Experimental and numerical investigation of heat transfer in a miniature heat sink utilizing silica nanofluid

In this paper, heat transfer characteristics of a miniature heat sink cooled by SiO₂–water nanofluids were investigated both experimentally and numerically. The heat sink was fabricated from aluminum and insulated by plexiglass cover plates. The heat sink consisted of an array of 4 mm diameter circular channels with a length of 40 mm. Tests were performed while inserting a 180 W/cm² heat flux to the bottom of heat sink and Reynolds numbers ranged from 400 to 2000. The three-dimensional heat transfer characteristics of the heat sink were analyzed numerically by solving conjugate heat transfer problem of thermally and hydrodynamically developing fluid flow. Experimental results showed that dispersing SiO₂ nanoparticles in water significantly increased the overall heat transfer coefficient while thermal resistance of heat sink was decreased up to 10%. Numerical results revealed that channel diameter, as well as heat sink height and number of channels in a heat sink have significant effects on the maximum temperature of heat sink. Finally, an artificial neural network (ANN) was used to simulate the heat sink performance based on these parameters. It was found that the results of ANN are in excellent agreement with the mathematical simulation and cover a wider range for evaluation of heat sink performance.     

Luminescence properties of Ge implanted SiO2:Ge and GeO2:Ge films

We have investigated cathodeluminescence (CL) of Ge implanted SiO₂:Ge and GeO₂:Ge films. The GeO₂ films were grown by oxidation of Ge substrate at 550 °C for 3 h in O₂ gas flow. The GeO₂ films on Ge substrate and SiO₂ films on Si substrate were implanted with Ge-negative ions. The implanted Ge atom concentrations in the films were ranging from 0.1 to 6.0 at%. To produce Ge nanoparticles the SiO₂:Ge films were thermally annealed at various temperatures of 600-900 °C for 1 h in N₂ gas flow. An XPS analysis has shown that the implanted Ge atoms were partly oxidized. CL was observed at wavelengths around 400 nm from the GeO₂ films before and after Ge⁻-implantation as well as from SiO₂:Ge films. After Ge⁻-implantation of about 0.5 at% the CL intensity has increased by about four times. However, the CL intensity from the GeO₂:Ge films was several orders of magnitude smaller than the intensity from the 800 °C-annealed SiO₂:Ge films with 0.5 at% of Ge atomic concentration. These results suggested that the luminescence was generated due to oxidation of Ge nanoparticles in the SiO₂:Ge films.     

Stabilized finite element method for incompressible flows with high Reynolds number

In the following paper, we discuss the exhaustive use and implementation of stabilization finite element methods for the resolution of the 3D time-dependent incompressible Navier–Stokes equations. The proposed method starts by the use of a finite element variational multiscale (VMS) method, which consists in here of a decomposition for both the velocity and the pressure fields into coarse/resolved scales and fine/unresolved scales. This choice of decomposition is shown to be favorable for simulating flows at high Reynolds number. We explore the behaviour and accuracy of the proposed approximation on three test cases. First, the lid-driven square cavity at Reynolds number up to 50,000 is compared with the highly resolved numerical simulations and second, the lid-driven cubic cavity up to Re = 12,000 is compared with the experimental data. Finally, we study the flow over a 2D backward-facing step at Re = 42,000. Results show that the present implementation is able to exhibit good stability and accuracy properties for high Reynolds number flows with unstructured meshes.     

Numerical study of mixed convection flow in a lid-driven cavity with sinusoidal heating on sidewalls using nanofluid

Mixed convection flow of Cu–water nanofluid inside a lid-driven square cavity with adiabatic horizontal walls and sinusoidal heating on sidewalls has been investigated numerically. The effects of increase in shear force for a fixed buoyancy force and effects of increase in buoyancy force for a fixed shear force were investigated. Effects of variations of Richardson number, phase deviation of sinusoidal heating, and volume fraction of nanoparticles on flow and temperature field were studied. The obtained results showed that for a constant Grashof number at all Richardson numbers, a clockwise eddy was developed inside the cavity, also the rate of heat transfer increases with decrease in Richardson number and increase of volume fraction of nanoparticles. For a constant Reynolds number the clockwise eddy is observed up to Ri = 1. For Ri = 10 a multicellular flow pattern is formed inside the cavity. Moreover it was found that when the Reynolds number is kept constant, the rate of heat transfer increases with increase in Richardson number.     

Heat transfer behaviour of nanofluids in a uniformly heated circular tube fitted with helical inserts in laminar flow

The CFD simulation of heat transfer characteristics of a nanofluid in a circular tube fitted with helical twist inserts under constant heat flux has been explained using Fluent version 6.3.26 in laminar flow. Al₂O₃ nanoparticles in water of 0.5%, 1.0% and 1.5% concentrations and helical twist inserts of twist ratios 2.93, 3.91 and 4.89 has been used for the simulation. All thermophysical properties of nanofluids are temperature dependent. The heat transfer enhancement increases with Reynolds number and decreases with twist ratio with maximum for the twist ratio 2.93. By comparing the heat transfer rates of water and nanofluids, the increase in Nusselt number is 5%–31% for different helical inserts and different volume concentrations. The maximum heat transfer enhancement is 31.29% for helical insert of twist ratio 2.93 and for the volume concentration of 1.5% corresponding to the Reynolds number of 2039. The data obtained by simulation match with the literature value of water with the discrepancy of less than ±10% for plain tube and tube fitted with helical tape inserts for Nusselt number.     

Comparative study on thermal performance of helical screw tape inserts in laminar flow using Al2O3/water and CuO/water nanofluids

This paper presents a comparison of thermal performance of helical screw tape inserts in laminar flow of Al₂O₃/water and CuO/water nanofluids through a straight circular duct with constant heat flux boundary condition. The helical screw tape inserts with twist ratios Y = 1.78, 2.44 and 3 were used in the experimental study using 0.1% volume concentration Al₂O₃/water and CuO/water nanofluids. Nanofluids with required volume concentration of 0.1% were prepared by dispersing specified amounts of Al₂O₃ and CuO nanoparticles in deionised water. The performance analysis of helical screw tape inserts in laminar flow of Al₂O₃/water and CuO/water nanofluids is done by evaluating thermal performance factor for constant pumping power condition. Thermal performance factor of helical screw tape inserts using CuO/water nanofluid is found to be higher when compared with the corresponding value using Al₂O₃/water. Therefore, the helical screw tape inserts show better thermal performance when used with CuO/water nanofluid than with Al₂O₃/water nanofluid.     

Intriguingly high convective heat transfer enhancement of nanofluid coolants in laminar flows

We reported on investigation of the convective heat transfer enhancement of nanofluids as coolants in laminar flows inside a circular copper tube with constant wall temperature. Nanofluids containing Al₂O₃, ZnO, TiO₂, and MgO nanoparticles were prepared with a mixture of 55 vol.% distilled water and 45 vol.% ethylene glycol as base fluid. It was found that the heat transfer behaviors of the nanofluids were highly depended on the volume fraction, average size, species of the suspended nanoparticles and the flow conditions. MgO, Al₂O₃, and ZnO nanofluids exhibited superior enhancements of heat transfer coefficient, with the highest enhancement up to 252% at a Reynolds number of 1000 for MgO nanofluid. Our results demonstrated that these oxide nanofluids might be promising alternatives for conventional coolants.     

Microstructural analysis of liquid and amorphous SiO2 nanoparticles

Microstructural properties of liquid and amorphous SiO₂ nanoparticles have been investigated via molecular dynamics (MD) simulations with the interatomic potentials that have weak Coulomb interaction and Morse-type short-range interaction under non-periodic boundary conditions. Structural properties of spherical nanoparticles with different sizes of 2, 4 and 6 nm obtained at 3500 K have been studied through partial radial distribution functions (PRDFs), coordination number and bond-angle distributions, and compared with those observed in the bulk. The core and surface structures of liquid SiO₂ nanoparticles have been studied in detail. We found significant size effects on structure of nanoparticles. Calculations also show that if the size is larger than 4 nm, liquid SiO₂ nanoparticles at the temperature of 3500 K have a lightly distorted tetrahedral network structure with the mean coordination number Z_Si-O≈4.0 and Z_O-Si≈2.0 like those observed in the bulk. Moreover, temperature dependence of structural defects and SiO_x stoichiometry in nanoparticles on cooling from the melt has been found and presented.     

Numerical study of heat transfer enhancement of counter nanofluids flow in rectangular microchannel heat exchanger

This paper reports a numerical analysis of the performance of a counter-flow rectangular shaped microchannel heat exchanger (MCHE) using nanofluids as the working fluids. Finite volume method was used to solve the three-dimensional steady, laminar developing flow and conjugate heat transfer in aluminum MCHE. The nanofluids used were Ag, Al₂O₃, CuO, SiO₂, and TiO₂ and the performance was compared with water. The thermal, flow fields and performance of the MCHE were analyzed using different nanofluids, different Reynolds numbers and different nanoparticle concentrations. Temperature profile, heat transfer coefficient, pressure profile, and wall shear stress were obtained from the simulations and the performance was discussed in terms of heat transfer rate, pumping power, effectiveness, and performance index. Results indicated enhanced performance with the usage of nanofluids, and slight penalty in pressure drop. The increase in Reynolds number caused an increase in the heat transfer rate and a decrease in the overall bulk temperature of the cold fluid. The increase in nanoparticle concentration also yielded better performance at the expense of increased pressure drop.     

Numerical investigation of natural convection heat transfer of nanofluids in a Γ shaped cavity

This paper analyzes the heat transfer and fluid flow of natural convection in a Γ shaped enclosure filled with Al₂O₃/Water nanofluid that operates under differentially heated walls. The Navier–Stokes and energy equations are solved numerically. Heat transfer and fluid flow are examined for parameters of non-uniform nanoparticle size, mean nanoparticle diameter, nanoparticle volume fraction, Grashof number and different geometry of enclosure. Finite volume method is used for discretizating positional expressions, and the forth order Rung-Kuta is used for discretizating time expressions. Also an artificial compressibility technique was applied to couple continuity to momentum equations. Results indicate that using nanofluid causes an increase in the heat transfer and the Nusselt number so that for R = 0.001 in Gr = 10³, the Nusselt number 25%, in Gr = 10⁴ 26%, and in Gr = 10⁵ 28% increases. Furthermore; by decreasing the mean diameters of nanoparticles, Nusselt number increases. By increasing R parameter (d_p,min/d_p,max) and nano particle volume fraction, Nusselt number increases.     

Fabrication, characterization and measurement of thermal conductivity of Fe3O4 nanofluids

Magnetite Fe₃O₄ nanoparticles were synthesized by a co-precipitation method at different pH values. The products were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, and transmission electronic microscopy. Their magnetic properties were evaluated on a vibrating sample magnetometer. The results show that the shape of the particles is cubic and they are superparamagnetic at room temperature. Magnetic nanofluids were prepared by dispersing the Fe₃O₄ nanoparticles in water as a base fluid in the presence of tetramethyl ammonium hydroxide as a dispersant. The thermal conductivity of the nanofluids was measured as a function of volume fraction and temperature. The results show that the thermal conductivity ratio of the nanofluids increases with increase in temperature and volume fraction. The highest enhancement of thermal conductivity was 11.5% in the nanofluid of 3 vol% of nanoparticles at 40 °C. The experimental results were also compared with the theoretical models.     

Studies on poly(vinylidene fluoride-co-hexafluoropropylene) based gel electrolyte nanocomposite for sodium-sulfur batteries

Experimental investigations on a sodium ion conducting gel polymer electrolyte nanocomposite based on poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP), dispersed with silica nanoparticles are reported. The gel nanocomposites have been obtained in the form of dimensionally stable, transparent and free-standing thick films. Physical characterization by X-ray diffraction (XRD), Fourier transform Infra-red (FTIR) spectroscopy and Scanning electron microscopy (SEM) have been performed to study the structural changes and the ion-filler-polymer interactions due to the dispersion of SiO₂ nanoparticles in gel electrolytes. The highest ionic conductivity of the electrolyte has been observed to be 4.1 × 10⁻³ S cm^− 1 at room temperature with ~ 3 wt.% of SiO₂ particles. The temperature dependence of the ionic conductivity has been found to be consistent with Vogel-Tammen-Fulcher (VTF) relationship in the temperature range from 40 to 70 °C. The sodium ion conduction in the gel electrolyte film is confirmed from the cyclic voltammetry, impedance analysis and transport number measurements. The value of sodium ion transport number (t_Na+) of the gel electrolyte is significantly enhanced to a maximum value of 0.52 on the 15 wt.% SiO₂ dispersion. The physical and electrochemical analyses indicate the suitability of the gel electrolyte films in the sodium batteries. A prototype sodium-sulfur battery, fabricated using optimized gel electrolyte, offers the first discharge capacity of ~165 mAh g^− 1 of sulfur.     

Photoluminescence of Si from Si nanocrystal-doped SiO2/Si multilayered sample

A multilayered Si nanocrystal-doped SiO₂/Si (or Si-nc:SiO₂/Si) sample structure is studied to acquire strong photoluminescence (PL) emission of Si via modulating excess Si concentration. The Si-nc:SiO₂ results from SiO thin film after thermal annealing. The total thickness of SiO layer remains 150 nm, and is partitioned equally into a number of sublayers (N = 3, 5, 10, or 30) by Si interlayers. For each N-layered sample, a maximal PL intensity of Si can be obtained via optimizing the thickness of Si interlayer (or d_Si). This maximal PL intensity varies with N, but the ratio of Si to O is nearly a constant. The brightest sample is found to be that of N = 10 and d_Si = 1 nm, whose PL intensity is ∼5 times that of N = 1 without additional Si doping, and ∼2.5 times that of Si-nc:SiO₂ prepared by co-evaporating of SiO and Si at the same optimized ratio of Si to O. Discussions are made based on PL, TEM, EDX and reflectance measurements.     

A versatile method for the preparation of end-functional polymers onto SiO2 nanoparticles by a combination of surface-initiated ATRP and Huisgen [3 + 2] cycloaddition

A versatile method was developed for the chain-end functionalization of the grafted polymer chains for surface modification of nanoparticles with functionalized groups through a combination of surface-initiated atom-transfer radical polymerization (ATRP) and Huisgen [3 + 2] cycloaddition. First, the surface of SiO₂ nanoparticles was modified with poly(methyl methacrylate) (PMMA) brushes via the “grafting from” approach. The terminal bromides of PMMA-grafted SiO₂ nanoparticles were then transformed into an azide function by nucleophilic substitution. These azido-terminated PMMA brushes on the nanoparticle surface were reacted with alkyne-terminated functional end group via Huisgen [3 + 2] cycloaddition. FTIR and ¹H NMR spectra indicated quantitative transformation of the chain ends of PMMA brushes onto SiO₂ nanoparticles into the desired functional group. And, the dispersibility of the end-functional polymer-grafted SiO₂ nanoparticles was investigated with a transmission electron microscope (TEM).     

Textural and electronic characteristics of mechanochemically activated composites with nanosilica and activated carbon

Nanosilicas (A-50, A-300, A-500)/activated carbon (AC, S_BET = 1520 m²/g) composites were prepared using short-term (5 min) mechanochemical activation (MCA) of powder mixtures in a microbreaker. Smaller silica nanoparticles of A-500 (average diameter d_av = 5.5 nm) can more easily penetrate into broad mesopores and macropores of AC microparticles than larger nanoparticles of A-50 (d_av = 52.4 nm) or A-300 (d_av = 8.1 nm). After MCA of silica/AC, nanopores of non-broken AC nanoparticles remained accessible for adsorbed N₂ molecules. According to ultra-soft X-ray emission spectra (USXES), MCA of silica/AC caused formation of chemical bonds Si-O-C; however, Si-C and Si-Si bonds were practically not formed. A decrease in intensity of OK_α band in respect to CK_α band of silica/AC composites with diminishing sizes of silica nanoparticles is due to both changes in the surface structure of particles and penetration of a greater number of silica nanoparticles into broad pores of AC microparticles and restriction of penetration depth of exciting electron beam into the AC particles.     

rf reactive co-sputtered Au-Ag alloy nanoparticles in SiO2 thin films

We have studied formation of Au-Ag alloy nanoparticles in sputtered SiO₂ thin films. Silica thin films containing Au-Ag nanoparticles were deposited on quartz substrates using rf reactive magnetron co-sputtering technique. The films heat-treated in reducing Ar + H₂ atmosphere at different temperatures. They were analyzed by using UV-vis spectrophotometry, atomic force microscopy and X-ray photoelectron spectroscopy (XPS) methods for their optical, surface morphological as well as structural and chemical properties. The optical absorption of the Au-Ag alloy nanoparticles illustrated one plasmon resonance absorption peak located at 450 nm between the absorption bands of pure Au and Ag nanoparticles at 400 and 520 nm, respectively, for the thin films annealed at 800 °C. XPS results showed that the alloys were in metallic state, and they had a greater tendency to lose electrons as compared to their corresponding monometallic state. Using lateral force microscopy analysis, we have found that the alloy particles were uniformly distributed on the surface with grain size of about 20 nm.     

Optical properties of sol-gel fabricated Mn/SiO2 nanocomposites: Observation of surface plasmon resonance in Mn nanoparticles

Manganese nanoparticles were grown in silica glass and silica film on silicon substrate by annealing of the sol-gel prepared porous silicate matrices doped with manganese nitrate. Annealing of doped porous silicate matrices was performed at various conditions that allowed to obtain the nanocomposite glasses with various content of metallic Mn. TEM of Mn/SiO₂ glass indicates the bimodal size distribution of Mn nanoparticles with mean sizes of 10.5 nm and 21 nm. The absorption and photoluminescence spectra of Mn/SiO₂ glasses were measured. In the absorption spectra at 300 nm (4.13 eV) we observed the band attributed to the surface plasmon resonance in Mn nanoparticles. The spectra proved the creation of Mn²⁺ and Mn³⁺ ions in silica glass as well. The absorption spectra of Mn/SiO₂ glasses annealed in air prove the creation of manganese oxide Mn₂O₃. The measured reflection spectra of Mn/SiO₂ film manifest at 240-310 nm the peculiarity attributed to surface plasmons in Mn nanoparticles.     

Electronic state of ruthenium deposited onto oxide supports: An XPS study taking into account the final state effects

The electronic state of ruthenium in the supported Ru/EO_x (EO_x = MgO, Al₂O₃ or SiO₂) catalysts prepared by with the use of Ru(OH)Cl₃ or Ru(acac)₃ (acac = acetylacetonate) and reduced with H₂ at 723 K is characterized by X-ray photoelectron spectroscopy (XPS) in the Ru 3d, Cl 2p and O 1s regions. The influence of the final state effects (the differential charging and variation of the relaxation energy) on the binding energy (BE) of Ru 3d_5/2 core level measured for supported Ru nanoparticles is estimated by comparison of the Fermi levels and the modified Auger parameters determined for the Ru/EO_x samples with the corresponding characteristics of the bulk Ru metal. It is found that the negative shift of the Ru 3d_5/2 peak which is observed in the spectrum of ruthenium deposited onto MgO (BE = 279.5-279.7 eV) with respect to that of Ru black (BE = 280.2 eV) or ruthenium supported on γ-Al₂O₃ and SiO₂ (BE = 280.4 eV) is caused not by the transfer of electron density from basic sites of MgO, as considered earlier, but by the differential charging of the supported Ru particles compared with the support surface. Correction for the differential charging value reveals that the initial state energies of ruthenium in the Ru/EO_x systems are almost identical (BE = 280.5 ± 0.1 eV) irrespectively of acid-base properties of the support, the mean size of supported Ru crystallites (within the range of 2-10 nm) and the surface Cl content. The results obtained suggest that the difference in ammonia synthesis activity between the Ru catalysts supported on MgO and on the acidic supports is accounted for by not different electronic state of ruthenium on the surface of these oxides but by some other reasons.     

High-temperature magnetic properties of noninteracting randomly oriented single-domain Fe3 − δO4 nanoparticles

Magnetic measurements have been performed on 40-nm sphere-like Fe_3 − δO₄ (δ=0.043) nanoparticles using a Quantum Design vibrating sample magnetometer. Coating Fe_3 − δO₄ nanoparticles with SiO₂ effectively eliminates magnetic interparticle interactions so that the coercive field HC in the high-temperature range between 300 K and the Curie temperature (855 K) can be well fitted by an expression for noninteracting randomly oriented single-domain particles. From the fitting parameters, the effective anisotropy constant K is found to be (1.38±0.11)×¹⁰⁵ erg/cm³, which is very close to the bulk magnetocrystalline anisotropy constant of 1.35×¹⁰⁵ erg/cm³. Moreover, the inferred mean particle diameter from the fitting parameters is in quantitative agreement with that determined from transmission electron microscope. Such a quantitative agreement between data and theory suggests that the ensemble of our SiO₂-coated sphere-like Fe_3 − δO₄ nanoparticles represents a good system of noninteracting randomly-oriented single-domain particles.     

Iron/iron oxides core-shell nanoparticles by laser pyrolysis: Structural characterization and enhanced particle dispersion

Well-dispersed nanoparticles with iron/iron carbide core and iron oxide shell structures may constitute an excellent magnetic material for different applications as magnetic nanofluids, contrast agents in magnetic resonance imaging, sensors and catalysts. Based on the ability of the CO₂ laser pyrolysis technique to synthesize nanoparticles of the Fe/Fe₂O₃ core-shell type, we further improve the powder dispersion by first collecting the nanoparticles in a toluene bubbler, positioned downstream and prior to the collection filter. Structural characterisation of the samples by electron microscopy and X-ray diffraction was performed. Conditions in which clusters contain a reduced number of nanoparticles (around 50) are evidenced. Mean core-shell particle sizes of 15 nm were estimated. Finally, preliminary results on the morphology of iron/iron oxide core-shell nanoparticles as hydrocarbon-based magnetic nanofluids are presented.