Thermal conductivity and viscosity of nanofluids: A review of recent molecular dynamics studies

Nanofluids for Advanced Applications: A Comprehensive Review on Preparation Methods, Properties, and Environmental Impact

Nanofluids, an advanced class of heat transfer fluids, have gained significant attention due to their superior thermophysical properties, making them highly effective for various engineering applications. This review explores the impact of nanoparticle integration on the thermal conductivity, viscosity, and overall heat transfer performance of base fluids, highlighting improvements in systems, such as heat exchangers, electronics cooling, PV/T systems, CSP technologies, and geothermal heat recovery. Key mechanisms such as nanolayer formation, Brownian motion, and nanoparticle aggregation are discussed, with a focus on hybrid nanofluids that show enhanced thermal conductivity. The increase in viscosity poses a trade-off, necessitating careful control of the nanoparticle properties to optimize heat transfer while reducing energy consumption. Empirical data show up to a 123% increase in the convective heat transfer coefficients, demonstrating the tangible benefits of nanofluids in energy efficiency and system miniaturization. The review also considers the environmental impacts of nanofluid use, such as potential toxicity and the challenges of sustainable production and disposal. Future research directions include developing hybrid nanofluids with specific properties, integrating nanofluids with phase change materials, and exploring new nanomaterials such as metal chalcogenides to enhance the efficiency and sustainability of thermal management systems.

Molecular dynamics simulations and experimental investigation of viscosity of CuO-oil nanolubricant at different temperatures and volume fractions of nanoparticles

Nanolubricant viscosity plays a crucial role in various industries due to its impact on pressure drop, pumping power, and heat transfer. The purpose of this research is to measure the viscosity of a (base oil) C30H62-CuO nano-lubricant experimentally using a viscometer and determine its viscosity using the equilibrium molecular dynamics (MD) simulation. In addition, the impacts of nano CuO particle volume fraction and temperature on the viscosity were investigated within different concentrations of nano CuO particles (0%, 0.25%, 0.5%, and 0.75%) and variable temperatures (300 K, 313 K, 323 K, and 373 K). The simulation results agreed with experimental results and depicted that the viscosity of base oil and nano lubricant of CuO-base oil decreased with increasing temperature. Additionally, increasing the concentration of nanoparticles increased the viscosity of the nano lubricant, but the effect of increasing the concentration of nanoparticles at high temperatures was not significant. For instance, the viscosity of the base oil increased by 1.2% and 1.5% after adding 0.5% and 0.75% copper oxide nanoparticles at 373 K. Based on our research; no study has been done to calculate the viscosity of nanolubricant (C30H62 (base oil) - CuO) and its influencing factors by molecular dynamics simulation and compare its results with experimental methods. The research findings have practical implications for using nano lubricants in various industries, such as the internal combustion engine industry or other industries that use lubricants, and it is a critical parameter in heat transfer.

Thermal conductivity and structural behavior of confined H2from molecular dynamics simulation

In this work, we perform equilibrium molecular dynamics simulation to study the thermal conductivity of hydrogen molecules (H2) under extreme confinement within graphene nanochannel. We analyze the structural behavior of H2molecules inside the nanochannel and also examine the effect of nanochannel height, the number of H2molecules, and temperature of the system on the thermal conductivity. Our results reveal that H2molecules exhibit a strong propensity for absorption onto the nanochannel wall, consequently forming a dense packed layer in close to the wall. This phenomenon significantly impacts the thermal conductivity of the confined system. We made a significant discovery, revealing a strong correlation between the mass density near the nanochannel wall and the thermal conductivity. This finding highlights the crucial role played by the density near the wall in determining the thermal conductivity behavior. Surprisingly, the average thermal conductivity for nanochannels with a height (h) less than 27 Å exhibited an astonishing increase of over 12 times when compared to the bulk. Moreover, we observe that increasing the nanochannel height, while the number of H2molecules fixed, leads to a notable decrease in thermal conductivity. Furthermore, we investigate the influence of temperature on thermal conductivity. Our simulations demonstrate that higher temperature enhance the thermal conductivity due to increased phonon activity and energy states, facilitating more efficient heat transfer and higher thermal conductivity. To gain deeper insights into the factors affecting thermal conductivity, we explored the phonon density of states. Studying the behavior of hydrogen in confined environments can offer valuable insights into its transport properties and its potential for industrial applications.

Coupling at the molecular scale between the graphene nanosheet and water and its effect on the thermal conductivity of the nanofluid

Graphene nanofluid is a promising way to improve heat transfer in many situations. As a two-dimensional material, graphene's anisotropic thermal conductivity influences the heat transfer of nanofluids. In the present study, a nonequilibrium molecular dynamics (MD) simulation is adopted to study the interaction between graphene nanosheets (GNSs) and liquid water in water-based graphene nanofluids. Consequently, the coupling interaction between the orientation and length of GNSs and the thermal conductivity of nanofluids is then investigated. We discover that the molecular thermal coupling between GNSs and water can effectively influence the orientation angle of the GNSs. A preferential orientation angle of the GNSs inside the nanofluid is then observed during heat transfer. The preferential orientation angle decreases with the GNS length and has no apparent relation with the size of heat flux in this study. The overall thermal conductivity of the nanofluid decreases as the orientation angle of the GNS rises. Increasing the GNS length not only reduces the preferential orientation angle but also improves the thermal conductivity along the graphene length direction. The thermal conductivity of the nanofluid along the graphene length direction increases from 0.414 to 4.085 W m K-1 as the length increases from 103 to 3274 A. Our results provide the fundamental knowledge of the heat transfer performance of graphene nanofluids.

Numerical investigation of nanofluid heat transfer in the wall cooling panels of an electric arc steelmaking furnace

Wall cooling panels are typically a kind of electric arc furnace equipment that has precisely influence on different aspects of the steelmaking process. This investigation employs a CFD method to evaluate the thermal performance of water cooling panels in real operating conditions to validate the numerical method followed by replacing cooling water with Al2O3/Water nanofluid coolant. The results are revealed that the high rate of receiving heat flux and generated vortexes with low-velocity cores lead to hot spots inducing on bends and elbows. In the operating flow rate, the maximum temperature of the hot-side wall decrease by 14.4% through increasing the nanoparticle concentration up to 5%, where the difference between maximum temperature and average temperature on the hot-side decrease to 12 degrees. According to the results, use of nanofluid coolant is a promising method to fade the hot spots out on the hot-side and gifting a lower and smoother temperature distribution on the panel walls of thereby prolonging the usage period of panels.

Experimental investigation of thermal efficiency and thermal performance improvement of compound parabolic collector utilizing SiO2/Ethylene glycol-water nanofluid

A compound parabolic collector has been used in the present study to lower operating costs per unit of heat increase compared to other tracker concentrators. This type of collector has been given more attention in industrial and domestic applications in the temperature range of 60 to 300 °C. Also, to increase the thermal efficiency, nanofluid containing SiO₂ nanoparticles in ethylene glycol-water hybrid base fluid (10-90 vol.%) have been used in three different volumetric fractions. The innovation of this present study includes the utilization of mentioned nanofluids for the first time in this collector, which has good stability and is cost-effective compared to other nanoparticles. In addition, the experimental measurement of thermal and hydraulic properties of nanofluids represents new aspects of the present study. The experiments used three volumetric fractions of 0.5%, 1%, and 1.5% under extensive solar radiation. Thermal performance of the collector at four volumetric flow rates of 1, 1.5, 2, and 2.5 Lit/min have been investigated according to ASHRAE standard 93-2010 (RA2014). According to the experimental data, the thermal efficiency of the collector improved by 5% to 11.6% when the nanofluid was applied. The maximum enhancement of the average Nusselt number of the nanofluid versus the base fluid at the volumetric flow rate of 1 Lit/min and the volumetric fraction of 1.5% was equal to 7.3%. Besides, nanofluid increased the pressure drop, and consequently, the pumping power slightly. Finally, considering both the impacts of heat transfer and pressure drop, performance evaluation criteria and overall efficiency for nanofluid have been analyzed. The results represented that in all volumetric fractions, the values of performance evaluation criteria and overall efficiency enhanced compared to the base fluid. This research provides researchers and engineers with important information to better understand the thermal and hydraulic parameters of the parabolic compound concentrator in the presence of nanofluid to improve its thermal performance. The results also highlight the potential of using SiO₂ nanoparticles to improve the thermal efficiency of solar collectors despite their low thermal conductivity compared to other conventional nanoparticles.

Synthesis and Characterization of Novel Ternary-Hybrid Nanoparticles as Thermal Additives

The performance of water as a heat transfer medium in numerous applications is limited by its effective thermal conductivity. To improve the thermal conductivity of water, herein, we report the development and thermophysical characterization of novel metal-metal-oxide-carbon-based ternary-hybrid nanoparticles (THNp) GO-TiO₂-Ag and rGO-TiO₂-Ag. The results indicate that the graphene oxide- and reduced graphene oxide-based ternary-hybrid nanoparticles dispersed in water enhance the base fluid (H₂O) thermal conductivity by 66% and 83%, respectively, even at very low concentrations. Mechanisms contributing to this significant enhancement are discussed. The experimental thermal conductivity is plotted against the existing empirical hybrid thermal conductivity correlations. We found that those correlations are not suitable for the metal-metal-oxide-carbon combinations, calling for new thermal conductivity models. Furthermore, the rheological measurements of the nanofluids display non-Newtonian behavior, and the viscosity reduces with the increase in temperature. Such behavior is possibly due to the non-uniform shapes of the ternary-hybrid nanoparticles.

Flow Characteristics of Heat and Mass for Nanofluid under Different Operating Temperatures over Wedge and Plate

BACKGROUND AND PURPOSE

Nanofluids are a new class of heat transfer fluids that are used for different heat transfer applications. The transport characteristics of these fluids not only depend upon flow conditions but also strongly depend on operating temperature. In respect of these facts, the properties of these fluids are modified to measure the temperature effects and used in the governing equations to see the heat and mass flow behavior. Design of Model: Consider the nanofluids which are synthesized by dispersing metallic oxides (SiO₂, Al₂O₃), carbon nanostructures (PEG-TGr, PEG-GnP), and nanoparticles in deionized water (DIW), with (0.025-0.1%) particle concentration over (30-50 °C) temperature range. The thermophysical properties of these fluids are modeled theoretically with the help of experimental data as a function of a temperature and volume fraction. These models are further used in transport equations for fluid flow over both wedge and plate. To get the solution, the equations are simplified in the shape of ordinary differential equations by applying the boundary layer and similarity transformations and then solved by the RK method.

RESULTS

The solution of the governing equation is found in the form of velocity and temperature expressions for both geometries and displayed graphically for discussion. Moreover, momentum and thermal boundary layer thicknesses, displacement, momentum thicknesses, the coefficient of skin friction, and Nusselt number are calculated numerically in tabular form.

FINDING

The maximum reduction and enhancement in velocity and temperature profile is found in the case of flow over the plate as compared to the wedge. The boundary layer parameters are increased in the case of flow over the plate than the wedge.

Statistical physics of inhomogeneous transport: Unification of diffusion laws and inference from first-passage statistics

Characterization of composite materials, whose properties vary in space over microscopic scales, has become a problem of broad interdisciplinary interest. In particular, estimation of the inhomogeneous transport coefficients, e.g., the diffusion coefficient or the heat conductivity, which shape important processes in biology and engineering, is a challenging task. The analysis of such systems is further complicated because two alternative formulations of the inhomogeneous transport equations exist in the literature-the Smoluchowski and Fokker-Planck equations, which are also related to the so-called Ito-Stratonovich dilemma. Using the theory of statistical physics, we show that the two formulations, usually regarded as distinct models, are physically equivalent. From this result we develop efficient estimates for the transverse space-dependent diffusion coefficient in fluids near a phase boundary. Our method requires only measurements of escape probabilities and mean exit times of molecules leaving a narrow spatial region. We test our estimates in three case studies: (i) a Langevin model of a Büttikker-Landauer ratchet; atomistic molecular-dynamics simulations of liquid-water molecules in contact with (ii) vapor, and (iii) soap (surfactant) film which has promising applications in physical chemistry. Our analysis reveals that near the surfactant monolayer the mobility of water molecules is slowed down almost twice with respect to the bulk liquid. Moreover, the diffusion coefficient of water correlates with the transition from hydrophilic to hydrophobic parts of the film.

Enhanced thermal conductivity of nanofluids by introducing Janus particles

The addition of nanoparticles to a base fluid (i.e., nanofluids) is an effective strategy to achieve a higher thermal conductivity of a fluid. In a common nanofluid, the suspended nanoparticles are mostly symmetrical spheres. In the present paper, we propose to add Janus nanoparticles into a fluid (termed as Janus nanofluids), to further enhance the thermal conductivity of nanofluids. By using molecular dynamics simulations, it is found that the thermal conductivity can be distinctly improved by introducing Janus particles into the nanofluids in contrast with common nanofluids. Based on the calculation results of the molecular radial distribution function around the nanoparticle, and the diffusion coefficient of the base fluid and the Janus nanoparticle, the enhancement in the thermal conductivity of Janus nanofluids is attributed to the enhanced Brownian motion of Janus nanoparticles, which increases the probability of inter-molecular collisions and leads to enhanced energy transfer in nanofluids. The Janus nanofluids proposed in this work provide insights for the design of nanofluids with high thermal conductivity.

Enhanced local viscosity around colloidal nanoparticles probed by equilibrium molecular dynamics simulations

Nanofluids-dispersions of nanometer-sized particles in a liquid medium-have been proposed for a wide variety of thermal management applications. It is known that a solid-like nanolayer of liquid of typical thicknesses of 0.5-1 nm surrounding the colloidal nanoparticles can act as a thermal bridge between the nanoparticle and the bulk liquid. Yet, its effect on the nanofluid viscosity has not been elucidated so far. In this article, we compute the local viscosity of the nanolayer using equilibrium molecular dynamics based on the Green-Kubo formula. We first assess the validity of the method to predict the viscosity locally. We apply this methodology to the calculation of the local viscosity in the immediate vicinity of a metallic nanoparticle for a wide range of solid-liquid interaction strength, where a nanolayer of thickness 1 nm is observed as a result of the interaction with the nanoparticle. The viscosity of the nanolayer, which is found to be higher than its corresponding bulk value, is directly dependent on the solid-liquid interaction strength. We discuss the origin of this viscosity enhancement and show that the liquid density increment alone cannot explain the values of the viscosity observed. Rather, we suggest that the solid-like structure of the distribution of the liquid atoms in the vicinity of the nanoparticle contributes to the nanolayer viscosity enhancement. Finally, we observe a failure of the Stokes-Einstein relation between viscosity and diffusion close to the wall, depending on the liquid-solid interaction strength, which we rationalize in terms of the hydrodynamic slip.

A Review on Thermophysical Properties for Heat Transfer Enhancement of Carbon‐Based Nanolubricant

In previous years, nanolubricants have gained attention in terms of their use as heat transfer fluids in applications such as engine cooling and air conditioning. Carbon‐based nanolubricants have attracted great interest due to their superior heat transfer performance. To date, the stability of a carbon‐based nanolubricant is crucial if the enhanced properties are to be retained after the fabrication process. In this context, the current research aims to review the stability and thermophysical properties of carbon‐based nanolubricants. Recent works on these nanolubricants are summarized herein, including the method of preparation and improvements in their properties. The factors that affect the thermal conductivity and viscosity are also discussed, and finally, the applications of nanolubricants are described. The recent studies of nanolubricants are summarized and opportunities for further improvements in their efficiency are suggested.

Ballistic Heat Transport in Nanocomposite: The Role of the Shape and Interconnection of Nanoinclusions

In this article, the effect on the vibrational and thermal properties of gradually interconnected nanoinclusions embedded in an amorphous silicon matrix is studied using molecular dynamics simulations. The nanoinclusion arrangement ranges from an aligned sphere array to an interconnected mesh of nanowires. Wave-packet simulations scanning different polarizations and frequencies reveal that the interconnection of the nanoinclusions at constant volume fraction induces a strong increase of the mean free path of high frequency phonons, but does not affect the energy diffusivity. The mean free path and energy diffusivity are then used to estimate the thermal conductivity, showing an enhancement of the effective thermal conductivity due to the existence of crystalline structural interconnections. This enhancement is dominated by the ballistic transport of phonons. Equilibrium molecular dynamics simulations confirm the tendency, although less markedly. This leads to the observation that coherent energy propagation with a moderate increase of the thermal conductivity is possible. These findings could be useful for energy harvesting applications, thermal management or for mechanical information processing.

Statistical analysis of thermal conductivity experimentally measured in water-based nanofluids

Nanofluids are suspensions of nanoparticles in a base heat-transfer liquid. They have been widely investigated to boost heat transfer since they were proposed in the 1990s. We present a statistical correlation analysis of experimentally measured thermal conductivity of water-based nanofluids available in the literature. The influences of particle concentration, particle size, temperature and surfactants are investigated. For specific particle materials (alumina, titania, copper oxide, copper, silica and silicon carbide), separate analyses are performed. The conductivity increases with the concentration in qualitative agreement with Maxwell’s theory of homogeneous media. The conductivity also increases with the temperature (in addition to the improvement due to the increased conductivity of water). Surprisingly, only silica nanofluids exhibit a statistically significant effect of the particle size, whereby smaller particles lead to faster heat transfer. Overall, the large scatter in the experimental data prevents a compelling, unambiguous assessment of these effects. Taken together, the results of our analysis suggest that more comprehensive experimental characterizations of nanofluids are necessary to estimate their practical potential.

Quantum bath effects on nonequilibrium heat transport in model molecular junctions

Quantum-classical dynamics simulations enable the study of nonequilibrium heat transport in realistic models of molecules coupled to thermal baths. In these simulations, the initial conditions of the bath degrees of freedom are typically sampled from classical distributions. Herein, we investigate the effects of sampling the initial conditions of the thermal baths from quantum and classical distributions on the steady-state heat current in the nonequilibrium spin-boson model-a prototypical model of a single-molecule junction-in different parameter regimes. For a broad range of parameter regimes considered, we find that the steady-state heat currents are ∼1.3-4.5 times larger with the classical bath sampling than with the quantum bath sampling. Using both types of sampling, the steady-state heat currents exhibit turnovers as a function of the bath reorganization energy, with sharper turnovers in the classical case than in the quantum case and different temperature dependencies of the turnover maxima. As the temperature gap between the hot and cold baths increases, we observe an increasing difference in the steady-state heat currents obtained with the classical and quantum bath sampling. In general, as the bath temperatures are increased, the differences between the results of the classical and quantum bath sampling decrease but remain non-negligible at the high bath temperatures. The differences are attributed to the more pronounced temperature dependence of the classical distribution compared to the quantum one. Moreover, we find that the steady-state fluctuation theorem only holds for this model in the Markovian regime when quantum bath sampling is used. Altogether, our results highlight the importance of quantum bath sampling in quantum-classical dynamics simulations of quantum heat transport.

A Toughening and Anti-Counterfeiting Benzotriazole-Based High-Performance Polymer Film Driven by Appropriate Intermolecular Coordination Force

It is of great significance to circumvent the inherent trade-off between strength and extensibility for epoxy resins. Herein dynamic Cu-benzotriazole cross-links are incorporated, as the appropriate intermolecular coordination interaction, into high performance epoxy networks, and the resulting epoxy resins exhibits outstanding thermal stability and mechanical properties, their strength and extensibility are simultaneously improved. Additionally, local manipulation of coordination crosslinking confers the film with anti-counterfeiting function.

Corrosion Behaviour of 316L Stainless Steel in CNTs-Water Nanofluid: Effect of Temperature

The inhibition behavior of carbon nanotubes (CNTs) and Gum Arabic (GA) on the corrosion of 316L stainless steel in CNTs–water nanofluid under the effect of different temperatures was investigated by electrochemical methods and surface analysis techniques. Thereby, 316L stainless steel samples were exposed to CNTs–water nanofluid under temperatures of 22, 40, 60 and 80 °C. Two concentrations of the CNTs (0.1 and 1.0 wt.% CNTs) were homogenously dispersed in deionized water using the surfactant GA and tested using three corrosion tests conducted in series: open circuit test, polarization resistance test, and potentiodynamic scans. These tests were also conducted on the same steel but in solutions of GA-deionized water only. Tests revealed that corrosion increases with temperature and concentration of the CNTs–water nanofluids, having the highest corrosion rate of 32.66 milli-mpy (milli-mil per year) for the 1.0 wt.% CNT nanofluid at 80 °C. In addition, SEM observations showed pits formation around areas of accumulated CNTs that added extra roughness to the steel sample. The activation energy analysis and optical surface observations have revealed that CNTs can desorb at higher temperatures, which makes the surface more vulnerable to corrosion attack.

Thermal Performance of Hybrid-Inspired Coolant for Radiator Application

Due to the increasing demand in industrial application, nanofluids have attracted the considerable attention of researchers in recent decades. The addition of nanocellulose (CNC) with water (W) and ethylene glycol (EG) to a coolant for a radiator application exhibits beneficial properties to improve the efficiency of the radiator. The focus of the present work was to investigate the performance of mono or hybrid metal oxide such as Al₂O₃ and TiO₂ with or without plant base-extracted CNC with varying concentrations as a better heat transfer nanofluid in comparison to distilled water as a radiator coolant. The CNC is dispersed in the base fluid of EG and W with a 60:40 ratio. The highest absorption peak was noticed at 0.9% volume concentration of TiO₂, Al₂O₃, CNC, Al₂O₃/TiO₂, and Al₂O₃/CNC nanofluids which indicates a better stability of the nanofluids’ suspension. Better thermal conductivity improvement was observed for the Al₂O₃ nanofluids in all mono nanofluids followed by the CNC and TiO₂ nanofluids, respectively. The thermal conductivity of the Al₂O₃/CNC hybrid nanofluids with 0.9% volume concentration was found to be superior than that of the Al₂O₃/TiO₂ hybrid nanofluids. Al₂O₃/CNC hybrid nanofluid dominates over other mono and hybrid nanofluids in terms of viscosity at all volume concentrations. CNC nanofluids (all volume concentrations) exhibited the highest specific heat capacity than other mono nanofluids. Additionally, in both hybrid nanofluids, Al₂O₃/CNC showed the lowest specific heat capacity. The optimized volume concentration from the statistical analytical tool was found to be 0.5%. The experimental results show that the heat transfer coefficient, convective heat transfer, Reynolds number and the Nusselt number have a proportional relationship with the volumetric flow rate. Hybrid nanofluids exhibit better thermal conductivity than mono nanofluids. For instance, a better thermal conductivity improvement was shown by the mono Al₂O₃ nanofluids than the CNC and TiO₂ nanofluids. On the other hand, superior thermal conductivity was observed for the Al₂O₃/CNC hybrid nanofluids compared to the other mono and hybrid ones (Al₂O₃/TiO₂).

Insights on (C, BN, Si, Ge, MoS2) Nanotubes in Reline Deep Eutectic Solvent

The properties of carbon, boron nitride, silicon, germanium, and molybdenum disulfide nanotubes in reline (cholinium chloride + urea) deep eutectic solvents were studied by using classical molecular dynamics simulations. These nanotubes + reline nanofluids provide a suitable platform for the development of sustainable thermal engineering applications. The reported results lead to the characterization of nanotube solvation and reline layering around the nanotube surfaces as well as the behavior of reline upon confinement inside the considered nanotube cavities. Changes in reline hydrogen bonding in the presence of the nanotubes are also analyzed and related with the development of stable nanotube dispersions, thus showing reline as a suitable vehicle for nanotubes.