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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]

CNT nanofluid shows better heat transfer rate due to their high thermal conductivity. Due to hydrophobic nature of CNT, preparation of stable nanofluid is a major challenge. Here nanofluid was prepared using modified two-step method using multi-walled CNT as nanomaterial, double distilled water as base fluid and SDS as surfactant. CNT concentration was varied from 0.1 to 0.3 wt%, surfactant/CNT ratio from 1 to 3 and ultrasonication time from 0 to 180 min. The results indicate that ultrasonication time and surfactant/CNT ratio have a significant effect on the dispersion of CNT in surfactant water solution. Prepared CNT nanofluid was found to be stable for a longer duration. [ABSTRACT FROM AUTHOR]

CNT nanofluids are getting attention in heat transfer applications due to their very high thermal conductivity in comparison with conventional fluids. For commercial exploitation of CNT nanofluids as heat transfer media, they must have long-term stability. In this study, the two-step method was modified to prepare dynamically stable CNT nanofluids by utilizing commercial grade multiwalled carbon nanotubes and SDBS as a surfactant. The modified technique consists of separation of coarse agglomerates of CNT from the CNT nanofluids by applying centrifugal action after its preparation. The effect of relative centrifugal force was also studied for the very first time on the stable concentration of CNT nanofluids. The stability of CNT nanofluids was analyzed by measurement of their CNT concentration and Zeta potential. Results showed that CNT nanofluids possess good stability and remain stable for more than 15 months. In addition to stability, thermo-physical properties such as thermal conductivity, density, and viscosity of CNT nanofluids were also measured. The results of this study elucidated the effect of RCF on the stable concentration of CNT nanofluids. It is expected that the results obtained in this study may significantly contribute to the proper tailoring of CNT nanofluids, by providing long-term stable CNT nanofluids which are suitable for industrial heat transfer applications. [ABSTRACT FROM AUTHOR]

In the present paper we examine the pulsatile hydromagnetic flow and heat transfer of a non-Newtonian biofluid through a saturated non-Darcian porous medium channel. The upper plate of the channel is heated and the lower plate is cooled. The Nakamura-Sawada rheological model is employed which provides a higher yield stress than the Casson model. A Darcy-Forchheimer porous medium drag force model is incorporated to simulate blood vessel blockage with deposits in the cardiovascular system. Viscous heating is also included in the energy equation. The governing conservation equations for mass, momentum and energy equation are transformed into a system of nonlinear, coupled ordinary differential equations and these are solved numerically using finite element method. The effect of other important parameters such as magnetohydrodynamic parameter (Nm), Reynolds number (Re), Eckert number (Ec), Darcian parameter (λ), Forchheimer parameter (NF) and Prandtl number on velocity and temperature profiles are studied graphically. Spatial-temporal velocity and temperature profile visualizations are also presented. Numerical results shows that normalized fluid velocity (U) increases throughout the channel (-1 < Y < 1) with an increase in Reynolds number, Darcian parameter, steady pressure gradient parameter and rheological parameter; conversely velocity is decreased with the increase in magnetic parameter and Forchheimer quadratic drag parameter. Higher Eckert number (Ec = 3) is also found to have a considerable effect on temperature (θ) profile. Finite difference numerical computations are also compared with the finite element solutions to verify efficiency and accuracy. [ABSTRACT FROM AUTHOR]

Abstract: Intent of present investigation is to improve the properties of graphite–polymer composite bipolar plate by nanostructuring. This involves the incorporation of different vol.% of multiwall carbon nanotubes (MWNTs) in graphite–polymer composite bipolar plate. It has been found that by inclusion of 1 vol.% of MWNTs in graphite composite plate, the electrical and thermal conductivity of nanocomposite increased by 100%. The thermal conductivity of nanocomposite plate increases from 1 W/m K to 13 W/m K in through-plane and in-plane from 25 W/m K to 50 W/m K at 1 vol.% of MWNTs. This significant enhancement is due to the orientation of MWNTs in all the directions of composite, positive synergistic effect of MWNTs and heat transfer along the axis directions. However, bending strength of nanocomposite increases by 25% and maximum augmentation is in case of 1 vol.% of MWNTs. The improvement in conductivity of nanocomposite plate is due to an increase in the electron transfer ability within the composite plate which influences the I–V performance of ultimate fuel cell. These observations confirm that the optimal content of MWNTs is 1 vol.%, in graphite–polymer composite. [Copyright &y& Elsevier]

In this paper, a simple two-dimensional theoretical model based on enthalpy formulation of a latent heat storage system has been developed to study the effects of thermo physical properties of heat exchanger container materials on the thermal performance of the storage system. Numerical results show that thermal conductivity, specific heat and density of the heat exchanger container materials increases, the melting time of the PCM decreases. Numerical results also show that high value of thermal conductivity of the heat exchanger container materials did not make significant contribution on the melt fraction. It is also found that initial temperature of the PCM does not have very important effects on the melting time, while the boundary wall temperature play an important role during melting. [ABSTRACT FROM AUTHOR]

• DNS of transitional supersonic boundary layers are performed. • JPDF distribution and covariance integrands' analyses are used to explain the turbulent flow topology. • Similarities with the case of the incompressible turbulent boundary layers are reported for turbulent shear stress behaviors. • The effects of wall temperature are mainly confined to the near-wall region. • Coherent structures' orientations are affected by the wall temperature. Direct numerical simulations (DNS) are performed for the supersonic boundary layers (SBLs) with a free-stream Mach number M ∞ = 2.2. Different cases including the adiabatic and the isothermal (cooled and heated) walls are investigated. The laminar boundary layer is excited by means of a blowing and suction strip with single-frequency and multiple spanwise wave-numbers. The incoming laminar flow is strongly perturbed with a perturbation intensity of 2.4% of the free-stream velocity to obtain the turbulent boundary layer. In the fully developed turbulent regions, the joint probability density function (JPDF) distribution and the covariance integrands' analyses of different parameters are performed to find out the contribution of various physical mechanisms towards different transfer processes. The results reveal that behavior of the turbulent shear stress is similar to its incompressible counterpart and the wall-temperature impacts are dominant in the buffer layer region (at y + = 10). The inclination angles of coherent structures show variations arising from the wall-temperature in both the buffer-layer and the log region. The covariance integrands' analyses of different components of the heat flux reveal the dominance of a different transfer process in case of the cooled wall, and as a result of this difference, the cooled wall acts as a heat sink. [ABSTRACT FROM AUTHOR]