Characterization and stability analysis of oil‐based copper oxide nanofluids for medium temperature solar collectors

Thermal oils are widely used as heat transfer fluids in medium temperature applications. Addition of small amounts of nanoparticles in such fluids can significantly improve their thermophysical properties. This paper presents experimental investigation of an oil‐based nanofluids prepared by dispersing different concentrations (0.25 wt%–1.0 wt%) of copper oxide nanoparticles in Therminol‐55 oil using two‐step method. Shear mixing and ultrasonication were used for uniform distribution and de‐agglomeration of nanoparticles to enhance the stability of the suspensions. The effect of nanoparticles concentrations on thermophysical properties of the nanofluids was analysed by measuring thermal conductivity, dynamic viscosity, effective density and specific heat capacity at different temperatures (25 °C–130 °C). Thermal conductivity exhibited increasing trend with rising temperature and increase in nanoparticles loading. A significant decrease in dynamic viscosity and effective density against increasing temperature makes it suitable for medium temperature applications. Nano‐oils with improved thermal properties are expected to increase the efficiency of concentrating solar thermal collectors.     

Experimental investigation on thermophysical properties of solar glycol dispersed with multi-walled carbon nanotubes

This article deals with well-dispersed solar glycol-based nanofluids containing multi-walled carbon nanotube (MWCNT) nanoparticles with different particle volume concentrations of 0.1%, 0.2%, 0.3%, and 0.4% prepared by typical two-step method. Thermal conductivity, viscosity and specific heat capacity of solar glycol (SG)-based MWCNT nanofluids, in the temperature range of 30°C–70°C were measured. The values of density showed a noticeable deviation from the predictions of Pak and Cho correlation. Hence, correlations are developed for thermal conductivity and viscosity from the experimental results obtained from the various range parameters of interest. The presence of MWCNT enhanced the thermal conductivity of the nanofluids by 17.26% at 0.4 vol.% particle concentration at 70°C. The relative viscosity of MWCNT nanofluids depends on the nanoparticles percentage concentration and decreases significantly with increase in temperature for higher concentrations. The presence of MWCNT enhances the specific heat of the nanofluids significantly, and this enrichment decreases with the increase of the MWCNT concentration. MWCNT/SG represents a new and innovative class of heat-transfer fluid, which possesses excellent thermophysical properties. The MWCNT/SG-based nanofluids could be suitable working fluids for solar thermal and automobile applications.     

Study of viscosity and specific heat capacity characteristics of water-based Al2O3 nanofluids at low particle concentrations

In this paper, the specific heat capacity and viscosity properties of water-based nanofluids containing alumina nanoparticles of 47 nm average particle diameter at low concentrations are studied. Nanofluids were prepared with deionised water as base fluid at room temperature by adding nanoparticles at low volume concentration in the range of 0.01%–1% to measure viscosity. The effect of temperature on viscosity of the nanofluid was determined based on the experiments conducted in the temperature range of 25°C to 45°C. The results indicate a nonlinear increase of viscosity with particle concentration due to aggregation of particles. The estimated specific heat capacity of the nanofluid decreased with increase of particle concentration due to increase in thermal diffusivity. Generalised regression equations for estimating the viscosity and specific heat capacity of nanofluids for a particular range of particle concentration, particle diameter and temperature are established.     

Experimental evaluation of the effect in the stability and thermophysical properties of water-Al2O3 based nanofluids using SDBS as dispersant agent

Nanofluids are thermo-fluids engineered by dispersing nanosized particles in conventional base fluids used in heat transfer applications. In this experimental study, the temporal stability and thermophysical properties of water-based alumina nanofluids, such as thermal conductivity, surface tension and viscosity, were evaluated at three different concentrations (0.1 wt%, 0.5 wt%, and 1.0 wt%) using sodium dodecyl benzene sulfonate (SDBS) as dispersant agent. The results showed that the nanofluid prepared with 1.0 wt% exhibits thermal conductivity enhancements between 11% and 15% compared to deionized water. Additionally, a pseudoplastic behaviour was identified by means of a rheological study, which increased as the nanoparticle concentration increased. Nevertheless, the nanofluids with SDBS showed a Newtonian behaviour and viscosity values close to those of water, which is suitable to avoid frictional losses in pump processes. In turn, surface tension increases with the amount of DI-water in Al₂O₃ concentrations, but it decreases with the addition of SDBS, which has an important effect on the boiling applications of nanofluids. This work was carried out to highlight the importance of nanofluids stability in function of the surfactant added and the nanoparticle concentration, in the measurement of significant thermophysical properties such as surface tension, viscosity and thermal conductivity, which could be used to explain the nanofluids behaviour in different thermal devices.     

Effect of sonication time on enhancement of effective thermal conductivity of nano TiO2–water,ethylene glycol,and paraffin oil nanofluids and models comparisons

This paper presents experimental and theoretical determination of the effective thermal conductivity of various base fluids and nano TiO₂ composition. Ultrasonically assisted sol–gel method was used for synthesising anatase TiO₂ nanoparticles and dispersing them into base fluids using sonication for the synthesis of nanofluids. It is observed that thermal conductivity enhancement is significantly higher than that of base fluid. The thermal conductivity shows an increment with the addition of nanoparticles and confirms a 22% enhancement achievable in base fluids. The effect of base fluids is a complex idea and difficult to understand; lower base fluid viscosities were supposed to contribute higher in enhancement of thermal conductivity, but another important factor; i.e. fluid nanoparticles surface interaction, nanoparticles crystal type also contributes in enhancement. In the further study, as the sonication time increases; an improvement in the thermal conductivity of nanofluids is also observed. Except water-based nanofluids, all others show reasonably good agreement with the data predicted by Bruggeman model and the prediction is in the range of 5%. This study is important since it covers base fluids with a wide range of thermal conductivity and viscosity.     

Experimental evaluation of thermophysical properties of oil-based titania nanofluids for medium temperature solar collectors

Thermal oils are widely used as working fluids in the medium temperature heat transfer applications including concentrating solar thermal collectors. However, the weak thermal characteristics of these oils are major drawbacks in their successful application in the medium-high temperature solar collectors. Fortunately, the emergence of nanotechnology has provided the opportunity to alter thermo-physical properties of base fluids by adding small amount of sub-micron size solid particles possessing better properties. This paper presents an experimental investigation of thermophysical properties of an oil-based nanofluid to be used in the medium temperature solar collector for enhanced thermal energy transport. The colloidal suspensions were prepared by dispersing different weight fractions (0.25 wt.%, 0.5 wt.%, 0.75 wt.% and 1.0 wt.%) of Titania nanoparticles in Therminol-55 oil using two-step method. Shear mixing and high energy ultrasonication was employed to achieve uniform mixing and de-agglomeration of the nanoparticles in order to enhance the stability of the colloidal suspensions. Thermophysical properties of the nanofluids were determined as a function of nanoparticles concentrations in the temperature range of 25 °C–130 °C. The experimental results demonstrated substantial improvement in thermal conductivity of the nano-oils with an increase in nanoparticles loading which further enhanced at higher temperatures. Dynamic viscosity and effective density displayed a decreasing trend against rising temperature which indicate the effectiveness of these nanofluids for medium temperature heat supply. Nano-oils with superior thermal properties can improve the performance of medium temperature solar thermal collectors.     

Stability,viscosity, thermal conductivity,and electrical conductivity enhancement of multi-walled carbon nanotube nanofluid using gum arabic

This experimental investigation discussed on the stability and rheological behavior of multi-wall carbon nanotubes (MWCNTs) nanofluids with and without gum arabic (GA). The stability of MWCNT in the presence of GA dispersant in solar glycol is systematically investigated by taking into account the combined effect of different parameters, such as sonication time, temperature, dispersant and particle concentration. The concentrations of MWCNT and GA have been varied from 0.2 to 0.6% volume concentration and from 0.25 to 1.25 wt%, respectively, and the sonication time has been varied in between 30 and 120 min. The effect of sonication time on viscosity was discussed. It was perceived that the shear thinning behavior is exhibited by all the nanofluid samples. The stability of nanofluid is measured in terms of MWCNT concentration as a function of sediment time using UV-Vis spectrophotometer. Rheological behavior of MWCNT nanofluids is measured using Bohlin CVO Rheometer in the temperature range of 30–50°C, with step sizes of 5°C. Optimal GA concentration is obtained for the entire range of MWCNT concentration and 0.25–1.25 wt% of GA is found to be sufficient to steady all MWCNT range in solar glycol. Rapid sedimentation of MWCNTs is observed at higher GA concentration and sonication time. The presence of MWCNT and GA enhanced the thermal conductivity of the nanofluids by 30.59% at 0.6 vol.% particle concentration and 1.25 GA wt% at 50°C. The electrical conductivity is enhanced in a linear manner with respect to the loading of MWCNT and GA. Nevertheless, the electrical conductivity is increased linearly with increasing the temperature of the nanofluid. At particle concentration of 0.6 vol.% of MWCNT and 1.25 wt% of GA, the electrical conductivity of the nanofluid is improved by 190.57% at a temperature of 50°C.     

MgO nanofluids: higher thermal conductivity and lower viscosity among ethylene glycol-based nanofluids containing oxide nanoparticles

Five kinds of oxides, including MgO, TiO₂, ZnO, Al₂O₃ and SiO₂ nanoparticles were selected as additives and ethylene glycol (EG) was used as base fluid to prepare stable nanofluids. Thermal transport property investigation demonstrated substantial increments in the thermal conductivity and viscosity of all these nanofluids with oxide nanoparticle addition in EG. Among all the studied nanofluids, MgO–EG nanofluid was found to have superior features, with the highest thermal conductivity and lowest viscosity. The thermal conductivity enhancement ratio of MgO–EG nanofluid increases nonlinearly with the volume fraction of nanoparticles. In the experimental temperature range of 10–60°C, thermal conductivity enhancement ratio of MgO–EG nanofluids appears to have a weak dependence on the temperature. Viscosity measurements showed that MgO–EG nanofluids demonstrated Newtonian rheological behaviour, and the viscosity significantly decreases with the temperature. The thermal conductivity and viscosity increments of the nanofluids are much higher than the corresponding values predicted by the existing classical models for the solid–liquid mixture.     

多壁碳纳米管水基纳米流体的对流换热特性

实验研究了纳米粉体浓度、雷诺数Re和热流密度对多壁碳纳米管水基纳米流体(MWNTs/H2O)对流换热性能的影响。纳米粉体浓度分别为0.05 g/L、0.1 g/L、0.2 g/L和0.4 g/L,雷诺数Re为500~900,热流密度为10~20 k W/m2。结果表明:1)纳米流体对流换热系数随着纳米粉体浓度、Re、热流密度的增加而增加。如在Re为631且纳米粉体浓度为0.4 g/L时,纳米流体对流换热系数比基液增大了17.6%;2)纳米流体对流换热系数的提高率明显大于对应的导热系数提高率,当纳米粉体浓度为0.05g/L时,其对流换热系数和导热系数的提高率分别为7.4%和0.15%;3)在Eubank-Proctor方程的基础上,建立了适合于低Re条件下的混和对流换热的实验关联式。     

Mechanism of heat transport in nanofluids

We have calculated thermal conductivity of alumina nanofluids (with water and ethylene glycol as base fluids) using temperature as well as concentration-dependent viscosity, η. The temperature profile of η is obtained using Gaussian fit to the available experimental data. In the model, the interfacial resistance effects are incorporated through a phenomenological parameter α. The micro-convection of the alumina nanoparticle (diameter less than 100 nm) is included through Reynolds and Prandtl numbers. The model is further improved by explicitly incorporating the thermal conductivity of the nanolayer surrounding the nanoparticles. Using this improved model, thermal conductivity of copper nanofluid is calculated. These calculations capture the particle concentration-dependent thermal conductivity and predict the dependence of the thermal conductivity on the size of the nanoparticle. These studies are significant to understand the underlying processes of heat transport in nanofluids and are crucial to design superior coolants of next generation.     

γ-Fe_2O_3/EG纳米流体热输运性质的研究

选用乙二醇(EG)为基液,运用两步法制得稳定性良好的γ-Fe2O3纳米流体。测量并研究了γ-Fe2O3纳米流体的导热系数和粘度等热输运性质。结果表明,γ-Fe2O3纳米粒子的加入使得纳米流体的导热系数较基液提高了,纳米流体的粘度在低温下较大,并随着温度的升高而减小,纳米流体在强化传热领域有着潜在的应用前景。     

Elaboration of thermophysical performance enhancement mechanism of functionalized boron nitride/graphite hybrid nanofluids

This study mainly investigated the physicochemical characteristics of ethylene glycol/ water (EG/W) based hydroxyl-functionalized boron nitride (BN-OH) and graphite (G) hybrid nanofluids. A novel simple and efficient annealing method was proposed to have hexagonal boron nitride (h-BN) nanoparticles functionalized to improve the synergistic role between hybrid G/BN-OH nanoparticles. Meanwhile, the dispersion stability, thermal stability, and rheological behavior of diverse nanofluids (h-BN, BN-OH, G, G/BN and G/BH-OH) were comprehensively evaluated. The results showed that the G/BN-OH hybrid nanofluids demonstrate both better dispersion stability and thermal stability, as well as a lower increase in viscosity. In addition, the thermal conductivity of G/BN-OH hybrid nanofluids was increased by up to 18.05% with a concentration of 0.2 wt% when compared to the base fluid. Ultimately, the complicated theoretical mechanism of thermophysical performance augment for G/BH-OH hybrid nanofluids was reliably presented. The enhanced thermal conductivity of nanofluids may be attributed to the formation of adsorption layers and the synergistic effect of the thermal conductivity network.     

The effects of temperature, volume fraction and vibration time on the thermo-physical properties of a carbon nanotube suspension (carbon nanofluid)

In this investigation, nanofluids of carbon nanotubes are prepared and the thermal conductivity and volumetric heat capacity of these fluids are measured using a thin layer technique as a function of time of ultrasonication, temperature, and volume fraction. It has been observed that after using the ultrasonic disrupter, the size of agglomerated particles and number of primary particles in a particle cluster was significantly decreased and that the thermal conductivity increased with elapsed ultrasonication time. The clustering of carbon nanotubes was also confirmed microscopically. The strong dependence of the effective thermal conductivity on temperature and volume fraction of nanofluids was attributed to Brownian motion and the interparticle potential, which influences the particle motion. The effect of temperature will become much more evident with an increase in the volume fraction and the agglomeration of the nanoparticles, as observed experimentally. The data obtained from this work have been compared with those of other studies and also with mathematical models at present proven for suspensions. Using a 2.5% volumetric concentration of carbon nanotubes resulted in a 20% increase in the thermal conductivity of the base fluid (ethylene glycol).The volumetric heat capacity also showed a pronounced increase with respect to that of the pure base fluid.     

Transport properties of alumina nanofluids

Recent studies have showed that nanofluids have significantly greater thermal conductivity compared to their base fluids. Large surface area to volume ratio and certain effects of Brownian motion of nanoparticles are believed to be the main factors for the significant increase in the thermal conductivity of nanofluids. In this paper all three transport properties, namely thermal conductivity, electrical conductivity and viscosity, were studied for alumina nanofluid (aluminum oxide nanoparticles in water). Experiments were performed both as a function of volumetric concentration (3-8%) and temperature (2-50?°C). Alumina nanoparticles with a mean diameter of 36?nm were dispersed in water. The effect of particle size was not studied. The transient hot wire method as described by Nagaska and Nagashima for electrically conducting fluids was used to test the thermal conductivity. In this work, an insulated platinum wire of 0.003?inch diameter was used. Initial calibration was performed using de-ionized water and the resulting data was within 2.5% of standard thermal conductivity values for water. The thermal conductivity of alumina nanofluid increased with both increase in temperature and concentration. A maximum thermal conductivity of 0.7351?W?m(-1)?K(-1) was recorded for an 8.47% volume concentration of alumina nanoparticles at 46.6?°C. The effective thermal conductivity at this concentration and temperature was observed to be 1.1501, which translates to an increase in thermal conductivity by 22% when compared to water at room temperature. Alumina being a good conductor of electricity, alumina nanofluid displays an increasing trend in electrical conductivity as volumetric concentration increases. A microprocessor-based conductivity/TDS meter was used to perform the electrical conductivity experiments. After carefully calibrating the conductivity meter's glass probe with platinum tip, using a standard potassium chloride solution, readings were taken at various volumetric concentrations. A 3457.1% increase in the electrical conductivity was measured for a small 1.44% volumetric concentration of alumina nanoparticles in water. The highest value of electrical conductivity, 314?μS?cm(-1), was recorded for a volumetric concentration of 8.47%. In the determination of the kinematic viscosity of alumina nanofluid, a standard kinematic viscometer with constant temperature bath was used. Calibrated capillary viscometers were used to measure flow under gravity at precisely controlled temperatures. The capillary viscometers were calibrated with de-ionized water at different temperatures, and the resulting kinematic viscosity values were found to be within 3% of the standard published values. An increase of 35.5% in the kinematic viscosity was observed for an 8.47% volumetric concentration of alumina nanoparticles in water. The maximum kinematic viscosity of alumina nanofluid, 2.901?42?mm(2)?s(-1), was obtained at 0?°C for an 8.47% volumetric concentration of alumina nanoparticles. The experimental results of the present work will help researchers arrive at better theoretical models.     

Influence of nanofluids application on contact length during hard turning

The length of tool–chip contact area (L_c) is considered as a considerable parameter in metal cutting process. Mechanical stresses and high temperature at this region may easily lead to abrasion or even breakage of cutting tool. Up to now, several solutions have been presented to overcome these limitations. Using cutting fluids is one of the solutions to reduce friction, stresses, and temperature over this area. This paper presents experimental investigation and finite element simulation of tool–chip interface in hard turning AISI 4140 using TiO₂ nanofluids. Nanofluids are newly class of engineering fluids developed by distributing nanometer solid particles in a base fluid. The main reason to use nanofluids in cutting process is to increase heat transfer capabilities and also its tribological attributes. At first, the effects of cutting speed, nanoparticles’ size, and nanofluid concentration on L_c have been experimentally investigated. Then, a numerical model has been developed to simulate the contact area length in case of nanofluids application. Comparing the results with that of the experimental tests shows that TiO₂ nanofluids are able to decrease L_c, about 35%, in feed rate of 0.11 mm/rev, nanoparticle size equal to 10 nm, and nanofluid concentration equal to 3.0 wt%.     

A Simple Investigation of the Thermal Effusivity of Silver Nanofluid Using Photopyroelectric Technique

We investigated the thermal effusivity of silver nanofluids using a microwave technique. During microwave irradiation, silver nanoparticles with a narrow particle size distribution were formed in water and in ethylene glycol, with a polyvinylpyrrolidone stabilizer. We designed and used a front-photopyroelectric technique that employed a metalized polyvinylidene difluoride (PVDF) pyroelectric sensor, with a thermally thick sensor and sample. Using this technique, we calculated the thermal effusivity of the silver nanofluids at a given frequency using the combination of the signal’s normalized amplitude–phase. The thermal effusivity of the nanofluids increased with the number of microwave irradiation cycles, which increased the nanoparticle concentration in the base fluids. A comparison with reported values illustrates the high accuracy obtained from the results of thermal diffusivity, the thermal effusivity of the PVDF sensor, and the thermal effusivity of ethylene glycol as a base fluid (differing by only 1.7 %, 0.5 %, and 2.3 %, respectively). Our method can therefore be used to study nanofluids with varying nanoparticle properties, such as concentration, size, and shape.     

Particle shape effect on the viscosity and thermal conductivity of ZnO nanofluids

The viscosity and thermal conductivity of ZnO nanofluids with nanoparticle shapes of nearly rectangular and of sphere, were experimentally investigated under various volume concentrations of the nanoparticles, ranging from 0.05 to 5.0 vol.%. The viscosity of the nanofluids increased with increases in the volume concentration by up to 69%. In addition, the enhancement of the viscosity of the nearly rectangular shape nanoparticles was found to be greater by 7.7%, than that of the spherical nanoparticles. The thermal conductivity of the ZnO nanofluids increased by up to 12% and 18% at 5.0 vol.% for the spherical and the nearly rectangular shape nanoparticles, respectively, compared to that of the base fluid (water). The shape of the particles is found to have a significant effect on the viscosity and thermal conductivity enhancements.     

Stability and thermal conductivity enhancement of carbon nanotube nanofluid using gum arabic

This experimental study reports on the stability and thermal conductivity enhancement of carbon nanotubes (CNTs) nanofluids with and without gum arabic (GA). The stability of CNT in the presence of GA dispersant in water is systematically investigated by taking into account the combined effect of various parameters, such as sonication time, temperature, dispersant and particle concentration. The concentrations of CNT and GA have been varied from 0.01 to 0.1?wt% and from 0.25 to 5?wt%, respectively, and the sonication time has been varied in between 1 and 24?h. The stability of nanofluid is measured in terms of CNT concentration as a function of sediment time using UV-Vis spectrophotometer. Thermal conductivity of CNT nanofluids is measured using KD-2 prothermal conductivity meter from 25 to 60°C. Optimum GA concentration is obtained for the entire range of CNT concentration and 1–2.5?wt% of GA is found to be sufficient to stabilise all CNT range in water. Rapid sedimentation of CNTs is observed at higher GA concentration and sonication time. CNT in aqueous suspensions show strong tendency to aggregation and networking into clusters. Stability and thermal conductivity enhancement of CNT nanofluids have been presented to provide a heat transport medium capable of achieving high heat conductivity. Increase in CNT concentrations resulted in the non-linear thermal conductivity enhancement. More than 100–250% enhancement in thermal conductivity is observed for the range of CNT concentration and temperature.     

Application of CNT nanofluids in a turbulent flow heat exchanger

Nanofluids have received much attention since its discovery owing to its enhanced thermal conductivity and heat transfer characteristics which makes them a promising coolant in heat transfer application. In this study, the enhancement in heat transfer of carbon nanotube (CNT) nanofluids under turbulent flow conditions is investigated experimentally. The CNT concentration was varied from 0.051 to 0.085 wt%, respectively. The nanofluid suspension was stabilised by gum arabic through a process of homogenisation and water bath sonication at 25 °C. The flow rate of cold fluid (water) is varied from 1.7 to 3 L/min, while flow rate of the hot fluid is varied between 2 and 3.5 L/min. Thermal conductivity, density, and viscosity of the nanofluids are also measured as a function of temperature and CNT concentration. The experimental results were validated with theoretical correlations for turbulent flow available in the literature. Results showed an enhancement in heat transfer between 9% and 67% as a function of temperature and CNT concentration.     

Investigation on Stability and Optical Properties of Titanium Dioxide and Aluminum Oxide Water-Based Nanofluids

Water is regarded as a poor absorber of solar energy. This affects the efficiency of solar thermal systems. The addition of nanoparticles to heat transfer fluids used in solar thermal systems can enhance their optical properties. These new-generation heat transfer fluids are known as nanofluids. The present study investigates the stability and optical properties of three nanofluids, including aluminum oxide (13 nm and <50 nm) and titanium dioxide (21 nm) nanofluids. The stability of the nanofluids was observed through a photo-capturing method and zeta potential measurements. Ultraviolet–visible (UV–Vis) spectrophotometer was used to measure the absorbance and transmittance of the prepared nanofluids. The effect of factors such as type of particle, type of surfactant, and pH of the solution on the optical properties of the nanofluids was also investigated. We found that the titanium dioxide nanofluid had better optical properties but lower stability compared to aluminum oxide nanofluids.