Synthesis of nanofluids containing eco-friendly functionalized carbon nanomaterials for improving heat dissipation / Rad Sadri

Conventional working fluids used in high heat flux systems such as heat exchangers and solar collectors typically have low thermal conductivities which provides low heat transfer efficiency. Many studies have been performed to improve the heat transfer of the conventional working fluids by dispersing higher thermally conductive particles into them. In this study, highly conductive carbon nanostructures are dispersed in distilled (DI) water, and the thermal properties are significantly enhanced at the lowest particle concentration. This study is focused on the development of eco-friendly, facile, functionalization technique to synthesize a new generation of water-based carbon nanostructure nanofluids for use as coolants in order to improve the convective heat transfer and hydrodynamic properties of a single-tube heat exchanger. The approach is green since it does not involve the use of toxic, corrosive acids. The graphene nanoplatelets (GNPs) and multi-walled carbon nanotubes (MWCNTs) were covalently functionalized using clove buds and gallic acid, respectively, using the one-pot method. Next, the functionalized GNPs and MWCNTs were dispersed in DI water to synthesize the clove-treated GNP, gallic acid-treated GNP, gallic acid-treated MWCNT nanofluids. Moreover, graphene oxide was reduced using saffron in one pot to synthesize water-based saffron-reduced graphene oxide (SrGO). The nanofluids were produced at various particle concentrations (0.025, 0.075, and 0.1 wt %). The effectiveness of the covalent treatment and reduction method were evaluated using Fourier transform infrared spectroscopy, Raman spectroscopy, thermogravimetric analysis, X-ray photoelectron spectroscopy, transmission electron microscopy, and ultraviolet-visible spectroscopy. Ultraviolet-visible spectroscopy and zeta potential measurements were also conducted to verify the colloidal stability and the presence of hydrophilic groups on the surface of the functionalized nanoparticles. The thermo-physical characteristics of the nanofluids were investigated experimentally and the results indicate there is significant thermal conductivity enhancement (up to 29.2%) for the water-based SrGO at 45°C. The turbulent convective heat transfer was studied using a heat exchanger subjected to constant heat flux (12,752 W/m2) within a Reynolds number range of 6,371–15,927. Preliminary experiments were conducted with DI water and the results were compared with those determined from empirical correlations. The results indicate good reliability and accuracy of the set-up. Experiments were conducted for the nanofluids flowing through the loop under fully-developed turbulent condition and the results show that the addition of a low fraction of green-functionalized carbon nanostructures into DI water significantly enhances the Nusselt number and convective heat transfer coefficient (up to 40% for water-based SrGO). The increase in friction factor and relative pumping power (1) indicates that these nanofluids have great potential for use as heat transfer fluids considering the overall thermal performance and energy savings. The computational fluid dynamics (CFD) simulation are performed using shear stress transport k-ω turbulence model to predict the heat transfer performance of water-based CGNPs nanofluids in three-dimensional heated tube. The results conform well to those from experiments with an average relative deviation of ±10. The experimental results confirm the applicability of the numerical model to simulate heat transfer performance of nanofluids in turbulent flow conditions

Study on thermophysical properties of fluids containing carbon nanotubes / Rad Sadri

In the verge of acute energy demand the heat transfer fluids were required to modify and develop a high performance liquid as nanofluid. The properties of the fluids are not yet fully obtained. The carbon nanotubes (CNT`s) are able to enhance the thermal performance of the conventional heat transfer liquids, nonetheless the majority of past works have been focused on the impacts of concentrations of carbon nanotubes on the thermo-physical properties of the nanofluids. No considerable researches have been performed to straightly indicate the influence of preparation methods on stability, thermal conductivity and viscosity of carbon nanotubes suspensions. The thermo-physical properties of CNT` nanofluids differ under different preparation techniques and the precise measurements were performed in this current experiment to figure out the impacts of these methods. The effect of variation of ultrasonication time and different surfactants on the thermal properties of Multi-walled carbon nanotubes(MWCNT`s) were studied. The thermo-physical properties measured were thermal conductivity and viscosity under different temperatures. Gum Arabic (GA), Sodium dodecylbenzenesulfonate (SDBS) and Sodium dodecyl sulfate (SDS) were used as surfactants. Addition of GA showed superior thermal conductivity of nanofluids than that of SDBS and SDS dispersants in it. Samples of 0.5 wt% MWCNT, 0.25% GA and distilled water as the base liquid were prepared at different ultrasonication times in this study. Imaging was carried out by TEM technique to view the dispersing characteristics of samples and observed both the reduction of CNT agglomerations and length. The reduction in agglomerations was found to be more important than that of CNT length. The results exhibited that the ultrasonication affects thermal conductivity, viscosity and dispersion. The maximum thermal conductivity enhancement was found to be 22.31% (the ratio of 1.22) at 45 o C temperature of the sample sonication bath for 200 minutes. The thermal conductivity enhanced with the increase of both temperature and sonication time. In the consideration of viscosity, the nanofluids treated as shear thinning and non-Newtonian fluids. The viscosity was raised to the maximum for the sample sonication of 50 minutes and the subsequently decreased with the further increase of sonication time. Thus the lowest viscosity and the highest thermal conductivity ratios were achieved by utilizing prolonged sonication time, which could be useful in heat transfer applications

Numerical simulation of laminar to turbulent nanofluid flow and heat transfer over a backward-facing step

This paper presents a numerical study of heat transfer to turbulent and laminar Cu/water flow over a backward-facing step. Mathematical model based on finite volume method with a FORTRAN code is used to solve the continuity, momentum, energy and turbulence equations. Turbulence was modeled by the shear stress transport (SST) K-ω Model. In this simulation, three volume fractions of nanofluid (0%, 2% and 4%), a varying Reynolds number from 50 to 200 for the laminar range and 5000 to 20,000 for the turbulent range, an expansion ratio of 2 and constant heat flux of 4000 W/m2 were considered. The results show the effect of nanofluid volume fraction on enhancing the Nusselt number in the laminar and turbulent ranges. The effect of expansion ratio was clearly observed at the downstream inlet region where the peak of the Nusselt number profile was referred to as enhanced heat transfer due to the generated recirculation flow. An increase of pressure drop was evident with an increasing Reynolds number and decreasing nanofluid volume fraction, while the maximum pressure drop was detected in the downstream inlet region. A rising Reynolds number caused an increasing Nusselt number, and the highest heat transfer augmentation in the present investigation was about 26% and 36% for turbulent and laminar range, respectively compared with pure water

Numerical Study of Entropy Generation in a Flowing Nanofluid Used in Micro- and Minichannels

This article mainly concerns theoretical research on entropy generation influences due to heat transfer and flow in nanofluid suspensions. A conventional nanofluid of alumina-water (Al2O3-H2O) was considered as the fluid model. Due to the sensitivity of entropy to duct diameter, mini- and microchannels with diameters of 3 mm and 0.05 mm were considered, and a laminar flow regime was assumed. The conductivity and viscosity of two different nanofluid models were examined with the help of theoretical and experimentally determined parameter values. It was shown that order of the magnitude analysis can be used for estimating entropy generation characteristics of nanofluids in mini- and microchannels. It was found that using highly viscous alumina-water nanofluid under laminar flow regime in microchannels was not desirable. Thus, there is a need for the development of low viscosity alumina-water (Al2O3-H2O) nanofluids for use in microchannels under laminar flow condition. On the other hand, Al2O3-H2O nanofluid was a superior coolant under laminar flow regime in minichannels. The presented results also indicate that flow friction and thermal irreversibility are, respectively, more significant at lower and higher tube diameters

Experimental study on thermo-physical and rheological properties of stable and green reduced graphene oxide nanofluids: Hydrothermal assisted technique

In this study a dehydration hydrothermal technique has been used to introduce a simple, environmentally friendly and facile method for manufacturing highly dispersed reduced graphene oxide for improving the thermo-physical and rheological properties of heat transfer liquids. The hydrothermal reduction of graphene oxide was verified by various characterizations methods such as UV-visible absorption spectroscopy, Zeta potential, Raman spectroscopy, X-ray photoemission spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and transmission electron microscopy. A thorough investigation was conducted on the thermo-physical properties of reduced graphene oxide at concentrations of 0.02, 0.04, 0.06, and 0.08 wt% under different temperatures. Significant improvements in electrical and thermal conductivity were obtained by adding a small amount of hydrothermal-assisted reduced graphene oxide (h-rGO) in the suspension. The viscosity and density remained relatively unchanged with the increase of concentrations where the pH was maintained within the desirable value, despite the fact that no additive was used during the reduction process. It is noteworthy to highlight that the h-rGO aqueous suspensions have shown Newtonian behavior. Results indicated that the h-rGO could be employed as a promising additive for conventional heat transfer liquids for different thermal applications.This research has been financially supported by High Impact Research (MOHE-HIR) grants UM. C/625/1/HIR/MOHE/ENG/45 and UM. C/625/1/HIR/MOHE/ENG/23 and UMRG RP012A-13AET Faculty of Engineering, University of Malaya

CFD modeling of turbulent convection heat transfer of nanofluids containing green functionalized graphene nanoplatelets flowing in a horizontal tube: Comparison with experimental data

In this research, a series of numerical simulations were conducted utilizing computational fluid dynamics (CFD) software in order to predict the heat transfer performance of queues nanofluids containing clove-treated graphene nanoplatelets (CGNPs) flowing in a horizontal stainless steel heated pipe. The GNPs were covalently functionalized with clove buds using free radical grafting reaction using an eco-friendly process. The advantage of this synthesis method was that it did not use hazardous acids, which are typically used in traditional treatment methods of carbon nanostructures. The thermo-physical properties of the aqueous nanofluids obtained experimentally were used as inputs for the CFD simulations for solving the governing equations of heat transfer and fluid motion. The shear stress transport (SST) k-ω turbulence model was also used in these simulations. The corresponding convective heat transfer coefficient and friction factor of aqueous nanofluids for nanoparticle weight concentrations of 0.025, 0.075, and 0.1% were evaluated. The simulation results for both heat transfer coefficient and friction factor were shown to be in agreement with the experimental data with an average relative deviation of about ±10%. The presented results confirmed the applicability of the numerical model for simulating the heat transfer performance of CGNPs aqueous nanofluids in turbulent flow regimes

Effect of various refining processes for Kenaf Bast non-wood pulp fibers suspensions on heat transfer coefficient in circular pipe heat exchanger

Heat transfer coefficients were obtained for a range of non-wood kenaf bast pulp fiber suspensions flowing through a circular pipe heat exchanger test loop. The data were produced over a selected temperature and range of flow rates from the flow loop. It was found that the magnitude of the heat transfer coefficient of a fiber suspension is dependent on characteristics, concentration and pulping method of fiber. It was observed that at low concentration and high flow rates, the heat transfer coefficient values of suspensions were observed higher than that of the heat transfer coefficient values of water, on the other hand the heat transfer coefficient values of suspensions decreases at low flow rates and with the increase of their concentration. The heat transfer were affected by varying fiber characteristics, such as fiber length, fiber flexibility, fiber chemical and mechanical treatment as well as different pulping methods used to liberate the fibers. Heat transfer coefficient was decreased with the increase of fiber flexibility which was also observed by previous researchers. In the present work, the characteristics of fibers are correlated with the heat transfer coefficient of suspensions of the fibers. Deviations in fiber properties can be monitored from the flowing fiber suspensions by measuring heat transfer coefficient to adjust the degree of fiber refining treatment so that papers made from those fibers will be more uniform, consistent, within the product specification and retard the paper production loss

A facile, bio-based, novel approach for synthesis of covalently functionalized graphene nanoplatelet nano-coolants toward improved thermo-physical and heat transfer properties

In this study, we synthesized covalently functionalized graphene nanoplatelet (GNP) aqueous suspensions that are highly stable and environmentally friendly for use as coolants in heat transfer systems. We evaluated the heat transfer and hydrodynamic properties of these nano-coolants flowing through a horizontal stainless steel tube subjected to a uniform heat flux at its outer surface. The GNPs functionalized with clove buds using the one-pot technique. We characterized the clove-treated GNPs (CGNPs) using X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). We then dispersed the CGNPs in distilled water at three particle concentrations (0.025, 0.075 and 0.1 wt%) in order to prepare the CGNP-water nanofluids (nano-coolants). We used ultraviolet–visible (UV–vis) spectroscopy to examine the stability and solubility of the CGNPs in the distilled water. There is significant enhancement in thermo-physical properties of CGNPs nanofluids relative those for distilled water. We validated our experimental set-up by comparing the friction factor and Nusselt number for distilled water obtained from experiments with those determined from empirical correlations, indeed, our experimental set-up is reliable and produces results with reasonable accuracy. We conducted heat transfer experiments for the CGNP-water nano-coolants flowing through the horizontal heated tube in fully developed turbulent condition. Our results are indeed promising since there is a significant enhancement in the Nusselt number and convective heat transfer coefficient for the CGNP-water nanofluids, with only a negligible increase in the friction factor and pumping power. More importantly, we found that there is a significant increase in the performance index, which is a positive indicator that our nanofluids have potential to substitute conventional coolants in heat transfer systems because of their overall thermal performance and energy savings benefits