Experimental investigation of turbulent flow and convective heat transfer characteristics of alumina water nanofluids in fully developed flow regime

In this paper the convective heat transfer and friction factor of the nanofluids in a circular tube with constant wall temperature under turbulent flow conditions were investigated experimentally. Al₂O₃ nanoparticles with diameters of 40 nm dispersed in distilled water with volume concentrations of 0.1–2 vol.% were used as the test fluid. All physical properties of the Al₂O₃–water nanofluids needed to calculate the pressure drop and the convective heat transfer coefficient were measured. The results show that the heat transfer coefficient of nanofluid is higher than that of the base fluid and increased with increasing the particle concentrations. Moreover, the Reynolds number has a little effect on heat transfer enhancement. The experimental data were compared with traditional convective heat transfer and viscous pressure drop correlations for fully developed turbulent flow. It was found that if the measured thermal conductivities and viscosities of the nanofluids were used in calculating the Reynolds, Prandtl, and Nusselt numbers, the existing correlations perfectly predict the convective heat transfer and viscous pressure drop in tubes.     

Heat transfer augmentation in concentric elliptic annular by ethylene glycol based nanofluids

In this article, laminar mixed convective heat transfer at different nanofluids flow in an elliptic annulus with constant heat flux boundary condition has been numerically investigated. The three dimensional governing equations (continuity, momentum and energy) are solved using the finite volume method (FVM). The investigation covers Reynolds number and nanoparticle volume fraction in the ranges of 200–1000 and 0–4% respectively. In the present work, four different types of nanofluids are examined in which Al₂O₃, CuO, SiO₂ and ZnO are suspended in the base fluid of ethylene glycol (EG) with different nanoparticle sizes 20, 40, 60 and 80 nm. The results show that SiO₂-EG nanofluid has the highest Nusselt number, followed by Al₂O₃-EG, ZnO-EG, CuO-EG, and lastly pure ethylene glycol. The Nusselt number increased as the nanoparticle volume fraction and Reynolds number increased; however, it decreased as the nanoparticle diameter increased. It is found that the glycerine-SiO₂ shows the best heat transfer enhancement compared with other tested base fluids. Comparisons of the present results with those available in the literature are presented and discussed.     

Effect of Volume Fraction of γ‐Al2O3 Nanofluid on Heat Transfer Enhancement in a Concentric Tube Heat Exchanger

In this work, γ-Al₂O₃ nanoparticles with mean diameter of 10 nm are dispersed in deionized water with four nanoparticle volume concentrations of 0.25, 0.5, 0.75, and 1%. The effect of γ-Al₂O₃/water nanofluids on the heat transfer enhancement of heat exchangers is investigated under turbulent regime for four different volumetric flow rates of 150, 200, 250, and 300 L/h. The experimental results showed that the convective heat transfer is increased by increasing particles volume fraction as well as flow rate. The maximum enhancement obtained in Nusselt number and heat transfer coefficient was 20 and 22.8%, respectively, at Reynolds number of 6026 and particle volume fraction of 1%. The experimental Nusselt numbers of nanofluids showed good agreement with the available empirical correlation at particle volume fractions of 0.25 and 0.5%. An empirical correlation is obtained to estimate the Nusselt number of nanofluid under the conditions of this work.     

Effect of CuO-, SiO2-, and Al2O3-based nanofluids on automotive cooling systems from laminar to turbulent flow regime—A CFD study

In recent studies, much attention has been given to nanofluids suggesting that adding nanoparticles in base fluids offers a higher heat transfer rate compared with conventional fluids. This study is based on the numerical investigation of different types of nanofluids, consisting of CuO (50 nm), SiO₂ (40 nm), and Al₂O₃ (15 nm) nanoparticles at different volume concentrations. Several simulations were performed from low to high Reynolds numbers, corresponding to laminar and turbulent flow regimes using ANSYS-Fluent CFD solver. Results suggest that under a laminar flow regime with the same Reynolds number of 2000, CuO-based nanofluids perform better as compared with SiO₂ and Al₂O₃-based nanofluids with Nusselt number (Nu) having percentage increase of 90% and 60% comparing with SiO₂- and Al₂O₃-based nanofluids, respectively. However, at higher Reynolds numbers when the flow is turbulent, Al₂O₃-based nanofluids demonstrate better performance having a percentage increase in Nusselt numbers equal to 40% and 23% as compared with CuO and SiO₂-based nanofluids respectively under the same Reynolds number of 15,000. This implies that turbulence has a significant effect on heat transfer rate, and is not only related to thermal conductivity. This study will help in designing more compact cooling systems for engines and the internal environment of motor vehicles.     

Numerical study of turbulent flow and heat transfer characteristics of nanofluids considering variable properties

Turbulent flow and heat transfer of three different nanofluids (CuO, Al₂O₃ and SiO₂) in an ethylene glycol and water mixture flowing through a circular tube under constant heat flux condition have been numerically analyzed. New correlations for viscosity up to 10% volume concentration for these nanofluids as a function of volume concentration and temperature are developed from the experiments and are summarized in the present paper. In our numerical study, all the thermophysical properties of nanofluids are temperature dependent. Computed results are validated with existing well established correlations. Nusselt number prediction for nanofluids agrees well with Gnielinski correlation. It is found that nanofluids containing smaller diameter nanoparticles have higher viscosity and Nusselt number. Comparison of convective heat transfer coefficient of CuO, Al₂O₃ and SiO₂ nanofluids have been presented. At a constant Reynolds number, Nusselt number increases by 35% for 6% CuO nanofluids over the base fluid.     

Conjugate heat transfer enhancement of laminar slot jets with various nanofluids on an array of protruding hot sources using MPM approach

Conjugate heat transfer in laminar slot jets impinging on multiple protruding hot sources using various nanofluids has been investigated numerically by employing (i) a mass-based modeling and an (ii) Eulerian-based multi-phase modeling (MPM). Various parameters such as streamline contours, isotherm profiles, local Nusselt number (Nu), average Nusselt number (Nuavg) are evaluated for different nanofluids (Ag–water, Al₂O₃–water, CuO–water and TiO₂–water), various range of Reynolds number (Re), particle volume fraction (?), diameter of the nanoparticle (d) and thermal conductivity ratio (kr). The steady, laminar, incompressible and two-dimensional flows are considered for the analysis. Finite-volume method with SIMPLE algorithm is used to solve continuity, momentum and energy equations along with boundary conditions. The highest heat transfer rate is achieved at ??=?0.05 for any protruding blocks and Reynolds number. Conjugate heat transfer rate of nanofluids increases with decreasing the diameter of nanoparticles. Here, Al₂O₃–water nanofluid is found to exhibit highest average Nusselt number compared to other nanofluids. The mixture based MPM approach with considering slip velocity yields higher heat transfer rate compared to the results obtained by single phase modeling approach.     

Heat transfer and nanofluid flow characteristics through a circular tube fitted with helical tape inserts

Numerical investigations are performed using finite volume method to study laminar convective heat transfer and nanofluids flows through a circular tube fitted with helical tape insert. The wall of tube was subjected to a uniform heat flux boundary condition. The continuity, momentum and energy equations are discretized and the SIMPLE algorithm scheme is applied to link the pressure and velocity fields inside the domain for plain tube. Four different twist ratios of 1.95–4.89, two different types of nanoparticles, Al₂O₃ and SiO₂ with different nanoparticle shapes of spherical, cylindrical and platelets, and 0.5–2.0% volume fraction in base fluid (water) and nanoparticle diameter in the range of 20–50 nm were used to identify their effect on the heat transfer and fluid flow characteristics through a circular tube fitted with helical tape insert geometries. The results indicate that the four types of nanofluid achieved higher Nusselt number than pure water. Nanofluid with Al₂O₃ particle achieved the highest Nusselt number. For all the cases studied, the Nusselt number increased with the increase of Reynolds number and with the decrease of twist ratio of helical tape insert.     

A review and analysis on influence of temperature and concentration of nanofluids on thermophysical properties,heat transfer and pumping power

The Prandtl number, Reynolds number and Nusselt number are functions of thermophysical properties of nanofluids and these numbers strongly influence the convective heat transfer coefficient. The pressure loss and the required pumping power for a given amount of heat transfer depend on the Reynolds number of flow. The thermophysical properties vary with temperature and volumetric concentration of nanofluids. Therefore, a comprehensive analysis has been performed to evaluate the effects on the performance of nanofluids due to variations of density, specific heat, thermal conductivity and viscosity, which are functions of nanoparticle volume concentration and temperature. Two metallic oxides, aluminum oxide (Al₂O₃), copper oxide (CuO) and one nonmetallic oxide silicon dioxide (SiO₂), dispersed in an ethylene glycol and water mixture (60:40 by weight) as the base fluid have been studied.     

Experimental investigation of nanofluids in confined laminar radial flows

This paper presents an experimental investigation of heat transfer enhancement capabilities of coolants with suspended nanoparticles (Al₂O₃ dispersed in water) inside a radial flow cooling device. Steady, laminar radial flow of a nanofluid between a heated disk and a flat plate with axial coolant injection has been considered. An experimental test rig was built. Results show that heat transfer enhancements are possible in radial flow cooling systems with the use of nanofluids. In general, it was noticed that the Nusselt number increases with particle volume fraction and Reynolds number and decreases with an increase in disk spacing.     

Estimation of heat transfer coefficient and friction factor in the transition flow with low volume concentration of Al2O3 nanofluid flowing in a circular tube and with twisted tape insert

Experiments to evaluate heat transfer coefficient and friction factor for flow in a tube and with twisted tape inserts in the transition range of flow with Al₂O₃ nanofluid are conducted. The results showed considerable enhancement of convective heat transfer with Al₂O₃ nanofluids compared to flow with water. It is observed that the equation of Gleninski applicable in transitional flow range for single-phase fluids showed considerable deviation when compared with values obtained with nanofluid. The heat transfer coefficient of nanofluid flowing in a tube with 0.1% volume concentration is 23.7% higher when compared with water at number of 9000. Heat transfer coefficient and pressure drop with nanofluid has been experimentally determined with tapes of different twist ratios and found to deviate with values obtained from equations developed for single-phase flow. A regression equation is developed to estimate the Nusselt number valid for both water and nanofluid flowing in the transition flow Reynolds number range in circular plain tube and with tape inserts. The maximum friction factor with twisted tape at 0.1% nanofluid volume concentration is 1.21 times that of water flowing in a plain tube.     

Numerical Simulation of Nanoparticles Concentration Effect on Forced Convection in a Tube With Nanofluids

The present study aims to identify effects due to convection heat transfer in a tube. Turbulent and laminar forced convection flow of a water–Al₂O₃ nanofluid in a tube subjected to a constant and uniform temperature at the wall was numerically analyzed. The single-phase model was employed to simulate the nanofluid convection, taking into account appropriate thermophysical properties. Particles are assumed spherical with a diameter equal to 24 nm. Simulations have been carried out for the pertinent parameters in the following ranges: Reynolds number from 10³ to 10⁵ and volumetric fraction of alumina nanoparticles between 0 to 4%. It is found that convective heat transfer coefficient for nanofluids is greater than that of the base liquid. Heat transfer enhancement is increasing with the particle volume concentration and Reynolds number. As for the friction factor, it shows a good agreement with the classical correlation used for normal fluid, such as the Blasius formula. Moreover, a study on wall shear stress was attempted.     

Heat transfer enhancement of nanofluids flow in microtube with constant heat flux

In this paper, laminar convective heat transfer in a two-dimensional microtube (MT) with 50 μm diameter and 250 μm length with constant heat flux is numerically investigated. The governing (continuity, momentum and energy) equations were solved using the finite volume method (FVM) with the aid of SIMPLE algorithm. Different types of nanofluids Al₂O₃, CuO, SiO₂ and ZnO, with different nanoparticle size 25, 45, 65 and 80 nm, and different volume fractions ranged from 1% to 4% using ethylene glycol as a base fluid were used. This investigation covers Reynolds number in the range of 10 to 1500. The results have shown that SiO₂–EG nanofluid has the highest Nusselt number, followed by ZnO–EG, CuO–EG, Al₂O₃–EG, and lastly pure EG. The Nusselt number for all cases increases with the volume fraction but it decreases with the rise in the diameter of nanoparticles. In all configurations, the Nusselt number increases with Reynolds number.     

Heat transfer enhancement for combined convection flow of nanofluids in a vertical rectangular duct considering radiation effects

In this paper, combined convective heat transfer and nanofluids flow characteristics in a vertical rectangular duct are numerically investigated. This investigation covers Rayleigh numbers in the range of 2 × 10⁶ ≤ Ra ≤ 2 × 10⁷ and Reynolds numbers in the range of 200 ≤ Re ≤ 1000. Pure water and five different types of nanofluids such as Ag, Au, CuO, diamond, and SiO₂ with a volume fraction range of 0.5% ≤ φ ≤ 3% are used. The three‐dimensional steady, laminar flow, and heat transfer governing equations are solved using finite volume method (FVM). The effects of Rayleigh number, Reynolds number, nanofluids type, nanoparticle volume fraction of nano‐ fluids, and effect of radiation on the thermal and flow fields are examined. It is found that the heat transfer is enhanced using nanofluids by 47% when compared with water. The Nusselt number increases as the Reynolds number and Rayleigh number increase and aspect ratio decreases. A SiO₂ nanofluid has the highest Nusselt number and highest wall shear stress while the Au nanofluid has the lowest Nusselt number and lowest wall shear stress. The results also revealed that the wall shear stress increases as Reynolds number increases, aspect ratio decreases, and nanoparticle volume fraction increases. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20354     

Comparison between single-phase and two-phases CFD modeling of laminar forced convection flow of nanofluids in a circular tube under constant heat flux

In this article, forced convection heat transfer with laminar and developed flow for water-Al₂O₃ nanofluid inside a circular tube under constant heat flux from the wall was numerically investigated using computational fluid dynamics method. Both single and two-phase models are accomplished for either constant or temperature dependent properties. For this study nanofluids with size particles equal to 100 nm and particle concentrations of 1 and 4 wt% were used. It is observed that the nanoparticles when dispersed in base fluid such as water enhance the convective heat transfer coefficient. The Nusselt number and heat transfer coefficient of nanofluids were obtained for different nanoparticle concentrations and various Reynolds numbers. Heat transfer was enhanced by increasing the concentration of nanoparticles in nanofluid and Reynolds number. Also, a correlation based on the dimensionless numbers was obtained for the prediction the Nusselt number. The modeling results showed that the predicted values were in very good agreement with reference experimental data.     

Numerical studies on heat and fluid flow of nanofluid in a partially heated vertical annulus

A numerical investigation on natural convective heat transfer of nanofluid (Al₂O₃+water) inside a partially heated vertical annulus of high aspect ratio (352) has been carried out. The computational fluid dynamics solver Ansys Fluent is used for simulation and results are presented for various volume fraction of nanoparticles (0‐0.04) at different heat flux values (3‐12 kW/m²). Two well‐known correlations for evaluating thermal conductivity and viscosity have been used. Thus different combinations of the available correlations have been set to form four models (I, II, III, and IV). Therefore, a detailed analysis has been executed to identify effects of thermophysical properties on heat transfer and fluid flow of nanofluids using different models. The results show enhancement in heat transfer coefficient with volume fraction of nanoparticles. Highest enhancement achieved is found to be 14.17% based on model III, while the minimum is around 7.27% based on model II. Dispersion of nanoparticles in base fluid declines the Nusselt number and Reynolds number with different rates depending on various models. A generalized correlation is proposed for Nusselt number of nanofluids in the annulus in terms of volume fraction of nanoparticles, Rayleigh number, Reynolds number, and Prandtl number.     

Convective Heat Transfer and Resistance Characteristics of Nanofluids with Cylindrical Particles

Numerical research on convective heat transfer and resistance characteristics of TiO₂/water nanofluids with cylindrical particles in laminar channel flow are performed by solving the governing equations of fluid flow with the additional term of cylindrical nanoparticles, the equation of probability density functions for cylindrical nanoparticle orientation, and general dynamics equation for nanoparticle volume concentration. The nonuniformity of nanoparticle distribution is considered and the effects of both particle volume concentration and Reynolds number on friction factor and local Nusselt number are mainly analyzed. The results show that the friction factor of nanofluid flow increases with an increase in particle volume concentration. And the friction factor decreases with increasing Reynolds number and is not dependent on the volume concentration at high Reynolds numbers. The Nusselt number declines when the Reynolds number decreases, and finally approaches an asymptotic value after the Reynolds number falls to a certain value. The Nusselt number is higher in the entrance region than at the downstream locations, and will become steady at somewhere downstream when the flow is thermally and hydraulically developed.     

Asymmetric Forced Convection of Nanofluids in a Channel with Symmetrically Mounted Rib Heaters on Opposite Walls

Laminar forced convection of nanofluids in a vertical channel with symmetrically mounted rib heaters on surfaces of opposite walls is numerically studied. The fluid flow and heat transfer characteristics are examined for various Reynolds numbers and nanoparticles volume fractions of water-Al₂O₃ nanofluid. The flow exhibits various structures with varying Reynolds number. Even though the geometry and heating is symmetric with respect to a channel vertical mid-plane, asymmetric flow and heat transfer are found for Reynolds number greater than a critical value. Introduction of nanofluids in the base fluid delays the flow solution bifurcation point, and the critical Reynolds number increases with increasing nanoparticle volume fraction. A skin friction coefficient along the solid-fluid interfaces increases and decreases sharply along the bottom and top faces of the heaters, respectively, due to sudden acceleration and deceleration of the fluid at the respective faces. The skin friction coefficient, as well as Nusselt numbers in the channel, increase with increasing volume fraction of nanoparticles.     

Experimental investigation of oxide nanofluids laminar flow convective heat transfer

In the present investigation nanofluids containing CuO and Al₂O₃ oxide nanoparticles in water as base fluid in different concentrations produced and the laminar flow convective heat transfer through circular tube with constant wall temperature boundary condition were examined. The experimental results emphasize that the single phase correlation with nanofluids properties (Homogeneous Model) is not able to predict heat transfer coefficient enhancement of nanofluids. The comparison between experimental results obtained for CuO / water and Al₂O₃ / water nanofluids indicates that heat transfer coefficient ratios for nanofluid to homogeneous model in low concentration are close to each other but by increasing the volume fraction, higher heat transfer enhancement for Al₂O₃ / water can be observed.     

An experimental study of convective heat transfer in silicon microchannels with different surface conditions

An experimental investigation has been performed on the laminar convective heat transfer and pressure drop of water in 13 different trapezoidal silicon microchannels. It is found that the values of Nusselt number and apparent friction constant depend greatly on different geometric parameters. The laminar Nusselt number and apparent friction constant increase with the increase of surface roughness and surface hydrophilic property. These increases become more obvious at larger Reynolds numbers. The experimental results also show that the Nusselt number increases almost linearly with the Reynolds number at low Reynolds numbers (Re<100), but increases slowly at a Reynolds number greater than 100. Based on 168 experimental data points, dimensionless correlations for the Nusselt number and the apparent friction constant are obtained for the flow of water in trapezoidal microchannels having different geometric parameters, surface roughnesses and surface hydrophilic properties. Finally, an evaluation of heat flux per pumping power and per temperature difference is given for the microchannels used in this experiment.     

Thermal Enhancement of Triangular Ducts Using Compound of Vortex Generators and Nanofluids

Laminar flow and heat transfer of three different types of nanofluids; Al₂O₃, CuO, and SiO₂ suspended in ethylene glycol, in a triangular duct using delta-winglet pair of vortex generator are numerically simulated in three dimensions. The governing equations of mass, momentum and energy are solved using the finite volume method. The effects of types, concentrations, and diameter of solid nanoparticles and Reynolds number on thermal and hydraulic performance of triangular duct are examined. The range of Reynolds number, volume fraction and nanoparticles diameters is 100–1200, 1–4%, and 25–85 nm, respectively. The results indicate that the average Nusselt number increases with the particles volume fraction and Reynolds number associated with an increase in the pressure drop. The heat transfer enhancement and pressure drop penalty reduce with increasing the particles diameters. However, a reduction in the pumping power required is observed to force the nanofluids when the volume fraction increases, assuming the heat transfer coefficient remains constant.