Experimental investigation on thermo-physical properties and heat transfer characteristics of green synthesized highly stable CoFe2O4/rGO nanofluid

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Highlights

Abstract

The application of spinel ferrite decorated graphene hybrids in heat transfer has been limited mostly due to their poor stability in water. This study reports a straightforward strategy to synthesis a cobalt ferrite/reduced graphene oxide (CoFe2O4/rGO) nanocomposite with high water dispersibility characteristics through the treatment of synthesized hybrid material with Gallic acid (GA) as a green agent at room temperature. A π-π interaction between graphene and the aromatic ring of GA results in a suspension with high stability. The structure of the nanocomposite was investigated in detail by FTIR, XRD, TGA, VSM, Raman spectroscopy, FESEM, and TEM. Also, UV–vis spectroscopy is used to examine the stability of the stability of the CoFe2O4/rGO in the base fluid. The highly stable aqueous nanofluids were subsequently prepared from the products. Thermo-physical analyses of nanofluids revealed that the thermal conductivity ratio of ferrofluid prepared by the obtained CoFe2O4/rGO with 0.9 wt.% load of nanoparticles at 60 °C is rose to 1.46. Besides, the convective heat transfer in laminar flow regime conditions which passes a circular tube under uniform heat flux was examined. The nanofluid with the highest load of nanoparticles (0.9 wt.%) shown the maximum enhancement in Nusselt number of 27.8 % at Reynolds number of 1713.

Introduction

A wide variety of industrial processes are based on the exchange of heat, meaning that heat transfer plays a vital role in the manufacturing processes of industrial applications such as electronics cooling, oil and gas, as well as distillation and nuclear reactors. Generally, small enhancement in the efficiency of heat exchanger equipment can lead to the saving of energy and operational costs [1]. One feasible solution to this aim is using stable nanofluid with high thermal conductivity characteristics [2]. Prior studies demonstrated that NPs possess the capability to boost effective thermal conductivity of the base fluid and provide new possibilities to improve thermal efficiency relative to traditional working fluids [3]. Thus far, carbon-based materials have been employed for heat transfer applications owing to their high thermal conductivity [4,5]. So, many investigations are performed on the utilization of carbon allotropes and their derivatives such as multi-wall carbon nanotube [6,7] and graphene nanoplatelet (GNP) [8].

Graphene is a flat monolayer sheet of hexagonally sp2-bonded carbon atoms tightly packed into a crystal lattice of honeycomb [9], which has drawn tremendous attention owing to its exceptional mechanical properties, extremely high intrinsic thermal and electric conductivity, and impressive structural flexibility [10]. Graphene oxide (GO) has a variety of oxygenated-containing groups, such as epoxide, hydroxyl, carboxyl, and carbonyl groups. These groups can not only enhance the hydrophilicity but also serve reactive sites for chemical modification [11]. Kim and Bang [12] studied the thermal efficiency of GO NPs in a heat pipe. They claimed that the temperature of the heat pipe filled with GO/water is ca. 25 % lower than that of filled with pure water. In a similar study, Akhavan-Zanjani et al. [13] investigated using graphene/water nanofluid in tubes under uniform flux. Findings exhibit in Re = 1850, by adding an appropriate dose of graphene NPs into the water, heat transfer coefficient and thermal conductivity would be enhanced up to 14.2 % and 10.3 %, respectively. Moreover, Zhao et al. observed a 109.42 % increase in thermal conductivity of the phase change material containing graphene oxide in comparison with that without GO [14].

The combination of nanocrystal ferrites (MFe2O4, M = Mn, Co, Ni, or Zn) having spinel-type structure and GO also has initiated interesting research because of their outstanding properties [15]. It has been stated that such materials belong to a non-toxic category and are relatively stable against oxidation [16]. Among spinel ferrites, magnetic cobalt ferrite (CoFe2O4) NPs are well-known hard magnetic materials due to their moderate coercivity, high saturation magnetization, mechanical hardness, and excellent chemical stability [17]. Despite, several studies on the synthesis of CoFe2O4 decorated graphene hybrids, most of them suffer from poor stability in water, limiting their applications for thermal management. Karimi et al. [18] reported a maximum enhancement of 17.2 % for thermal conductivity of ferrofluid containing 2 vol% of nickel-ferrite at the temperature of 55 °C.

Recent reports demonstrated that several natural materials, particularly the plant-based components rich in polyphenolic substances are promising compounds for the production of water-soluble materials especially reduced graphene oxide (rGO). Wang et al. [19] prepared a water-soluble rGO through the treatment of GO in tea solution. Gallic acid (GA), a natural water-soluble phenolic compound present in a variety of plants (such as grape, spinach, and green tea), has also been used as a stabilizer and reductant agent for rGO [20]. Progress in chemistry provided a new class of ferrofluids and recent studies indicate that graphene-based nanofluids, even in very low concentrations, could provide higher convective heat transfer coefficient enhancement in comparison with the conventional nanofluids, e.g., TiO2, SiO2, Al2O3 etc. Graphene, a 2D single layer of sp2 carbon atoms has high theoretical surface area (∼2600 m2/g, [21]) which is desirable in thermal management applications [4]. Although graphene nanoparticles are excellent materials for nanofluid, the synthesis of graphene in large batch is difficult and expensive. Considering these facts, it could be conceived that the use of graphene in a hybrid component could be more effective, and therefore nanofluid containing CoFe2O4/rGO nanocomposite is selected as a working fluid in the current work.

Inspired by the prior studies and following our recently published work [22], herein, a novel facile synthesis method is introduced to prepare a water-soluble CoFe2O4/rGO nanocomposite which held functional characteristics to achieve efficient cooling performance in critical equipment. One of the key parameters in nanofluids is the stability of NPs. Thereby, an appropriate lipophilic modification procedure is used to produce a high stable nanofluid. Accordingly, gallic acid was utilized as a stabilizer and reductant agent in this work. There is no study on the GA preparation of stabilized aqueous ferrofluid containing CoFe2O4/rGO nanocomposite, to the best of the authors’ knowledge. Investigation on thermophysical properties of nanofluids prepared with various loads of CoFe2O4/rGO (GA-treated) nanocomposite exhibited that resultant hybrid material can be considered as a promising candidate for cooling applications. The usage of such ferrofluids is not limited to cooling applications owing to its high stability, magnetic characteristics, and environmentally friendly production process.

Section snippets

Chemicals and reagents

Purified graphite (50 mesh), sodium nitrate (NaNO3), hydrogen peroxide (H2O2, 30 % aq.), sulfuric acid (H2SO4, 98 %), GA (C7H6O5), potassium permanganate (KMnO4), sodium hydroxide (NaOH), hydrogen chloride (HCl, 5% aq.) were all acquired from Merck, and ferric chloride nonahydrate (FeCl3·9H2O, 98 %) and cobalt chloride hexahydrate (CoCl2·6H2O, 98 %) were also acquired from Sigma-Aldrich. All chemicals were used ‘as received’ without further purification. DI-water is also used throughout all

Characterization of CoFe2O4/rGO

Fig. 2 represents the schematic of the synthesis route of CoFe2O4/rGO nanocomposite which has been coated by GA. Owing to the presence of many oxygen-containing groups such as epoxy, carboxyl, and hydroxyl groups on the surface of GO, its charge comes to be negative whereby iron ions with positive charge have been attracted to it [25].

By increasing pH and temperature, the attracted metal ions result in the redox effect which provides nucleation sites and finally, CoFe2O4 is decorated on the GO

Conclusions

The purpose of the present work is to investigate a facile method of synthesis and preparation of highly water dispersible CoFe2O4/rGO. Besides, the heat transfer characteristic of CoFe2O4/rGO nanofluids in the laminar flow regime was investigated thoroughly. The obtained ferrofluid exhibited promising thermal performance and potential capability for heat transfer intensifications. The increase of temperature in all nanofluid concentrations led to thermal conductivity enhancement. It was also

CRediT authorship contribution statement

Soheil Omiddezyani: Conceptualization, Methodology, Formal analysis, Writing - original draft. Samira Gharehkhani: Methodology, Funding acquisition, Supervision. Vahid Yousefi-Asli: Writing - review & editing, Formal analysis, Data curation. Iman Khazaee: Supervision, Methodology, Resources. Mehdi Ashjaee: Methodology, Supervision, Project administration. Reyhaneh Nayebi: Investigation, Methodology. Farzaneh Shemirani: Methodology, Resources. Ehsan Houshfar: Writing - review & editing,

Declaration of Competing Interest

The authors report no declarations of interest.

Acknowledgement

The support provided to our team by Dr. Mohammad Behshad Shafii at Sharif University of Technology is much appreciated.

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