Carbon nanotube glycol nanofluids: Photo-thermal properties,thermal conductivities and rheological behavior

The efficiency and effectiveness of solar energy capture and storage are to a large extent functions of the heat transfer and storage capacity of the medium used. This paper investigates the potential of using carbon nanotube (CNT)-glycol nanosuspension as such a medium, prepared by freeze drying-ultrasonic dispersing after oxidation treatment with HNO₃. The influences of the mass fraction of CNTs glycol nanofluids and temperatures on photo-thermal properties, thermal conductivities and rheological behavior were investigated. The results show that CNTs with oxidation treatment exhibited good dispersing performance. Strong optical absorption of the CNTs glycol nanofluids was detected in the range of 200–2500 nm. At room temperature, 18% enhancement was found in the photo-thermal conversion efficiency of the 0.5% mass fraction CNTs glycol nanofluids in comparison to the basic fluids, without significant increase in viscosity. At 55 °C, CNTs glycol nanofluids with 4.0% mass fraction exhibited much lower viscosity and 25.4% higher thermal conductivity in comparison to that of pure glycol at room temperature.     

Prediction of thermal conductivity of composites with spherical fillers by successive embedding

In this study, a new model to predict the thermal conductivity of composites with spherical fillers is proposed. The original Eshelby model is extended to a finite filler volume fraction by successively embedding small filler volume fraction. The predicted results by the present model are compared with bounds such as parallel, series, and Hashin–Shtrikman models, results by modified Eshelby model, generalized self-consistent model, and effective medium theory, and the experimental results from the literature. It is found that the present model always lies between the bounds and shows better agreement with the experimental results than the other models for various filler volume fractions and thermal conductivity ratios.     

Prediction of thermal conductivity of air voided-fiber-reinforced composite laminates part II: 3D simulation

Applying the fundamental definition of thermal conductivity to a unit cell of unidirectional fiber reinforced composite with air voids, one can deduce simple empirical formula to predict the thermal conductivity of the composite material with estimated air void volume percent. The inherent 3-D problem is modeled using finite element analysis. The model is tested at different fiber to resin volume ratios and various fibers to resin thermal conductivity ratios for three different air void volume percent. The air voids are modeled as cylindrical shapes with different lengths aligned with fiber direction. Two prediction schemes have been developed through the present work. One is to predict the longitudinal thermal conductivity and the other is to predict the transverse thermal conductivity of the fibers. Also, the model can be used to estimate the voids volume percent if the fiber thermal conductivity has been provided. Such expression can, also, serve as useful guides for quality and perfect bonding for material development.     

Investigation of viscosity and thermal conductivity of alumina nanofluids with addition of SDBS

The present study focused on thermal conductivity and viscosity of alumina nanoparticles, at low volume concentrations of 0.01–1.0 % dispersed in the mixture of ethylene glycol and water (mass ratio, 60:40). Sodium dodeobcylbenzene sulfonate (SDBS) was applied for better dispersion and stability of alumina nanoparticles and study of its influence on both thermal conductivity and viscosity. The thermal conductivity established polynomial enhancement pattern with increase of volume concentration up to 0.1 % while linear enhancement was obtained at higher concentrations. In addition, thermal conductivity was enhanced with the rise of temperature. However, the augmentation was negligible compared to that obtained with increase of volume concentration. In contrast, viscosity data showed remarkable reduction with increase of temperature. Meanwhile, viscosity of nanofluids enhanced with loading of alumina nanoparticles. Thermal conductivity and viscosity measurements showed higher values over theoretical predictions. Results showed SDBS at different concentrations has distinct influence on thermal conductivity and viscosity of nanofluid.     

Role of interfacial layer and clustering on the effective thermal conductivity of CuO-gear oil nanofluids

Copper oxide nanoparticles (∼40 nm) are dispersed in gear oil (IBP Haulic-68) at different volume fractions (0.005-0.025) with oleic acid added as a surfactant to stabilize the system. Prepared nanofluids are characterized by Fourier Transform Infrared spectroscopy (FTIR) and Dynamic light scattering (DLS) measurements. DLS data confirmed the presence of agglomerated nanoparticles in the prepared nanofluids. Thermal conductivity measurements are performed both as a function of CuO volume fraction and temperature between 5 and 80 °C. An enhancement in thermal conductivity at 30 °C of 10.4% with 0.025 volume fraction of CuO nanoparticle loading is observed. Measured volume fraction dependence of the thermal conductivity enhancement at room temperature is predicted fairly well considering contributions from both nanolayer at the solid-liquid interface and particle agglomeration in the suspension, as visualized by Feng et al.     

Numerical prediction of the thermal conductivity of fibers

The need to determine the thermal conductivity of fibers for design purposes of new composite materials and the inherent difficulties in the direct measurement of the thermal conductivity of fibers motivated the present work due to its importance for energy conservation purposes. In this work, a correlation formula is developed to predict the thermal conductivities of fiber as function of the effective thermal conductivity of a fiber-reinforced composite laminates and their constituents which are easy to measure. The parallel and series thermal models of composite walls have been utilized in developing this correlation equation. The coefficients of this formula can be given as functions of the voids volume fraction for each fiber to resin volume ratio considered. The validity of the models is verified through finite element analysis. This model also shows excellent agreement with the available experimental values.     

Wall slip phenomena in concentrated ionic liquid-based magnetorheological fluids

Ionic liquids (ILs) have been recently proposed as carrier for magnetorheological (MR) fluids. Their special properties, such as very low vapor pressure and high thermal stability, make ILs highly suitable dispersion media to increase the broad range of technological applications that magnetorheological fluids already have. It has been just reported that using ILs as carriers in MR fluids an improvement in the colloidal stability and suspension redispersibility is obtained. In this work, the magnetorheological behavior of highly concentrated suspensions in ILs is studied. Two kinds of suspensions were analyzed: using an ionic liquid of low conductivity and a mineral oil as carriers. In both cases, silica-coated iron microparticles were used as solid phase, being the solid volume concentration of 50% vol. A complete magnetorheological analysis focused on the wall slip phenomenon was performed. Steady-state and oscillatory experiments were carried out. In order to study wall slip effects, all experiments were performed with a plate–plate system, using both smooth and rough measuring surfaces. A significant effect of wall slip was observed when the experiments were performed using smooth surfaces. The novelty of this paper is mainly based on (1) the use of an ionic liquid as carrier to prepare magnetic suspensions, and?(2) the analysis of wall slip phenomena in MR fluids with a particle content close to the maximum packing fraction.     

Measurement of temperature-dependent thermal conductivity and viscosity of TiO2-water nanofluids

Nanofluid is an innovative heat transfer fluid with superior potential for enhancing the heat transfer performance of conventional fluids. Many attempts have been made to investigate its thermal conductivity and viscosity, which are important thermophysical properties. No definitive agreements have emerged, however, about these properties. This article reports the thermal conductivity and dynamic viscosity of nanofluids experimentally. TiO₂ nanoparticles dispersed in water with volume concentration of 0.2–2 vol.% are used in the present study. A transient hot-wire apparatus is used for measuring the thermal conductivity of nanofluids whereas the Bohlin rotational rheometer (Malvern Instrument) is used to measure the viscosity of nanofluids. The data are collected for temperatures ranging from 15 °C to 35 °C. The results show that the measured viscosity and thermal conductivity of nanofluids increased as the particle concentrations increased and are higher than the values of the base liquids. Furthermore, thermal conductivity of nanofluids increased with increasing nanofluid temperatures and, conversely, the viscosity of nanofluids decreased with increasing temperature of nanofluids. Moreover, the measured thermal conductivity and viscosity of nanofluids are quite different from the predicted values from the existing correlations and the data reported by other researchers. Finally, new thermophysical correlations are proposed for predicting the thermal conductivity and viscosity of nanofluids.     

A prediction model for the effective thermal conductivity of nanofluids considering agglomeration and the radial distribution function of nanoparticles

Conventional heat transfer fluids usually have low thermal conductivity, limiting their efficiency in many applications. Many experiments have shown that adding nanosize solid particles to conventional fluids can greatly enhance their thermal conductivity. To explain this anomalous phenomenon, many theoretical investigations have been conducted in recent years. Some of this research has indicated that the particle agglomeration effect that commonly occurs in nanofluids should play an important role in such enhancement of the thermal conductivity, while some have shown that the enhancement of the effective thermal conductivity might be accounted for by the structure of nanofluids, which can be described using the radial distribution function of particles. However, theoretical predictions from these studies are not in very good agreement with experimental results. This paper proposes a prediction model for the effective thermal conductivity of nanofluids, considering both the agglomeration effect and the radial distribution function of nanoparticles. The resulting theoretical predictions for several sets of nanofluids are highly consistent with experimental data.     

The effect of carbon nanotubes in enhancing the thermal transport properties of PCM during solidification

This study is aimed to prepare a novel class of nanofluid phase change material (NFPCM) by dispersing a small amount of multi-walled carbon nanotubes (MWCNT) in liquid paraffin, to enhance the heat transfer properties and examine the characteristics of the NFPCM during the solidification process. The stable NFPCMs are prepared by dispersing the MWCNT in liquid paraffin at 30°C with volume fractions of 0.15, 0.3, 0.45 and 0.6% without any dispersing agents. The rheology measurement illustrates the Newtonian fluid behavior in the shear stress range of 1–10?Pa. The differential scanning calorimetric results showed that there is no observable variation in the freezing/melting temperature of the NFPCM, and only a small observable change in the latent heat values. The thermal conductivity of various NFPCM is measured. The enhancement in thermal conductivity increases with the increased volume fraction of the MWCNT, and shows a weak dependence on the temperature. Further, for the NFPCM with a volume fraction of 0.6%, there is an appreciable increase in heat transfer with a reduction in the solidification time of 33.64%. The enhancement in the heat transfer performance would alleviate the major problems that have been encountered in the conventional phase change materials since several years.     

Experimental measurement of thermophysical properties of oxide-water nano-fluids down to ice-point

This paper presents the measurement of the thermal conductivity and the dynamic viscosity of Al₂O₃-water (1-4% particle volume fraction) and TiO₂-water (1-6% particle volume fraction) nano-fluids carried out at atmospheric pressure in the temperature range from 1 to 40 °C, which is particularly interesting for the application of nano-fluids as thermal medium in refrigeration and air-conditioning.The thermal conductivity measurement was performed by using a Transient Hot Disk TPS 2500S apparatus instrumented with a 7577 probe (2.001 mm in radius) having a maximum uncertainty (k = 2) lower than ±5.0% of the reading. The dynamic viscosity measurement and the rheological analysis were carried out by a rotating disc type rheometer Haake Mars II instrumented with a single cone probe (60 mm in diameter and 1° angle) having a maximum uncertainty (k = 2) lower than ±5.0% of the reading.The thermal conductivity measurements of the tested nano-fluids show a great sensitivity to particle volume fraction and temperature and a weak sensitivity to cluster average size: TiO₂-water and Al₂O₃-water nano-fluids show a thermal conductivity enhancement (with reference to pure water) from −2 to 16% and from −2 to 23% respectively.TiO₂-water and Al₂O₃-water nano-fluids exhibit a Newtonian behaviour in all the investigated ranges of temperature and nano-particle volume fraction. The relative viscosity shows a great sensitivity to particle volume fraction and cluster average size and no sensitivity to temperature: TiO₂-water and Al₂O₃-water nano-fluids show a dynamic viscosity increase with respect to pure water from 17 to 210% and from 15 to 150% respectively.Al₂O₃-water nano-fluid seems to be more promising as thermal medium than TiO₂-water nano-fluid, particularly at low thermal level (between ambient temperature and ice point) where TiO₂-water is not suitable showing worse performance than pure water.Present experimental measurements were compared both with available measurements carried out by different researchers and computational models for thermophysical properties of suspensions.     

Thermophoresis and Brownian motion effects on boundary layer flow of nanofluid in presence of thermal stratification due to solar energy

The problem of laminar fluid flow,which results from the stretching of a vertical surface with variable stream conditions in a nanofluid due to solar energy,is investigated numerically.The model used for the nanofluid incorporates the effects of the Brownian motion and thermophoresis in the presence of thermal stratification.The symmetry groups admitted by the corresponding boundary value problem are obtained by using a special form of Lie group transformations,namely,the scaling group of transformations.An exact solution is obtained for the translation symmetrys,and the numerical solutions are obtained for the scaling symmetry.This solution depends on the Lewis number,the Brownian motion parameter,the thermal stratification parameter,and the thermophoretic parameter.The conclusion is drawn that the flow field,the temperature,and the nanoparticle volume fraction profiles are significantly influenced by these parameters.Nanofluids have been shown to increase the thermal conductivity and convective heat transfer performance of base liquids.Nanoparticles in the base fluids also offer the potential in improving the radiative properties of the liquids,leading to an increase in the efficiency of direct absorption solar collectors.     

Heat- and mass-transport in aqueous silica nanofluids

Using the transient hot wire and pulsed field gradient nuclear magnetic resonance methods we determined the thermal conductivity and the solvent self-diffusion coefficient (SDC) in aqueous suspensions of quasi-monodisperse spherical silica nanoparticles. The thermal conductivity was found to increase at higher volume fraction of nanoparticles in accordance with the effective medium theory albeit with a smaller slope. On the other hand, the SDC was found to decrease with nanoparticle volume fraction faster than predicted by the effective medium theory. These deviations can be explained by the presence of an interfacial heat-transfer resistance and water retention by the nanoparticles, respectively. We found no evidence for anomalous enhancement in the transport properties of nanofluids reported earlier by other groups.     

Prediction of the thermal conductivity of the constituents of fiber reinforced composite laminates

Three empirical formulas are developed to predict the thermal conductivities of fiber-reinforced composite laminates (FRCL) and its constituents. The inherent two or three-dimensional problem that is common in composites is simplified to a one-dimensional problem. The validity of the models is verified through finite element analysis. This method utilizes the parallel and series thermal models of composite walls. The models are tested at different fiber-to-resin volume ratios (30:70–75:25) and various fiber-to-resin thermal conductivity ratios (0.2–5). The predicted thermal conductivity of the fiber can be accurately predicted throughout the spectrum via two models. The first model is a first-order formula (R ² = 0.94) while the second model is a second-order formula (R ² = 0.976). These two models can be used to predict the fiber thermal conductivity based on the easily measured resin and laminate values. A third model to predict the overall laminate thermal conductivity is introduced. The thermal conductivity of the composite panel is predicted with very high accuracy (R ² = 0.995). The thermal conductivity predicted via the use of these models has an excellent agreement with the experimental measurements. Another use of these models is to determine the fiber-to-resin volume ratio (if all thermal conductivities of fiber, resin and laminate are known).     

Influence of temperature dependent conductivity of a nanofluid in a vertical rectangular duct

Natural convective heat transfer and fluid flow in a vertical rectangular duct filled with a nanofluid is studied numerically assuming the thermal conductivity to be dependent on the fluid temperature. The transport equations for mass, momentum and energy formulated in dimensionless form are solved numerically using finite difference method. Particular efforts have been focused on the effects of the thermal conductivity variation parameter, Grashof number, Brinkman number, nanoparticles volume fraction, aspect ratio and type of nanoparticles on the fluid flow and heat transfer inside the cavity. It is found that the flow was enhanced for the increase in Grashof number, Brinkman number and aspect ratio for any values of conductivity variation parameter and for regular fluid and nanofluid. The heat transfer rate for regular fluid is less than that for the nanofluid for all governing parameters.     

The effect of real viscosity on the heat transfer of water based Al2O3 nanofluids in a two-sided lid-driven differentially heated rectangular cavity

The heat transfer and fluid flow behavior of water based Al₂O₃ nanofluids are numerically investigated inside a two-sided lid-driven differentially heated rectangular cavity. Physical properties which have major effects on the heat transfer of nanofluids such as viscosity and thermal conductivity are experimentally investigated and correlated and subsequently used as input data in the numerical simulation. Transport equations are numerically solved with finite volume approach using SIMPLEC algorithm. It was found that not only the thermal conductivity but also the viscosity of nanofluids has a key role in the heat transfer of nanofluids. The results show that at low Reynolds number, increasing the volume fraction of nanoparticles increases the viscosity and has a deteriorating effect on the heat transfer of nanofluids. At high Reynolds number, the increase in the viscosity is compensated by force convection and the increase in the volume fraction of nanoparticles which results in an increase in heat transfer is in coincidence with experimental results.     

Heat Transfer and Particle Behaviours in Dispersed Two-Phase Flow with Different Heat Conductivities for Liquid and Solid

Liquid-solid two-phase flow with heat transfer is directly simulated, to investigate the effects of the ratios of heat conductivities (solid to liquid) and bulk solid volume fraction from dense to dilute situations. The interaction between fluid and particles is solved by our original immersed solid approach on a rectangular grid system. A discrete element method with a soft-sphere collision model is applied for particle-particle and particle-wall interactions. Governing equation of temperature is time-updated with an implicit treatment for the diffusion term, which enables robust simulation with particles of very high/low ratios of heat conductivities (from 1/1000 to 1000) to the fluid. The local heat flux at the fluid-solid interface is modelled by a new flux decomposition technique, and incorporated into the implicit scheme of the temperature. The method is applied to a 2-D particulate flow in a natural convection in a square domain at a relatively low Rayleigh number. In the dense condition, for the cases with high ratios of heat conductivity, the heat transfer is promoted by strong convection, while the particles of low ratios of heat conductivity tend to hinder the development of the temperature rise in the flow field, causing a weak convection and low Nusselt number. Under a condition of relatively low solid volume fraction, fixed particles only depress the heat convection as the number of particles and heat conductivity ratio increase. For the cases with freely-moving particles, on the other hand, heat conductivity of particles has a stronger influence on the heat transfer of the system than the number of particles. The above simulation results highlight the effect of temperature distributions within the particles and liquid.     

Numerical study of solidification of a nano-enhanced phase change material (NEPCM) in a thermal storage system

The effects of nanoparticle dispersion on solidification of a Cu-n-hexadecane nanofluid inside a vertical enclosure are investigated numerically for different temperatures of the left vertical wall. An enthalpy porosity technique is used to trace the solid-liquid interface. The resulting nanoparticle-enhanced phase change materials (NEPCMs) exhibit enhanced thermal conductivity in comparison to the base material. The effect of the wall temperature and nanoparticle volume fraction are studied in terms of the solid fraction and the shape of the solid-liquid phase front. It has been found that a lower wall temperature and a higher nanoparticle volume fraction result in a larger solid fraction. The increase in the heat release rate of the NEPCM shows its great potential for diverse thermal energy storage applications.     

Experimental characterization of convective heat transfer with MWCNT based nanofluids under laminar flow conditions

This study describes an investigation on the convective heat transfer performance of aqueous suspensions of multiwalled carbon nanotubes. The results suggested an increase on heat transfer coefficient of 47 % for 0.5 % volume fraction. Moreover, the enhancement observed during thermal conductivity assessment, cannot fully explain the heat transfer intensification. This could be associated to the random movements among the particles through a fluid, caused by the impact of the base fluid molecules.     

Multilayer film casting of modified Giesekus fluids Part 2. Linear stability analysis

A linear stability analysis of the multilayer film casting of polymeric fluids has been conducted. A modified Giesekus model was used to characterize the rheological behaviors of the fluids. The critical draw ratio at the onset of draw resonance was found to depend on the elongational and shear viscosities of the fluids. Extensional-thickening has a stabilizing effect, whereas shear-thinning and extensional-thinning have destabilizing effects. The critical draw ratios for bilayer films of various thickness fractions are bounded by those for single layer films of the two fluids. When the two fluids have a comparable elongational viscosity, the critical draw ratio at a given Deborah number varies linearly with thickness fraction. When one fluid has a much larger elongational viscosity, it dominates the flow and the critical draw ratios at most thickness fractions remain close to its critical draw ratio as a single layer film. When the dominating fluid exhibits extensional-thickening, a film with a certain thickness fraction has more than one critical draw ratio at a given Deborah number and may not exhibit draw resonance within some range of the Deborah number.