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The heat transfer coefficient and friction factor of Fe3O4/vacuum pump oil nanofluids flowing in a tube with twisted tape inserts were analyzed experimentally. The experiments were conducted for different mass flow rates (0.04 kg/s to 0.208 kg/s), volume concentrations (0.05% to 0.5%), Prandtl numbers (440 to 2534), Graetz numbers (500 to 3000) and twisted tape inserts (H/D = 5, 10 and 15). Results reveal that the Nusselt number is enhanced by 8.94% and 13.48% for the 0.5% nanofluid for the mass flow rates of 0.0416 kg/s and 0.208 kg/s, respectively, relative to the base fluid. For the mass flow rate of 0.208 kg/s with 0.5 vol. % nanofluid, the friction factor penalty is 1.21-times compared to the base fluid. By using twisted tape insert of H/D = 5, the Nusselt number is further enhanced by 23.86% and 39.53% for the 0.5% nanofluid for mass flow rates of 0.0416 kg/s and 0.208 kg/s, respectively, relative to the base fluid. For the twisted tape insert of H/D = 5 with mass flow rate of 0.208 kg/s and 0.5 vol. % nanofluid, the friction factor penalty is 1.44-times compared to the base fluid. New Nusselt number and friction factor correlations are proposed. [ABSTRACT FROM AUTHOR]

The heat transfer and friction factor of a new kind of nanodiamond-nickel (ND-Ni) hybrid nanofluids flow in a tube with and without longitudinal strip inserts were studied experimentally. The hybrid ND-Ni nanoparticles were synthesized using in-situ and chemical coprecipitation method and characterized with different techniques. The bulk quantity of hybrid ND-Ni nanofluids were prepared and used for heat transfer and friction analysis. The experiments were conducted in the Reynolds number range from 3000 to 22,000, for the volume concentrations of 0.1% and 0.3%, and for different aspect ratios (AR = 1, 2 and 4) of the longitudinal strip inserts. The Nusselt number is enhanced by 35.43% (without insert) and further enhanced by 93.30% with longitudinal strip insert (AR = 1) for 0.3% volume concentration of nanofluid and at Reynolds number of 22,000 compared to water data. The friction factor penalty of 1.12-times without inserts and further penalty of 1.248-times with longitudinal strip insert (AR = 1) at 0.3% volume concentration of nanofluid and at Reynolds number of 22,000 compared to water data. The obtained experimental Nusselt number of hybrid nanofluids was compared with that of other kind of hybrid nanofluids available in the literature. The Nusselt number and friction factor correlations were developed based on the experimental data. [ABSTRACT FROM AUTHOR]

The thermal effectiveness of solar water heaters can be enhanced if passive heat transfer enhancement techniques are used. Among the most effective passive heat transfer enhancement techniques are the increase of the working fluid thermal conductivity and of its flow turbulence. In this paper, Al 2 O 3 nanofluids and twisted tape inserts are the passive techniques used to enhance the heat transfer and, consequently the thermal effectiveness of the solar water heater. In the solar water heating system considered in this study, the collector is essentially mimicked by a tube with or without a twisted tape, with water or nanofluids flowing through it. Results of the heat transfer experiments indicate that for a Reynolds number of 13000 the heat transfer enhancement for 0.3% volume concentration of nanofluid is 21% for the plain tube and it is further enhanced to 49.75% when a twisted tape of H/D = 5 is inserted in the tube. The maximum friction penalty of 1.25-times was observed for 0.3% nanofluid with H/D = 5 when compared to water in a plain collector. The thermal effectiveness of the plain collector is enhanced to 58%, when the 0.3% nanofluid is used and it is further enhanced to 76% with a twisted tape of H/D = 5 at a mass flow rate of 0.083 kg/s. Solar water heaters, in which the collectors have twisted tape inserts and use nanofluids, have thermal performance increases that largely outweigh pressure drop losses. Under the same operating conditions, the nanofluids/twisted tape inserts collector outperforms that with water and no twisted tapes. [ABSTRACT FROM AUTHOR]

Turbulent heat transfer and friction factor of nanodiamond-nickel (ND-Ni) hybrid nanofluids flow in a horizontal tube has been investigated experimentally. The ND-Ni nanoparticles were synthesized using in-situ growth and chemical co-precipitation method and characterized by XRD, TEM and VSM. The hybrid nanofluids were prepared by dispersing ND-Ni hybrid nanoparticles in distilled water. The thermal conductivity and viscosity enhancements were observed as 29.39% and 23.24% at 0.3% volume concentration of hybrid nanofluid at 60 °C compared to distilled water. The heat transfer and friction factor experiments were conducted at different Reynolds numbers (3000–22,000) and particle volume concentrations (0.1% and 0.3%). The Nusselt number enhancement of 0.3% volume concentration of hybrid nanofluid is 35.43% with a friction factor penalty of 1.12-times at a Reynolds number of 22,000 compared to distilled water data. The obtained experimental Nusselt number of hybrid nanofluids was compared with other kind of hybrid nanofluids available literature. New Nusselt number and friction factor correlations were proposed based on the experimental data. [ABSTRACT FROM AUTHOR]

Abstract: In the present experimental work, the convective heat transfer coefficient and friction factor for fully developed turbulent flow of MWCNT–Fe3O4/water hybrid nanofluids flowing through a uniformly-heated-at-constant-heat-flux circular tube are estimated. The MWCNT–Fe3O4 nanocomposite was prepared by in-situ method, which includes the dispersion of carboxylated carbon nanotubes in distilled water and mixing of ferrous chloride and ferric chloride. Sodium hydroxide was used as reducing agent to form MWCNT–Fe3O4 hybrid nanocomposite. The detailed surface and magnetic properties were performed by X-ray diffraction and scanning electron microscopy, and using a vibrating sample magnetometer. The stable hybrid nanofluids were prepared by dispersing nanocomposite in distilled water, and the heat transfer and friction factor experiments were conducted for particle loadings of 0.1% and 0.3%. The results indicate a maximum of 31.10% enhancement in Nusselt number with a penalty of 1.18-times increase of pumping power for the particle loading of 0.3% at a Reynolds number of 22,000 as compared to base fluid data. The empirical correlations were proposed for the estimation of Nusselt number and friction factor to match well with the experimental data. [Copyright &y& Elsevier]

Abstract: A magnetic nanofluid was prepared by dispersing magnetic Ni nanoparticles in distilled water. The nanoparticles were synthesized by chemical co-precipitation method and characterised by X-ray diffraction and atomic force microscopy. The average particle size was measured by the dynamic light scattering method. Thermal conductivity and absolute viscosity of the nanofluid were experimentally determined as a function of particle concentration and temperature. In addition, the Nusselt number and friction factor were experimentally estimated as a function of particle concentration and Reynolds number for constant heat flux condition in forced convection apparatus with no phase change of the nanofluid flowing in a tube. The experiments were conducted for a Reynolds number range of 3000–22,000, and for a particle concentration range from 0% to 0.6%. The results indicate that both Nusselt number and friction factor of the nanofluid increase with increasing particle volume concentration and Reynolds number. For 0.6% volume concentration, the enhancement of Nusselt number and friction factor is 39.18% and 19.12%, respectively, as compared to distilled water under the same flow conditions. It was verified the classical Gnielinski and Notter–Rouse correlations under predict the Nusselt number of the nanofluid; therefore, new generalized correlations are proposed for the estimation of the Nusselt number and friction factor based on the experimental data. [Copyright &y& Elsevier]

Abstract: In the heat transfer area researches have been carried out over several years for the development of convective heat transfer enhancement techniques. The use of additives in the base fluid like water or ethylene glycol is one of the techniques applied to augment the heat transfer. Recently an innovative nanometer sized particles have been dispersed in the base fluid in heat transfer fluids. The fluids containing the solid nanometer size particle dispersion are called ‘nanofluids’. The dispersed solid metallic or nonmetallic nanoparticles change the thermal properties like thermal conductivity, viscosity, specific heat, density, heat transfer and friction factor of the base fluid. Nanofluids are having high thermal conductivity and high heat transfer coefficient compared to single phase fluids. The enhancement in heat transfer coefficient with the effect of Brownian motion of the nanoparticles present in the base fluid. In this paper, a comprehensive literature on the correlations developed for heat transfer and friction factor for different kinds of nanofluids flowing in a plain tube under laminar to turbulent flow conditions have been compiled and reviewed. The review was also extended to the correlations developed for the estimation of heat transfer coefficient and friction factor of nanofluid in a plain tube with inserts under laminar to turbulent flow conditions. However, the conventional correlations for nanofluid heat transfer and friction factor are not suitable and hence various correlations have been developed for the estimation of Nusselt number and friction factor for both laminar and turbulent flow conditions inside a tube with inserts. [Copyright &y& Elsevier]

• The twisted tape inserts in a thermosyphon Cu/water nanofluids in a collector has been analyzed. • The collector efficiency is increased with increase of particle loadings and decrease of twist ratio of the twisted tape inserts. • New Nusselt number correlations were proposed at different phases. • The collector size is reduced to 11.53% with the use of 0.3 vol. % of Cu/water nanofluid. • The collector size is further reduced to 25% for 0.3 vol. % of Cu/water nanofluid with twisted tape insert of H/D = 5. Heat transfer, friction factor and efficiency of a thermosyphon type flat plate collector with and without twisted tape inserts were analyzed experimentally. Water based Cu nanofluids were used as the absorber fluid to receive the heat from solar radiation into the flat plate collector. The experiments were conducted for volume concentrations of 0.1% and 0.3% and twist ratios of H/D = 5, 10 and 15. Results reveal that the heat transfer rate and efficiency of nanofluids increase with the increase of particle volume concentrations. The Nusselt number of 0.3% nanofluid is enhanced up to 20.46%, and it is further enhanced up to 46.90% with twisted tape insert of H/D = 5 with a maximum friction factor penalty of 1.477-times compared to plain collector. The efficiency of the solar collector with water is 52% and it is enhanced to 58% for 0.3% nanofluid, whereas it is further enhanced to 64% for 0.3% nanofluid with twisted tape insert of H/D = 5. Empirical correlations are proposed based on the experimental data for the friction factor and Nusselt number with a standard deviation of less than ±6.5%. The study indicates that for 100 units of solar flat plate collector the use of 0.1% and 0.3% Cu nanofluids leads to weight savings of 284 kg and 567 kg, respectively. The total weight of 100 units of solar flat plate collector is further saved by 767 kg, 1050 kg and 1250 kg for 0.3% nanofluid with twisted tape inserts of H/D = 15, H/D = 10 and H/D = 5, respectively. The collector cost is reduced by 5.66% and 11.33% for 0.1% and 0.3% nanofluids, and it is further reduced to 25% for 0.3% nanofluid with a twisted tape insert of H/D = 5. [ABSTRACT FROM AUTHOR]

The work is focused on the estimation of convective heat transfer and friction factor of vacuum pump oil/Fe3O4 magnetic nanofluids flow in a tube under laminar flow at high Prandtl numbers experimentally. The thermophysical properties also studied experimentally at different particle concentrations and temperatures. The Fe3O4 nanoparticles were synthesized using the chemical reaction method and characterized using X-ray powder diffraction (XRD) and vibrating sample magnetometer (VSM) techniques. The experiments were conducted at mass flow rate from 0.04 kg/s to 0.208 kg/s, volume concentration from 0.05% to 0.5%, Prandtl numbers from 440 to 2534 and Graetz numbers from 500 to 3000. The results reveal that, the thermal conductivity and viscosity enhancements are 9% and 1.75-times for 0.5 vol. % of nanofluid at a temperature of 60℃, respectively, compared with base fluid data. The heat transfer enhancement is 13.1% and 17.8%, the Nusselt number enhancement is 8.95% and 13.48% for 0.5 vol. % of nanofluid at mass flow rates of 0.0416 kg/s and 0.208 kg/s, respectively, compared with base fluid data with a friction factor penalty of 1.21-times. The correlations of Nusselt number and friction factor were proposed based on the experimental data at high Prandtl numbers. [ABSTRACT FROM AUTHOR]

In the past decade, research on nanofluids has been increased rapidly and reports reveal that nanofluids are beneficial heat transfer fluids for engineering applications. The heat transfer enhancement of nanofluids is primarily dependent on thermal conductivity of nanoparticles, particle volume concentrations and mass flow rates. Under constant particle volume concentrations and flow rates, the heat transfer enhancement only depends on the thermal conductivity of the nanoparticles. The thermal conductivity of nanoparticles may be altered or changed by preparing hybrid (composite) nanoparticles. Hybrid nanoparticles are defined as nanoparticles composed by two or more different materials of nanometer size. The fluids prepared with hybrid nanoparticles are known as hybrid nanofluids. The motivation for the preparation of hybrid nanofluids is to obtain further heat transfer enhancement with augmented thermal conductivity of these nanofluids. This review covers the synthesis of hybrid nanoparticles, preparation of hybrid nanofluids, thermal properties, heat transfer, friction factor and the available Nusselt number and friction factor correlations. The review also demonstrates that hybrid nanofluids are more effective heat transfer fluids than single nanoparticles based nanofluids or conventional fluids. Notwithstanding, full understanding of the mechanisms associated with heat transfer enhancement of hybrid nanofluids is still lacking and, consequently it is required a considerable research effort in this area. [ABSTRACT FROM AUTHOR]

The heat transfer and friction factor characteristics of multi-walled carbon nanotubes–Fe 3 O 4 (MWCNT–Fe 3 O 4 ) nanocomposite nanofluids flow in a tube with longitudinal strip inserts were studied experimentally. The MWCNT–Fe 3 O 4 nanocomposite was synthesized by in-situ growth and chemical co-precipitation method and characterized by various techniques; after that stable nanofluids were prepared by dispersing MWCNT–Fe 3 O 4 in distilled water. The heat transfer and friction factor experiments were conducted for nanofluids in the Reynolds number from 3000 to 22,000, volume concentrations from 0% to 0.3%, and longitudinal strip inserts of aspect ratios 1, 2, 4 and 12. The maximum Nusselt number and friction factor enhancements were observed for 0.3% nanofluid flow in a tube with longitudinal strip insert of aspect ratio 1. From the results, the Nusselt number enhancement for 0.3% nanofluid flow in a tube without inserts is 32.72% and with inserts of aspect ratio 1 is 50.99% at a Reynolds number of 22,000. Similarly, friction factor enhancement for 0.3% nanofluid flow in a tube without inserts is 1.15-times and with inserts of aspect ratio 1 is 1.26-times for a Reynolds numbers of 22,000 compared to base fluid. A new Nusselt number and friction factor regression equations were proposed based on the experimental data. [ABSTRACT FROM AUTHOR]

Highlights • Heat transfer, friction factor, and effectiveness-number of transfer units were estimated experimentally for Fe 3 O 4 nanofluid flow in an inner tube with wire coil with core-rod (WCCR) inserts. • The Nusselt number is enhanced up to 9.76% and 14.76% at Reynolds number of 16545 and 28954 for 0.06% volume concentration of nanofluid. • The Nusselt number is further enhanced up to 25.39% and 37.90% for 0.06% nanofluid with wire coil with core-rod insert of p / d = 1 at Reynolds number of 16545 and 28954. • Nusselt number and friction factor correlations were proposed based on the experimental data. Abstract Heat transfer, friction factor, and effectiveness-number of transfer units were estimated experimentally for different volume concentrations of Fe 3 O 4 nanofluid flow in an inner tube of a double pipe U-bend heat exchanger with different pitch ratios (p / d) of wire coil with core-rod (WCCR) inserts. The experiments were conducted at different particle volume concentrations (0.005%, 0.01%, 0.03% and 0.06%), different Reynolds numbers (16000–29000) and different pitch ratios of WCCR inserts p / d = 1 , 1.34 and 1.79 . The Nusselt number, for a 0.06% volume concentration of nanofluid, is enhanced up to 9.76% and 14.76% at Reynolds number of 16545 and 28954, respectively when compared to water data with a pumping penalty of 10%. Similarly, the Nusselt number is further enhanced to 25.39% and 37.90% for 0.06% nanofluid with wire coil with core-rod insert of p / d = 1 at Reynolds number of 16545 and 28954, respectively compared to water. Nusselt number and friction factor correlations were proposed based on the experimental data. The effectiveness-number of transfer units (NTU) were calculated for water/nanofluids flow in a tube with WCCR inserts. [ABSTRACT FROM AUTHOR]

The convective heat transfer, friction factor, effectiveness and number of transfer units (NTU) of Fe 3 O 4 /water nanofluids flow in a double pipe U-bend heat exchanger and with twisted tape inserts were estimated experimentally. The experiments were conducted in the Reynolds number range from 16,000 to 32,000, particle volume concentrations range from 0.005% to 0.06% and twisted tape inserts of H/D = 10, 15 and 20 were used. The Nusselt number of nanofluids increases with increase of particle volume concentrations and Reynolds number and it further increases with decrease of twist ratio of twisted tape inserts. The Nusselt number is enhanced to 14.76% (no insert) and it is further increased to 38.75% (with twisted tape inserts of H/D = 10) at 0.06% volume concentration and at Reynolds number of 30,000 compare to water data. Similarly, the friction factor penalty of 1.092-times (no insert) and further friction penalty of 1.251-times (with twisted tape inserts of H/D = 10) at Reynolds number of 30,000 compared to water data. New Nusselt number and friction factor correlations have been proposed based on the experimental data. Finally the effectiveness and NTU of heat exchanger is enhanced for nanofluids flow in a double pipe heat exchanger with twisted tape inserts. [ABSTRACT FROM AUTHOR]

Heat transfer, friction factor, effectiveness and number of transfer units (NTU) were determined experimentally for a Fe 3 O 4 nanofluid flowing through the inner tube with longitudinal strip inserts of a double pipe U-bend heat exchanger. Different concentrations of the Fe 3 O 4 nanofluid, which is the hot fluid, were used in the present study and cold water circulates in the annulus region of the double pipe heat exchanger. The heat transfer and friction factor experiments were conducted for the Reynolds number range from 15,000 to 30,000 with the Fe 3 O 4 nanofluid volume concentrations of 0.005%, 0.01%, 0.03% and 0.06%. The effect on heat transfer and friction factor of longitudinal strip inserts in the inner tube is studied for three different strip aspect ratios (AR) with the values of 1, 2 and 4, respectively. The results indicate the Nusselt number on the nanofluid side increases with increasing Reynolds number and particle concentration, and with decreasing aspect ratio of the longitudinal strip inserts. The Nusselt number enhancement, compared to the water data, for the 0.06% volume concentration of the nanofluid is 14.7% and it further increases to 41.29% for the same 0.06% concentration with the longitudinal strip insert with AR equal to 1 for the Reynolds number of 28,954. Compared to water data, the friction factor for the 0.06% volume concentration of the nanofluid increases by 1.092-times and it further increases to 1.267-times for the same concentration with the longitudinal strip insert with AR equal to 1 for the Reynolds number of 28,954. The overall performance of the double pipe heat exchanger with longitudinal strip inserts in the nanofluid side is expressed in terms of effectiveness and number of transfer units (NTU). New correlations for the Nusselt number and friction factor are reported and they are based on the obtained experimental data. [ABSTRACT FROM AUTHOR]

The paper reports on the convective heat transfer, friction factor, effectiveness and number of transfer units ( NTU ) of Fe 3 O 4 /water nanofluids flow in a double pipe U-bend heat exchanger with and without wire coil inserts under turbulent flow conditions. The heat transfer and friction factor experiments were conducted at different Reynolds numbers (16,000–30,000), different particle concentrations (0.005–0.06%) and different wire coil inserts of coil pitch to tube inside diameter (p/d) ratios (1, 1.34 and 1.79). The experimental results indicate that, Nusselt number increases with increasing Reynolds number and particle concentrations, and decreasing p/d ratio of wire coil inserts. The Nusselt number enhancement for 0.06% volume concentration of nanofluid is 14.7% and it further increases to 32.03% for the same 0.06% nanofluid with wire coil inserts of p/d = 1 with a friction penalty of 1.162-times at the Reynolds number of 28,954 compared to water data. The experimental data was fitted using regression analysis to estimate the Nusselt number and friction factor of nanofluid as a function of the Reynolds number, Prandtl number, volume concentration, and wire coil helix ratio. The effectiveness of heat exchanger is formulated in terms of number of transfer units (NTU) for the base fluid and nanofluids. [ABSTRACT FROM AUTHOR]

The convective heat transfer, friction factor and effectiveness of different volume concentrations of Fe 3 O 4 nanofluid flow in an inner tube of double pipe heat exchanger with return bend has been estimated experimentally and turbulent flow conditions. The test section used in this study is of double pipe type in which the inner tube diameter is 0.019 m, the annulus tube diameter is 0.05 m and the total length of inner tube is 5 m. At a distance of 2.2 m from the inlet of the inner tube the return bend is provided. The hot Fe 3 O 4 nanofluid flows through an inner tube, where as the cold water flows through an annulus tube. The volume concentrations of the nanoparticles used in this study are 0.005%, 0.01%, 0.03% and 0.06% with Reynolds number range from 15,000 to 30,000. Based on the results, the Nusselt number enhancement is 14.7% for 0.06% volume concentration of nanofluid flow in an inner tube of heat exchanger at a Reynolds number of 30,000 when compared to base fluid data; the pumping penalty of nanofluid is < 10%. The effectiveness of heat exchanger for water and nanofluid flow is explained in terms of number of transfer units (NTU) in order to estimate the overall performance of the double pipe heat exchanger. New correlations for Nusselt number and friction factor have been developed based on the experimental data. [ABSTRACT FROM AUTHOR]

Heat transfer and friction factor analysis of CuO/water nanofluid flowing through a tube under turbulent flow conditions and with twisted tape (TT) and wire coil (WC) inserts were presented in this paper. The experimental investigations were performed in the Reynolds number range from 4000 to 20,000, volume concentrations of 0.1% and 0.3%, twisted tape inserts of h/d = 5 and 10 and wire coil inserts of p/d = 1.97 and 2.95. The experimental results indicated that under same operating conditions and flow rates, heat transfer coefficient, friction factor and thermal performance factor associated with nanofluid in a tube with wire coil inserts are higher than those with the twisted tape inserts. The Nusselt number enhancement for 0.3% nanofluid in a tube without inserts is 17.62%, 0.3% nanofluid in a tube with TT-2 is 31.88% and 0.3% nanofluid in a tube with WC-2 is 44.45% at a Reynolds number of 20,000 compared to water. Whereas, the friction factor enhancement for 0.3% nanofluid in a tube without inserts is 1.149-times, 0.3% nanofluid in a tube with TT-2 is 1.179-times and 0.3% nanofluid in a tube with WC-2 is 1.198-times at a Reynolds number of 20,000 compared to water. The thermal performance factor of 0.3% nanofluid in tube with twisted tape and wire coil inserts are 1.24 and 1.36 compared against water data respectively. [ABSTRACT FROM AUTHOR]