Enhanced specific heat and thermal conductivity of ternary carbonate nanofluids with carbon nanotubes for solar power applications

Specific heat and thermal conductivity are important thermal properties of high-temperature heat transfer fluids and thermal storage materials for supercritical solar power plants. In the present work, nanofluids composed of ternary carbonate Li₂CO₃-K₂CO₃-Na₂CO₃ (4:4:2, mass ratio) and 1.0 wt.% carbon nanotubes (CNT) were prepared to obtain high-temperature heat transfer and storage media with enhanced specific heat and thermal conductivity. The dispersion of CNTs in the nanofluids was tuned by changing the evaporation temperature (100, 140, 180 and 220 °C) and adding surfactants such as sodium dodecyl benzene sulfonate (SDBS), sodium dodecyl sulfate (SDS), or gum Arabic (GA). The results showed that GA and SDS facilitate good dispersion of CNT in nanofluids at the evaporation temperatures of 140 °C and 180 °C, resulting in the formation of more needle-like nanostructures. The higher increase in the specific heat and thermal conductivity of the nanofluids with SDS at 500 °C was 78.3% and 149.2%, respectively. Additionally, the specific heat of as-prepared ternary carbonate nanofluids exhibits a good thermal stability after 30 cycles of thermal shock experiments.     

Thermal performance of multi-walled carbon nanotubes (MWCNTs) in aqueous suspensions with surfactants SDBS and SDS

The surfactants of sodium dodecylbenzene sulfonate (SDBS) and sodium dodecyl sulfate (SDS) are used in multi-walled carbon nanotubes (MWCNT) aqueous solution respectively due to the hydrophobic nature of MWCNT. Thermal conductivities of nanofluid solutions are measured via the LAMBDA measuring system by transient hot wire method and compared as function of dispersing two different surfactants. MWCNT (hereinafter sometime referred to as CNTs) nanofluid gets a good dispersion and long time stability with both surfactants within 3/1 relative ratio mixture. However, the thermal conductivity of nanofluid decreases with increasing the concentration of both surfactants, and CNT nanofluid with SDBS exhibits better thermal conductivity than that with SDS dispersant. Finally the proper mixture ratio of CNT nanofluid with SDBS and pH value is examined and results show that 0.5 wt.% CNT nanofluids with 0.25 wt.% SDBS, at pH ≈ 9.0 condition display the best thermal performance which increases by 2.8% totally on thermal conductivity compared with that of base fluid distilled water (DW).     

Pool boiling characteristics of multiwalled carbon nanotube (CNT) based nanofluids over a flat plate heater

This paper is mainly concerned about the pool boiling heat transfer behavior of multi-walled carbon nanotubes (CNTs) suspension in pure water and water containing 9.0% by weight of sodium lauryl sulphate anionic surfactant (SDS). Three different concentrations of 0.25%, 0.5% and 1.0% by volume of CNT dispersed with water and water containing 9.0% by weight of sodium lauryl sulphate anionic surfactant (SDS) were prepared and boiling experiments were conducted over a stainless steel flat plate heater of size 30 mm² and 0.44 mm thickness. The test results exhibit that the addition of carbon nanotubes increases boiling heat transfer coefficients of the base fluids. At a given heat flux of 500 kW/m², the enhancement of heat transfer coefficient was found to be 1.5, 2.6 and 3.0 times of water corresponding to 0.25%, 0.5% and 1.0% concentration of CNT by volume in water, respectively. In water–CNT–surfactant nanofluid, it was found that 0.5% of CNT concentration gives the highest enhancement of 1.7 compared with water. In both water and water–surfactant base fluids, it was observed that the enhancement factor for 0.25% of CNT first increases up to the heat flux of 66 kW/m² and then decreases for higher heat fluxes. Further, the overall heat transfer coefficient enhancement in the water–CNT nanofluids is approximately two times higher than that in the water–CNT–surfactant nanofluids. With increasing heat flux, however, the enhancement was concealed due to vigorous bubble generation for both water–CNT and water–CNT–surfactant nanofluids. Foaming was also observed over the liquid-free surface in water–CNT–surfactant nanofluids during the investigation. No fouling over the test-section surface was observed after experimentation.     

Investigation of Thermal Conductivity and Viscosity of Carbon Nanotubes–Ethylene Glycol Nanofluids

Conventional fluids used for heat transfer applications in automobiles limit the performance enhancement and compactness of the heat exchangers. These problems can be overcome by using the technology of nanofluids. The objectives of this work are to prepare nanofluids and to study their dynamic viscosity and thermal conductivity. Chemically treated carbon nanotubes (CNTs) were added with ethylene glycol (EG) and sonicated using a bath sonicator to have a homogeneous dispersion of CNTs in EG. In this study, the nanofluids were prepared with different concentrations of CNTs varying from 0.12 to 0.4 wt%. The dynamic viscosity of nanofluids was measured using a rheometer over a temperature range of 25°C to 60°C. It was observed that the viscosity of nanofluids decreases with an increase of temperature and enhances with CNT concentration. The nanofluid follows the characteristic behavior of Newtonian fluids. A linear rise in thermal conductivity of ethylene glycol was observed with an increase of CNT concentration. It is concluded that EG–CNT nanofluids are promising to meet the challenges required by automobile systems.     

Effect of CNT concentration and agitation on surface heat flux during quenching in CNT nanofluids

In this article, the effect of Carbon Nanotube (CNT) concentration and agitation on the heat transfer rate has been studied during immersion quenching in CNT nanofluids. For this purpose, CNT nanofluids were prepared by suspending chemically treated CNTs (TCNT) at four different concentrations in deionized (D.I) water without using any surfactant. Quench probes with a diameter of 20 mm and a length of 50 mm were machined from 304L stainless steel (SS) and quenched in water and CNT nanofluids with the CNT concentration ranging from 0.25 to 1.0 wt.%. The heat flux and temperature at the quenched surface were estimated based on the Inverse Heat Conduction (IHC) method using the temperature data recorded at 2 mm below the probe surface during quenching. The computation results showed that the peak heat flux increased with an increase in the CNT concentration up to 0.50 wt.% and started decreasing with further increase in the CNT concentration. The enhanced heat transfer performance of CNT nanofluids during quenching at lower concentration of CNTs is attributed to their higher effective thermal conductivity. The reduced heat transfer performance of CNT nanofluids having higher concentration of CNTs is due to the increased viscosity of CNT nanofluids. The effect of agitation on heat transfer rate during quenching has also been studied in this work by stirring the CNT nanofluid prepared with 0.50 wt.% of CNTs which recorded the maximum peak heat flux among the four concentrations. The effect of CNT nanofluid agitation was counter-intuitive and resulted in decreased heat transfer rate with the increase in agitation rate.     

Thermal conductivity of carbon nanotube nanofluid—Experimental and theoretical study

In this work, thermal conductivity of carbon nanotubes' (CNTs) nanofluid is studied both experimentally and theoretically. CNT nanofluids were stabilized using gum arabic (GA). The concentration of CNTs was varied from 0.01–0.1 wt% while the concentration of GA was varied from 1–2.5 wt%, respectively. The effect of particle volume fraction and temperature on the thermal conductivity enhancement of the nanofluids was also studied. A simple thermal conductivity model which demonstrates the effect of diameter and aspect ratio of the CNTs and takes into account the effect of temperature on thermal conductivity enhancement is presented. Good agreement between experimental and estimated values proves that the proposed model can provide precise prediction of the thermal conductivity of fluid containing CNTs. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20405     

Rheology and thermal conductivity of non-porous silica (SiO2) in viscous glycerol and ethylene glycol based nanofluids

Nanofluids are advanced fluids with novel properties useful for diverse applications in heat transfer. This article reports the experimental determination of thermal conductivity and viscosity for silica (SiO₂) nanofluids in ethylene glycol (EG) and glycerol (G) as base fluids. A two-step method was applied to disperse the nanoparticles in the base fluids for the particle volume concentration of 0.5–2.0%. The dispersion stability of the nanofluids was evaluated by zeta potential analysis. All the measurements were performed in the temperature interval from 30 °C to 80 °C. It was found that the thermal conductivity increases with temperature. The SiO₂-EG showed higher conductivity enhancement than SiO₂-G nanofluids. Rheological analyses confirm Newtonian behavior for silica nanofluids within shear rate range of 20–100 s^− 1. Viscosity decreases with an increase in operating temperature. The SiO₂-EG demonstrated very weak temperature dependence compared to the SiO₂-G nanofluids. Based on these measured properties, the criterion for heat transfer performance was determined. Furthermore, equations have been proposed with accuracy within ± 10% deviations to predict the thermal conductivity and dynamic viscosity of EG and G-based SiO₂ nanofluids.     

Numerical Calculation of the Effective Thermal Conductivity of Nanocomposites

The effective thermal conductivity (K _eff) of carbon nanotube (CNT) composites is affected by the thermal boundary resistance (TBR), the dispersion pattern, and geometry distribution of the CNTs. Traditional effective medium theories assume that CNTs are perfectly dispersed without considering TBR. In this work, we report the development of a new algorithm using CNTs with 3-D worm-like geometry and different persistence lengths. We describe how to obtain K _eff using simulations of these realistic CNT configurations with off-lattice Monte Carlo simulation. The results are compared with straight cylinder models without effects of persistence length.     

Experimental thermal conductivity of ethylene glycol and water mixture based low volume concentration of Al2O3 and CuO nanofluids

Thermal conductivity of ethylene glycol and water mixture based Al₂O₃ and CuO nanofluids has been estimated experimentally at different volume concentrations and temperatures. The base fluid is a mixture of 50:50% (by weight) of ethylene glycol and water (EG/W). The particle concentration up to 0.8% and temperature range from 15 °C–50 °C were considered. Both the nanofluids are exhibiting higher thermal conductivity compared to base fluid. Under same volume concentration and temperature, CuO nanofluid thermal conductivity is more compared to Al₂O₃ nanofluid. A new correlation was developed based on the experimental data for the estimation of thermal conductivity of both the nanofluids.     

Study of synthesis,stability and thermo-physical properties of graphene nanoplatelet/platinum hybrid nanofluid

In the present study a new synthesis method has been introduced for the decoration of platinum (Pt) on the functionalized graphene nanoplatelet (GNP) and also highlighted the preparation method of nanofluids. GNP–Pt uniform nanocomposite was produced from a simple chemical reaction procedure, which included acid treatment for functionalization of GNP. The surface characterization was performed by various techniques such as XRD, FESEM and TEM. The effective thermal conductivity, density, viscosity, specific heat capacity and stability of functionalized GNP–Pt water based nanofluids were investigated in different instruments. The GNP–Pt hybrid nanofluids were prepared by dispersing the nanocomposite in base fluid without adding any surfactant. The examined nanofluids were stable and no significant sedimentation was observed for a long time (22 days). Thermal conductivity of GNP–Pt nanocomposite dispersed in distilled water nanofluids shows an enhancement of 17.77% at 40 °C and 0.1% weight concentration.     

Solvent effect in the synthesis of nanostructured Pt–Sn/CNT as electrocatalysts for the electrooxidation of ethanol

Pt and Pt–Sn nanoparticles were synthesized and supported onto carbon nanotubes (CNT), the electrocatalytic activity towards the ethanol oxidation reaction was analyzed. The effect of the solvent employed for the synthesis was evaluated. Metal nanoparticles synthesis was made using water (Pt–Sn/CNT-W) or ethanol (Pt–Sn/CNT-E) as a solvent. Pt–Sn/CNT-W material presented only Pt–Sn alloy nanoparticles homogeneously distributed on the carbon nanotubes support. Pt–Sn/CNT-E sample showed non well-dispersed nanoparticles forming agglomerates along the CNTs surface with predominantly Sn⁴⁺ superficial species (SnO₂) as show the XPS, FTIR and electrochemical results. These surface arrangements had important effects on the electrocatalytic properties. Pt–Sn/CNT-W shows higher ethanol electrooxidation activity than the Pt–Sn/CNT-E, which is attributed to: a) the double catalytic effect and the intrinsic electronic mechanism favored by the presence of Sn; b) the good particle dispersion of the bimetallic active phase on the CNT and; c) the absence of SnO₂ species.     

Experimental investigation on pool boiling heat transfer of ZnO,and CuO water-based nanofluids and effect of surfactant on heat transfer coefficient

Comparison of boiling performance of nanofluids and mixtures of nanofluids with surfactant is an objective of this research. Experimental investigation has been performed with different heat flux and concentrations of nanoparticles and surfactant. CuO and ZnO water-based nanofluids are used and sodium dodecyl sulfate (SDS) is used as surfactant. The size of nanoparticles is measured from Field Emission Scanning Electron Microscopy pictures. Roughness of rod heater is calculated by using Atomic Force Microscopy picture. The pure water is tested after each run with nanofluid and mixture of nanofluid with SDS and the results are presented. Result from experiments demonstrates that the addition of SDS to nanofluids solution resulted in improving boiling performance. Experimental results also show an inefficient process by excluding surfactant. An optimum value for heat transfer coefficient is found by increasing of surfactant concentration within CuO nanofluid (0.01wt%CuO). Pictures of coated and clean surface in boiling pure water are employed for understanding the dynamics of bubbles. Gorenflo constant (h₀) is utilized to show the effect of addition of nanoparticles and SDS in boiling performance of base fluid.     

Phase change characteristics of ethylene glycol solution‐based nanofluids for subzero thermal energy storage

Nanofluids, particularly water‐based nanofluids, have been extensively studied as liquid–solid phase change materials (PCMs) for thermal energy storage (TES). In this study, nanofluids with aqueous ethylene glycol (EG) solution as the base fluid are proposed as a novel PCM for cold thermal energy storage. Nanofluids were prepared by dispersing 0.1–0.4 wt% TiO₂ nanoparticles into 12, 22, and 34 vol.% EG solutions. The dispersion stability of the nanofluids was evaluated by Turbiscan Lab. The liquid–solid phase change characteristics of the nanofluids were also investigated. Phase change temperature (PCT), nucleation temperature, and half freezing time (HFT) were investigated in freezing experiments. Subcooling degree and HFT reduction were then calculated. Latent heat of solidification was measured using differential scanning calorimetry. Thermal conductivity was determined using the hot disk thermal constant analyzer. Experimental results show that the nanoparticles decreased the PCT of 34 vol.% EG solution but minimally influenced the PCT of 12 and 22 vol.% EG solutions. For all nanofluids, the nanoparticles decreased the subcooling degree, HFT, and latent heat but increased the thermal conductivity of the EG solutions. The mechanism of the improvement of the phase change characteristics and decrease in latent heat by the nanoparticles was discussed. The nanoparticles simultaneously served as nucleating agent that induced crystal nucleation and as impurities that disturbed the growth of water crystals in EG solution‐based nanofluids. Copyright © 2016 John Wiley & Sons, Ltd.     

An experimental study on the pressure drop of nanofluids containing carbon nanotubes in a horizontal tube

This article reports an experimental study on the flow characteristics of the aqueous suspensions of carbon nanotubes (CNTs). Stable nanotube suspensions were made for pressure drop measurements by two different methods. One of them is to disperse nanotubes using a surfactant, and the other is to introduce oxygen-containing functional groups on the CNT surfaces by acid treatment. The pressure drops in a horizontal tube and viscosities of nanofluids were measured and the effects of CNT loading and different preparation methods were investigated. Viscosity measurements show that both CNT nanofluids prepared by two methods are shear thinning fluids and at the same volume fraction, the nanofluids prepared by the acid treatment have much smaller viscosity than the ones made with surfactant. Under laminar flow conditions, the friction factor of CNT nanofluids stabilized by adding surfactant is much larger than that of CNT nanofluids prepared by acid treatment, and both nanofluids show larger friction factors than distilled water. In contrast to this, under turbulent flow conditions, the friction factors of both nanofluids become similar to that of the base fluids as the flow rate increases. It is also shown that as CNT loading is increased, laminar regime of nanofluids has been extended to further higher flow rates, therefore, nanofluids could have low friction factors than pure water flows at certain range of flow rates.     

Synthesis and characterization of Pt nanoparticles on sulfur-modified carbon nanotubes for methanol oxidation

Pt nanoparticles supported on carbon nanotubes (Pt/CNTs) have been synthesized from sulfur-modified CNTs impregnated with H₂PtCl₆ as Pt precursor. The dispersion and size of Pt nanoparticles in the synthesized Pt/CNT nanocomposites are remarkably affected by the amount of sulfur modifier (S/CNT ratio). The results of X-ray diffraction and transmission electron microscopy indicate that an S/CNT ratio of 0.3 affords well dispersed Pt nanoparticles on CNTs with an average particle size of less than 3 nm and a narrow size distribution. Among different catalysts, the Pt/CNT nanocomposite synthesized at S/CNT ratio of 0.3 showed highest electrochemically active surface area (88.4 m² g⁻¹) and highest catalytic activity for methanol oxidation reaction. The mass-normalized methanol oxidation peak current observed for this catalyst (862.8 A g⁻¹) was ∼ 6.5 folds of that for Pt deposited on pristine CNTs (133.2 A g⁻¹) and ∼ 2.3 folds of a commercial Pt/C (381.2 A g⁻¹). The results clearly demonstrate the effectiveness of a relatively simple route for preparation of sulfur-modified CNTs as a precursor for the synthesis of Pt/CNTs, without the need for tedious pretreatment procedures to modify CNTs or complex equipments to achieve high dispersion of Pt nanoparticles on the support.     

A numerical comparison among different water-based hybrid nanofluids on their influences on natural convection heat transfer in a triangular solar collector for different tilt angles

Natural convection inside a triangular solar collector is investigated numerically for different nanofluids and hybrid nanofluids in this study. The individual effects of Al₂O₃–water, carbon nanotubes (CNT)–water, and Cu–water nanofluids are observed for different solid volume fractions of nanoparticles (0%–10%). Three types of hybrid nanofluids are prepared using different ratios of Al₂O₃, CNT, and Cu nanoparticles in water. A comparison is made varying the Rayleigh numbers within laminar range (10³–10⁶) for different tilt angles (0°, 30°, 60°, and 90°) of the solar collector. The inclined surface of the triangular solar collector is isothermally cold and the bottom wall (absorber plate) is isothermally hot, whereas the vertical wall with respect to the absorber plate is considered adiabatic. Average Nusselt numbers along the hot wall for different parameters are observed. Streamlines and isotherm contours are also plotted for different cases. Dimensionless governing Navier–Stokes and thermal energy conservation equations are solved by Galerkin weighted residual finite element method. Better convective heat transfer is found for higher Rayleigh number, solid volume fraction, and tilt angle. In the case of hybrid nanofluid, increasing the percentage of the nanoparticle that gives better heat transfer performance individually results in enhancing natural convection heat transfer inside the enclosure.     

Performance analysis of hybrid/single nanofluids on augmentation of heat transport in lid-driven undulated cavity

This numerical study reveals the heat transfer performance of hybrid/single nanofluids inside a lid-driven sinusoidal trapezoidal-shaped enclosure. The right and left inclined surfaces of the trapezium have been considered as insulated, whereas the bottom sinusoidal wavy and the flat top surfaces of the enclosure as hot and cold, respectively. The governing partial differential equations of fluid's velocity and temperature have been resolved by applying the finite element method. The implications of Prandtl number (4.2-6.2), Richardson number (0.1-10.0), undulation number (0-3), nanoparticles volume fraction (0%-3%), and nanofluid/base fluid (water, water–copper (Cu), water–Cu–carbon nanotube, water–Cu–copper oxide (CuO), water–Cu–TiO₂, and water–Cu–Al₂O₃) on the velocity and temperature profiles have been studied. Simulated findings have been represented by means of streamlines, isothermal lines, and average Nusselt number of above-mentioned hybrid nanofluids for varying the governing parameters. The comparison of heat transfer rates using hybrid nanofluids and pure water has been also shown. The heat transfer rate is increased about 15% for varying Richardson number from 0.1 to 10.0. Blending of two nanoparticles suspension in base fluid has a higher heat transfer rate—approximately 5% than a mononanoparticle. Moreover, a higher average Nusselt number is obtained by 14.7% using the wavy surface than the flat surface of the enclosure. Thus, this study showed that applying hybrid nanofluid may be beneficial to obtain expected thermal performance.     

Counterflow HE analysis of Cu and SiO2 nanofluids in the developing flow region

Three concentrations of 0.2, 0.6, and 1.0 vol.% Copper/25 nm and silica/22 nm nanofluids are prepared in a base liquid glycerol–water mixture of 30:70 ratio by volume (GW70). The thermophysical properties of Cu and SiO₂ nanofluids are determined with a TPS500S hot disc thermal analyzer and Brookfield viscometer in the temperature range of 20–80°C. The maximum enhancement in Cu and SiO₂ nanofluid viscosity (63.4%, 35.7%), thermal conductivity (100.4%, 71.3%), and density (7.5%, 1.5%) while specific heat (7.8%, 2.3%) determined for 1.0% concentration at 80°C compared to base liquid GW70. Heat transfer experiments are conducted in a short-length double pipe heat exchanger. The flow rates resulted in the lamifnar entry length region. A maximum enhancement in the overall heat transfer coefficient (HTC; 25.0%, 19.7%) and convective HTC (46.2%, 34.8%), respectively for Cu and SiO₂ nanofluids is estimated at 1.0% concentration compared to base liquid at a bulk temperature of 35°C.     

Direct numerical simulation of thermal conductivity of nanofluids: The effect of temperature two-way coupling and coagulation of particles

An Eulerian–Lagrangian based direct numerical simulations (DNS) model was developed to investigate the effective thermal conductivity of nanofluids. A two-way coupling term to resolve the temperature interactions between the solid particles and fluid field was considered. The model also considered various forces acting on the nanoparticles. Cu/water nanofluids with 100 nm particles and Al₂O₃/water nanofluids with 80 nm particles were simulated at different volume fractions and the effective thermal conductivity of nanofluids was calculated. The present results suggest that the particle conductivity and forces acting on nanoparticle are necessary while predicting the effective thermal conductivity of nanofluids.     

The effect of functionalized group concentration on the stability and thermal conductivity of carbon nanotube fluid as heat transfer media

In this research, the pristine CNTs sample synthesized by the CCVD method contains catalytic particles and the carbonaceous impurities, and then the special purification procedure was done. By different methods CNT functionalized with various concentration of COOH was prepared. The carboxylated CNTs were analyzed by back titration method for determining the COOH concentrations on the surface of the oxidized CNTs. Thermal conductivity of difference carbon nanotube fluid has been measured under the stable condition by KD₂ prob. For the first time, we have compared the effect of difference COOH concentration as important parameter in stability and heat transfer behavior of nanofluid. The results show that increasing the functionalized group causes better stability and higher thermal conductivity if the surface of MWNT does not damage in functionalize process.