Investigations of Heat Transfer Augmentation for Turbulent Nanofluids Flow in a Circular Tube: Recent Literature Review

AASCIT Journal of Nanoscience 2015; 1(4): 60-65 Published online September 20, 2015 (http://www.aascit.org/journal/nanoscience) Investigations of Heat Transfer Augmentation for Turbulent Nanofluids Flow in a Circular Tube: Recent Literature Review Fouad A. Saleh 1, Laith J. Habeeb 2, Bassim M. Maajel 1 1 Mechanical Engineering Department, University of Almustansiriya, Baghdad, Iraq 2 Mechanical Engineering Department, University of Technology, Baghdad, Iraq Email address laithhabeeb1974@gmail.com Keywords Heat Transfer, Heat Transfer Coefficient, Nanofluids, Volume Fraction, Thermal Conductivity Received: August 22, 2015 Revised: August 29, 2015 Accepted: August 30, 2015 Citation Fouad A. Saleh, Laith J. Habeeb, Bassim M. Maajel. Investigations of Heat Transfer Augmentation for Turbulent Nanofluids Flow in a Circular Tube: Recent Literature Review. AASCIT Journal of Nanoscience. Vol. 1, No. 4, 2015, pp. 60-65. Abstract The literature in enhanced heat transfer is growing faster. At least fifteen percent of the heat transfer literature is directed towards the techniques of heat transfer augmentation now. A lot of research has been done in heat transfer equipment by using nanofluid, both experimentally and numerically. Nanofluids are this new class of heat transfer fluids and are engineered by suspending nanometer-sized particles in conventional heat transfer fluids. The average size of particles used in nanofluids is below 50 nm. Choi coined the term Nanofluids for this new class of heat transfer fluids. Nanofluids appear to be a very interesting alternative heat transfer fluids for many advanced thermal applications, as Electronics, Power generation, transmission, Renewable energy, HVAC and space. The purpose of this literature review is to go through the main topics of interest and address a number of experimental and numerical studies which focused on the nanofluids performance in tube and carried out to cover the enhancement in heat transfer by the use of nanofluids in all applications and the effect of concentration, Reynolds number, type of nanofluid, and diameter of nanoparticle and other parameters. 1. Introduction As a new research and technology frontier, nanofluids are used to enhance heat transfer. Nanofluids are engineered colloids which are made of a basefluid and nanoparticles (1-100) nm. The convective heat transfer can be enhanced passively by changing the flow geometry, boundary conditions, or by enhancing the thermal conductivity of the fluid. Researchers tried to increase the heat transfer rate by increasing the thermal conductivity of the fluid. Thermal conductivity of nanofluids is found to be an attracting characteristic for many applications. It represents the ability of a material to conduct or transmit heat. Considerable researches have been carried out on measuring thermal conductivity of nanofluid. It has been noticed that most authors agreed that nanofluids provide higher thermal conductivity compared to basefluids. Its value increases with the increase in particle concentration, temperature, particle size, dispersion and stability. Therefore, it is expected that other factors are also responsible for the convective heat transfer enhancement of nanofluids. Many review articles have described the factors affecting the enhancement of heat transfer of nanofluids in details. This literature review will address a number of experimental and numerical studies which focused on the nanofluids performance of tube and carried out to cover the enhancement in heat transfer by the use

AASCIT Journal of Nanoscience 2015; 1(4): 60-65 61 of nanofluids in all applications. 2. Literature Review 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. The convective heat transfer of nanofluids has 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: Fotukian and Esfahany [1] investigated experimentally turbulent convective heat transfer of dilute γ-al 2 O 3 / water inside circular tubes. The nanofluid γ-al 2 O 3 /water with dilute loading of 0.03 %, 0.054 %, 0.135 % were studied. The Reynolds number was varied from 6000 to 31000. The experimental results indicated that addition of small amount of nanoparticles to pure water improves the heat transfer performance significantly. The maximum value of 48 % increase in the heat transfer coefficient compared to pure water for 0.054 % volume concentration at Reynolds number of 10000 was observed. Increasing the particle concentration did not show much heat transfer enhancement in the turbulent region. The ratio of convective heat transfer coefficient of nanofluid to that of pure water decreases with the Reynolds number. The pressure drop of the nanofluid with 0.135 % volume concentration with showed 30 % increase at Reynolds number of 20000 compared to pure water. Bayat and Hossein [2] studied numerically the thermal performance and the pressure drop of nanofluids in turbulent forced convection. The involves the axisymmetric steady, forced turbulent convective flow of nanofluid through the circular tube by mathematical modeling. The set of coupled non-linear Navier-Stoke differential equations have been discretized using finite volume technique. The experimentation was performed on water/ Ethylene Glycol (60:40) by mass with Al 2 O 3 for wide range of Reynolds number of 104 < Re < 105 and constant wall heat flux condition. The result shows that the volume fraction has great impact on heat transfer, pressure drop, Prandtl number, pumping power. There is large difference in the pressure drop by using the nanofluid and the base fluid for same pumping power. Nanofluids provide higher thermal enhancement at higher Reynolds number but not recommended for the practical application in the turbulent regimes, as pumping power is considerable. Amani et al. [3] studied experimentally the effect of TiO 2 /water nanofluid on heat transfer and pressure drop. The particle size selected was of 30 nm. The experimentation was performed for the volume fraction of 0.002 and 0.02, the Reynolds number was in between 8000 to 51000. The apparatus was in the form of horizontal double tube counterflow heat exchanger. It is observed that by increasing the Reynolds number or nanoparticle volume fraction, the Nusselt number increases. Meanwhile all nanofluids have a higher Nusselt number compared to distilled water. It is observed that by use the nanofluid at high Reynolds number (say greater than 30,000) more power compared to low Reynolds number needed to compensate the pressure drop of nanofluid, while increments in the Nusselt number for all Reynolds numbers are approximately equal. Therefore using nanofluids at high Reynolds numbers compared with low Reynolds numbers, have lower benefits. It is also seen that, the maximum thermal performance factor of 1.8 is found with the simultaneous use of the TiO 2 / water nanofluid with 0.02 % volume and at Reynolds number of 47,000. Maïga et al. [4] studied numerically the turbulent flow of γ-al 2 O 3 /water and γ-al 2 O 3 /Ethylene glycol nanofluids inside the single uniformly heated tube at the Reynolds numbers of 10,000 and 50,000. It showed that the γ-al 2 O 3 /Ethylene glycol nanofluid offered a better heat transfer enhancement than the γ-al 2 O 3 /water nanofluid. However, the wall shear stress in the γ-al 2 O 3 /Ethylene glycol was higher than that in the γ-al 2 O 3 /water. B. Farajollahi et al. [5] measured heat transfer characteristics of γ-al 2 O 3 /water and TiO 2 /water nanofluids in a shell and tube heat exchanger under turbulent flow condition. The effects of Peclet number, volume concentration of suspended nanoparticles, and particle type on the heat characteristics were investigated. Based on the results, adding of nanoparticles to the base fluid causes the significant enhancement of heat transfer characteristics. For both nanofluids, two different optimum nanoparticle concentrations exist. Comparison of the heat transfer behavior of two nanofluids indicates that at a certain Peclet number, heat transfer characteristics of TiO 2 /water nanofluid at its optimum nanoparticle concentration are greater than those of γ-al 2 O 3 /water nanofluid while γ-al 2 O 3 /water nanofluid possesses better heat transfer behavior at higher nanoparticle concentrations. Pak and Cho [6] studied experimentally the heat performance of Al 2 O 3 and TiO 2 nanoparticles dispersed in water flowing in horizontal circular tube. Alumina and Titanium dioxide (TiO 2 ) nanoparticles with diameter 13 nm and 27 nm respectively were used in their. They found that the Nusselt number of nanofluid increased with increases the Reynolds number as well as volume fraction. However they still found that the convective heat transfer coefficient of nanofluid with 3 % by vol nanoparticles was 12 % lower that of pure water at agaves Reynolds number. This may cause the nanofluid to have larger viscosity than that of pure water, especially at high particle volume fraction. Weerapun and Somchai [7] studied experimentally the force convective heat transfer and flow characteristics of a nanofluid consisting of water and 0.2 vol. % TiO 2 nanoparticles. The heat transfer coefficient and friction factor of the TiO 2 /water nanofluid flowing in a horizontal doubletube counter flow heat exchanger under turbulent flow conditions are investigated. The Degussa P25 TiO 2 nanoparticles of about 21 nm diameter are used in the present

62 Fouad A. Saleh et al.: Investigations of Heat Transfer Augmentation for Turbulent Nanofluids Flow in a Circular Tube: Recent Literature Review. 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. Ravikanth and Kulkarni [8] investigated experimentally the convective heat transfer and pressure loss characteristics of three nanofluids (Aluminum Oxide, Copper Oxide and Silicon dioxide dispersed in 60 % ethylene glycol and 40 % water by mass) flowing in a circular tube in the turbulent regime. The thermo physical properties such as viscosity, density, specific heat and thermal conductivity were measured at different temperatures for varying particle volume concentrations. Heat transfer coefficient of nanofluids showed an increase with the particle volumetric concentration. For example, at a Reynolds number of 7240, the percentage increase in the heat transfer coefficient over the base fluid for a 10 % Al 2 O 3 nanofluid is 81.74 %. The pressure loss of nanofluids also increases with an increase in particle volume concentration. The increase of pressure loss for a 10 % Al 2 O 3 nanofluid at a Reynolds number of 6700 is about 4.7 times than that of the base fluid. This is due to the increase in the viscosity of the nanofluid with concentration. The pressure loss was also measured and a new correlation was developed to represent friction factor for nanofluid. He et al. [9] carried out an experimental on the flow and heat transfer behavior of aqueous TiO 2 nanofluids flowing through a straight vertical pipe under both laminar and turbulent flow conditions. They observed that for a given Reynolds number and particle size, the convective heat transfer coefficient increased with volume concentration in both the laminar and turbulent flow regimes and was insensitive to the changes in particle size. As the convective heat transfer coefficient, h, can be approximately given by (h=kf/δt) with (δt) is the thickness of thermal boundary layer, they attributed that both an increase in (kf) and a decrease in (δt) increased the convective heat transfer coefficient of nanofluid. Their measured pressure drop of nanofluids was very close to that of the base liquid for a given Reynolds number. Tanguturi et al. [10] analyzed numerically turbulent flow and heat transfer of three different nanofluids (CuO, Al 2 O 3 and SiO 2 ) in an Ethylene glycol and water mixture flowing through a circular tube under constant heat flux condition. 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. In this, 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, Al 2 O 3 and SiO 2 nanofluids have been presented. At a constant Reynolds number, Nusselt number increases by 35 % for 6 % CuO nanofluids over the base fluid. Hojjat et al. [11] investigated experimentally the forced convective heat transfer using dispersing γ-al 2 O 3, CuO, and TiO 2 nanoparticles in an aqueous solution of carboxymethyl cellulose (CMC). Their experimental apparatus included a uniformly heated circular tube under turbulent flow conditions passing through these nanofluids. Their results showed that the local and average heat transfer coefficients of nanofluids were larger than those of the base fluid. A new correlation was therefore proposed for the prediction of the Nusselt number of non-newtonian nanofluids as a function of the Reynolds and the Prandtl numbers. Williams et al. [12] published an interesting finding that the convective heat transfer and pressure loss behavior of the alumina/water and zirconia/water nanofluids tested in fully developed turbulent flow can be predicted by means of the traditional correlations and models, as long as the effective nanofluid properties are used in calculating the dimensionless numbers. They also stated that there is no abnormal heat transfer enhancement with nanofluids. Naik et al. [13] analyzed turbulent convection flow of CuO nanofluids of propylene glycol water (PGW) as the base fluid and flowing in a circular tube, subjected to a constant and uniform heat flux at the wall, numerically. The effects of nanoparticles concentrations and Reynolds number are investigated on the flow and the convective heat transfer behavior of CuO nanofluids. They found that nanofluids containing more concentrations have shown higher heat transfer coefficient. The analysis is carried out in the nanoparticles volume concentration range from 0.1 % to 1.2 %. The heat transfer coefficient increases by 9 % for 1.2 % CuO nanofluids over the base fluid. They compared the numerical results with the experimental data and reasonable good agreement is achieved. Yu et al. [14] measured the heat transfer rates in the turbulent flow of SiC/water nanofluid consisting of a volume concentration of 3.7 % with 170 nm silicon carbide particles. Heat transfer coefficient increase of 50 60 % above the base fluid water was obtained when compared on the basis of constant Reynolds number. Heat transfer mechanisms that involve particle interactions are believed for heat transfer enhancement. Khodadadi et al. [15] analyzed numerically the turbulent flow of nanofluids with different volume concentrations of nanoparticles flowing through a two-dimensional duct under constant heat flux condition. The nanofluids considered are mixtures of copper oxide (CuO), alumina (Al 2 O 3 ) and oxide titanium (TiO 2 ) nanoparticles and water as the base fluid. All the thermophysical properties of nanofluids are temperature dependent. The viscosity of nanofluids is obtained on basis of experimental data. The predicted Nusselt numbers exhibit

AASCIT Journal of Nanoscience 2015; 1(4): 60-65 63 good agreement with Gnielinski's correlation. The results show that by increasing the volume concentration, the wall shear stress and heat transfer rates increase. For a constant volume concentration and Reynolds number, the effect of CuO nanoparticles to enhance the Nusselt number is better than Al 2 O 3 and TiO 2 nanoparticles. Nasiri et al. [16] investigated experimentally the heat transfer perfor-mance of Al 2 O 3 /H 2 O and TiO 2 /H 2 O nanofluids through an annular channel with constant wall temperature boundary condition under turbulent flow regime. The constant temperature is applied on the outer wall of the channel. investigation was done for nanoparticle concentrations 0.1, 0.5, 1.0 and 1.5 % and Reynolds number range (4000-13000). Based on the experimental results, for specific Peclet number, Nusselt number of nanofluids is higher than that of the base fluid. The enhancement increases with increase of nanparticle concentration for both employed nanofluids. Based on the results of this investigation there is no significant difference on the heat transfer enhancement associated with two employed nanofluids. Sajadi [17] investigated experimentally turbulent heat transfer beha-vior of TiO 2 /water nanofluid in a circular pipe with volume fraction of nanoparticles in the base fluid was in the range (0.05-0.25 %) and Reynolds number range (5000-30000). The results indicated that addition of small amounts of nanoparticles to the base fluid augmented heat transfer remarkably. The measurements also showed that the pressure drop of nanofluid was slightly higher than that of the base fluid and increased with increasing the volume concentration. results have been compared with the existing correlations for nanofluid convective heat transfer coefficient in turbulent regime. Syam Sundar et al. [18] experimentally evaluated the convective heat transfer coefficient and friction factor characteristics of Fe 3 O 4 magnetic nanofluid for flow in a circular tube is evaluated experimentally in the range of 3000 < Re < 22,000 and the volume concentration range of 0 < φ < 0.6 % using a stable colloidal suspension of magnetite Table (1). Summary of important investigations of nanofluids in turbulent flow. Fotukian and Esfahany [1] Constant wall temperature Bayat and Hossein [2] Numerical Amani et al. [3] Maïga et al. [4] B. Farajollahi et al. [5] Numerical Double tube counterflow heat exchanger Constant and uniform heat flux Shell and tube heat exchanger (Fe 3 O 4 ) nanoparticles of average diameter 36 nm. Nanofluid heat transfer is higher compared to water and increases with volume concentration. Correlations are developed based on the experimental data useful for the estimation of Nusselt number and friction factor of water and nanofluid for flow in a tube. The heat transfer coefficient is enhanced by 30.96 % and friction factor by 10.01 % at 0.6 % volume concentration compared to flow of water at similar operating conditions. Mohamed H. Shedid [19] studied numerically the thermal behavior of nanofluids for annular flow. The flow is subjected to a constant wall temperature at the outer wall. The nanofluids considered are alumina (Al 2 O 3 ) and oxide titanium (TiO 2 ) nanoparticles and water as the base fluid. Validated model was used for different concentration ratios of Al 2 O 3 and TiO 2 for different Peclet numbers. The results were compared with many correlations for convection of nanofluids flow and revealed better agreement with Spalart- Allmaras model rather than k-ε model. Results of numerical simulations are compared and showed an enhancement of Nusslet number as Peclet number grows with increasing concentration ratio. Aghaei et al. [20] investigated numerically the flow field and heat transfer of Al 2 O 3 /water nanofluid turbulent forced convection in a tube. The surface of the tube is hot (T h = 310 K). Simulations are carried out for constant water Prandtl number of 6.13, Reynolds numbers from 10,000, 20,000, 30,000 to 100,000, nanoparticles volume fractions of 0, 0.001, 0.1, 0.2, 0.4 and nanoparticles diameter of 25, 33, 75, and 100 nm. The finite volume method and SIMPLE algorithm are utilized to solve the governing equations numerically. The numerical results showed that with enhancing Reynolds numbers, average Nusselt number increases. The variations of the average Nusselt number relative to volume fractions are not uniform. For all of the considered volume fractions, by increasing the Reynolds number the skin friction factor decreases and with increasing volume fractions and Reynolds number the pressure drop increases. A summary is follow in Table (1). Water Water-EG Water EG Water Addition of small amount of nanoparticles to pure water improves the heat transfer performance significantly. The ratio of convective heat transfer coefficient of nanofluid to that of pure water decreases with the Re. The volume of fraction has great impact on heat transfer, pressure drop, Prandtl number, pumping power. Nanofluids provide higher thermal enhancement at higher Reynolds number. By increasing the Reynolds number or nanoparticle volume fraction, the Nusselt number increases. The EG nanofluid offered a better heat transfer enhancement than the water nanofluid. Adding of nanoparticles to the base fluid causes the significant enhancement of heat transfer characteristics. Heat transfer characteristics of TiO2/water nanofluid are greater than those of water nanofluid.

64 Fouad A. Saleh et al.: Investigations of Heat Transfer Augmentation for Turbulent Nanofluids Flow in a Circular Tube: Recent Literature Review Table (1). Continued. The Nu number of nanofluid increased with increases the Re Pak and Cho [6] number as well as volume fraction. The convective heat transfer coefficient of nanofluid is slightly Weerapun and Double tube heat higher than that of the base liquid by about 6 11 %. Somchai [7] exchanger The heat transfer coefficient of the nanofluid increases with an increase in the mass flow rate of nanofluid. Ravikanth and Kulkarni [8] He et al. [9] Tanguturi et al. [10] Numerical Al2O3,CuO,Si O2 / EGW Al2O3, CuO, SiO2 / EGW Table (1). Continued. Duangthongsuk and Wongwises [11] Williams et al. [12] Naik et al. [13] Yu et al. [14] Khodadadi et al. [15] Nasiri et al. [16] Numerical Numerical Uniform heat flux Constant wall temperature CMC TiO2 / CMC CuO / CMC Zirconia/Water CuO / PGW SiC / Water CuO / Water Table (1). Continued. Sajadi [17] Syam Sundar et al. [18] Mohamed H. Shedid [19] Aghaei et al. [20] Numerical Numerical Uniform heat flux Uniform heat flux Constant wall temperature Constant and uniform wall temperature Fe3O4 magnetic nanofluid Heat transfer coefficient of nanofluids showed an increase with the particle volumetric concentration; a new Nu correlation has been developed. The convective heat transfer coefficient increased with volume concentration and was insensitive to the changes in particle size. Heat transfer coefficient increases with volume concentration and Re. Viscosity increases with particle diameter. The local and average heat transfer coeffi-cients of nanofluids were larger than those of the base fluid. A new correlation was proposed for the prediction of the Nu as a function of the Reynolds and the Prandtl numbers. Heat transfer coefficient enhancement is not abnormal, but due to the different mixture properties. Nanofluids containing more concentra-tions have shown higher heat transfer coefficient. The heat transfer coefficient increases by 9 % for 1.2 % CuO nanofluids over the base fluid. Heat transfer coefficient increase of 50-60 % above the base fluid water was obtained when compared on the basis of constant Reynolds number. By increasing the volume concentration, the wall shear stress and heat transfer rates increase. The effect of CuO nanoparticles to enha-nce the Nu number is better than Al2O3 and TiO2 nanoparticles at constant Re. The enhancement increases with increase of nanparticle concentration for both employed nanofluids. Addition of small amounts of nanoparticles to the base fluid augmented heat transfer remarkably. The pressure drop of nanofluid was slightly higher than that of the base fluid and increased with increasing the volume concentration. The heat transfer coefficient is enhanced by 30.96 % and friction factor by 10.01 % at 0.6 % volume concentration compared to flow of water at similar operating conditions. Results of numerical simulations are compared and showed an enhancement of Nusslet number as Peclet number grows with increasing concentration ratio. With enhancing Reynolds numbers, average Nusselt number increases. Nu relative to φ are not uniform. 3. Conclusions 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. Nanofluid is considered as an innovative heat transfer fluid with superior potential for enhancing the heat transfer performance of conventional fluids. 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. According to the above researches, we can summarize the conclusions as follows: 1. 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.

AASCIT Journal of Nanoscience 2015; 1(4): 60-65 65 2. The higher the nanoparticles weight fraction, the more the rate of heat transfer enhancement. 3. 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. 4. The heat transfer rate is directly proportional to Nusselt and Peclet number of the fluid. 5. Besides of many attempts made to determine the nanofluids thermal conductivity and viscosity, being important thermophysical properties no definitive agreements have emerged on these properties. 6. The fine grade of nanoparticles increases the surface area which results in increase in the heat transfer rate. 7. 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. 8. 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