dm^?3 Al₂O₃ nanoparticles to sea water and compares the performance with the base fluid without nanoparticles. Studies are conducted on carbon steel, using a hydrodynamically smooth-rotating cylinder electrode in turbulent flow at 298 K. The pure corrosion rate and erosion rate of carbon steel in the fluids free of nanoparticles are, respectively, higher (up to 82 %) and lower (ca. 11 %) than in the nanofluids. The synergistic effect of erosion and corrosion in a nanofluid is much higher (up to 237 %) than in the base fluid. These results indicate that the presence of nanoparticles in a flowing fluid could lead to considerable rates of material loss.     

A comparative study on particle�Cfluid interactions in micro and nanofluids of aluminium oxide

Stable dispersions of micro and nanosized Al₂O₃ particles in ethylene glycol are prepared with the aid of sonication. The temperature dependant acoustic properties such as ultrasonic velocity, adiabatic compressibility, attenuation and acoustic impedance are studied and reported in this paper. In microfluids the particle–fluid interaction is observed to decrease with increase of concentration of particles whereas in nanofluids it is observed to increase up to the critical concentration (0.6 Wt%) and above which the particle–particle interaction dominates due to agglomeration of particles. A range of concentration with significant particle–fluid interaction is identified for effective nanofluid applications.     

Turbulent mass transfer of Al2O3 and TiO2 electrolyte nanofluids in circular tube

Experimental study was performed to investigate turbulent mass transfer in straight circular tube. Electrochemical limiting diffusion current technique was used to measure the mass transfer coefficient in fully developed hydrodynamics and under developed mass transfer region. TiO₂ and γ-Al₂O₃ nanoparticles were added into the electrolyte solution (ES) to make electrolyte nanofluids (ENF). Measurements revealed that enhancement in mass transfer reaches 10 % in a 0.01 vol% γ-Al₂O₃/electrolyte nanofluid while 18 % in a 0.015 vol% TiO₂/electrolyte nanofluid relative to the base ES. Mass transfer coefficients increased with nanoparticles concentration up to an optimum concentration (0.01 % in γ-Al₂O₃/electrolyte nanofluid and 0.015 % in TiO₂/electrolyte nanofluid) while decreased by increasing nanoparticles concentration further. Enhancement ratio which is the ratio of the mass transfer coefficient of nanofluid to that of the base fluid was a function of nanoparticle concentration and was independent of Reynolds number. The mechanisms of nanoparticles Brownian motion and nanoparticles clustering were used to describe the behavior of the enhancement ratio in ENF.     

Nanofluid flow over a shrinking sheet with surface slip

In the present study, the effects of partial slip on mixed convection stagnation point flow and heat transfer of nanofluid impinging normally toward a shrinking sheet are investigated numerically. In particular, focus is on Cu–water and Al₂O₃–water nanofluids. Similarity transformation technique is adopted to obtain the self-similar ordinary differential equations and then solved numerically using Runge–Kutta–Fehlberg method with shooting technique. A parametric study is performed to explore the effects of various governing parameters on the fluid flow and heat transfer characteristics. Both the cases of assisting and opposing flows are considered. The physical aspects of the problem are highlighted and discussed.     

Analysis of magnetohydrodynamic flow and heat transfer of Cu–water nanofluid between parallel plates for different shapes of nanoparticles

The present study analyzes the heat transfer in the flow of copper–water nanofluids between parallel plates. For effective thermal conductivity of nanofluids, Hamilton and Crosser's model has been utilized to examine the flow by considering different shape factors. By employing the suitable similarity transformations, the equations governing the flow are transformed into a set of nonlinear ordinary differential equations. The resulting set of equations is solved numerically with the help of Runge–Kutta–Fehlberg numerical scheme. The graphical simulation presents the analysis of variations, in velocity and temperature profiles, for emerging parameters. A comprehensive discussion also accompanies the graphical results. Moreover, the effects of relevant parameters, on skin friction coefficient and Nusselt number, are highlighted graphically. It is noticed that the velocity field is an increasing function of all the parameters involved. Furthermore, the temperature of the fluid is maximum for the platelet-shaped particles followed by the cylinder- and brick-shaped particles.

     

Enhanced sensing characteristics in MEMS-based formaldehyde gas sensors

This paper presents a novel micro fabrication for formaldehyde gas sensors to enhance sensitivity and detection resolution capabilities. Therefore, two different types of fabrication sequences of gas sensors were considered, different positions of micro heaters and sensing layers to compare the effects of different areas of the sensing layers contact with the surrounding gas. The MEMS-based formaldehyde gas sensor consists of a quartz substrate, a thin-film NiO/Al₂O₃ sensing layer, an integrated Pt micro-hotplate, and Pt inter-digitized electrodes (IDEs) to measure the resistance variation of sensing layers caused by formaldehyde oxidation at the oxide surface. This abstract offers comparisons of the characteristics of sensors in different areas of the sensing layers contacting the surrounding gas as well as those to decrease the thickness of the sensing layer and deposits of the sensing layer using co-sputtering technology with NiO/Al₂O₃ to improve the sensitivity limits of the sensors. The experimental data indicated that increasing the area of the sensing layer that contacts with the surrounding gas and decreasing the thickness of the sensing layer in the sputtering process and then co-sputtered NiO/Al₂O₃ sensing layers, significantly enhanced the sensing characteristics of the developed formaldehyde sensor.     

Phase equilibria of the Al2O3–CaO–SiO2-(0%, 5%, 10%) MgO slag system for non-metallic inclusions control

Al₂O₃–CaO-(MgO–SiO₂) inclusions are one of the dominant inclusions in Al-deoxidized spring steel, the compositions changes of which are closely related to refining slags and deoxidization process. The Al₂O₃–CaO–SiO₂–MgO system can represent the primary ingredients of the Al₂O₃–CaO inclusions. According to analyzed compositions and predicted liquidus temperature ranges of inclusions and refining slag, equilibra experiments under high temperature, water quenching technique and subsquent electron probe X-ray microanalysis (EPMA) has been conducted to ascertain detailed thermodynamic database for inclusions control. Liquidus temperatures within the dominant phase fields of Ca₃SiO₅, Ca₂SiO₄, CaAl₂O₄, Ca₃Al₄O₉, spinel and MgO with the intervals of 20 °C from 1350 to 1560°C were identified. To further promote inclusions control, the influences of mass ratios of Mass(Al2O3)Mass(Al2O3+SiO2+CaO) and MgO contents on equilibrated phases and liquidus temperature changes have been explored. To further enhance modification levels of Al₂O₃–CaO-(MgO–SiO₂) system inclusions, it is suggested that refining time could be suitably prolonged.     

Thermochemical analysis for the reduction behavior of FeO in EAF slag via Aluminothermic Smelting Reduction (ASR) process: Part II. Effect of aluminum dross and lime fluxing on Fe and Mn recovery

We investigated Fe recovery from EAF slag by means of aluminothermic smelting reduction (ASR) at 1773 K with Al dross as the reductant, especially the effect of the added amount of the fluxing agent CaO on the Fe recovery. The maximum reaction temperature calculated using FactSage™ 7.0 decreased with increasing CaO addition, but the experimentally measured maximum temperatures increased with increasing CaO addition. We calculated the amounts of various phases before and after Al dross addition under different conditions of added CaO. FeO and Al₂O₃ contents in molten slag sharply varied within the first 5 min of the reaction, stabilizing soon thereafter. The aluminothermic reduction of FeO appeared to proceed rapidly and in good stoichiometric balance, based upon the mass balance between the consumption of FeO and MnO (ΔFeO and ΔMnO) and the production of Al₂O₃ (∆Al₂O₃). Iron recovery from EAF slag was maximized at about 90% when 40 g of CaO was added to 100 g slag. Furthermore, Mn could also be reduced from the EAF slags by the metallic Al in the Al dross reductant. The solid compounds of spinel (MgO∙Al₂O₃) and MgO were precipitated from the slag during the FeO reduction reaction, as confirmed by means of XRD analysis and thermochemical computations. To maximize Fe recovery from EAF slag, it is crucial to control the slag composition, namely to ensure high fluidity by suppressing the formation of solid compounds.     

Phase equilibria study in the system “Fe2O3”-ZnO-Al2O3-(PbO+CaO+SiO2) in air

Phase equilibria studies have been undertaken first time in the system PbO-“FeO_x”-CaO-SiO₂-ZnO-Al₂O₃. Pseudo-ternary section of the phase diagram “Fe₂O₃”-ZnO-(PbO + CaO + SiO₂) with CaO/SiO₂ weight ratio of 0.93 and PbO/(CaO + SiO₂) weight ratio of 2.0 at fixed 4 wt% Al₂O₃ has been constructed by equilibration and quenching technique. Zincite, spinel, melilite and Ca₂SiO₄ are the major primary phases in the temperature and composition ranges investigated. It was found that, Al₂O₃ stabilizes the spinel and melilite phases and extents their primary phase fields. The liquidus temperatures are increased in the spinel primary phase filed and decreased in the zincite primary phase field with increasing Al₂O₃. Significant difference of the liquidus temperatures is observed between the experimental data and FactSage calculations. Accurate compositions of the solid solutions have been determined in the present study together with the accurate temperature and liquid compositions. This study can provide direct support for industrial practice and optimization of the thermodynamic database.     

Investigation on the structural and spectral characteristics of deposited FBG stacks at elevated temperature

A composite nano-crystalline structured thin film was realized by depositing mixed Al₂O₃ and MgO coating material using physical vapor deposition approach and then annealing at high temperature. The film thus fabricated retains a high transmission even after annealing at 1500 °C. The grain size of less than 100 nm was measured by atomic force microscopy and the composite nano-crystalline structure of spinel and corundum was confirmed by the X-ray diffraction pattern analysis. Based on this, Bragg grating stacks were fabricated by depositing alternating quarter-wave layers of Al₂O₃ and Al₂O₃/MgO at the end of a sapphire crystal fiber at first and then the layers of NiO and Al₂O₃/MgO on the surface of a sapphire slice. The performance of the grating stacks at high temperature or after high temperature annealing was measured. It was found that the reflection peak measured from the grating stacks can survive a high temperature up to 1050 °C but will disappear after annealing at temperature of 1100 °C or above. A conclusion of inter-diffusion between layers of stack was obtained to explain the phenomenon of reflection peak disappearing after annealing at high temperature.     

Predicting the effects of nanoparticles on compressive strength of ash-based geopolymers by gene expression programming

In the present work, the effect of SiO₂ and Al₂O₃ nanoparticles on compressive strength of ash-based geopolymers with different mixtures of rice husk ash, fly ash, nanoalumina and nanosilica has been predicted by gene expression programming. The models were constructed by 12 input parameters, namely the water curing time, the rice husk ash content, the fly ash content, the water glass content, NaOH content, the water content, the aggregate content, SiO₂ nanoparticle content, Al₂O₃ nanoparticle content, oven curing temperature, oven curing time and test trial number. The value for the output layer was the compressive strength. According to the input parameters in gene expression programming models, the data were trained and tested, and the effects of SiO₂ and Al₂O₃ nanoparticles on compressive strength of the specimens were predicted with a tiny error. The results indicate that gene expression programming model is a powerful tool for predicting the effect of nanoparticles on compressive strength of the geopolymers in the considered range.     

Structural and physicochemical properties of silver-rich Ag–Al alloys

The X-ray diffraction, Scanning Electron Microscopy, Differential Scanning Calorimetry, dilatometric and electrical conductivity measurements were used to study the structural and physicochemical properties of selected silver-rich alloys from Ag–Al system. All the studied alloys, containing from 10 to 37 at. % of Al (Ag₉₀Al₁₀, Ag₈₅Al₁₅, Ag₇₇Al₂₃, Ag₇₅Al₂₅, Ag₇₂Al₂₈, Ag₇₀Al₃₀, Ag₆₃Al₃₇), were prepared from high purity metals by melting in a glove-box filled with a high purity argon atmosphere. The obtained X-ray diffraction patterns and microstructure observation of alloys containing up to 15 at. % of Al suggested that in this range only solid solution of silver exists. The thermal analysis showed heat effects related to phase transitions in Ag–Al system. In addition, the thermal expansion studies revealed an anomalous behavior in expansion for some composition of alloys associated with the phase transition. The electrical conductivity values rapidly changed, which may be associated with the formation of different phase areas in the Ag–Al system.Based on the results obtained in this work and critically reviewed literature data a thermodynamic re-optimization of the binary Ag–Al system using CALPHAD method was proposed. A good agreement between calculation and experiment was found.     

An investigation of dielectric material selection of RF-MEMS switches using Ashby’s methodology for RF applications

This paper presents a dielectric material selection methodology for RF-MEMS switch used for radio frequency applications. Here Ashby’s material selection approach is used to optimize the performance indices of RF-MEMS switch such as dielectric charging, stability, hold down voltage and RF performance. In this work, dielectric constant (ɛ _r), electrical resistivity (ρ), thermal conductivity (λ), thermal expansion coefficient (α), Young’s Modulus (E) are chosen as material indices of dielectric layer in RF-MEMS switch to evaluate the various performance indices. The Ashby’s material selection charts shows that Al₂O₃ and SiN are the best suitable material for dielectric layer in RF-MEMS switches to exhibit improved performance for radio frequency applications.

     

Phase equilibria studies of CaO-SiO2-Al2O3-Fe2O3-MgO system using CALPHAD

A detailed CALPHAD (CALculation of PHAse Diagram) based thermodynamic study on CaO- SiO₂-Al₂O₃-Fe₂O_3-MgO system (C-A-S-F-M) has been carried out to study the evolution of phases during heating and to investigate the effects of individual oxides. The important phases present at high temperature in the system are tricalcium silicate (C₃S), dicalcium silicate (C₂S), tricalcium aluminate (C₃A), dicalcium ferrite (C₂F) and a liquid phase. The evolution of liquid phase and other important phases predicted by CALPHAD closely matches with the experimental results. It was observed that in C-A-S-F-M system, there is a critical percentage of CaO and SiO₂ at which C₃S is maximum. Both Fe₂O₃ and Al₂O₃ significantly increase the amount of liquid phase. Both Fe₂O₃ and Al₂O₃ reduce C₃S and increase C₂S in the system. Up to 1.5 wt%, MgO also helps in increasing liquid phase. A mathematical model to predict phases has been developed as an alternative to the Bougue's equation which is very effective in predicting phase fractions.     

Microdroplet formation of water and nanofluids in heat-induced microfluidic T-junction

This paper reports experimental investigations on the droplet formation and size manipulation of deionized water (DIW) and nanofluids in a microfluidic T-junction at different temperatures. Investigations of the effect of microchannel depths on the droplet formation process showed that the smaller the depth of the channel the larger the increase of droplet size with temperature. Sample nanofluids were prepared by dispersing 0.1 volume percentage of titanium dioxide (TiO₂) nanoparticles of 15 nm and 10 nm × 40 nm in DIW for their droplet formation experiments. The heater temperature also affects the droplet formation process. Present results demonstrate that nanofluids exhibit different characteristics in droplet formation with the temperature. Addition of spherical-shaped TiO₂ (15 nm) nanoparticles in DIW results in much smaller droplet size compared to the cylindrical-shaped TiO₂ (10 nm × 40 nm) nanoparticles. Besides changing the interfacial properties of based fluid, nanoparticles can influence the droplet formation of nanofluids by introducing interfacial slip at the interface. Other than nanofluid with cylindrical-shaped nanoparticles, the droplet size was found to increase with increasing temperature.