Bother two-dimensional (2D) and three-dimensional (3D) simulations on two example melting problems, i.e., melting in a differentially-heated rectangular cavity and constrained melting in a horizontal cylindrical capsule, were carried out to investigate the rationality of 2D simplification. The effects of thermophysical properties of the phase change material, size of the container along the direction perpendicular to the 2D cross-section, as well as wall superheat were taken into consideration for a systematic and detailed comparison. It was shown that a small length of the container perpendicular to 2D plane will result in a confine space to limit the development of velocity distribution (i.e., parabolic velocity profile) due to the end effects, leading to to an almost identical melting rate to that obtained by the 2D simplified case. A larger size indicates stronger thermal convection (bulk uniform velocity profile) and faster melting rate. When fixing a large size of the container perpendicular to the 2D plane, decreasing the heating temperature and increasing the viscosity of liquid PCM (e.g., by adding nanoparticles) reduce the discrepancy between 2D and 3D simulation results.

Phase change materials (PCMs) are commonly used as energy storage mediums in solar thermal systems. This paper investigates the mixture of PCM doped with nanoparticles to be used as HTFs directly integrated in a U-pipe ETC to be applied in solar thermal collectors. The selected type of PCM-HTF in this study is erythritol (C 4 H 10 O 4 ), with high specific heat capacity in liquid form, as well as its unique sub-cooling behavior. In order to overcome the low thermal conductivity of erythritol and further enhance specific heat capacity, a weight concentration of 1% multi-walled carbon nanotubes (MWCNT) is added. Additionally, to insure even distribution of MWCNT and consistent properties of the HTF, triethanolamine (TEA) is proposed to be incorporated as a dispersant. The samples were each tested in a Thermogravimetric Analyzer (TGA) and Differential Scanning Calorimeter (DSC) to analyze their thermal properties. The results from the DSC tests show 12.4% enhancement of specific heat capacity of the proposed HTF mixture as well as nearly 5° C depression of freezing onset temperature. This study allows for the optimization of the operating temperature range of the collector when integrated with these materials, where direct heat gain can be obtained in the collector.

The arguments in favor of using nanofluids in thermal applications have been increasing substantially for the last few decades. Nanofluids provide improved performance in heat transfer processes in comparison to their base fluids as a result of their superior thermal properties. Even though nanofluids exhibit better thermal properties, their usage has been limited due to their stability issues. The stability of the nanofluid greatly affects its thermal properties over a period of time. The stability and thermal properties of nanofluids can be affected by parameters like surfactants used and their concentrations, and also on the nanomaterial used in the nanofluid. In this study, surfactant material and nanoparticle material are selected as process variables and for each variable two levels are selected. For surfactant material, Sodium Lauryl Sulfate (SLS) and Cetyl Trimethyl Ammonium Bromide (CTAB) are selected. Surfactant concentration ratios are taken as 1:2 for CuO and 1:4 for Fe 3 O 4 material. Four nanofluid samples are prepared with 0.1% weight of nanoparticles and their stability and properties are studied. The feasibility of turbidity as an indicator for stability is also explored in this work. The results show that the zeta potential and hydrodynamic characteristics are largely dependent on the surfactant material. Both surfactants show good stability of nanofluids. In-line with earlier observations, it is also observed that the nanoparticle material has a dominant effect on the thermal conductivity of the nanofluids. Comparing the turbidity of the nanofluids to the zeta potential, it is observed that the turbidity measurement gives first-hand information about the stability of nanofluids and can act as an index for stability. But still, more exploration is necessary for this field so a quantitive relation can be established between turbidity and zeta potential of different nanofluid materials and concentrations.

In this paper a numerical investigation on mixed convection in confined slot jets impinging on a porous media is accomplished. The working fluids are pure water or Al2O3/water based nanofluids and a single-phase model approach has been adopted in order to describe their behavior. A two-dimensional configuration is analized and different Peclet numbers and Rayleigh numbers are considered. The thermal non-equilibrium energy condition is assumed to execute two-dimensional simulations on the system. The examined foams are characterized by distinct values of pores per inch, PPI, equal to 5, 10, 20 and 40. The particle volume concentrations range from 0% to 4% and the particle diameter is equal to 30 nm. The target surface is heated by a constant temperature value, calculated according to the value of Rayleigh number. The distance of the target surface is five times greater than the slot jet width. The aim consists into study the thermal and fluid-dynamic behaviour of the system. Results show increasing values of the convective heat transfer coefficients for increasing values of Peclet number and nanoparticle concentration. Furthermore, the heat transfer coefficient presents a different behavior at varying PPI numbers for different Peclet numbers.

Near-field electromagnetic coupling between nanoparticles in dielectric media has led to new advances in next generation technologies such as photonic integrated circuits and photonic sintering. Understanding the light confinement between nanoparticles at subwavelength dimensions is vital in such applications. For instance, the wavelength of the incident light, dielectric medium properties, nanoparticle size, interparticle spacing, and relative orientation between the nanoparticles affect the near-field plasmonic interactions that characterize the thermal energy transport. In this study, we perform a field intensity analysis by looking at the relative orientation of gold (Au) and copper (Cu) nanoparticles which have 25, 50 and 100 nm radius under 532 nm laser irradiation in different surrounding medium to understand the thermal energy transport between the nanoparticles at different relative orientations.

In addition to the increase of thermal conductivity, heat transferring for nanofluids strengthening mechanism also includes the changes of the flow characteristics, therefore it is needed to take an in-depth research on nanofluids flow characteristics. However previous visualization experiment do not quantitatively analyze the change of flow characteristics after nano-particles is added, do not reveal the mechanism of nanofluids changing the characteristics of the fluid in intense turbulent flow condition. Therefore in this paper, by means of particle image velocimetry, quantitatively study SiO 2 -water nanofluids flow characteristics in intense turbulent flow condition and analyze the influence of SiO 2 -water nanofluids on turbulent flow energy by measuring the pressure drop caused by fluid flowing through the channel. Fluid flow through rectangular convex channel (channel composed of continuous staggered rectangular convex platform) to obtain the steady intense turbulent flow. The rectangular convex channel makes the fluid flow through obstacles for several times, so that the flow direction changes for several times, and vortexes are generated in the local scope, which makes turbulence enhance and increases minor loss. In this way, flow can be in the intense turbulent state under a low flow rate, which meets the experiment requirement and is convenient to compare the influence of nano-particles on flow resistance and energy loss. The experiment takes the quantitative PIV experimental research on pure water and the volume fraction of 0.5% SiO 2 -water nanofluids respectively in the Reynolds number is 2300, 2500, 3000, 4000. Through the experiment, we can obtain nanofluids turbulent flow condition fluctuating velocity, turbulence kinetic, energy loss and so on, and the fluid flow velocity vector, streamline and vorticity graph. Through the quantitative comparison of the spiral numbers, the vorticity distribution, and energy loss, analyze strengthening effect and influence of flow resistance on basic fluid after adding nanoparticles.

Nanoparticle packed beds are of interest because of their extremely low thermal conductivities. However, thermal nanoparticle packed beds exposed to ambient air show unusual increases in conductivity. The causes for that increase in conductivity are examined with experimental measurements. Sorption isotherms and thermal conductivity measurements for 20 nm and 200 nm silica nanoparticle packed beds exposed to a range of humidity are presented. Both nanoparticle beds demonstrate hygroscopic behavior, absorbing significant amounts of moisture upon exposure to humid air. The thermal conductivity of the nanoparticle beds are shown to increase monotonically with relative humidity. More than an order of magnitude increase in conductivity is seen, with a large increase at very low humidity. The measurements illuminate the effects of moisture on the thermal properties of nanoparticle-based materials.

Nanoparticle-based materials have demonstrated extremely low thermal conductivities, a property that has made them attractive candidates in a variety of macroscale and microscale applications. Understanding the thermal transport between nanoparticles is necessary for the further development of these materials. Molecular dynamics simulation is an effective method to investigate thermal transport on these scales because no assumption about phonon transmission at the nanoparticle interface, nor prior knowledge of thermal transport of the system is necessary. In this work, the total thermal resistance between adjacent amorphous silica nanoparticles is calculated using non-equilibrium molecular dynamics simulations (NEMD). Numerical results show that interparticle resistance depends strongly on the forces between particles, in particular the presence or absence of chemical bonds between nanoparticles. In addition, the effect of interfacial force strength on thermal resistance increases as nanoparticle diameter decreases. Numerical results are compared to interparticle resistances determined from the predictions of the analytical constriction resistance model. The simulation results are shown to be in good agreement the constriction resistance theory depending on the choice of surface energy.

Stable homogeneous colloidal suspensions of nanoparticles in a liquid solvents are termed as nanofluids. In this review the results for the forced convection heat transfer of nanofluids are gleaned from the literature reports. This study attempts to evaluate the experimental data in the literature for the efficacy of employing nanofluids as heat transfer fluids (HTF) and for Thermal Energy Storage (TES). The efficacy of nanofluids for improving the performance of compact heat exchangers were also explored. In addition to thermal conductivity and specific heat capacity the rheological behavior of nanofluids also play a significant role for various applications. The material properties of nanofluids are highly sensitive to small variations in synthesis protocols. Hence the scope of this review encompassed various sub-topics including: synthesis protocols for nanofluids, materials characterization, thermo-physical properties (thermal conductivity, viscosity, specific heat capacity), pressure drop and heat transfer coefficients under forced convection conditions. The measured values of heat transfer coefficient of the nanofluids varies with testing configuration i.e. flow regime, boundary condition and geometry. Furthermore, a review of the reported results on the effects of particle concentration, size, temperature is presented in this study. A brief discussion on the pros and cons of various models in the literature is also performed — especially pertaining to the reports on the anomalous enhancement in heat transfer coefficient of nanofluids. Furthermore, the experimental data in the literature indicate that the enhancement observed in heat transfer coefficient is incongruous compared to the level of thermal conductivity enhancement obtained in these studies. Plausible explanations for this incongruous behavior is explored in this review. A brief discussion on the applicability of conventional single phase convection correlations based on Newtonian rheological models for predicting the heat transfer characteristics of the nanofluids is also explored in this review (especially considering that nanofluids often display non-Newtonian rheology). Validity of various correlations reported in the literature that were developed from experiments, is also explored in this review. These comparisons were performed as a function of various parameters, such as, for the same mass flow rate, Reynolds number, mass averaged velocity and pumping power.

In this study, effects of particle morphology (shape and size) on nanofluids laminar forced convection in helically coiled tubes are investigated numerically using Eulerian-Lagrangian two-phase approach. The laminar forced convective heat transfer and pressure drop of Al 2 O 3 -water nanofluids containing nanoparticles with various particle shapes (sphere, platelet, blade, cylinder and brick) and sizes at different volume fractions in the developing and fully developed regions are investigated using the validated two-phase model. It is found that the nanofluids containing platelet particle shape has the highest heat transfer enhancement, which is followed by nanofluids containing cylinder, blade, sphere and brick nanoparticle shapes, respectively. Non-spherical nanoparticles with larger aspect ratio, small particle size and a suitable particle volume concentration are beneficial for heat transfer enhancement of forced convection. Heat transfer efficiency reaches minima at Re of 1250 for laminar forced convection with 1% volume fraction. The correlations of Nusselt number and pressure drop with nanoparticle shape and size were developed to predict convective heat transfer of nanofluids containing spherical nanoparticles and non-spherical nanoparticles.

Wet chemistry approaches have been widely used to synthesize nanoparticle suspensions with different size and shape. Controlling particle size is crucial for tailoring the properties of the nanofluid. In this study, we simulated the particle size growth during a thermal-chemical nanofluid synthesis routine. The simulation was based on the population balance model for aggregation kinetics, which is coupled with thermal decomposition, nucleation and crystal growth kinetics. The simulation result revealed a typical burst nucleation mechanism towards self-assembly of supersaturated monomers in the nanoparticle formation process and the shift from monodispersed particles to polydispersed particles by the particle-particle coagulation.

Fabrication of micro and nanoscale electronic components has become increasingly demanding due to device and interconnect scaling combined with advanced packaging and assembly for electronic, aerospace and medical applications. Recent advances in additive manufacturing have made it possible to fabricate microscale, 3D interconnect structures but heat transfer during the fabrication process is one of the most important phenomena influencing the reliable manufacturing of these interconnect structures. In this study, optical absorption and scattering by three-dimensional (3D) nanoparticle packings are investigated to gain insight into micro/nano heat transport within the nanoparticles. Because drying of colloidal solutions creates different configurations of nanoparticles, the plasmonic coupling in three different copper nanoparticle packing configurations were investigated: simple cubic (SC), face-centered cubic (FCC) and hexagonal close packing (HCP). Single-scatter albedo (ω) was analyzed as a function of nanoparticle size, packing density, and configuration to assess effect for thermo-optical properties and plasmonic coupling of the Cu nanoparticles within the nanoparticle packings. This analysis provides insight into plasmonically enhanced absorption in copper nanoparticle particles and its consequences for laser heating of nanoparticle assemblies.

Molecular dynamics simulation was performed to investigate pool boiling heat transfer of nanofluids on rough walls. Nanoparticle movement was calculated to investigate the physical mechanisms of boiling heat transfer. The simulated system consisted of four regions: vapor argon, liquid argon, solid copper, and copper nanoparticles, and three cases were considered: base fluids (case A), nanoparticles far from the wall (case B), and nanoparticles near the wall (case C). Boiling heat transfer was enhanced by the addition of nanoparticles, and the enhancement increased with increasing heating temperature. Case C showed that nanoparticles were adsorbed on the nonevaporated film and did not move with the fluids. Thus, nanoparticles enhanced heat and energy transfer between the wall and fluids. Case B showed that nanoparticles moved randomly in the fluid area, which enhanced heat transfer within the fluid.

Literature review of molten salt nanofluids is performed in this study with focus on the thermo-fluidic properties and performance in thermal management applications. The colloidal mixture of nanoparticles in a base liquid phase is called nanofluid. Molten salts such as alkali nitrate eutectics, alkali carbonate eutectics and alkali chloride eutectics have high melting temperatures. These materials are suitable for various high temperature applications, including as Heat Transfer Fluid (HTF), Thermal Energy Storage (TES), Concentrated Solar Power (CSP) plants, nuclear power, etc. The major drawback of molten salt materials is their low thermal conductivity and specific heat capacity. Enhancing the thermo-physical properties of molten salt materials can lower the cost of power production involving these materials (e.g., as HTF and/ or TES in CSP or nuclear power plants. Mixing molten alt eutectics with nanoparticles (e.g., molten salt nanofluids) can provide a cost-effective technique for enhancing the specific heat capacity and thermal conductivity which in turn can enable the reduction in the cost of power production. In this review - the following topics involving molten salt nanofluids were explored: thermo-physical property measurements, numerical modeling (e.g., Molecular Dynamics/ MD simulations), materials characterization (e.g., using electron microscopy techniques — such as SEM and TEM). For example, SEM studies in conjunction with MD simulation results confirm the formation of a dense layer of fluid molecules on the surface of nanoparticles that can enhance the specific heat capacity of these molten salt nanomaterials. Subsequently the concepts of nanofins was explored (which involves the study of interfacial thermal impedance, such as resistance, capacitance and diodicity). The contribution of these interfacial thermal impedances to the enhancement of specific heat capacity and thermal conductivity are also explored. Specific heat enhancement as high as 100% has been observed for various molten salt eutectics when doped with 1.5% (weight) silica nanoparticles. Various synthesis protocols such as one-step, two-step and three-step methods as well as conventional experimental methods used for specific heat capacity measurement are compared and examined. A review of the effects of concentration, nanoparticle size, temperature, base fluid, and nanofluid chemical properties is also performed. Other topics of interest are the anomalous enhancement of thermal conductivity in molten salt nanofluids which contradict typical predictions obtained from using the effective medium theory. The available data in literature shows enhancement in thermal conductivity by as much as 35–45% for carbonate eutectics doped with silica nanoparticles at 1% mass fraction. The possible mechanisms suggested for this improvement are briefly discussed and compared with experimental observations (e.g., using SEM). In addition, nanofluids often display non-Newtonian rheological behavior. This necessitates a rigorous study, since the applications of nanofluids will impact the required pumping power. Studies show that the rheological properties of molten salt nanofluids are a function of base salt composition, shape of nanoparticles selected, chemical formula of nanoparticles, concentration of nanoparticles, size of nanoparticles, temperature, shear rate and synthesis protocol of the nanofluid. Several models are introduced to predict the viscosity variation along with their advantageous and disadvantages. SEM results show agglomeration of nanoparticles can be reduced by doping the nanofluids with very small values of mass fractions of additives such as Gum Arabic.

Metal nanoparticle has been a promising option for fillers in thermal interface materials due to its low cost and ease of fabrication. However, nanoparticle aggregation effect is not well understood because of its complexity. Theoretical models, like effective medium approximation model, barely cover aggregation effect. In this work, we have fabricated nickel-epoxy nanocomposites and observed higher thermal conductivity than effective medium theory predicts. Smaller particles are also found to show higher thermal conductivity, contrary to classical models indicate. A two-level EMA model is developed to account for aggregation effect and to explain the size-dependent enhancement of thermal conductivity by introducing local concentration in aggregation structures.

We theoretically demonstrate a novel, efficient and cost effective thermal emitter using a Mie-resonance metamaterial for thermophotovoltaic (TPV) applications. We propose for the first time the design of a thermal emitter which is based on nanoparticle-embedded thin film. The emitter consists of a thin film of SiO 2 on the top of tungsten layer deposited on a substrate. The thin film is embedded with tungsten nanoparticles which alter the refractive index of the film. This gives rise to desired emissive properties in the wavelength range of 0.4 μm to 2 μm suitable for GaSb and InGaAs based photovoltaics. Effective dielectric properties are calculated using Maxwell-Garnett-Mie theory. Our calculations indicate this would significantly improve the efficiency of TPV cells. We introduce a new parameter to gauge the efficacy of thermal emitters and use it to compare different designs.

Phonon scattering from media with embedded spherical nanoparticles has been studied extensively over the last decade due to its application to reducing the thermal conductivity of thermoelectric materials. However, similar studies of thermal transport in fiber-embedded media have received little attention. Calculating the thermal conductivity tensor from microscopic principles requires knowledge of the scattering cross section spanning all possible incident elastic wave orientations, polarizations and wavelengths including the transition from Rayleigh to geometric scattering regimes. In this paper, we use continuum mechanics to develop an analytic treatment of elastic wave scattering for an embedded cylinder and show that a classic treatise on the subject contains important errors for oblique angles of incidence, which we correct. We also develop missing equations for the scattering cross section at oblique angles and study the sensitivity of the scattering cross section as a function of elas-todynamic contrast mechanisms. In particular, we find that for oblique angles of incidence, both elastic and density contrast are important mechanisms by which scattering can be controlled, but that their effects can offset one another, similar to the theory of reflection at flat interfaces. The solution developed captures the scattering physics for all possible incident elastic wave orientations, polarizations and wavelengths including the transition from Rayleigh to geometric scattering regimes, so long as the continuum approximation holds. The method thus enables incorporation of coherent scattering models into calculations of the thermal conductivity tensor for media with nanofibers.

The oscillatory flows are often in order to augment heat transfer rates in various processes. It is also well known fact that nanofluids provide significant enhancement in heat transfer at certain conditions. In this research, heat transfer in an oscillatory pipe flow of both water and water-alumina nanofluid were studied experimentally under low frequency regime flow conditions. The aim of the conducted research is parametric experimental investigation of the convective heat transfer in the oscillatory pipe flow. Firstly, the nanofluids were prepared and thermophysical properties weare measured. The experimental apparatus consist of a capillary pipe bundle connecting two reservoirs which are placed at the top and bottom side of the capillary pipe bundle. Upper reservoir contains the hot fluid while lower reservoir and capillary pipe bundle filled with cold fluid. The oscillatory flow in the pipe bundle is driven by the periodic vibrations of a surface mounted on the bottom end of the cold reservoir. The effects of the maximum displacement amplitude of the vibrations and volumetric concentration of nanoparticles on heat transfer were evaluated based on the measured temperature and acceleration data. It is found that heat transfer rate increases with increasing vibration displacement in the fluid.

Microscopic thin films are known to have narrow band selective thermal radiation whch have potential applications in selective emitter and absorbers. We propose a methodology to shift the wavelength selectivity in the desired location. Calculation of emissivity and near field radiation for different materials and structures is presented. Emissivity calculations are performed using the standard expressions for Fresnel reflection coefficients. For the near field thermal radiation, we use the analytical expression for transitivity between two half spaces obtained using the dyadic Greens function formalism. We use Maxwell-Garnett-Mie theory to calculate effective dielectric function of media doped with nanoparticles. We observe that spectrum of thermal radiation in far-field and in near-field can be altered using nanoparticles. Moreover spectral properties of mixtures can be characterized using refractive indices. Influence of nanoparticle size and concentration is also studied.

Nanofluids — colloidal suspensions of nanoparticles in base fluids — are known to possess superior thermal properties compared to the base fluids. Various theoretical models have been suggested to explain the often anomalous enhancement of these properties. Liquid layering around the nanoparticle is one of such reasons. The effect of the particle size on the extent of liquid layering around the nanoparticle has been investigated in the present study. Classical molecular dynamics simulations have been performed in the investigation, considering the case of a copper nanoparticle suspended in liquid argon. The results show a strong dependence of thickness of the liquid layer on the particle size, below a particle diameter of 4nm. To establish the role of liquid layering in the enhancement of thermal conductivity, simulations have been performed at constant volume fraction for different particle sizes using Green Kubo formalism. The thermal conductivity results show 100% enhancement at 3.34% volume fraction for particle size of 2nm. The results establish the dominant role played by liquid layering in the enhanced thermal conductivity of nanofluids at the low particle sizes used. Contrary to the previous findings, the molecular dynamics simulations also predict a strong dependence of the liquid layer thickness on the particle size in the case of small particles.