Thermal Properties Of Nanofluids Research Articles

The influence of magneto-hydrodynamics and vortex generators on nanofluids heat transfer

This paper investigates the effects of a vortex generator (VG) and magneto-hydrodynamics (MHD) on the thermal properties of nanofluids in a microchannel. A variety of test scenarios within the laminar flow regimes are numerically examined: nanofluids in a hot-walled simple channel (SC); nanofluids in a heated-walled channel where a circular cylinder is embedded within the channel and functions as a vortex generator (VG); nanofluids in a heated-walled channel that incorporates both a vortex generator and magneto-hydrodynamics (VG-MHD). The channel has a circular cylinder with a diameter of D that is fixed inside the channel with various gap ratios (GR = G/D) ranging from 0.5 to 4.0. Numerical investigations for laminar flow with Re ≤ 1,000 are carried out. The nanoparticle volume fractions range from 0 % to 7 %, and the magnetic field is set at 0.05 T. The best heat transmission conditions with the lowest skin friction coefficient are evaluated for the optimal GR. It is found that heat transfer is enhanced by approximately 20 % when MHD and VG effects are employed, in contrast to a traditional channel that does not utilize the MHD effect.

Thermal properties of nanofluids using hydrophilic and hydrophobic LiYF4:Yb/Er upconverting nanoparticles.

Luminescent nanoparticles have shown great potential for thermal sensing in bio-applications. Nonetheless, these materials lack water dispersibility that can be overcome by modifying their surface properties with water dispersible molecules such as cysteine. Herein, we employ LiYF4:Er3+/Yb3+ upconverting nanoparticles (UCNPs) capped with oleate or modified with cysteine dispersed in cyclohexane or in water, respectively, as thermal probes. Upconversion emission was used to sense temperature with a relative thermal sensitivity of ∼1.24% K-1 (at 300 K) and a temperature uncertainty of 0.8 K for the oleate capped and of 0.5 K for cysteine modified NPs. To study the effect of the cysteine modification in the heat transfer processes, the thermal conductivity of the nanofluids was determined, yielding 0.123(6) W m-1 K-1 for the oleate capped UCNPs dispersed in cyclohexane and 0.50(7) W m-1 K-1 for the cysteine modified UCNPs dispersed in water. Moreover, through the heating curves, the nanofluids' thermal resistances were estimated, showing that the cysteine modification partially prevents heat transfer.

Enhanced thermal conductivity in Ag-H2O nanofluids by nanoparticles of different shapes: Insights from molecular dynamics simulation

The mechanism of thermal conductivity enhancements and influential factors make a lot of sense in the intensive investigation of nanofluids. While numerous studies have explored the effect of solid–liquid interfacial layers on thermal conductivity improvement in water-based nanofluids with diverse applications, investigations into the intrinsic mechanisms underlying interfacial layers in relation to nanoparticle shape remain limited. The thermal conductivity of water-based Ag-H2O nanofluids was investigated using molecular dynamics simulations of non-equilibrium molecular dynamics (NEMD), with emphasis placed on the shape and volume fraction of nanoparticles. The influence of nanofluid volume fraction on thermal conductivity was revealed through visual analysis of 3D smoothed surface plots, highlighting the positive correlation between thermal properties and increasing volume fraction of nanoparticles. Moreover, the contribution of different shapes of silver nanoparticles to thermal conductivity enhancement demonstrated an increasing trend with the corresponding growth in surface-to-volume ratio (S/V values associated with different particle shapes). For the volume fraction of 1.5%, the minimum thermal conductivity enhancement of 22.75% was observed for the spherical shape, while the maximum thermal conductivity enhancement of 25.51% was achieved with the triple-platelet particles. Importantly, the comprehensive analysis of radial distribution function (RDF) and mean square displacement (MSD) indicated the positive effect of interfacial nanolayers influenced by nanoparticles of various shapes on the thermal conductivity of nanofluids. This study provides valuable insights into the impact of nanoparticle shape on the thermal properties of nanofluids, opening up promising prospects for future molecular dynamics simulations.

Investigation of nanoparticles shape that influence the thermal conductivity and viscosity in argon-based nanofluids: A molecular dynamics simulation

The shape of nanoparticles plays an essential role in the thermophysical properties of nanofluids, yet their mechanisms and characteristics remain lacking in comprehensive studies. Molecular dynamics simulations of non-equilibrium molecular dynamics (NEMD) and reversing perturbation non-equilibrium molecular dynamics (RNEMD) calculation methods were used to study thermal conductivity and viscosity of Cu/Au-Argon based nanofluids, taking into account a variety of influencing factors, as well as nanoparticles shape and volume fraction. Through the analysis of the number density distribution, radial distribution function (RDF) and mean square displacement (MSD), the influences of nanoparticles shape (represented by the surface-to-volume ratio) were described and investigated: the highest value was 0.182 WK−1m−1 containing columnar particles of Au at 2% volume fraction, while the thermal conductivity of Cu nanofluids was 0.162 WK−1m−1 under the same conditions; the average values of viscosity had very close values of 2.82 × 10−4 Pa·s (Cu-Ar) and 2.86 × 10−4 Pa·s (Au-Ar) at 1.5% particle volume fraction for nanofluids containing spherical particles, respectively. Simulations of argon-based nanofluids containing five different shapes of Au nanoparticles indicate that the thermal conductivity improves with the growth of the surface-to-volume ratio. And the enhancement of the interfacial nanolayer density turns out to be the main factor that nanoparticles with higher S/V value facilitated the enhancement of the thermal conductivity of argon-based nanofluids. This study enriches the molecular dynamics studies of nanofluids and provides important insights for understanding the effect of nanoparticle's shape on the thermal properties of nanofluids.

A Simulation Study on Electrical and Thermal Properties of Nanofluids Based Mineral Oils for Potential Transformer Applications

The present report demonstrates the efficacy and capabilities of transformer oil due to their growing demand for the production of good insulation. In this work, we have enhanced the electrical and thermal properties of nanofluid (NF)-based mineral oils using the Quickfield simulation in the presence of the different types of nanoparticles (NPs), namely, magnesium oxide (MgO), titanium dioxide (TiO2), and iron oxide (Fe3O4), which are classified based on their conductivity. Different concentrations of NPs of 1%, 5%, and 10% have been used to prepare the NFs. The test cell model and the dummy model of a three-phase transformer have been designed to investigate the electrical and thermal properties of the NFs. It is found that the addition of NPs enhanced both the electrical and thermal properties of the fluid. The voltage at the center of the electrodes decreased by 46% in the case of TiO2 at the highest percentage concentration (10%) of NPs, while for the same concentration of MgO and Fe3O4, it is reduced by 18% and 24%, respectively, whereas the temperature of the oil decreased by approximately 20%, for all NFs at the highest concentration (10%). It is envisaged that the method used can be very useful in determining the properties of NPs before testing them in real life.

Thermal performance of nanofluids based on tungsten disulphide nanosheets as heat transfer fluids in parabolic trough solar collectors

Nanofluids are considered as a new generation of heat transfer fluids since they exhibit thermophysical properties improvements compared with conventional heat transfer fluids. The high thermal conductivity of nanofluids and even the isobaric specific heat enhancements over conventional liquids make these colloidal suspensions very attractive in many research areas, including solar energy. In this work, nanofluids based on tungsten disulphide (WS 2 ) nanosheets have been prepared from the thermal oil currently used as heat transfer fluid in Concentrating Solar Power (CSP) plants. The high aspect ratio of WS 2 bidimensional nanostructures provides high long-term colloidal stability to the nanofluids and facilitates heat transport. Cetyltrimethylammonium bromide and polyethylene glycol have been used as surfactants to improve the exfoliation process and enhance the colloidal stability of the nanomaterial dispersions. Some properties such as density and viscosity of the base fluid have not been significantly altered by the presence of WS 2 nanosheets in the base fluid. However, studies on the thermal properties of nanofluids have shown promising results with increases in thermal conductivity of up to 33% and heat transfer coefficient by 21% over the base fluid. Furthermore, it has been estimated that the overall efficiency of the CSP system could be improved by 31% by replacing the conventional thermal fluid with 2D-WS 2 -based nanofluids. • The efficiency of PTC solar collectors is increased by 31% by using WS 2 nanofluids. • Nanofluids improved the heat transfer coefficient of the base fluid by 21%. • Thermal conductivity was enhanced by 33% in WS 2 -based nanofluids. • WS 2 nanofluids favours the elimination of selective coatings on the absorber tube.

BN white graphene well-dispersed solar salt nanofluids with significant improved thermal properties for concentrated solar power plants

Solar salt is the most commonly used medium for thermal storage and transfer in concentrated solar power (CSP) plants, and the utilization efficiency of solar energy is depended on the thermal properties of solar salt. In this work, atomic-layer-thick hexagonal boron nitride nanosheet (BNNSs, so called white graphene) were obtained and dispersed into solar salt uniformly to produce composites nanofluids. Attributed to the ultra-high thermal conductivity and a large specific surface area of BNNSs, which reduce the thermal resistance, and facilitate forming of more semi-solid boundary layer and nucleation sites, the as-prepared composites nanofluids show superior thermal properties. The properties enhancement mechanism and effects of the mass fraction of BNNSs on the thermal properties of nanofluids was investigated carefully. Compared with the pure solar salt, the solid-phase thermal conductivity, solid-phase and liquid-phase specific heat capacity was elevated by 76.79%, 29.8% and 12.82%, respectively. At the same time, the supercooling degree was significantly decreased from 12.2 °C to 4.7 °C. The enhanced thermal conductivity and specific heat capacity offer significant cost savings, and the reduced supercooling degree prevent phase separation and pipe blockage. Thus, the developed composites nanofluids can be used as a superior medium for CSP plants.

Radiative Heat Transfer of Hybrid Nanofluid Flow Over an Expanding Surface with the Interaction of Joule Effect

The current research examines the characteristic of dissipative heat energy owing to the inclusion of a magnetic field here on the two-dimensional flow of an electrically conducting hybrid nanofluid past an expanding surface. Additionally, the free convection of hybrid nanofluid thermal properties is enhanced with the inclusion of the Joule heating effect as well as the thermal radiation in the heat transfer phenomenon. These physical properties were influenced as a result of the combination of the nanoparticles Al2O3 and Cu into the base liquid ethylene glycol. The novelty arises due to the interaction of thermal conductivity employing the Mintsa model and the viscosity using the Gharesim model. The transformed governing set of nonlinear equations obtained with the assistance of suitable similarity transformations are solved numerically using the Runge-Kutta fourth-order shooting base technique. A good correlation between the earlier studies is obtained in specific cases showing the convergence criteria of the present procedure. Further, the physical significance of the contributive parameters is presented through graphs and tables. The observation shows that the particle concentration for the hybrid nanofluid augments the fluid velocity. Moreover, the inclusion of dissipative heat favors enhancing the fluid temperature for the involvement of the particle concentration.

Review on Thermal Performance of Nanofluids With and Without Magnetic Fields in Heat Exchange Devices

Addition of nanoparticles into a fluid can improve the heat transfer performance of the base fluid in heat exchangers. In this work, the preparation method and process of nanofluids are introduced, and thermal properties of nanofluids, such as thermal conductivity and viscosity, are discussed deeply. This paper summarizes various theoretical models of thermal conductivity and viscosity of nanofluids. A comprehensive literature survey on applications and limitations of nanofluids has been compiled. This paper also aims to review the natural and forced convective heat transfer characteristics of nanofluids with and without magnetic fields. The discussion for the natural convective heat transfer of nanofluids focuses on the heat transfer performance of non-conventional enclosures and electric heaters. The effects on heat transfer due to variations of heated walls are also investigated. Specific applications of nanofluids in a tube with trapezoidal ribs, double-tube heat exchangers, and plate heat exchangers have been reviewed and presented in a discussion about forced convective heat transfer. The previous results show that the inlet temperature of nanofluids obviously affects the heat transfer characteristics of double-tube heat exchangers, whereas a multi-walled carbon nanotube–water nanofluid shows significant advantages in plate heat exchangers. Finally, this paper studies natural convective heat transfer of magnetic fluids in a square cavity and forced convection heat transfer in a straight tube and a corrugated structure under the action of magnetic fields. It is found that the heat transfer performance of an Fe3O4–water nanofluid is enhanced when a magnetic field is applied to the corrugated plate heat exchangers, and the pressure drop can be reduced by around 10%. It is recommended that natural convection of magnetic fluids needs to be investigated experimentally in a real cavity and a corrugated channel under the influence of a magnetic field. In addition, studies of alternating magnetic field are recommended to reveal any improvements of thermal performance of magnetic fluids in heat exchange devices. This review puts forward an effective solution for improvement of the thermal performance of heat transfer equipment and serves as a basic reference for applications of nanofluids in heat transfer fields.

Simulation on influences of Al2O3 nanofluids as coolant for nuclear power plant based on modified empirical formula of nanofluids thermal properties at high temperature and high pressure

ABSTRACT The thermal properties of nanofluids are variable with working temperature and pressure and ignored by the existing empirical formula. In the present work, based on the application of nanofluids in pressurized water reactor (PWR) nuclear power plants, different concentrations of Al2O3/H2O nanofluids are prepared for measuring its thermal properties and then modifying empirical formulas at high temperature and high pressure (HTHP). In addition, the primary heat transfer process of PWR nuclear power plants is simulated by numerical method. The results show that, when volume fraction within 2.0%, the heat transfer performance of Al2O3 nanofluid increases with the increase of concentrations; however, it is on the contrary when the volume fraction of nanoparticles is larger than 2%. Comparing with water as the coolant, Al2O3 nanofluids with 2% volume fraction give the largest heat transfer enhancement of 103%. Therefore, Al2O3 nanofluids as the coolant can achieve the goals of heat transfer enhancement and safety limit improvement.

Experimental study of hydrogen production using electrolyte nanofluids with a simulated light source

In this research, we conducted water electrolysis experiments of a carbon black (CB) based sodium sulfate electrolyte using a Hoffman voltameter. The main objective was to investigate hydrogen production in such systems, as well as analyse the electrical properties and thermal properties of nanofluids. A halogen lamp, mimicking solar energy, was used as a radiation source, and a group of comparative tests were also conducted with different irradiation areas. The results showed that by using CB and light, it was possible to increase the hydrogen production rate. The optimal CB concentration was 0.1 wt %. At this concentration, the hydrogen production rate increased by 30.37% after 20 min of electrolysis. Hence, we show that using CB in electrolytes irradiated by solar energy could save the electrical energy necessary for electrolysis processes.

THERMAL-HYDRAULIC PERFORMANCE OF Al2O3 NANOFLUIDS UNDER LAMINAR FLOW IN A MINITUBE

The improvement of performance and applications that include the convection heat transfer process is extremely related to the thermophysical properties of the fluids. However, it was found that adding nanoparticles to the conventional fluids (known as nanofluids) is an advanced way to enhance their thermophysical properties leading to an enhanced heat transfer process. This study presents a numerical approach to investigate the laminar flow-based convective heat transfer of Al2O3 nanofluids in a mini-tube under constant heat flux boundary conditions (15 kW/m2) and at an inlet temperature of 25°C. For this study, the nanofluid samples are prepared for several concentrations of Al2O3 nanoparticles and ethylene glycol/water mixture as a base fluid. Also, the thermal properties of nanofluids and base fluid are characterized. Computational fluid dynamics tools are used to develop a numerical model and validate it in comparison to experimental data. The results are discussed based on the heat transfer coefficient, considering the entrance area, pressure drop, and friction factor to determine the performance of nanofluids in comparison to the base fluid. A considerable enhancement in the heat transfer, especially in the entrance area up to 17.8%, with a corresponding increase in the pressures drop, friction factor, and the required pumping power of the nanofluids was observed.

Numerical analysis of convective heat transfer on straight finned tube with different nano fluids

The major idea of this effort is to study and analyse heat transfer through a straight finned duct with different nanofluids. Dynamics and simulations for fluid flow at the start of fully developed laminar flow with nanofluids in a circular duct with and without fins under constant wall temperature conditions are performed by using fluent commercial software. Heat transfer behaviours of the duct are investigated for constant wall temperature conditions for oxides of nanofluid. The thermal properties of nanofluid are considered from the literature. Commercial software ANSYS 2018 was used to compute the velocity and thermal analysis between the inlet and outlet of the tube. The finite volume method will be used to discretize the whole domain in quadratic elements considered with the turbulence model. Turbulence differential equations system coupled with the heat convection. The flow and pressure, the temperature in the exit will be calculated as a function of initial conditions in the entrance of the duct. The fluid properties such as conductivity and viscosity of the base fluid are taken as temperature independent. The enhancement in heat transfer rate are presented graphically with the effect of effects of nanoparticle volume fraction, nanoparticle size.

Numerical duality of MHD stagnation point flow and heat transfer of nanofluid past a shrinking/stretching sheet: Metaheuristic approach

In the present era of Industry 4.0, automation, and nanotechnology, many industrial applications require the simultaneous control of heat and fluid flow for better efficiency of machines. The polymeric excursion process, manufacturing of plastic films, glass, metallic equipment, all need control over friction forces and heat flow rates. Considering the importance of the subject, the present study deals with the stagnation point flow and heat transfer of water-based nanofluid over a stretching/shrinking sheet with a non-aligned axis of symmetry. The permeable sheet is considered in the presence of a uniform magnetic field. The non-linear partial differential equations governing the flow and heat transfer are converted into non-linear coupled ordinary differential equations using similarity transformations. The resultant equations are solved using “Particle Swarm Optimization” and Runge-Kutta method based hybrid shooting technique. Considering the nanoparticle size, concentration and shape-dependence on physical and thermal properties of nanofluid more realistic aggregation based mathematical models are used for both viscosity and thermal conductivity. Numerical results for the velocity profile, non-alignment function, temperature distribution and local skin friction are discussed with respect to the physical parameters like nanoparticle concentration, nanoparticle size, magnetic field, suction, and stretching/shrinking velocity. The existence of dual solutions are observed for the shrinking velocity parameter λ<−1, and the practically realizable and stable solutions are filtered out by computing the eigenvalues for the formulated perturbed Eigen problem. The results for the stable solutions unveiled that, by changing the stretching velocity parameter λ from 0 to 0.5, there is approximately 38% enhancement in heat transfer rate and 42% decrement in skin friction coefficient. Moreover, in the case of shrinking velocity parameter λ=−1.25, with an increase in nanoparticle concentration from 0 to 5%, the skin friction coefficient reduces by 53%, whereas the change in nanoparticle size from 10 nm to 50 nm has a skin friction reduction of only 10%.

The impacts of utilizing nano‐encapsulated PCM along with RGO nanosheets in a pulsating heat pipe, a comparative study

Heat pipes are useful devices in heat transfer and particularly, in cooling systems. Given the high demand for cooling systems in various applications, an improvement in the performance of heat pipes has gained much attraction in recent years. In this study, the effects of utilizing working fluids with different thermal properties on the performance of pulsating heat pipes (PHP) are experimentally studied. Hence, nano-encapsulated phase change material (NPCM), reduced graphene oxide nanosheets, and their mixture, as a novel hybrid nanofluid, are prepared and dispersed in water as a working fluid. NPCM at 3 concentrations of 5, 10, and 20 g/L, as well as nanosheets at three concentrations of 0.3, 0.6, and 1.2 g/L, are synthesized and investigated. Moreover, both additives are mixed at five various concentrations to form a hybrid nanofluid. All experiments are conducted in the vertical orientation and filling ratio of 50%. It is found that due to the increase in viscosity with the increment of concentration, the highest concentration is not always the optimum concentration and the PHP's performance for each additive is optimized in a specific concentration. NPCMs could prevent the working fluid's temperature to rise by changing the phase, as a result, the effective specific heat of the working fluid is slightly augmented. Also, the thermal conductivity of the working fluid increased up to 13% using nanosheets. It is illustrated that using the mixture of nanosheets and NPCM leads to a 38% decline in thermal resistance. Meanwhile, it is found that the enhancement in the thermal performance of the PHP using nanofluids is not only purely due to the increase in thermal properties of nanofluids but also due to the increase in turbulence intensity and boiling nucleation sites created by the nanoparticles. Highlights Nano-encapsulated phase change material (PCM), reduced graphene oxide (RGO) nanosheets, and their mixture are used as working fluid in a pulsating heat pipe (PHP). The thermal performance of PHP is improved using PCM nanocapsules, RGO nanosheets, and their mixture. Nanosheets have a higher impact on the performance of PHP compared with nano-encapsulated PCM. The better performance of PHP using RGO nanosheets is due to the higher thermal conductivity and mixing of fluid containing them compared with pure water. The better performance of PHP using nanocapsules is attributed to the better mixing of the fluid.

A review on the application of hybrid nanofluids for parabolic trough collector: Recent progress and outlook

By far, the Parabolic trough collector (PTC) is the most established solar concentrating technology worldwide. This paper draws a comparison among various studies done earlier in the domain to bring forth different models and mediums used to evaluate the effect of nanofluids on thermal parameters for PTC providing an optimum output, resultantly. The study also reveals that by using nanofluid and hybrid nanofluids as heat transfer fluid (HTF) inside the absorber tube of solar collector, the thermal and optical characteristics can be enhanced. Reported papers in literature mention stability, effect on PTC’s performance by implementing various novel designs, collector efficiency enhancement by using various inserts, and selecting proper hybrid nanomaterial into the base fluid. Varied experimental studies report that using hybrid nanofluid results in a more significant enhancement in thermal properties viz-a-viz than the mono fluids. A close look at the results of some of the notable works of the domain explicitly implies that for enhancing the thermal conductivity of nanofluid, metal oxide hybrid nanofluids are more promising than the oxide ones in a rather ‘pecking order’. The study also verges on the proposition that the thermal properties of nanofluid can be accelerated by the usage of hybrid nanofluid instead of mono nanofluid, as a medium, into a more efficient hybrid structure. Conclusively, this review advances the understanding of optimizing alteration parameters for obtaining maximum increase in outcomes and offers substantially adequate understanding for choosing specific techniques to enhance the overall efficiency of PTC, which can be used as a guideline for future trends. • Current and past studies of parabolic trough collectors have been reviewed. • Nanofluid techniques are delineated in performance enhancement. • The implementation of inserts has resulted in Brownian motion enhancement. • The efficiency of the collector using nanofluids is higher than that of the base fluid. • This review provides research gaps for researchers to investigate further.

An experimental/numerical hydrothermal-Second law analysis of a finned/tubular heat exchanger using Bhatnagar–Gross–Krook Lattice Boltzmann (BGKLBM) and rheological-thermal behavior of Fe2O3-water

Purpose The purpose of this study is analysis of the natural convection and entropy production in a two-dimensional section of the considered heat exchanger. For this purpose, the lattice Boltzmann method which is equipped with Bhatnagar–Gross–Krook model is used. This model proposes a significant accurate prediction for thermal and hydro-dynamical behaviors over free convection phenomenon. The heat exchanger is filled with Fe2O3-water nanofluid. To improve the accuracy of prediction, it is neglected to use the theoretical models for properties of nanofluid. At this end, some experimental observations are conducted, and the required rheological and thermal properties of nanofluid are measured based on laboratory work.. Design/methodology/approach The present work focuses on the influence of different factors on the thermal behaviors and entropy production of a heat exchanger. The heat exchanger is consisted by an inner tube, an outer tube and some fins which are implanted at the surface of inner tube. Findings The effects of various factors like structure of inner fins, nanoparticle concentration and Rayleigh number over the heat transfer rate, local and volumetric entropy production, Bejan number, flow configuration and temperature distributions are provided. Originality/value The originality of this work is using a new-developed numerical method for treating natural convection and experimental measurements for thermal and rheological properties of nanofluid.

Experimental studies on evaporation kinetics of gold nanofluid droplets: Influence of nanoparticle sizes and coating on thermal performance

In this work, a series of experiments investigate the evaporation of gold nanofluid sessile droplets on a perfluorodecyltrichlorosilane (PFTS) silicon substrate heated to 77 °C. The evaporation processes of different nanofluid droplets of the same initial volume, all for a 1% Cv volume concentration prepared from the 0.1 mM original suspension, are visualized to examine the size effect of gold nanoparticles (2.2, 5 and 10 nm) and the impact of surface coating (with and without Citrate capping in phosphate-buffered saline solvent) on enhancing heat transfer. This study open access to understand the size effect of gold nanoparticle in very small scale and the different types of surfactant on heat and mass transfer during the evaporation of droplet. Two methods are used to analyze the evaporation process, an optical one coupled to an infrared thermography method and an acoustic method. These complementary methods have the ability to investigate clearly the solid/liquid and liquid/vapor interfaces at the same time. From the optical observation, using a Drop Shape Analyzer (Kruss system), the evolution in time of the shape of the droplets (contact angle, base diameter and volume) are measured under controlled conditions (Humidity = 50%, Tatm = 23 °C). Then, the evaporation rate is deduced from the measurements of the evolution of volume in time. At the same time, an infrared camera is used to observe the droplet surface gradient temperature, air/liquid interface, due to thermal Marangoni flow. The acoustic method, based on a high- frequency echography principle, allowed to monitor the stability of nanoparticles inside the droplets (Au-water mixture) during the process of evaporation. From the kinetics of the reflection coefficient at the interface of the silicon substrate over which the nanofluids are deposited, the results show that the evaporation rate of 5 nm Au-water mixture (phosphate buffered saline solvent only) was faster than that of 10 nm Au-water mixture. While with citrate capping-PBS, the evaporation rate of the largest particle sizes (10 nm Au-water mixture) was the fastest than that of the smaller sizes (2.2 and 5 nm). Also, the 5 nm gold nanofluids with PBS showed the highest evaporation rate (+35%) than that of citrate capping gold nanofluids (+15%). As results, the addition of a surfactant (citrate capping) decreases the effectiveness of the heat transfer of gold nanofluids. This seems to be due to the absence of Marangoni cells in droplets made of Au-water mixtures (Citrate capping-PBS). Moreover, reducing in the thermal conductivity of gold nanofluid due to the surfactant coating by covering the gold nanoparticles surface and this eventually affected the nanofluid thermal properties. This is further supported by the acoustic method, which clearly shows worse stability of the nanoparticles in the latter case.

Effect of different concentration of functionalized graphene on charging time reduction in thermal energy storage system

Abstract The experimental work was carried out on the thermal energy storage system to test the thermal performance with the use of carboxyl functionalized Graphene (–COOH) doped with phase change material (Paraffin wax). The volume concentrations were varied from 0.25 to 1% at an increment of 0.25%. The HTF (Heat transfer fluid) which was water had its temperature varied between 80 °C and 90 °C at a 5 °C interval and the flow rate of water (HTF) was kept as 12.5 ml/sec or 50 ml in 4 s. In this experiment charging time reduces with increase of the Graphene concentration in terms of vol%. This research also included measuring nanofluid thermal properties at varying concentration and inlet temperature of HTF. Based on research from the experiments. It is observed that, due to an increase in thermal conductivity, the charge time reduction rate increase by up to 20% to 30% because of variance in the HTF inlet temperature when used with doped PCM material. Better performance metrics such as charging power, greater overall efficiency and increased energy amount are all recorded. When the charging time is reduced by 30% to 50% using the nanomaterial in PCM at 1 vol%, the large amount of energy was stored in the short duration of the time.

Numerical Investigations of parabolic trough collectors using different nanofluids

This paper presents three-dimensional numerical simulations of parabolic trough collectors (PTC) based on twolow-Reynolds eddy viscosity turbulence models, namely; Launder and Sharma k-epsilon and k-omega SST models. Forthe simulations, water was used as the Heat Transfer Fluid (HTF) with four different nanoparticles; Al2O3, TiO2, CuO andCu. Different volume fractions (φ) of the nanoparticles were investigated for various Reynolds (Re) numbers with uniformheat flux. Results showed that the overall performance of the system is more sensitive to changes in the thermal properties of nanofluids than the thermal properties of the HTF. At a volume fraction of 6% and a Re number of 70,000, the Nusselt number (Nu) enhancement of nanofluids TiO2-water, Al2O3-water, CuOwater, and Cu-water were found to be 21.5%, 20.2%, 18.11%, and 15.7% respectively while the performance evaluation criteria (PEC) were 1.214, 1.2, 1.18 and 1.155 respectively.