forced convective heat transfer of nano fluids: a review...

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!American Journal of Oil and Chemical Technologies; ISSN (online): 2326-6589; ISSN (print): 2326-6570

Volume 2, Issue 3, March 2014

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Forced Convective Heat Transfer of Nano fluids: A Review of the Recent Literature

M. Motevasel1, A. Soleimanynazar1, M. Jamialahmadi 2

1. Chemical Engineering Department, University of Isfahan, Isfahan , Iran 2. Petroleum Engineering Department, Petroleum University of Technology, Ahwaz, Iran

Corresponding Author E-mail: [email protected]

Abstract:

Numerous researchers have investigated the forced convection of fluids, both experimentally and numerically. A good understanding of characteristics of nanofluid flow has thoroughly been investigated in these studies. In recent years, many researchers have tried to fill the gaps on this subject in the literature. This paper reports on most of the forced convective heat transfer literature occurring both in-tubes and in-channels regarding the use and preparation of nanofluids. The peer reviewed papers published in citation index journals up to 2013 have been selected for review in the paper. Classification of the papers has been performed according to the publication years. The critical information on the theoretical, experimental and numerical works is presented comprehensively for each paper.

Keywords: Nano fluids, Rheological, Instability, Heat transfer coefficient.

1. Introduction

Performance of heat transfer equipment can be improved with studies related to a significant increase in heat flux and miniaturization. In many industrial applications such as power generation, microelectronics, heating processes, cooling processes and chemical processes, water, mineral oil and ethylene glycol are used as heat transfer fluid. Effectiveness and high compactness of heat exchangers are obstructed by the lower heat transfer properties of these common fluids as compared to most solids. It is obvious that solid particles having thermal conductivities several hundred times higher than these conventional fluids must be used in the heat transfer applications. To improve thermal conductivity of a fluid, suspension of ultrafine solid particles in the fluid can be a creative idea. Different types of particles (metallic, non-metallic and polymeric) can be added into fluids to form slurries. Due to the fact that sizes of these suspended particles are in the millimeter or even micrometer scale, some serious problems such as the clogging of flow channels, erosion of pipelines and an increase in pressure drop can occur. Moreover, they often suffer from rheological and instability problems. Especially, the



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particles tend to settle rapidly. For that reason, though the slurries have better thermal conductivities but they are not practical.

Many published articles show that the heat transfer coefficient of nanofluids is much higher than the common-base fluid giving little or no penalty in pressure drop. The main reasons for the heat transfer enhancement of the nanofluids may roughly be listed as follow: the suspended nanoparticles increase the thermal conductivity of the fluids, and the chaotic movement of ultrafine particles increases fluctuation and turbulence of the fluids that accelerates the energy exchange process. Furthermore, that the numerous correlations for single-phase fluid have clearly failed

to predict the heat transfer coefficients of nanofluids that might have been caused by the complicated phenomena coexisting in the main flow. However, some sophisticated correlations for predicting the heat transfer performance of nanofluids have been established. At present, the uses of the nanofluids are still in early stages of development. Therefore, urgent theoretical and experimental research works is needed to clearly understand and accurately predict their hydrodynamic and thermal characteristics. Generally, many researchers indicated that nanofluids behave like pure fluids because the suspended particles are ultra-fine. However, at present, no formulated advanced theory exists to explain the behavior of nanofluids by considering them as multi-component materials. Moreover, several attempts have been discussed to determine the physical properties of nanofluids in the literature.

2. LITERATURE REVIEW OF RECENT YEARS

A research group at the Argonne National Laboratory first continuously conducted the studies on the use of particles of nanometer dimension approximately a decade ago. Choi [1] was possibly the first researcher who called the fluids with particlesof nanometer scale ‘nano-fluids’.

Nanofluids have better rheological and stability properties, significantly higher thermal conductivities and no penalty in pressure drop as compared with suspended particles of millimeter-ormicrometer scale.

In recent years, the nanofluid has emerged as an alternative heat transfer fluid for heat transfer applications showing a significant potential for heat transfer improvement. It has been expected to be suitable for the engineering application without severe problems in pipeline and with little or no penalty in pressure drop. According to earlier researches, thermal conductivity has come into the limelight as most studied transport properties of the nanofluid. The convective heat



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transfer of nanofluids ha comparatively been less acclaimed in literature therefore the number of the publications dealing with the convective heat transfer of nanofluids is limited. The most productive research has been continuously carried out by the following studies.

2.1. Some studies of 2013

Davarnejad et al. [2] reported the CFD simulation of heat flux was considered using Fluent software (version 6.3.26) in the laminar flow. Al2O3 nanoparticles in water with concentrations of 0.5%, 1.0%, 1.5%, 2% and 2.5% were used in this simulation. All of the thermo-physical properties of nanofluids were assumed to be temperature independent. Two particle sizes with average size of 20 and 50 nm were used in this research. They concluded that heat transfer coefficient increased by increasing the Reynolds number and the concentration of nanoparticles. The maximum convective heat transfer coefficient was observed at the highest concentration of nano-particles in water (2.5%). Furthermore, the two nanofluids showed higher heat transfer than the base fluid (water) although the nanofluid with particles size of 20 nm had the highest heat transfer coefficient.

Elias et al. [3] investigated the effect of different nanoparticle shapes (such as cylindrical, bricks, blades, platelets, and spherical) on the performance of a shell and tube heat exchanger operating with nanofluid analytically.Boehmite alumin(�-AlOOH) nanoparticles of different shapes were dispersed in a mixture of water/ethylene glycol as the nanofluid. The thermodynamic performance of the shell and tube heat exchanger that was used in a waste heat recovery system was analysed in terms of heat transfer rate and entropy generation. They used Established correlations to measure the thermal conductivity, heat transfer coefficient and rate and entropy generation of nanofluid. The results showed an increase in both the heat transfer and thermodynamic performance of the system. However, among the five nanoparticle shapes, cylindrical shape exhibited better heat transfer characteristics and heat transfer rate. On the other hand, they observed entropy generation for nanofluids containing cylindrical shaped nanoparticles was higher in comparison with the other nanoparticle shapes. However, the increased percentage of entropy was below 1%.Therefore they found this greater entropy generation could be deemed negligible and cylindrical shaped nanoparticles were recommended to be utilized in heat exchanger systems working with nanofluids.

Fan et al. [4] investigated the laminar fully developed nanofluid flow and heat transfer in a horizonal channel.They obtained highly accurate solutions for the temperature and nanoparticle concentration distributions.They discussed the effects of the Brownian motion parameter Nb, the thermophoresis parameter Nt , and the Lewis number Le on the temperature and nanoparticle concentration distributions. The current analysis showed that the nanoparticles can improve the heat transfer characteristics significantly for this flow problem.

Ghozatloo et al. [5] prepared nanofluids from functionalized Graphene by alkaline method.The nanosheet Graphene was synthesized by using CVD method.



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For dispersion of Graphene in water, it was hydrophilic by treating with the new alkaline method. Herein, it was reported as a facile and effective approach in preparing water-soluble Graphene by considering the potassium carboxylate (COOK) as a mild oxidation process using potassium persulfate (KPS).They studied different parameters of time and temperature effects on thermal conductivity variations of alkaline functionalized Graphene (AFG) at different concentrations. The best result showed that enhancement of thermal conductivity around 14.1% for the sample with 0.05 wt.% of AFG compared to water at 25 �C and 17% at 50 �C.

Huminic et al. [6] used a three-dimensional analysis to study the heat transfer performance of nanofluid flows through a flattened tube in a laminar flow regime and constant heat flux boundary condition. CuO nanoparticles dispersed in ethylene glycol with particle volume concentrations ranging between 0 and 4 vol.% were used as working fluids for simulating the heat transfer of nanofluids.They determined and discussed indetails effects of some important parameters such as nanoparticle volume concentration, particles Brownian motions, and Reynolds number on heat transfer coefficient.Their results showed that the heat transfer coefficient increases with increase in the volume concentration level of the nanoparticle, Brownian motion and the Reynolds number. Numerical results validated by comparison of simulations with those available in the literature.

Jahanbin et al. [7] reported nemerical investigation on forced convective heat transfer of nanofluids in laminar flow inside a mini-channel with circular cross-section under constant heat flux boundary condition at walls.They used nanofluid that contained CuO nanoparticles with diameter of 50 nanometer in water base fluid. At the entrance of channel, profiles of uniform velocity & temperature prevail. In order to obtain fully developed profiles, geometry of problem considered as L/D = 100. Problem was solved by means of 4 different models, including Homogeneous and Dispersion models in both of constant and variable thermophysical properties through the finite-volume method. The temperature-dependent properties was used for the first time in nanofluids dispersion model. It was regarded in the presence of nanoparticles the heat transfer coefficient will be increased to some considerable extent and the heat transfer enhancement strongly depends on the volume concentration of nanoparticles and Peclet number. Also,they carried out comparison with experimental data and literatures’ correlations which indicated the Dispersion model in both cases was more precise and Homogeneous model (single phase) underestimates the Nusselt number in constant thermo physical properties.

Javadi et al. [8] applied , SiO2, TiO2 and Al2O3 nanoparticles in a plate heat exchanger and the effects on thermophysical properties and heat transfer characteristics were compared with the base fluid. Since it was desired to minimize the pressure drop, the influence of nanofluid application on pressure drop and entropy generation was investigated. It was concluded that the thermal conductivity, heat transfer coefficient and heat transfer rate of the fluid increased by adding the nanoparticles and TiO2 and Al2O3 result in higher thermophysical properties in comparison with SiO2. They obtained the highest overall heat transfer coefficient was achieved by Al2O3 nanofluid, which was 308.69 W/m2.K in 0.2% nanoparticle concentration. They observed the related heat transfer rate was improved around 30% compared to SiO2 nanofluid. In



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terms of pressure drop, SiO2 showed the lowest pressure drop, and it was around 50% smaller than the pressure drop in case of using TiO2 and Al2O3.

Jokar et al. [9] investigated the effect of Al2O3 nanofluids in a corrugated plate heat exchanger (PHE) were investigated in this study using computational fluid dynamics (CFD).They investigated a three-channel corrugated PHE with a width of 127 mm, length of 56 mm and channel thickness of 2 mm. The hot fluid in the system flowed through the middle channel while the cold fluid flowed through the two side channels. Three chevron angle configurations were considered for the simulation: 60 deg/60 deg, 27 deg/60 deg, and 27 deg/27 deg.They used commercially available CFD software (ansys fluent) for the simulations. Numerical simulations were conducted for four Al2O3-water nanofluid concentrations: 1%, 2%, 3%, and 4% by volume. In addition, plain water was simulated for comparison. The simulation results showed that although the thermal conductivity increased with increasing nanofluid volume fraction, heat transfer decreased slightly with increasing nanofluid volume fraction. This decrease can be attributed to increased fluid viscosity with increasing volume fraction and the complex flow regimes of nanofluids in the three-dimensional geometries of PHEs.

Ma et al. [10] studied the forced convective heat transfer (FCHT) properties of nanofluids, made of Fe3O4 nanomaterials and deionized water, were firstly measured by a self-made forced convective heat transfer apparatus. The nanofluid flows through a horizontal copper tube in the transition region with Reynolds numbers in the range of 2500–5000. Also they investigated some parameters including Reynolds number, axial distance, and mass concentration. The preliminary results were firstly presented that the heat transfer coefficients of Fe3O4 nanofluids systematically decreased with increasing concentration of nanoparticles under transition region which contradicted the initial expectation.

Mohammed et al. [11] studied numerically the effect of using louvered strip inserts placed in a circular double pipe heat exchanger on the thermal and flow fields utilizing various types of nanofluids. The continuity, momentum and energy equations were solved by means of a finite volume method (FVM). The top and the bottom walls of the pipe were heated with a uniform heat flux boundary condition. Two different louvered strip insert arrangements (forward and backward) were used in their study with a Reynolds number range of 10,000 to 50,000. They also investigated the effects of various louvered strip slant angles and pitches.They used four different types of nanoparticles, Al2O3, CuO, SiO2, and ZnO with different volume fractions in the range of 1% to 4% and different nanoparticle diameters in the range of 20 nm to 50 nm, dispersed in a base fluid (water). The numerical results indicated that the forward louvered strip arrangement can promote the heat transfer by approximately 367% to 411% at the highest slant angle of � = 30� and lowest pitch of S = 30 mm.They obtained the maximal skin friction coefficient of the enhanced tube was around 10 times than that of the smooth tube and the value of performance evaluation criterion (PEC) lies in the range of 1.28�1.56. It was found that SiO2 nanofluid had the highest Nusselt number value, followed by Al2O3, ZnO, and CuO while pure water had the lowest Nusselt number. The results showed that the Nusselt number increased with decreasing the nanoparticle diameter and it increased slightly with increasing the volume fraction



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of nanoparticles. The results revealed that there was a slight change in the skin friction coefficient when nanoparticle diameters of SiO2 nanofluid were varied.

Mohammadian et al.[12]investigated experimentally forced convective heat transfer from a vertical circular tube conveying deionized (DI) water or very dilute Ag-DI water nanofluids (less than 0.02% volume fraction) in a cross flow of air.They performed some experiments in a wind tunnel and heat transfer characteristics such as thermal conductance, effectiveness, and external Nusselt number had been measured at different air speeds, liquid flow rates, and nanoparticle concentrations. The cross flow of air over the tube and the liquid flow in the tube were turbulent in all cases.They compared the experimental results and it had been found that suspending Ag nanoparticles in the base fluid increased thermal conductance, external Nusselt number, and effectiveness. Furthermore, they observed by increasing the external Reynolds number, the external Nusselt number, effectiveness, and thermal conductance increase. Also,they found by increasing internal Reynolds number, the thermal conductance and external Nusselt number enhance while the effectiveness decreases.

Moraveji et al. [13] reported CFD modeling of laminar forced convection on Al2O3 nanofluid with size particles equal to 33 nm and particle concentrations of 0.5, 1 and 6 wt.% within 130 < Re < 1600 in mini-channel heat sink was executed by four individual models (single phase, VOF, mixture, Eulerian). Three-dimensional steady-state governing partial differential equations was discretized using finite volume method. They investigated influences of some important parameters such as nanoparticle concentration and Reynolds number on the enhancement of nanofluid heat transfer. They observed the difference between the two-phase models results was marginal, and they were more precise by comparison with experimental reference data than single phase model. Besides with regard to the most precise and less CPU usage and run time, mixture model was chosen to obtain a correlation based on dimensionless numbers for the Nusselt number and friction factor estimation.

Naphon et al. [14] studied the heat transfer characteristics of nanofluids cooling in the mini-rectangular fin heat sink. They fabricated the heat sinks with three different channel heights from the aluminum by the wire electrical discharge machine with the length, width and base thickness of 110, 60, and 2 mm, respectively. The nanofluids were the mixture of de-ionized water and nanoscale TiO2 particles. They compared the results obtained from the nanofluids cooling in mini-rectangular fin heat sink with those from the de-ionized water cooling method.They considered the effects of the inlet temperature of nanofluids, nanofluid Reynolds number, and heat flux on the heat transfer characteristics of mini-rectangular fin heat sink. It was found that average heat transfer rates for nanofluids as coolant are higher than those for the de-ionized water as coolant. The results of this study were of technological importance for the efficient design of cooling systems of electronic devices to enhance cooling performance.

Rabeti et al. [15] analyzed numerically forced convection heat transfer of nanofluids over a horizontal flat plate embedded in a porous medium saturated with a nanofluid. In the boundary layer, heat can be generated or absorbed. It was assumed that the nanoparticles were uniformly



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dispersed in the base fluid. A similarity approach was used to reduce the governing partial differential equation to an ordinary differential equation. The resulting ordinary differential equation was numerically solved for a type of porous medium, sand, and three types of nanoparticles, namely, alumina (Al2O3), copper (Cu), and titanium dioxide (TiO2). They investigated theoretically effect of heat generation/absorption as well as volume fraction of nanoparticles on the heat transfer enhancement of nanofluids .

Saberi et al. [16] investigated numerically, laminar forced convective heat transfer of nanofluids consisted of alumina/water and zirconia/water through a vertical tube under constant heat flux boundary condition. They used single phase and two phase mixture models for analyzing thermal behavior of nanofluids. Furthermore, they studied effects of Reynolds number, nanoparticle types and nanoparticles volume fraction on the convective heat transfer coefficient. They compared the results of single phase and mixture models with the experimental data. The results of the mixture model for prediction of the convective heat transfer coefficient showed better agreement with the experimental data, while the prediction of nanofluid mean bulk temperature distribution inside the tube by the single phase model was better than the mixture model compared to the experimental data. In addition, according to the results of numerical data, the convective heat transfer of nanofluids was higher than that of water similar to the experimental data. The average relative error for predicting convective heat transfer coefficient between experimental data and single-phase model was 13% and 8% for alumina/water and zirconia/water nanofluids, respectively while for mixture model was 8% and 5%.

Sohel et al. [17] discussed analytically, the thermal performance of a circular shaped copper microchannel heat sink by using three types of nanofluids . Al2O3 Water, TiO2�water and CuO�water nanofluids used in this analysis and the comparative thermal performance of these three nanofluids also discussed. The hydraulic diameter of the circular channel was 400 �m and the total block dimension was 10 mm � 10 mm � 4 mm. A steady, laminar and incompressible flow with constant heat flux was assumed in the circular channel. The analyses were done at various volume fractions ranging from 0.5 vol.% to 4 vol.% and at a constant inlet velocity of 1.5 m/s. The results showed that the thermal performance can be increased significantly by using CuO�water nanofluid as a coolant for cooling of electronic heat sink when Al2O3�water and TiO2�water nanofluids showed less improvement. Compared to pure water, the highest improvement (13.15%) in the heat flux occurred for 4 vol.% CuO�water nanofluid when Al2O3�water and TiO2�water nanofluids showed 6.80% and 6.20% improvements respectively. They found this improvement in heat flux was calculated without considering the additional required pumping power due to the increased viscosity of nanofluids. Therefore, CuO�water nanofluid can be recommended to obtain maximum heat transfer performance in a circular microchannel heat sink.

Syam Sundar et al. [18] reported experimental investigations and theoretical determination of effective thermal conductivity and viscosity of magnetic Fe3O4/water nanofluid. They prepared the nanofluid by synthesizing Fe3O4 nanoparticles using the chemical precipitation method, and then dispersed in distilled water using a sonicator. Both experiments were conducted in the



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volume concentration range 0.0% to 2.0% and the temperature range 20 �C to 60 �C. They observed the thermal conductivity and viscosity of the nanofluid were increased with an increase in the particle volume concentration.They obtained viscosity enhancement was greater compared to thermal conductivity enhancement under at same volume concentration and temperature.They proposed theoretical equations were developed to predict thermal conductivity and viscosity of nanofluids without resorting to the well established Maxwell and Einstein models, respectively. The proposed equations showed reasonably good agreement with the experimental results.

Syam Sundar et al. [19] estimated experimentally thermal conductivity of ethylene glycol and water mixture based Al2O3 and CuO nanofluids at different volume concentrations and temperatures. The base fluid was a mixture of 50:50% (by weight) of ethylene glycol and water (EG/W). They found the particle concentration up to 0.8% and temperature range from 15 �C�50 �C were considered. Both the nanofluids were exhibiting higher thermal conductivity compared to base fluid.They observed under same volume concentration and temperature, CuO nanofluid thermal conductivity is more compared to Al2O3 nanofluid. Finally they obtained a new correlation was developed based on the experimental data for the estimation of thermal conductivity of both the nanofluids.

Tavassolpour et al. [20] studied numerically Laminar flow forced convection of water/Al2O3 nanofluid between two parallel plates with constant wall temperature. The flow assumed to be 2D, steady, incompressible and homogenous. The single phase model used for simulating flow and heat transfer process of nanofluid. The heat transfer coefficient, Nusselt number and shear stress obtained for different nanoparticles volume fraction as well as Reynolds numbers and wall temperature. Results indicated that the rate of convective Heat transfer coefficient and shear stress increases with increase in flow as well as increase in solid volume fraction of nanofluid. It showed that in spite of increase in heat transfer with increase in wall temperature, the wall shear stress does not significantly change.

In the studies made by Vakili et al. [21], nanofluids with different TiO2 nanoparticle concentrations were synthesized and measured in different constant heat fluxes for their heat transfer behavior upon flowing through a vertical pipe.Also they observed addition of nanoparticles into the base fluid enhances the forced convective heat transfer coefficient. Their results showed that the enhancement of the convective heat transfer coefficient in the mixture consisting of ethylene glycol and distilled water is more than distilled water as a base fluid.

Vasefi et al. [22] studied experimentally and numerically the effect of nanofluids on convectiveheat transfer through a triangular straight tube with a constant heat flux boundary condition in the laminar flow regime. The convective heat transfer coefficient of nanofluids obtained for different nanoparticle concentrations and Re number. Their experimental results showed heat transfer coefficient increases by increasing the concentration of nanoparticles in nanofluid. They observed the increase in heat transfer coefficient due to presenceof nanoparticles is much higher than the prediction of single phase heat transfer correlation used with nanofluid properties. They obtained numerically the axial development of temperature and convective heat transfer coefficient at the outer wall. The results showed a good agreement between numerical



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and experimental data. The velocity profiles in transverse plane at the fully developed region. They discussed about the effect of nanoparticles concentration on temperature, velocity and convective heat transfer coefficient .


Alizad et al. [25] investigated thermal performance, transient behavior and operational start-up characteristics of flat-shaped heat pipes by using nanofluids. They used three different primary nanofluid of CuO, Al2O3, and TiO2. They benefitted from a comprehensive analytical model having detailed heat transfer characteristics inside the condensation and evaporation sections of the tube. The thermal performance of either the flat-plate or diskshaped heat pipes was increased by higher concentration of nanoparticles.

Bobbo et al. [24] showed some viscosity data for nanofluids.They used water as base fluid and two different primary nanofluids such as single wall carbonnanohorn (SWCNH) and titanium dioxide (TiO2). They determined viscosity of nanofluids using arhometer experimentally and benefitted from the function of the nanoparticles’ mass fraction and the shear rate. Therefore they accounted the possible non-Newtonian behavior for the nanofluid. As a result, different empirical correlations were proposed in their study.

Buschmann [25] studied on ceramic nanofluid’s thermal conductivity and heat transfer characteristics. He observed that heat transfer could be increased by using nanofluids in the thermal entrance with test rig. It was revealed that when the thermal conductivity of five different ceramic nano fluids were measured by means

of a sophisticated ring gap apparatus the dynamic viscosity was found to be enhanced successfully.

Yang et al. [26] enhanced the heat and mass transfer on the ammonia water absorption refrigeration system. They yielded three different nanofluids by inserting the mixture of carbon black with emulsifier OP-10, ZnFe2O4 with sodium dodecyl benzene sulfonate (SDBS), and Fe2O3 with SDBS to the ammonia water solution, respectively. It was found that key parameters for this study werethe content of surfactant and nanoparticles, their interaction, and also the dispersion type that affected the viscosity of ammonia water nanofluid. They correlated mainly two models to predict the viscosity of ammonia water nanofluid.



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Colangelo et al. [27] intended to observe the diathermic oil based nanofluids being used in many areas such as renewable energy, cogeneration and cooling systems. They used the diathermic oil with CuO, Al 2O3, ZnO and Cu particles having different shapes and concentrations from 0% to 3.0%. It was found that the heat transfer performance of diathermic oil increased more than that of the water for the same nanoparticle concentrations.

Jamshidi et al. [28] investigated the heat transfer rate and pressure drop in helical coils in their study. They assumed that thermophysical properties of nanofluid did not depend on volume fraction and temperature. They utilized from a numerical simulation and Taguchi method to obtain the optimum condition for desired parameter values. Results showed that the use of nanofluids enhanced the thermal-hydraulic performance of helical tube. Although, temperature dependent properties changed with the optimum particle volume fraction, nanofluids were not varied by the optimized shape factor.

Giraldo et al. [29] investigated the thermal behavior of a nanofluid loaded with alumina nanoparticles and developed a directnumerical simulation of the flow. They utilized from the boundary element method to solve moving boundary problems. They observed that strong convective currents occurred as a result of the presence of the nanoparticles, which in time increased the total heat flow in the cavity particle concentration. Particle concentration increased the total heat flow influencing both the conductive part of heat transfer and the convective part expectedly.

Özerinç et al. [30] studied the application of some empirical correlations of forced convection heat transfer developed for the flow of pure fluids and nanofluids. They compared their results with existing theories in the literature and their model underestimated the heat transfer as a result of the numerical solution pointing that single- phase assumption with temperature-dependent thermal conductivity and thermal dispersion being the certain way of heat transfer enhancement analysis of nanofluids in convective heat transfer.

Yu et al. [31] investigated the thermophysical properties and convective heat transfer of Al2O3-polyalphaolefin (PAO) nanofluids includingboth the spherical and the rod-like nanoparticles. They determined the viscosity and thermal conductivity of the nanofluids and compared their predictions with several existing theories in the literature. Their results showed that in a convective flow, the shearinduced alignment and specific motion of the particles should be regarded to truly interpret the experimental data of the nanofluids including non-spherical nanoparticles.



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Farahani and Kowsary [32] intended to clarify a heat conduction method used for defining the local convective boiling heat transfer coefficient of pure water and copper based nanofluid flowing in a mini channel by utilizing three different concentrations of nanoparticles being 5 mg/L, 10 mg/L and 50 mg/L. They tried to solve the IHCP using sequential specification function method by calculating the space-variable convective heat transfer coefficient. Their results acknowledged two dimensional SFS method to have the optimum experiments for the determination of boiling heat transfer coefficients of pure water and nanofluid flowing in mini channel.

Cimpean and Pop [33] studied on the steady fully developed mixed convection flow of a nanofluid in a channel filled with a porous medium. They used some equations to solve the problem those being non-dimensional and based mainly on the mixed convection

parameter, the Péclet number, the inclination angle of the channel to the horizontal and the nanoparticle volume fraction. Their results showed that nanofluid greatly enhanced the heat transfer, even for small additions of nanoparticles in the base water fluid.

Leong et al. [34] studied on the flow of nanofluids in shell and tube heat recovery exchangers in a biomass heating plant. They benefited from the literature that included heat exchanger specification, nanofluid properties and mathematical formulations. They found that use of nanofluids augmented the convective and overall heat transfer coefficient as compared to the ethylene glycol or water based fluids .

Mahbubul et al. [35] investigated different characteristics of viscosity of nanofluids such as nanofluid preparation methods, temperature, particle size and shape, and volume fraction effects. They found that viscosity’s augmentation depended on volume concentration, increase and decrease of the temperature rise. They observed that viscosity improved with volume fraction for different concentration of Al2O3, TiO2.

Wenzheng et al. [36]studied a Molecular Dynamics simulation on Couette flow of nanofluids and showed the microscopic flow characteristics through visual observation and statistic analysis. They presented the even-distributed liquid argon atoms near solid surfaces of nanoparticles those which might have been appeared as a reform to base liquid had contributed to heat transfer enhancement. Nanoparticles moved rapidly in the shear direction accompanying with motions of rotation and vibration in the other two directions in the process of Couette flow. The motions of nanoparticles



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were reinforced significantly when the shearing velocity was grown. More enhanced heat transferring in nanofluids obtained by the motions of nanoparticles could disturb the continuity of fluid and strengthen partial flowing around nanoparticles.

3. Conclusion

Nanofluid has been classified as a new class of heat transfer fluids engineered by dispersing metallic or non-metallic nanoparticles with a typical size of less than 100 nm in the conventional heat transfer fluids. Their use remarkably augments the heat transfer potential of the base liquids thus offering an opportunity to the engineers for the development of highly compact and effective heat transfer equipment for many industrial applications, nuclear reactors, transportation, electronics as well as biomedicine and food.

Nanofluid is considered as an innovative heat transfer fluid with superior potential for enhancing the heat transfer performance of conventional fluids. This review presents the recent studies of single phase and two-phase nanofluid flows in tubes and channels and also helps to determine the physical properties of nanofluids chronologically. According to the most recent experimental papers, heat transfer performance of the base fluid can significantly be increased by the suspended nanoparticles since heat transfer coefficient of the nanofluid was found to be larger than that of its base fluid for the same Reynolds number. The volume fraction of nanoparticles increases the heat transfer feature of a nanofluid. Pressure drop and friction factors of nanofluids are also larger than its base fluids. Besides of many attempts made to determine the nanofluids’ thermal conductivity and viscosity, being important thermo physical properties no definitive agreements have emerged on these properties. Moreover, there is a lack of studies on the mixture flows of nano particles with the refrigerants as pressurized flows in tubes or channels due to the hardness in the experimental conditions.

4. References: [1] Choi, U.S. Enhancing thermal conductivity of fluids with nanoparticle. ASME FED 231, 1995, 231, 99-103. [2] Davarnejad ,R.; Barati ,S.; Kooshki, M . ; CFD simulation of the effect of particle size on the nanofluids convective heat transfer in the developed region in a circular tube . Int. J. Springer plus , 2013, 2 , 192. [3] Elisa, M.M .; Migdad ,M .; Mahbubul ,I.M .; Saidur,R .; Kamalisarvestani, M .; Sohel,M.R .; Hepbasli ,A,; Rahim ,N.A .; Amalina ,M.A .; Effect of nanoparticle shape on the heat transfer and thermodynamic performance of a shell and tube heat exchanger . Int. Commun. Heat Mass Trans., 2013, 44, 93-99 . [4] Fan,T .; XU,H .; Pop,I .; Mixed convection heat transfer in horizontal channel filled with nanofluids. Int. J. Springer plus , 2013, 34, 339-350 .



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[5] Ghozatloo,A.; Shariaty-Niasar,M .; Rashidi , A.M .; Preparation of nanofluids from functionalized Graphene by new alkaline method and study on the thermal conductivity and stability . Int. Commun. Heat Mass Trans., 2013, 42, 89-94 . [6] Huminic,G .; Huminic, A.; Numerical analysis of laminar flow heat transfer of nanofluids in a flattened tube . Int. Commun. Heat Mass Trans. 2013, 44, 55-57. [7] Jahanbin,A.H .; Javaherdeh,K .; Numerical investigation of CuO nanoparticles effect on forced convective heat transfer inside a mini-channel : Comparison of different approaches . Int. J. Life Science. 2013 , 10(8s). [8] Javadi,F.S .; Sadeghipour,S .; Saidur,R. ; BoroumandJazi,G.;Rahmati,B.;Elias,M.M.;Sohel,M.R.;The effects of nanofluid on thermophysical properties and heat transfer characteristics of a plate heat exchanger . Int. Commun. Heat Mass Trans. 2013 , 44, 58-63. [9] Jokar,A .; O Halloran,S.P .; Heat transfer and fluid flow analysis of nanofluids in corrugated plate heat exchangers using computational fluid dynamics simulation. J .Thermal Sci. Eng .Appl 2013 , 5(1), 011002. [10] Ma, J .; Xu,Y.; Li, W.; Zhao,J .; Zhang,S .; Basov,S. Experimental investigation into the forced convective heat transfer of aqueous Fe3O4 nanofluids under transition region. J. Nanoparticle . 2013 , 601363. [11] Mohammed, H.A .; Hasan,H.A .; Wahid, M.A .; Heat transfer enhancement of nanofluids in a double pipe heat exchanger with louvered strip inserts . Int. Commun. Heat Mass Trans., 2013 . 40, 36-46 . [12] Mohammadian, S.K .; Layeghi, M.; Hemmati,M .; Experimental study of forced convective heat transfer from a vertical tube conveying dilute Ag/DI water nanofluids in a cross flow of air. Int. Nano Letters. 2013 , 3: 15. [13] Keshavarz Moravej ,M .;Mohammadi Ardehali, R. CDF modeling(comparing single and two-phase approaches) on thermal performance of Al2O3 / water nanofluid in mini-channel heat sink. . Int. Commun. Heat Mass Trans., 2013, 44, 157-164 . [14] Naphon , P .; Nakharintr , L. Heat transfer of nanofluids in the mini-rectangular fin heat sinks . Int. Commun. Heat Mass Trans., 2013 ,40, 25-31. [15] Rabeti, M .; Noghreabadi, A.; Ghanbarzade, A.; Forced convection heat transfer over a horizontal plate embedded in a porous medium saturated with a nanofluid in the presence of heat Sources .J. Advances in Energy Engineering (AEE).2013 ,1(3). [16] Saberi, M .; Kalbasi, M.; Alipourzade,A . Numerical study of forced convective heat transfer of nanofluids inside a vertical tube . Int. J. Therm. Tech. 2013 , 3,1. [17] Sohel ,M.R .; Saidur , R.; Mohd Sabri, M.F .; Kamalisarvestani, M .; Elisa, M.M.; Ijam , A. Investigating the heat transfer performance and thermophysical properties of nanofluids in a circular micro-channel . Int. Commun. Heat Mass Trans. 2013 , 42, 75 – 81. [18] Syam Sundra, L.; Singh,M.K .; Sousa, A.C.M. Investigation of thermal conductivity and viscosity of Fe3O4 nanofluid for heat transfer applications . Int. Commun. Heat Mass Trans. 2013 , 44, 7-14. [19] Syam Sundra, L.; Hashim Farooky,Md.; Naga Sadra,S .; Singh, M.K . Experimental thermal conductivity of ethylene glycol and water mixture based low volume concentration of Al2O3 and CuO nanofluids . Int. Commun. Heat Mass Trans. 2013 , 41, 41-46 . [20]Tavassolpour,S.;Khodamrezaee ,F.; Noghrehabadi, A.R.; Abaiebaghri, A. Numerical study of developing laminar forced convection of a nanofluid flow through the parallel plates with constant wall temperature. American.J.Adv.Sci.Research .2013 , 1(7), 286-294. [21] Vakili, M.; Mohebbi, A.; Hashemipour, H . Experimental study on convective heat transfer of TiO2 nanofluids. J. Heat Mass Transfer, 2013, 49(8), 1159-1165. [22] Zabihi, K.; Gholamian ,F.; Vasefi, S.I. Experimental and numerical investigation of Al2O3-water nanofluid inside a triangular tube.Int. J. Word Appl.Sci.2013 ,22(5),601-607 .



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[23] Alizad, K.; Vafai, K.; Shafahi M. Thermal performance and operational attributes of the startup characteristics of flat-shaped heat pipes using nanofluids. Int. J. Heat Mass Transfer, 2012, 55, 140-155. [24] Bobbo, S.; Fedele, L.; Benetti, A.; Colla, L.; Fabrizio, M.; Pagura, C.; Barison,S. Viscosity of water based SWCNH and TiO2 nanofluids. Exp. Therm. Fluid Sci., 2012, 36, 65-71. [25] Buschmann, M. H.; Thermal conductivity and heat transfer of ceramic nanofluids. Int. J. Therm. Sci., 2011, Article in Press-Corrected Proof. [26] Yang, L.; Du, K.; Ding, Y. H.; Cheng, B.; Li, Y. J. Viscosity-prediction models of ammonia water nanofluids based on various dispersion types. Powder Technol., 2012, 215-216, 210-218. [27] Colangelo, G.; Favale E.; Risi A.; Laforgia, D. Results of experimental investigations on the heat conductivity of nanofluids based on diathermic oil for high temperature applications. Appl. Energy, 2011, Article in Press- Corrected Proof.. [28] Jamshidi, N.; Farhadi, M.; Sedighi, K.; Ganji, D. D. Optimization of design parameters for nanofluids flowing inside helical coils. Int. Commun. Heat Mass Trans., 2011, 39, 311-317. [29] Giraldo, M.; Sanin, D.; Flórez, W. F. Heat transfer in nanofluids: A computational evaluation of the effects of particle motion. Appl. Math. Comput., 2011, Article in Press-Corrected Proof. [30] S. Özerinç, A .G. Yazıcıoglu, S. Kakaç, Numerical analysis of laminar forced convection with temperature-dependent. Int. J. Therm. Sci., 2011, Article in Press-Corrected Proof. [31] Yu, L.; Liu, D.; Botz, F. Laminar convective heat transfer of aluminapolyalphaolefin nanofluids containing spherical and non-spherical nanoparticles. Exp. Therm. Fluid Sci., 2011, 37, 72-83. [32] Farahani, S.D.; Kowsary, F. Estimation local convective boiling heat transfer coefficient in mini channel. Int. Commun. Heat Mass Trans., 2011, 39, 304-310 [33] Cimpean, D. S.; Pop, I. Fully developed mixed convection flow of a nanofluid through an inclined channel filled with a porous medium. Int. J. Heat Mass Transfer, 2012, 55, 907-914. [34] Leong, K.Y.; Saidur, R.; Mahlia, T.M.I.; Yau, Y.H. Modeling of shell and tube heat recovery exchanger operated with nanofluid based coolants. Int. J. Heat and Mass Transfer, 2012, 55, 808-816.

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Forced Convective Heat Transfer of Nano fluids: A Review of the Recent Literature

M. Motevasel1, A. Soleimanynazar1, M. Jamialahmadi 2

1. Chemical Engineering Department, University of Isfahan, Isfahan , Iran 2. Petroleum Engineering Department, Petroleum University of Technology, Ahwaz, Iran

Corresponding Author E-mail: [email protected]

Abstract:

Numerous researchers have investigated the forced convection of fluids, both experimentally and numerically. A good understanding of characteristics of nanofluid flow has thoroughly been investigated in these studies. In recent years, many researchers have tried to fill the gaps on this subject in the literature. This paper reports on most of the forced convective heat transfer literature occurring both in-tubes and in-channels regarding the use and preparation of nanofluids. The peer reviewed papers published in citation index journals up to 2013 have been selected for review in the paper. Classification of the papers has been performed according to the publication years. The critical information on the theoretical, experimental and numerical works is presented comprehensively for each paper.

Keywords: Nano fluids, Rheological, Instability, Heat transfer coefficient.

1. Introduction

Performance of heat transfer equipment can be improved with studies related to a significant increase in heat flux and miniaturization. In many industrial applications such as power generation, microelectronics, heating processes, cooling processes and chemical processes, water, mineral oil and ethylene glycol are used as heat transfer fluid. Effectiveness and high compactness of heat exchangers are obstructed by the lower heat transfer properties of these common fluids as compared to most solids. It is obvious that solid particles having thermal conductivities several hundred times higher than these conventional fluids must be used in the heat transfer applications. To improve thermal conductivity of a fluid, suspension of ultrafine solid particles in the fluid can be a creative idea. Different types of particles (metallic, non-metallic and polymeric) can be added into fluids to form slurries. Due to the fact that sizes of these suspended particles are in the millimeter or even micrometer scale, some serious problems such as the clogging of flow channels, erosion of pipelines and an increase in pressure drop can occur. Moreover, they often suffer from rheological and instability problems. Especially, the



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particles tend to settle rapidly. For that reason, though the slurries have better thermal conductivities but they are not practical.

Many published articles show that the heat transfer coefficient of nanofluids is much higher than the common-base fluid giving little or no penalty in pressure drop. The main reasons for the heat transfer enhancement of the nanofluids may roughly be listed as follow: the suspended nanoparticles increase the thermal conductivity of the fluids, and the chaotic movement of ultrafine particles increases fluctuation and turbulence of the fluids that accelerates the energy exchange process. Furthermore, that the numerous correlations for single-phase fluid have clearly failed

to predict the heat transfer coefficients of nanofluids that might have been caused by the complicated phenomena coexisting in the main flow. However, some sophisticated correlations for predicting the heat transfer performance of nanofluids have been established. At present, the uses of the nanofluids are still in early stages of development. Therefore, urgent theoretical and experimental research works is needed to clearly understand and accurately predict their hydrodynamic and thermal characteristics. Generally, many researchers indicated that nanofluids behave like pure fluids because the suspended particles are ultra-fine. However, at present, no formulated advanced theory exists to explain the behavior of nanofluids by considering them as multi-component materials. Moreover, several attempts have been discussed to determine the physical properties of nanofluids in the literature.

2. LITERATURE REVIEW OF RECENT YEARS

A research group at the Argonne National Laboratory first continuously conducted the studies on the use of particles of nanometer dimension approximately a decade ago. Choi [1] was possibly the first researcher who called the fluids with particlesof nanometer scale ‘nano-fluids’.

Nanofluids have better rheological and stability properties, significantly higher thermal conductivities and no penalty in pressure drop as compared with suspended particles of millimeter-ormicrometer scale.

In recent years, the nanofluid has emerged as an alternative heat transfer fluid for heat transfer applications showing a significant potential for heat transfer improvement. It has been expected to be suitable for the engineering application without severe problems in pipeline and with little or no penalty in pressure drop. According to earlier researches, thermal conductivity has come into the limelight as most studied transport properties of the nanofluid. The convective heat



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transfer of nanofluids ha comparatively been less acclaimed in literature therefore the number of the publications dealing with the convective heat transfer of nanofluids is limited. The most productive research has been continuously carried out by the following studies.


Davarnejad et al. [2] reported the CFD simulation of heat flux was considered using Fluent software (version 6.3.26) in the laminar flow. Al2O3 nanoparticles in water with concentrations of 0.5%, 1.0%, 1.5%, 2% and 2.5% were used in this simulation. All of the thermo-physical properties of nanofluids were assumed to be temperature independent. Two particle sizes with average size of 20 and 50 nm were used in this research. They concluded that heat transfer coefficient increased by increasing the Reynolds number and the concentration of nanoparticles. The maximum convective heat transfer coefficient was observed at the highest concentration of nano-particles in water (2.5%). Furthermore, the two nanofluids showed higher heat transfer than the base fluid (water) although the nanofluid with particles size of 20 nm had the highest heat transfer coefficient.

Elias et al. [3] investigated the effect of different nanoparticle shapes (such as cylindrical, bricks, blades, platelets, and spherical) on the performance of a shell and tube heat exchanger operating with nanofluid analytically.Boehmite alumin(�-AlOOH) nanoparticles of different shapes were dispersed in a mixture of water/ethylene glycol as the nanofluid. The thermodynamic performance of the shell and tube heat exchanger that was used in a waste heat recovery system was analysed in terms of heat transfer rate and entropy generation. They used Established correlations to measure the thermal conductivity, heat transfer coefficient and rate and entropy generation of nanofluid. The results showed an increase in both the heat transfer and thermodynamic performance of the system. However, among the five nanoparticle shapes, cylindrical shape exhibited better heat transfer characteristics and heat transfer rate. On the other hand, they observed entropy generation for nanofluids containing cylindrical shaped nanoparticles was higher in comparison with the other nanoparticle shapes. However, the increased percentage of entropy was below 1%.Therefore they found this greater entropy generation could be deemed negligible and cylindrical shaped nanoparticles were recommended to be utilized in heat exchanger systems working with nanofluids.

Fan et al. [4] investigated the laminar fully developed nanofluid flow and heat transfer in a horizonal channel.They obtained highly accurate solutions for the temperature and nanoparticle concentration distributions.They discussed the effects of the Brownian motion parameter Nb, the thermophoresis parameter Nt , and the Lewis number Le on the temperature and nanoparticle concentration distributions. The current analysis showed that the nanoparticles can improve the heat transfer characteristics significantly for this flow problem.

Ghozatloo et al. [5] prepared nanofluids from functionalized Graphene by alkaline method.The nanosheet Graphene was synthesized by using CVD method.