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This experimental work investigated, the effect of ZnO particles' size on the water-based nanofluid viscosity. Nanofluid samples with 0.5, 0.75, and 1% volume concentrations were prepared using 20 and 50∼150 nm ZnO nanoparticle sizes. Their viscosity was determined at 20, 30, 40, 50, and 60°C. Scanning electron microscopy was employed to investigate the morphology of the nanoparticles. The maximum relative viscosity was measured for 1% ZnO (50∼150 nm) as 1.35 times water. The stability of samples was evaluated for 1% ZnO (20 nm) and 1% ZnO (50∼150 nm) by measuring their Zeta potential values which were −21.4 mV and –23.1 mV, respectively. The correlation for the dynamic viscosity using measured data was compared with well-known ones. The offered correlation has R2 = 0.988, R adj 2 = 0.987, and ±5.58% maximum deviation. The results showed that 12.8% reduction in viscosity is possible by varying nanoparticle sizes. The current study proposes additional new findings on the nanofluids' usability. [ABSTRACT FROM AUTHOR]

The heat transfer performance of a closed-loop pulsating heat pipe (CLPHP) having 2.2 mm inner diameter is experimentally studied at different filling ratios (40%, 50%, 60% and 70%) in a heat load range of 50–300 W using distilled water as the PHP fluid in vertical orientation with evaporator at the bottom. This experimental study validate that the optimum charging ratio is 50% for CLPHP while 40% filling ratio is good at lower heat inputs. Optimum filling ratio gives an average reduction of 26.0% and 14.8% in thermal resistance compared to 70% and 60% fill ratios. The heat transfer performance of the CLPHP is also studied at various volume fractions of graphene–water nanofluid using 50% filling ratio. This study shows that the optimum volume fraction for this CLPHP is 0.045%. The volume fraction of 0.045% gives an average reduction of 27.3% in thermal resistance for all heat inputs and a maximum temperature reduction of 24 °C for the evaporator wall temperature at 275 W heat input. When optimum values of filling ratio and volume fraction are used, CLPHP gives the best heat transfer performance. A correlation is also developed for CLPHP by considering the influential parameters. [ABSTRACT FROM AUTHOR]

Preparing and defining of thermal properties of new type hybrid nanofluids are essential to understand the fluidity mechanism of hybrid nanofluids and select suitable nanofluids in terms of application. This research aims to provide an alternative fluid for different applications and complete the new type of nanofluid necessity in the literature that has been reported by different research groups. In this current investigation, water-based ZnO–SiO2 hybrid nanofluid is prepared by using the two-step method, and thermal conductivity and dynamic viscosity values are experimentally specified. ZnO–SiO2 hybrid nanofluid has 0.5%, 0.75%, and 1% with 50% ZnO-50% SiO2; 33.3% ZnO-66.6% SiO2, and 66.6% ZnO-33.3% SiO2 nanoparticle mixtures. Thermal conductivity and dynamic viscosity are experimentally measured from 20 to 60 °C. Maximum thermal conductivity rising is 2.26%, and it is obtained for 1% ZnO0.66–SiO20.33 at 50 °C. Maximum dynamic viscosity increment is measured as 1.36 times of base fluid for 1% ZnO0.33–SiO20.66 at 50 °C. Changes in thermal properties are reasonable to use ZnO–SiO2 hybrid nanofluid in different thermal applications to increase system heat transfer rate and efficiency and reduce pressure drop and power consumption. Finally, two different regression equations are developed to predict the thermal conductivity and dynamic viscosity, respectively. [ABSTRACT FROM AUTHOR]

We investigate, in this analysis, the entropy generation characteristics associated with the transport of nanofluid in a microfluidic channel under the combined influences of applied pressure gradient and electrical forcing. In our study, the nanofluid is subjected to an asymmetric cooling at the channel walls, while the conductive transport of heat through the channel walls is also taken into account. We show that the underlying thermo-electro-hydrodynamics of nanofluids in the channel lead to entropy generation, attributed to the irreversibilities associated with heat transfer, viscous dissipation, and Joule heating effects. We establish that a non-trivial interplay among these irreversibilities gives rise to an optimum value of the geometrical parameter, viz. the channel wall thickness (δ ), and the thermophysical parameters, viz. the thermal conductivity of the wall (γ) , Biot number (Bi), and the modified Peclet number ( Pe ¯ ), leading to a minimum entropy generation rate of the system. Also, we unveil through this study that changes in the electroosmotic parameter κ ¯ (representative of the EDL thickness) or the composition of the fluid (ϕ , the volume fraction of nanoparticles agglomerates) non trivially alter the optimum values of these parameters. Inferences drawn from this analysis may have consequences in the optimum design of thermal systems/devices, typically used for thermal management in micro-heat exchangers, micro-reactors, and micro heat pipes. [ABSTRACT FROM AUTHOR]

Thermal performance evaluation of TiO2-coated copper mesh wick in a cylindrical heat pipe with graphite nanofluid is experimentally analyzed at various inclinations, nanoparticle concentrations and power levels. Boiling heat transfer from the evaporator of a heat pipe depends on both thermal conductivity of the nanoparticle and suspension stability of the nanofluid. The lower the density of the nanoparticle, the better the suspension stability. Spherical graphite nanoparticles having lower density and good thermal conductivity quicken the heat transfer rate and hence the vaporization of base fluid. A hydrophilic coating of TiO2 is done on the copper wick structure to reduce the contact angle of graphite nanofluid. Results showed that the heat pipe worked well at 60° inclination compared to the other tested orientations. For this optimum condition, a reduction in 168.75% of thermal resistance is obtained compared with DI water with uncoated wick at horizontal position and also an improvement in thermal efficiency of 94.07% for 1.0 mass% particle loading. The enhancement in equivalent thermal conductivity is 90.87% for 1.0 mass% compared with DI water. Results from the repeatability test also confirm that the hydrophilic coating over the wick is stable, and results are repeatable. [ABSTRACT FROM AUTHOR]

Over many years, great efforts have been made to develop new fluids for heat transfer applications. In this paper, the thermal conductivity (TC) and viscosity of SiO2–P25 TiO2 (SiO2–P25) hybrid nanofluids were investigated for different nanoparticle volume concentrations (0.5, 1.0 and 1.5 vol%) at five various temperatures (20, 30, 40, 50 and 60 °C). The mixture ratio (SiO2:P25) in all prepared hybrid nanofluids was 1:1. Besides, pure SiO2, P25 nanofluids were prepared with the same concentrations for comparison with the hybrid nanofluids. The base fluid used for the preparation of nanofluids was a mixture of deionized water and ethylene glycol at a ratio of 5:1. Before preparing the nanofluids, the nanoparticles were analyzed with energy-dispersive X-ray analysis, scanning electron microscope, X-ray powder diffraction, and Fourier transform infrared spectroscopy. The zeta potentials of the prepared nanofluids except SiO2 nanofluids were above 30 mV. These nanofluids were visually observed for stability in many days. The TC enhancement of the hybrid nanofluid was higher than the pure nanofluid. In particular, with 1.0 vol% concentration, the maximum enhancement of SiO2, P25 and SiO2–P25 nanofluids were 7.5%, 9.9% and 10.5%, respectively. The rheology of the nanofluids was Newtonian. The viscosity increment of SiO2, P25 and hybrid nanofluids were 19%, 32% and 24% with 0.5 vol% concentration. A new correlation was developed for the TC and dynamic viscosity of SiO2–P25 hybrid nanofluid. [ABSTRACT FROM AUTHOR]

• The results showed that, under the MDP scheme, the particle-fluid interaction force calculation model directly affected the heat transfer rate through the two-phase fluid. • In the MDP scheme, the results showed that the interactions between the solving models for the fluid and the particles must be revised and corrected due to differences in the nature of solving equations for the particles and the fluid. • The thermophoresis or DEMP model offered a unique description of the particle flow behavior. The cornerstone of a new method to modeling nanofluids is particle movement. There are three primary forms of nanofluid simulation. The single-phase modeling with nanofluid properties (SPU), the multiphase fluid with discrete particle (MDP) and the discrete element method with particle (DEMP). In this study, by comparing three approaches to modeling the natural convection heat transfer in nanofluid, this method's (discrete element method with particle) ability was examined to predict properties and processes, such as heat transfer and friction factor. With a high range of Rayleigh numbers (0 ≤ Ra ≤ 100, 000), this method tries to model the natural convection process in a conical cavity filled with Al 2 O 3 nanofluid and nanoparticles. For modeling the MDP and thermophoresis effect (DEMP), a unique code with a novel grid generation algorithm was developed. The results demonstrated that the initial conditions determined the accuracy of the three methods for modeling the final parameters of natural convection within the conical cavity, the accuracy of the used equations, the procedure through which the boundary conditions were applied and the Rayleigh number. Accordingly, the DEMP and MDP methods were recommended for low Rayleigh numbers and those beyond 5500, respectively. [Display omitted] [ABSTRACT FROM AUTHOR]

The heat exchanger pipe diameter has a significant effect on the flow characteristics as well as on the initial investment, operation and overall cost. Increasing fin dimensions increases the annulus hydraulic diameters. Even though the total volume of the heat exchanger remains unchanged between the finned and bare designs, the heat duty increases with increased heat transfer area for the finned design. The fins should be designed, and the dimensions should be calculated with special attention for different flow rates and heat exchanger dimensions. In this study, number, geometry and dimensions of the fins are determined using the algorithms available in the literature. The operational condition optimization is carried out accompanied with the cost analysis. In addition, the effects of the types of working fluids and fouled and clean cases are investigated for the total heat transfer enhancement in parallel with performance, lifetime and cost issues. A detailed analysis is presented for finned and unfinned double-pipe heat exchanger models for pure engine oil and its nanofluid mixtures with Ti, TiO2, Cu, CuO, Al and Al2O3 nanoparticles, multi-wall carbon nanotubes and graphene nanosheet having a constant particle concentration in the liquid phase. The nanofluid is flowing in annulus side, whereas the seawater is flowing in the tube side. It is observed that both the pressure drop and the pumping power increase with the increasing fin number and decrease with the cleanliness factor, whereas the total tube number decreases with increasing fin number. It is found that different types of nanofluids affect the cost and optimum annulus side velocity significantly. The results are summarized in several figures that consider the increasing Reynolds number with the cleanliness factor, the heat transfer coefficient and the pressure drop, the friction factor with changing mass flow rate and the cost values with corresponding annulus side velocities. Finally, the overall characteristics of the trend lines are provided in the figures. [ABSTRACT FROM AUTHOR]

The hybridization of nanoparticles is a concept employed for the improvement of the thermal properties of nanofluids. Presently, there is a scarcity of studies in the open literature concerning the influence of particle mass ratios of hybrid nanofluids on the thermal properties. Thus, this paper investigated the effect of temperatures (15–55 °C) and particle mass ratios (90:10, 80:20, 60:40, 40:60, and 20:80) on the viscosity and electrical conductivity of deionized water (DIW)-based γ-Al2O3 and MWCNT hybrid nanofluids. A two-process strategy was deployed to prepare the hybrid nanofluids at a volume concentration of 0.1%. The hybrid nanofluids were characterized for their morphology using a transmission electron microscope. Hybrid nanofluid stability was monitored using UV visible spectrophotometer, viscosity, and visual inspection methods. The prepared nanofluids were observed to be stable with relatively constant viscosity and absorbance values. At 55 °C, maximum enhancements of 442.9% and 26.3%, and 288.0% and 19.3% were recorded for the electrical conductivity and viscosity of Al2O3–MWCNT/DIW nanofluids at particle mass ratios of 90:10 and 20:80, respectively, in relation to DIW. Temperature increase was observed to significantly reduce the viscosity of hybrid nanofluids while the particle mass ratio considerably and positively impacted the electrical conductivity. The relatively low viscosity of the hybrid nanofluids coupled with its reduction under increasing temperature and its insignificance increase as the particle mass ratio of the Al2O3 nanoparticles increased to make them viable coolants for engineering applications. New correlations were proposed to accurately estimate the viscosity and electrical conductivity of the hybrid nanofluids. [ABSTRACT FROM AUTHOR]

Numerous researches to prepare and stabilize graphene-based nanofluids have been developed. [ABSTRACT FROM AUTHOR]

Heat is an inevitable by-product of every electronic device and thermal management is very much essential for them to improve reliability and to prevent premature failure. Liquid coolants are one of the best switching options to achieve higher cooling rates. However, by adopting them in cooling systems such as microchannel heat sinks in which flow enters in to and out of the system may cause leakage of the coolant, which results in the complete firing of the electronic device. Hence, in the present study, this problem is avoided by adopting lid-driven square cavity as a cooling system. A 2-D numerical study has been carried out to obtain the convective heat transfer efficiency (CHTE) of gold and carbon nanofluid (NF) in a lid-driven square cavity over silicon solid block under the influence of a variety of magnetic fields. The geometrical domain consists of a square cavity containing NF that is driven by means of lid moving in one direction. This circulating NF will extract an enormous amount of heat from the solid block underneath the cavity resulting in conjugate heat transfer. A finite volume method-based conjugate homogeneous heat transfer model has been developed, validated, and used in the present study. The heat efficient Gold (Au) and single-walled carbon nanotube (SWCNT) NF pairs obtained by Nimmagadda and Venkatasubbaiah are used in the present study. Moreover, efficiently varied magnetic fields identified by Nimmagadda et al. are also implemented and compared with the uniform magnetic field. Furthermore, the magnetic field is applied over the geometrical domain along the two axial directions separately and the effective CHTE is obtained. The significant impact of broad parameters, such as magnetic field type, location and strength, nanoparticle concentration and type, and Reynolds number on the CHTE, is systematically analyzed and presented. [ABSTRACT FROM AUTHOR]

The thermal conductivity of TiC/water nanofluid is found out at different weight fractions of 0.1%, 0.2% and 0.3% and the results are compared with water (base fluid). It has been shown that the thermal conductivity of TiC/water nanofluid is improved when compared with water. Then, an inexpensive cryogenic treatment is adopted to find the thermal conductivity of cryo-TiC/water nanofluid. It has been found that the cryogenic treatment on TiC nanopowder is further enhancing the thermal conductivity of TiC/water nanofluid. The lattice vibration of the particles is responsible for the improvement of thermal conductivity. [ABSTRACT FROM AUTHOR]

In this study, convective heat transfer performance of a nanofluids containing graphite is studied in an industrial microchannel. In the experiments, initially, to prepare nanofluids at the volume fraction values of 0.5, 1, 1.5, 2%, distilled water has been employed as the base liquid. To provide sedimentation and stabilization of nanofluids in distilled water, Cetyltrimethylammonium bromide (CTAB) is utilized as surfactant. Thermophysical properties of nanofluids such as thermal conductivity, dynamic viscosity, and specific heat are determined experimentally. Furthermore, by building an experimental setup, in the temperature range of 20–30 °C and with temperature intervals of 2 °C, performance experiments are carried out in a microchannel of which hydraulic diameter is 1.6 × 10−3 m. Additionally, experiments have been conducted using nanofluids at different volumetric rates from 1 to 7 l min−1, heat fluxes from 100 to 1100 W, and volume fractions from 0.5 to 2%. Measuring heat flux, temperature, and flow rate, outcomes such as convective heat transfer coefficient, Reynolds number, and Nusselt number are calculated. The validation process of the experimental results has been performed by plotting the figures of Nusselt numbers vs Reynolds ones, and heat transfer coefficient vs supplied heat considering distilled water and nanofluids having various volumetric proportions. Regarding with the performance of nanofluids against distilled water under similar operating conditions, some proportional positive increase are acquired. Using outcomes attained from experiments, new correlations for Nusselt number have been derived with the R2 values around 0.96, and afterward by means of those correlations experimental data have been compared with those in the literature. A large number of measured and calculated data are given in the paper for other researchers to validate their theoretical models. [ABSTRACT FROM AUTHOR]

Summary: In a comprehensive study, the thermal conductivity, dynamic viscosity, and the rheological behavior of a SiO2/water nanofluid are investigated experimentally at the temperatures, solid concentrations, and the shear rates of 25°C to 50°C, 0% to 1.5%, and 400 to 1400(s−1), respectively. The Response Surface Methodology (RSM) is utilized to obtain regression models for the thermal conductivity and the dynamic viscosity. Subsequently, the sensitivity of the aforementioned models to 10% changes in the temperature, and the nanofluid concentration is analyzed. Afterward, Nondominated Sorting Genetic Algorithm II (NSGA‐II) is utilized to find the maximum thermal conductivity and the minimum viscosity. The nondominated optimal points are presented through a fitted correlation on a Pareto front to make the results more practical. The measurements of the investigated nanofluid could be summarized as a paper of a handbook. The workability of the investigated nanofluid is also examined in both laminar and turbulent flow regimes through analysis of the heat transfer merit graphs. To this end, the ratio of the dynamic viscosity enhancement to the thermal conductivity enhancement and the Mouromtseff number are chosen as two criteria of the laminar and turbulent flow regimes, respectively. Finally, the results are compared with those for SiO2/glycerin and SiO2/ethylene glycol nanofluids to check the workability in different base fluids. From a thermal‐efficiency point of view, the SiO2/water nanofluid is not suggested for use in both laminar and turbulent pipe flows, except in temperatures higher than 30°C and volume concentrations lower than 1% for the case of laminar flow. This is because the favorable heat transfer enhancement of the nanofluid is more than the unfavorable increase of the pumping power. From the rheological point of view, though, a SiO2/water nanofluid would be a good choice in lubricating moving surfaces for both laminar and turbulent flow regimes. It is found that in higher nanofluid concentrations, the thermal conductivity of a SiO2/water nanofluid is highly influenced by temperature. Moreover, adding nanoparticles at temperatures of 35°C to 40°C would have the highest increasing effect on the thermal conductivity. It is also revealed that increasing the temperature does not significantly affect the viscosity when 1% SiO2 nanoparticles are suspended within the water. [ABSTRACT FROM AUTHOR]

The natural heat convection within a square enclosure filled with water–Al2O3 nanofluid has been studied numerically in the presence of a magnetic field. The effect of heat source geometry attached to the bottom wall on the Nusselt number was investigated by changing its nondimensional width and height, and side walls thickness of the enclosure ranging from 0.1 to 0.5, 0.1 to 0.8 and 0.05 to 0.2, respectively. A regression model has been obtained along with conducting a sensitivity analysis seeking an optimal heat transfer. Results, reveal that Nusselt number increases by enlarging the fin, and reaching a peak point before it declines. Thus, interestingly, the ever-increasing heat transfer by means of fin size does not retain and there is an optimal point wherein the maximum heat transfer occurs. Moreover, the thermal performance of the system largely depends on the fin size rather than the relative side walls thickness. However, its effect intensifies as the fin width increases. Results of optimization show that the maximum heat transfer occurs at W = 0.4615 , H = 0.6467 and L b = 0.2 . [ABSTRACT FROM AUTHOR]

This article studies buoyancy-driven natural convection of a nanofluid affected by a magnetic field within a square enclosure with an individual conductive pin fin. The effects of electromagnetic forces, thermal conductivity, and inclination angle of pin fin were investigated using non-dimensional parameters. An extensive sensitivity analysis was conducted seeking an optimal heat transfer setting. The novelty of this work lies in including different contributing factors in heat transfer analysis, rigorous analysis of design parameters, and comprehensive mathematical analysis of solution domain for optimization. Results showed that magnetic strength diminished the heat transfer efficacy, while higher relative thermal conductivity of pin fin improved it. Based on the problem settings, we also obtained the relative conductivity value in which the heat transfer is optimal. Higher sensitivity of heat transfer was, though, noticed for both magnetic strength and fin thermal conductivity in comparison to fin inclination angle. Further studies, specifically with realistic geometrical configurations and heat transfer settings, are urged to translate current findings to industrial applications. [ABSTRACT FROM AUTHOR]

This paper assesses the contribution of TiO2 nanoparticles on thermal performance of a 0.5 mass% aqueous solution of carboxymethyl cellulose (CMC) in a square channel. In this regard, a neural network model is firstly developed for modeling of power law index, consistency index, and thermal conductivity of the aqueous solution of TiO2/CMC-water non-Newtonian nanofluid in terms of the nanoparticle concentration and temperature. Then, an attempt is made to evaluate the friction factor and heat transfer coefficient relative values. According to the results, it is found that the friction factor ratio is directly proportional to the temperature and nanoparticle content, while it inversely varies relative to the shear rate. Moreover, heat transfer coefficient ratio is improved at elevated nanoparticle content, and this improvement is much more profound at higher temperature conditions. For practical purposes, the nanofluid hydrothermal performance index is examined since the addition of nanoparticles increases both heat transfer and friction factor. The corresponding data disclose that the performance index is directly proportional to the nanoparticle content, especially at decreased shear rate and elevated temperature conditions. The application of TiO2/CMC-water nanofluid is found to be more favorable for applications with elevated shear rate conditions. [ABSTRACT FROM AUTHOR]

The present work deals with numerical investigations on heat transfer characteristics and friction factor of aqueous CuO nanofluids flow in a set of four microchannels connected in parallel under laminar regime. For each single phase, volume of fluid, mixture and Eulerian models, a particular computer code is developed to carefully simulate this problem. The three-dimensional steady-state governing equations are solved through finite volume method. The primary aim of this study is to comparatively distinguish the most appropriate and accurate model for numerical studies of nanofluids in microchannels. The results are compared with one another and the data obtained from an experimental work. Regarding the results, an acceptable consistency is observed for all models with the experimental data. The current study truly demonstrates that applying single-phase model to simulate and evaluate the laminar flow of CuO-water nanofluid inside microchannels with uniform wall temperature is more modest, precise and reliable compared with two-phase models. [ABSTRACT FROM AUTHOR]

In this study, a comparison of the convective heat transfer, pressure drop, and performance index characteristics of heat sinks with a miniature circular pin-fin inline arrangement (MCFHS) and a zigzag flow channel with single cross-cut structures (CCZ-HS) is presented. SiO2-water nanofluids with different particle concentrations are used as the coolant. The effects of the heat sink type, particle concentration and fluid flow rate on the thermal and hydraulic performances are evaluated. The testing conditions are performed at the wall heat fluxes of 10 to 60 kW/m2 and at a mass flow rate ranging from 0.18 to 0.6 kg/s. The dimension of heat sinks is equally designed at 28 × 33 mm. The heat transfer area of MCFHS and of CCZ-HS is 1430 and 1238 mm2, respectively. Similarly, the hydraulic diameter of the flow channel of MCFHS and of CCZ-HS is 1.2 and 1.0 mm, respectively. The measured data indicate that the cooling performances of CCZ-HS are about 24-55% greater than that of MCFHS. The effects of the channel diameter and single cross-cut of the flow channel are more dominant than the effects of the fin structure and heat transfer area. [ABSTRACT FROM AUTHOR]

The thermal management of electronic devices such as high-end central processing units, graphic processing units, insulated gate bipolar transistor, and circuit breaker in low-voltage switchboard operating between 20 and 380 W is investigated using a miniature loop heat pipe (mLHP) having nanoparticle-coated evaporator with graphene–water nanofluid. The thermal evaporation method is used to deposit copper nanoparticles on the evaporator surface for a coating thickness of 400 nm. The experimental results show that the combination of nanoparticle coating and nanofluid gives the highest heat transfer compared to the mLHP having uncoated and coated evaporator with distilled water as a working fluid. The use of nanofluid enhances the heat transfer performance of mLHP with an average reduction of 45.2% in thermal resistance and an average enhancement of 113.4% in evaporator heat transfer coefficient for the optimum nanofluid volume concentration of 0.006%. Similarly, the lowest evaporator temperatures are also obtained with the same concentration of nanofluid for various heat loads. The experimental results of mLHP having nanoparticle coating and nanofluid are also found to be repeatable from the repeated tests and suitable for a long-term operation. [ABSTRACT FROM AUTHOR]

The present work investigated thermal performance and heat transfer characteristics of a solar parabolic trough collector using Al2O3/DI‐H2O nanofluids. Nanofluids of varying concentrations (0.05% ϕ ≤ 0.5%) with mass flows (0.0083–0.05 kg/s) were considered for turbulent regime (2401 ≤ Re ≤ 7202) analysis. Experiments were carried out as per ASHRAE 93(2010) standards. By thermal performance analysis using Al2O3 nanofluid, it was understood that the collector efficiency improved up to 56% at a maximum volume concentration of ϕ = 0.5% and flow rate of 0.05 kg/s. The heat transfer study comparing Al2O3 nanofluid with pure water showed appreciable reduction in temperature gradient and surface temperature of the absorber. The heat transfer characteristics such as Nusselt number and friction factor relating to Reynolds number fits the experimental and predicted data and found within the limits of ±5.35% and ±9.61% for Nusselt number and friction factor respectively. Moreover, a similar empirical correlation was developed for collector efficiency, which was identified to be within the limit of ±1.02%. © 2017 American Institute of Chemical Engineers Environ Prog, 37: 1149–1159, 2018 [ABSTRACT FROM AUTHOR]

This work presents the thermophysical properties of carbon nanotubes (CNT) and CNT/Al2O3 hybrid nanofluids for heat transfer applications. An equal proportion of nanoparticles CNT (50%) and Al2O3 (50%) were added to the base fluid for two different concentration of 0.05 and 0.1%. Results show that addition of Al2O3 nanoparticles with CNT nanofluid improves the thermophysical properties. The thermal conductivity of hybrid nanofluids improved by 20% with a maximum concentration of 0.1%, while the thermal conductivity of CNT alone improved by only 8% with the base fluid. Similarly, density and viscosity of hybrid nanofluid increased up to 7 and 10%, respectively, while comparing it with the base fluid. The result of specific heat energy capacity of hybrid nanofluids increases to about 138% than CNT nanofluid with a maximum concentration of 0.2%. [ABSTRACT FROM AUTHOR]

This research reports the thermal performance and flow characteristics of nanofluid flows in two different types of microchannel heat sink (MCHS) with multiple zigzag flow channel structures experimentally with regard to the continuous zigzag flow channel (CZ-HS) and the single cross-cutting zigzag flow channel (CCZ-HS). SiO 2 nanoparticles with particle loadings of 0.3, 0.6, and 0.8 vol.% and dispersed in deionized water (DI water) are used as the working medium. Both CZ-HS and CCZ-HS are made from copper material. Their dimensions are approximately 28 × 33 mm. Hydraulic diameter and number of flow channels are equally designed as seven 1-mm flow channels, respectively. The heat transfer area of CZ-HS is approximately 1176 mm 2 and that of CCZ-HS is 1238 mm 2 . The effects of single cross-cutting of the flow channel, Reynolds number, and particle concentration on the Nusselt number and pressure drop characteristics are investigated. The experimental data indicate that the nanofluid-cooled heat sink provided larger thermal performance than the heat sink cooled by water of approximately 3–15%. Similarly, the results indicated that the thermal performances of the CCZ-HS are larger than those of the CZ-HS by an average of 2–6%. For the pressure drop, the measured data showed that particle concentration and cross-cutting of the flow channel have a small effect on the pressure-drop data. [ABSTRACT FROM AUTHOR]

In this paper, the performance of a solar still equipped with a heat exchanger using nanofluids has been studied both experimentally and theoretically through three key parameters, i.e., freshwater yield, energy efficiency and exergy efficiency. First, experiments are performed on a set-up, which is mainly composed of two flat plate solar collectors connected in series, and a solar still equipped with a heat exchanger. After heated in the collectors, the nanofluid enters the heat exchanger installed in the solar still basin to exchange heat with brackish water. The research question is to know how much the effect of nanofluids on the evaporation rate inside the solar desalination system is. The experiments are conducted for different nanoparticle volume fractions, two sizes of nanoparticles (7 and 40 nm), two depths of water in the solar still basin (4 and 8 cm), and three mass flow rates of nanofluids during various weather conditions. It is found that the weather conditions (mainly the sun radiation intensity) have a dominant influence on the solar still performance. To discover the effects of nanofluids, a mathematical model is developed and validated by experimental data at given weather conditions. The results reveal that using the heat exchanger at temperatures lower than 60 °C is not advantageous and the corresponding yield is smaller than that of solar still without the heat exchanger; although in such a case, using nanofluids as the working fluid in the heat exchanger can enhance the performance indices about 10%. At higher temperatures (e.g. 70 °C), the use of heat exchanger is beneficial; however, using nanofluids instead of water can augment the performance indices marginally i.e. just around 1%. In addition, it is found that in high temperatures using SiO 2 /water nanofluids, which have a lower effective thermal conductivity than that of Cu/water nanofluids, provides higher performance indices. [ABSTRACT FROM AUTHOR]

Piping is one of the most important issues in the cost of process factories. It is known that 80% of bought equipment cost or 20% of overhead capital can belong to piping cost in a fluid-process factory. Pipe diameter and therefore flow velocity strongly affect the existing value of the factory regarding the consumed electric energy and fitting cost of pipes, pumps, and valves. We give a detailed cost analysis model for the pure fluids of water, motor oil, glycerin, ammonia, methanol, ethanol, ethylene glycol, and propane and their nanofluid mixtures with Ti and TiO2particles in liquid phase flowing in the tube side of a double-pipe heat exchanger. Pressure drop and pumping power values increase with flow velocity but total cost values show an arc with it. The clear outcome is that there is a minimum cost value as a result of the analyses for each investigated fluids. Moreover, validation of the model is performed by plotting the calculated items in figures such as total heat transfer coefficient versus Reynolds number, pressure drop versus Reynolds number, and friction factor versus mass flow rate. Characteristics of the trend lines in these figures are seen as they should be. [ABSTRACT FROM AUTHOR]

The present study deals with the entropy generation analysis of a flat-plate solar collector using SiO/ethylene glycol-water nanofluids. For this purpose, available experimental data on the performance of a flat-plate solar collector are exploited for estimating the entropy generation in the system. Ethylene glycol-water (EG-water) and EG-water-based nanofluids having three different nanoparticle volume fractions including 0.5, 0.75, and 1 % are considered as the working fluids. The results are presented in terms of exergy efficiency, entropy generation parameter, and Bejan number for three different mass flow rates and various solar radiation intensities. It is found that when nanofluid concentration increases from 0 to 1 %, exergy efficiency enhances up to 62.7 % for a mass flow rate of 1 L min, whereas the corresponding increases in mass flow rates of 1.75 and 2.5 L min are 45.2 and 39.7 %, respectively. The results also elucidate that entropy generation parameter, which is a function of entropy generation, ambient temperature, and solar radiation, reduces with increasing the nanofluid concentration. [ABSTRACT FROM AUTHOR]

The paper reports an experimental investigation into the thermal performance and pressure drop characteristics of nanofluid-cooled heat sinks and then compares the results with the data for water-cooled heat sinks. Heat sinks with miniature circular-fin (MCFHS) and square-fin (MSFHS) structures are used and made from aluminum material with dimensions of 28 × 33 mm. Similarly, SiO 2 nanoparticles dispersed in deionized water with particle concentrations of 0.2%, 0.4%, and 0.6% volume are used as working fluids. The effects of pin fin configuration, particle concentration, and flow rate on the heat transfer performance and flow behaviors are presented. Reynolds numbers based on the hydraulic diameter of each flow channel ranging between 700 and 3700, fluid temperature of 15 °C, and heat flux ranging from 2 and 5 W/cm 2 are tested. Hydraulic diameters based on each flow channel are equally designed at 1.2 mm for both heat sinks. The experimental results indicate that the heat transfer coefficient increased with increasing Reynolds numbers and particle concentrations. The MCFHS gave greater heat transfer performance than that of the MSFHS by about 6–9%. For pressure drop data, the measured data showed that the pin fin configuration and particle concentration had small effects on the pressure drop and pumping power. [ABSTRACT FROM AUTHOR]

This study presents the experimental thermal and hydraulic performance of heat sink with miniature circular pin fin structure using two different types of nanofluid as coolant. ZnO and SiO 2 nanoparticles dispersed in DI water with particle volume fraction of 0.2, 0.4 and 0.6 vol.% are tested and compared with the data for water. A heat sink with inline arrangement of circular pins is designed and made from aluminum material. The height, diameter, pitch, and number of pins are 1.2, 1.2, 2.4 mm and 143, respectively. Uniform heat flux at the bottom of the heat sink is performed. The present work is conducted at fluid temperature of 15 °C. The mass flow rate ranged from 0.65 to 3.32 kg/min and the heat flux ranged between 20 and 48 kW/m 2 . The effects of particle type, particle concentration, and mass flow rate on the thermal and hydraulic performances are reported. The measured data show that the heat transfer performance of the nanofluid-cooled heat sink is higher than that of the water-cooled heat sink. Comparison between ZnO and SiO 2 nanofluids, higher heat transfer performance for ZnO–water nanofluids is observed by about 3–9%. For hydraulic performance, the results show that the addition of nanoparticles in the base fluid have a small effect on the pumping power. Finally, new heat transfer and pressure drop correlations are proposed to predict the Nusselt number and pressure drop of nanofluids flow in heat sinks with pin fin configuration. [ABSTRACT FROM AUTHOR]

In the present paper, the thermal conductivity of hybrid nanofluids is experimentally investigated. The studied nanofluid was produced using a two-step method by dispersing Cu and TiO 2 nanoparticles with average diameter of 70 and 40 nm in a binary mixture of water/EG (60:40). The properties of this nanofluid were measured in various solid concentrations (0.1, 0.2, 0.4, 0.8, 1, 1.5, and 2%) and temperatures ranging from 30 to 60 °C. Next, two new correlations for predicting the thermal conductivity of studied hybrid nanofluids, in terms of solid concentration and temperature, are proposed that use an artificial neural network (ANN) and are based on experimental data. The results indicate that these two new models have great ability to predict thermal conductivity and show excellent agreement with the experimental results. [ABSTRACT FROM AUTHOR]

The addition of nanoparticles to a base fluid is one of the significant issues to enhance heat transfer. In this study, different nanofluids were developed by mixing a water base fluid with magnetic nanoparticles. Thermophysical properties such as thermal conductivity and viscosity of the obtained nanofluid were investigated. The effect of different nominal diameters of nanoparticles and concentrations of nanoparticles on the thermal conductivity and viscosity of nanofluids have been examined. Three different diameters of magnetic nanoparticles (about 37 nm, 71 nm, and 98 nm) have been tested in this experimental investigation. Experimental results indicate that thermal conductivity increases as volume fraction increases, and thermal conductivity of the nanofluid increases with a decrease of nanoparticle's size. Moreover, the nanofluid dynamics viscosity ratio increases with an increase in particle concentration and nanoparticle's diameter. This paper identifies several important issues that should be considered in future work. [ABSTRACT FROM AUTHOR]

A considerable number of studies can be found on the thermal conductivity of nanofluids in which AlO nanoparticles are used as additives. In the present study, the aim is to measure the thermal conductivity of very narrow AlO nanoparticles with the size of 5 nm suspended in water. The thermal conductivity of nanofluids with concentrations up to 5 % is measured in a temperature range between 26 and 55 °C. Using the experimental data, a correlation is presented as a function of the temperature and volume fraction of nanoparticles. Finally, a sensitivity analysis is performed to assess the sensitivity of thermal conductivity of nanofluids to increase the particle loading at different temperatures. The sensitivity analysis reveals that at a given concentration, the sensitivity of thermal conductivity to particle loading increases when the temperature increases. [ABSTRACT FROM AUTHOR]

Halloysite nanotube (HNT) which is cheap, natural, and easily accessible 1D clay, can be used in many applications, particularly heat transfer enhancement. The aim of this research is to study experimentally the pool boiling heat transfer (PBHT) performance of novel halloysite nanofluids at atmospheric pressure condition from typical horizontal heater. The nanofluids are prepared from halloysite nanotubes (HNTs) nanomaterials-based deionized water (DI water) with the presence of sodium hydroxide (NaOH) solution to control pH = 12 to obtain stable nanofluid. The nanofluids were prepared with dilute volume concentrations of 0.01–0.5 vol%. The performance of PBHT is studied via pool boiling curve and pool boiling heat transfer coefficient (PBHTC) from the typical heater which is the copper horizontal tube with a thickness of 1 mm and a diameter of 22 mm. The temperatures of the heated tube surface are measured to obtain the PBHTC. The results show an improvement of PBHTC for halloysite nanofluids compared to the base fluid. At 0.05 vol% concentration, HNT nanofluid has the best enhancement of 5.8% at moderate heat flux (HF). This indicates that HNT is a potential material in heat transfer applications. [ABSTRACT FROM AUTHOR]

A CFD model was performed with commercial software through the adoption of the finite volume method and a SIMPLE algorithm. SiO2-P25 particles were added to water/ethylene glycol as a base fluid. The result is considered a new hybrid nanofluid (HN) for investigating heat transfer (HT). The volume concentrations were 0.5, 1.0, and 1.5%. The Reynolds number was in the range of 5000–17,000. The heat flux (HF) was 7955 W/m2, and the wall temperature was 340.15 K. The numerical experiments were performed strictly following the rules that one should follow in HT experiments. This is important because many studies related to nanofluid HT overlook these details. The empirical correlations that contain the friction factor perform better with higher Reynolds numbers than the correlations based only on Reynolds and Prandtl numbers. When temperature differences are moderate, researchers may consider using constant properties to lower computational costs, as they may give results that are similar to temperature-dependent ones. Compared with previous research, our simulation results are in agreement with the experiments in real time. [ABSTRACT FROM AUTHOR]

In this article, the viscosity of two common nanofluids including Al2O3/water and TiO2/water is measured at high temperatures, and high concentrations of the nanofluids. The range of temperature is 15–60°C where the volume fraction of nanoparticles varies from 1 to 8%. Next, comparisons have been done with the most well-known theoretical and experimental reports in the literature. Finally, using the experimental data, a helpful correlation is presented. [ABSTRACT FROM PUBLISHER]

Experimental studies are performed to evaluate the stability of zinc oxide (ZnO) nanoparticles suspended in a mixture of ethylene glycol and water with weight ratio of 40–60 as the base fluid. Different methods have been employed to disperse ZnO nanoparticles. It is found that using Gum Arabic leads to clustering and settle the nanoparticles. Also, the use of DI ammonium hydrogen citrate with weight ratio 1:1 (surfactant:nanoparticles) gives the acceptable stability. The density of nanofluids is measured and the results are compared with theoretical results. A helpful correlation for the measured densities of the stable nanofluids in a temperature range of 25–40 °C is presented which can used in practical applications. Finally based on the correlation a sensitivity analysis has been done. It is found that at higher temperatures the density is more sensitive to the increases in volume fraction. [ABSTRACT FROM AUTHOR]

Abstract: Nanofluids are a suspension of particles with ultrafine size in a conventional base fluid that increases the heat transfer performance of the original base fluid. They show higher thermal performance than base fluids especially in terms of the thermal conductivity and heat transfer coefficient. During the last decade, many studies have been carried out on the heat transfer and flow characteristics of nanofluids, both experimentally and theoretically. The purpose of this article is to propose a dispersion model for predicting the heat transfer coefficient of nanofluids under laminar flow conditions. TiO2 nanoparticles dispersed in water with various volume fractions and flowing in a horizontal straight tube under constant wall heat flux were used. In addition, the predicted values were compared with the experimental data from He et al. . In the present study, the results show that the proposed model can be used to predict the heat transfer behaviour of nanofluids with reasonable accuracy. Moreover, the results also indicate that the predicted values of the heat transfer coefficient obtained from the present model differ from those obtained by using the Li and Xuan equation by about 3.5% at a particle volume fraction of 2.0%. [Copyright &y& Elsevier]

Abstract: A study of nucleate pool boiling heat transfer of TiO2–water nanofluids is experimentally conducted. Nanofluids with various concentrations of 0.00005, 0.0001, 0.0005, 0.005, and 0.01vol.% are employed. Horizontal circular plates made from copper and aluminium with different roughness values of 0.2 and 4μm are used as heating surfaces. The experiments are performed to explore the effects of nanofluids concentration as well as heating surface material and roughness on nucleate pool boiling characteristics and the heat transfer coefficient under ambient pressure. The results show that based on the copper heated surface which is tested with a concentration of 0.0001vol.%, higher nucleate pool boiling heat transfer coefficient is obtained when compared with the base fluid. A 15% increase is obtained for the surface roughness of 0.2μm and a 4% increase is obtained for roughness of 4μm. For concentrations higher than 0.0001vol.%, however, the higher the concentration, the lower the heat transfer coefficient. In the case of aluminium heated surface, the corresponding heat transfer coefficients are larger than for the copper surface by around 30% with a roughness of 0.2μm and around 27% with a roughness of 4μm. Moreover, the results also indicate that the heat transfer coefficient obtained based on a roughness of 4μm is higher than that for a roughness of 0.2μm by around 12% for aluminium and by around 13% for copper. [ABSTRACT FROM AUTHOR]

Abstract: This article reports a comparison of the differences between using measured and computed thermophysical properties to describe the heat transfer performance of TiO2–water nanofluids. In this study, TiO2 nanoparticles with average diameters of 21nm and a particle volume fraction of 0.2–1vol.% are used. The thermal conductivity and viscosity of nanofluids were measured by using transient hot-wire apparatus and a Bohlin rotational rheometer, respectively. The well-known correlations for calculating the thermal conductivity and viscosity of nanofluids were used for describing the Nusselt number of nanofluids and compared with the results from the measured data. The results show that use of the models of thermophysical properties for calculating the Nusselt number of nanofluids gave similar results to use of the measured data. Where there is a lack of measured data on thermophysical properties, the most appropriate models for computing the thermal conductivity and viscosity of the nanofluids are the models of Yu and Choi and Wang et al., respectively. [Copyright &y& Elsevier]

This article presents the measurement of thermal conductivity and viscosity of nanofluids experimentally. Silver nanoparticles dispersed in water with volume concentrations of 0.3, 0.4, 0.6, 0.8, 0.9, and 1.2 vol% are used in the present study. A transient hot-wire apparatus is used for measuring the thermal conductivity of nanofluids and a Cannon-Fenske viscometer is used to measure the kinematic viscosity of nanofluids. The data are collected for temperatures ranging from 50 to 90°C. The results have shown an increase in the measured thermal conductivity and viscosity of nanofluids as the particle concentrations increase, and the values are higher than the values of the base liquids. The minimum enhancement of 27% for 0.3 vol% and a maximum enhancement of 115% for 1.2 vol% are observed at an average temperature of 70°C when compared with pure water for the same temperature. Further, the thermal conductivity of nanofluids increases with the increase in nanofluid temperatures and, conversely, the viscosity of nanofluids decreases with the increase in temperature of nanofluids. An experimental correlation is developed based on the experimental data for thermal conductivity and viscosity that relates the particle volume concentration and nanofluid temperature. [ABSTRACT FROM AUTHOR]

Abstract: Nanofluid is a new class of heat transfer fluids engineered by dispersing metallic or non-metallic nanoparticles with a typical size of less than 100nm in the conventional heat transfer fluids. Their use remarkably augments the heat transfer potential of the base liquids. This article presents the heat transfer coefficient and friction factor of the TiO2-water nanofluids flowing in a horizontal double tube counter-flow heat exchanger under turbulent flow conditions, experimentally. TiO2 nanoparticles with diameters of 21nm dispersed in water with volume concentrations of 0.2–2vol.% are used as the test fluid. The results show that the heat transfer coefficient of nanofluid is higher than that of the base liquid and increased with increasing the Reynolds number and particle concentrations. The heat transfer coefficient of nanofluids was approximately 26% greater than that of pure vol.%, and the results also show that the heat transfer coefficient of the nanofluids at a volume concentration of 2.0vol.% was approximately 14% lower than that of base fluids for given conditions. For the pressure drop, the results show that the pressure drop of nanofluids was slightly higher than the base fluid and increases with increasing the volume concentrations. Finally, the new correlations were proposed for predicting the Nusselt number and friction factor of the nanofluids, especially. [Copyright &y& Elsevier]

Abstract: 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 0.2–2vol.% 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. [Copyright &y& Elsevier]

Abstract: This article reports an experimental study on the forced convective heat transfer and flow characteristics of a nanofluid consisting of water and 0.2vol.% TiO2 nanoparticles. The heat transfer coefficient and friction factor of the TiO2–water nanofluid flowing in a horizontal double-tube counter flow heat exchanger under turbulent flow conditions are investigated. The Degussa P25 TiO2 nanoparticles of about 21nm diameter are used in the present study. The results show that the convective heat transfer coefficient of nanofluid is slightly higher than that of the base liquid by about 6–11%. The heat transfer coefficient of the nanofluid increases with an increase in the mass flow rate of the hot water and nanofluid, and increases with a decrease in the nanofluid temperature, and the temperature of the heating fluid has no significant effect on the heat transfer coefficient of the nanofluid. It is also seen that the Gnielinski equation failed to predict the heat transfer coefficient of the nanofluid. Finally, the use of the nanofluid has a little penalty in pressure drop. [Copyright &y& Elsevier]