An experimental investigation on the thermophysical properties of 40% ethylene glycol based TiO2-Al2O3 hybrid nanofluids
Introduction
The efficient heat transfer process remains one of the significant challenges in the energy sectors of industry. Conventional fluids such as water, oil and ethylene glycol have an essential role to play in heat transfer. The thermal properties of fluids suggest the effectiveness of heat transfer in this situation. A number of researchers have tried to improve these liquids thermophysical characteristics. Various studies of the past decades have shown evidence to affect the efficiency of heat transfer by introducing nanoparticles to fluids [[1], [2], [3], [4], [5], [6]]. Thus, nanofluids have developed which include the immersion of nanometer-sized particles, tubes bars or fibres into the base fluid [7], and have attracted the attention of engineers in fluid mechanics and fluid flow [[8], [9], [10], [11]], machining [[12], [13], [14], [15], [16]], electronics [17,18], solar and nuclear energy [[19], [20], [21]], biomedicine [22,23], process of treatment of water [24], transportation [25], and heat exchangers [2,26] and especially in various cooling and lubrication purpose experiments and devices [6,27]. The two most common methods of nanofluid synthesis are one and two-step method [28,29]. In the one-step method, the synthesization and dispersion of nanoparticles into the base fluid co-occurs [30,31] while in the two-step method, this occurs separately [32]. Although it is comparatively beneficial in terms of getting the stability of nanofluids while one-step synthesis process is occupied, it is not recommended [30] due to its high expense and applicability of fluids with only lower vapour pressure [33,34].
On the contrary, compared to one-step method, two-step synthesis process is widely used in industries and research areas at a large scale because of its simplicity and lower production cost [29] despite having difficulties of agglomeration of nanoparticles [28]. Most of the researchers use this two-step synthesis process of nanofluids [1,4,35]. In addition, this method is primarily proposed by Choi and Eastman [7] for the synthesis of oxide-based nanofluids, perhaps metallic particles based nanofluids. It influences the thermophysical properties of base fluids such as viscosity, thermal conductivity, density and thus plays an essential role after introducing nanoparticles into the base fluids. For instance, the viscosity suggests that the pumping power or energy use and thermal conductivity contribute to the efficiency of heat transfer [3]. The thermal properties of the nanofluids influence various parameters such as the type of particles and base fluid, particle size, shape, concentration, temperature and so on [[36], [37], [38], [39], [40]]. In addition, the thermal conductivity of nanofluid was more affected by the thermal conductivity of the base fluids in conjunction with the temperature, concentration and dimension of nanoparticles [40].
Many experiments were carried out on different nanofluids. Philip, Shima and Raj [41] reported 300% enhancement of thermal conductivity when applying Fe3O4 nanoparticles (oleic acid-coated) to kerosene with 82 G magnetic field inclusion. In another study, Choi, Zhang, Yu, Lockwood and Grulke [42] found that thermal conductivity improved by 150% for synthetic poly oil (α-olefin) based MWCNT nanofluid. The addition of composite nanoparticles to a basic fluid is expected to achieve better thermophysical properties than single nanoparticles by the combined physical and chemical effects of nanoparticles. Hamid, Azmi, Nabil, Mamat and Sharma [4] examined the thermal conductivity and viscosity of water-EG based TiO2-SiO2 nanofluids and found 13.8% improvement of thermal conductivity at 700C for the mixing ratio of 20:80 and highest viscosity has been found for 50:50 mixing ratio of TiO2-SiO2. Kumar, Vasu and Gopal [43] analyzed the efficiency of various base fluids (vegetable oil, SAE oil and paraffin oil) using CuZn (50:50 ratio) nanoparticles into the base oil for 0.1, 0.3 and 0.5% volume concentrations. Highest thermal conductivity was observed for Cu-Zn/vegetable oil hybrid nanofluid trailed by paraffin oil and SAE oil. This study concluded that the highest thermal conductivity of Cu-Zn/ vegetable oil nanofluid is responsible for the internal repellent fluid force to flow and a comparatively higher thermal conductivity of vegetable oil. An improved thermal conductivity with low viscosity could be attained at high temperature while studying ND- Co3O4 /water nanofluid [44]. In another study, used EG-based 70: 30 ratio of SiO2-MWCNT hybrid nanofluid revealed that an improvement in thermal conductivity of 20.1% at 50 °C [45]. This study also established a correlation for thermal conductivity ratio. Besides, the author also concluded from the price-performance analysis that the hybrid nanofluids are more economical and efficient than single nanofluids in terms of heat transfer efficiency. The thermal oil-based Al2O3- MWCNT hybrid nanofluid at a temperature between 25 and 50 °C and concentrations between 0.125 and 1.5% was studied and revealed 45% improvement in thermal conductivity (50 °C, 1.5% volume concentration) and 81% improvement in viscosity (40 °C, 1.5% volume concentration) [3]. In addition, this study proposed a new correlation for both thermal conductivity and viscosity. The rising solid concentration of particles and temperature is responsible for higher thermal conductivity. However, in the case of viscosity, shows an increasing trend with increasing fractions of particles but a decreasing trend with increasing temperature. The viscosity enhancement for water-based SWCNT at a volume fraction of 0.73% and 25 0C is 320% [46]. In this study, the result of dynamic viscosity is also compared with Einstein [47], Brinkman [48] and Batchelor models [49]. Finally, it concluded that the dynamic viscosity of SWCNT / water could not be accomplished using these three models. Moreover, the dynamic viscosity of 30% EG-based MWCNT-TiO2 (20: 80) is explored for volume concentrations ranging from 0.05 to 0.85% at 10, 30 and 50 °C [50]. Highest viscosity enhancement (83%) was observed at 10 °C for the volume fraction of 0.85% whereas 0.05, 0.45% solid fraction of MWCNT-TiO2 behaved as Newtonian fluid and 0.85% as non-Newtonian fluid. The study also showed that changes in viscosity with concentration are more incredible at low temperature due to the number of particles and collisions among them.
Some studies have shown that Brownian motion has the effect of modifying nanofluid viscosity [11,51]. The rate of heat transfer also increases with the Brownian parameters, thermophoresis and Biot number [52]. The viscosity of the magneto-hydrodynamic nanofluid radial flow over a stretch plate is related to convective boundary conditions, the temperature and the radiation effect [53]. This study has observed that an increase in viscosity with concentration and temperature profiles increases but the nanofluids velocity decreases. The viscosity variations in nanofluid are due to the brownian motion and interactive force of nanoparticle ions when assessing the effect of ions in the brine solution on salt and silica nanoparticles. The variation of viscosity of nanofluid is related to Brownian motion and interaction force of the ions of nanoparticles while evaluating the impact of various ions in the brine solution on the salt and silica nanoparticles [51]. The heat transfer coefficients increase by almost 7% and 20%, and the thermal efficiencies increase by 8% and 15% for inlet temperature of 500 K and 600 K when evaluating the effects of a new parabolic trough collector using Al2O3-synthetic oil nanofluids [21]. Nonetheless, the thermal efficiency, pressure drop, heat transfer enhances insignificantly by adding nanoparticles to synthetic oil; however, these properties decrease clearly with an increasing inlet temperature of heat transfer fluid. In the field of heat transfer, however, from an extensive literary analysis, the rheological and thermophysical properties of nanofluids play a vital role.
Although various types of research work on nanofluid properties exist, the thermophysical properties of the new hybrid nanofluids, including rheological properties, are yet to be explored. The rheological aspect of the fluids, which is a critical issue for the study of nanofluids, is included with the limited number of studies. Nevertheless, there is no study to predict the thermal conductivity, viscosity and heat transfer efficiency for TiO2-Al2O3 with 40% EG-based hybrid nanofluids. Therefore, the present study focuses on rheology, viscosity and thermal conductivity and their enhancement ratio for different temperatures and concentrations. Hybrid nanofluids thermophysical properties are also compared with single nanofluids properties. In addition, two new models are developed for viscosity and thermal conductivity in concentration and temperature for the studied range. However, for their superior properties, TiO2 and Al2O3 nanoparticles are selected in this investigation. There are therefore supposed to be good stability with enhanced thermal properties and heat transfer performance with the 40% EG-based TiO2-Al2O3 hybrid nanofluids. The superior thermal properties and the potential and widespread use of various nanofluids in industries and research areas, however, motivate authors to pursue the study. In the field of heat transportation and nanotechnology, the new study of 40% EG-based TiO2-Al2O3 nanofluids might add substantial value.
Section snippets
Synthesization of hybrid nanofluids
The first and foremost process of experimental research in the area of nanofluids is nanofluid synthesization. Two types of TiO2 and Al2O3 nanoparticles have been used to synthesize the hybrid nanofluids. Table 1 shows the properties of the single nanoparticles of TiO2 and Al2O3. In this study, 40% aquatic solution of EG or the 60:40 (W: EG) combination of ethylene glycol and water (W) is used as base fluid. Table 2 reveals the properties of the base fluid. Eq. (1) has been used to determine
Zeta potential measurement
Zeta potential measurement is one of the essential quantitative methods for nanofluid stability assessment related to the electrophoretic behaviour of nanofluids [28,55]. The main benefit is that the test is simple and fast [60]. The higher the value of zeta, the higher repulsive forces among nanoparticles will lead to stability. Ghadimi, Saidur and Metselaar [61] state that zeta potential value of 30 mV or higher indicates good stability. The standard zeta potential value range is presented in
Conclusion
In this analysis, 40% EG-based TiO2-Al2O3 (80:20) hybrid nanofluid is synthesized using the most common and suggested two-step process, a new heat transfer fluid for its superior properties. The stability of 40% EG based TiO2-Al2O3 hybrid nanofluid is analyzed using visual sedimentation photograph, UV- Vis spectral analysis and zeta potential test. The findings of the three methods have shown that the prepared nanofluids have excellent stability for over more than two weeks. The thermophysical
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors would like to thank University Malaysia Pahang (UMP), Pekan, Malaysia for providing the laboratories facilities and the financial support under the University LEAP-3 FLAGSHIP Research Grant, UMP, Malaysia (No. RDU172203), University Internal Fundamental Research Grant No. RDU190367 and Postgraduate Research Grant scheme, UMP, Malaysia (PGRS1903167).
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