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In recent years, numerous investigations have been carried out in heat transfer applications for CNT nanofluid as it possesses a high thermal conductivity compared to conventional fluids. Hydrophobicity of CNT poses a challenge to disperse CNT in polar basefluid. Surfactants can solve this issue up to a certain extent only but have some associated disadvantages such as foam formation, stickiness and viscosity enhancement responsible for an increase in power required to pump Nanofluid in heat transfer devices. This work presents preparation of a hybrid nanofluid as an alternative. Cu−CNT hybrid nanoparticles were generated in this research using in‐situ preparation of Cu nanoparticles in the presence of CNT and dispersion in double distilled water without addition of surfactant to produce a stable nanofluid. FESEM with EDX confirms copper nanoparticles present on outer surface of MWCNT and Raman spectroscopy confirmed the covalent functionalization. Spectral analysis, Zeta potential, and DLS were used to evaluate the dispersibility of Cu−CNT hybrid nanofluid. The results showed that the samples were extremely stable, with maximum stability of about 168 days. XRD pattern of Cu−CNT hybrid nanoparticles confirmed the existence of Cu and CNT. Nanofluid demonstrated a minor increase in density and viscosity compared to basefluid water due to addition of nanoparticles. An increase in thermal conductivity was also observed, which is critical for heat transfer applications. [ABSTRACT FROM AUTHOR]

CNT nanoparticles have high tensile strength, excellent thermal transfer properties, and optimal chemical and physical stability. The lack of CNT stable dispersion in most of the fluids limits its industrial exploitation in heat transfer applications. Researchers are constantly making efforts for preparing stable dispersions of CNT. Luckily, the unique π-electron-rich structures of CNT open a variety of possibilities for modifications in their structure leading to alterations in their chemical and electronic properties. Normally, chemical and physical methods are used for CNT surface properties alterations to make it dispersible in various base fluids. This review provides a comprehensive survey of chemical and physical methods used to prepare stable CNT nanofluid as well as methods used to analyse CNT nanofluid stability. Chemical modifications are either done by covalent or non-covalent methods. Covalent methods utilized by researchers include reaction with acids, bases, organic and inorganic molecules, metals, metal complexes, polymers, etc. In non-covalent method, surfactants, biomolecules and natural products, polymers, IL and DES, polymers, etc. are used. Physical methods discussed herein include techniques like homogenization, crushing, etc. that deagglomerate CNT bundles. The application of extreme forces on CNT leads to distortion in electronic framework of CNT. Therefore, to avoid excess of physical and chemical treatments, a blend of techniques in appropriate ratio is proposed for CNT dispersion. The techniques that are used to analyse the stability of nanofluid such as UV–vis, TEM, SEM, turbiscan, zeta, and DLS are also reviewed. It could be concluded that there is need for development of low-cost and fast method for prediction of the stability of CNT nanofluid. [ABSTRACT FROM AUTHOR]

Hydrodynamic and heat transfer characteristics of long-term stable nanofluids are very crucial for their industrial applications. Also utilization of coils is beneficial for industries as it provides a high rate of heat transfer and is compact in size too. So, the main focus of this study is to investigate the hydrodynamic and heat transfer characteristics of long-term stable f-CNT nanofluids when flowing inside a coil-based heat exchanger. f-CNT nanofluids were prepared by utilizing modified two-step method. To analyze the effect of different parameters on hydrodynamic and convective heat transfer characteristics, f-CNT concentration, D c , and Re were varied from 0 to 0.048 vol%, 95 to 175 mm, and 2300 to 9500, respectively. According to results, f-CNT concentration, D c , and Re substantially influenced the hydrodynamic and heat transfer characteristics of f-CNT nanofluids. It was found that improvement in h (152%) was much higher than the enhancement in friction factor (49%) when f-CNT nanofluid at 0.048 vol% was flowing through the coil of 95 mm diameter. Based on the heat transfer and hydrodynamic data, performance index was evaluated. The maximum performance index was calculated ⁓ 2.5, suggesting that the utilization of helical coils and f-CNT nanofluids is an excellent choice in industrial applications. Based on the experimental data, empirical correlations have been proposed to calculate the friction factor and Nusselt number for f-CNT nanofluids when flowing inside coils of different diameters and at different f-CNT concentrations. The proposed correlations explain the present experimental data within ± 15% and ± 20%, for friction factor and Nu, respectively. [ABSTRACT FROM AUTHOR]