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Abstract:
The enhanced thermal properties being the prominent objective behind the usage of nanofluids. Hence it is necessary to study the base/nanofluid physical properties at various conditions. This article projects the experimental results of thermal conductivity and viscosity of two different nanofluids. The ratio was considered as 60:40 and 40:40 by volume in water and ethylene glycol respectively. The preparation of nanofluids was started by scattering SiO2 nano-particles in EG and “water” (W) blended in “60:40” (60EGW) and “40:60” (40EGW) ratio by volume. The ratio was considered as 60:40 and 40:40 by volume in water and ethylene glycol respectively. The regression analysis was conducted with available data and correlations were formulated for thermal properties. The nanofluids were used in evaluating “viscosity” and “thermal conductivity” experimentally. From the results, the SiO2 particles have achieved enhancement of 34% and 32% in thermal conductivity with the two base-fluids. Similarly, enhancement of 102% and 62% were reported in viscosity. Hence, it can be observed that SiO2 nanofluids in 40EGW nanofluid are a better heat transfer fluid when compared to SiO2 in 60EGW nanofluid.
Keywords: Nano-fluids; Base-fluids; Heat transfer analysis; Nano-particles; Silicon dioxide; Thermal conductivity, Viscosity

Introduction:

Some of the renowned conventional working fluids include refrigerants, oil, glycol, ethylene, and water. Indeed, nanofluids, which represent new fluid trends, can be used to enhance these conventional working fluids’ characteristics. Imperative to note is that nanofluids constitute nano-sized solid particle suspensions, which are found in liquids and have their sizes ranging between 1 nm to 100 nm. Some of the applications in which the new particles (nanofluids) could gain application include manufactured drug delivery systems, solar water heating, thermal storage, lubrications, drilling, refrigeration, cooling electronics, engine oil transmissions, and engine cooling [1–4]. Some of the enhanced thermal properties with which nanofluids are associated, which account for their increasing application, include heat transfer[5], thermal conductivity[6,7], specific heat conductivity[8], viscosity[9], and density [10]. With enhanced thermal conductivity, the eventuality is that there is likely to be a state of enhanced heat transfer. These mixed, but promising outcomes point to the criticality of examining the nanofluids and base fluids’ thermal physical properties at different experimental or operating conditions [11–13].
Several scholarly investigations have been conducted to understand the extent to which there could be enhanced convective heat transfer, especially regarding the use of different nano particles [14–20]. Some of the particles that have been investigated included Silicon Dioxide, Titanium Dioxide, Zinc Oxide, Copper Oxide, and Aluminum Dioxide. Upon dispersing these nano particles in water, most of the outcomes suggest good results. Apart from water as a base fluid that has been used, other materials that have continually been embraced included glycerin, ethylene glycol, and oil. In situations where the volume of water and ethylene glycol mixtures as base fluids has been set at 40:60 (40EGW) and 60:40 (60 EGW), promising results have been reported. An enhancement of 40% was observed by Vajjha [21] for a particle concentration varying from 1.0%-10.0% concertation range and 0oC to 100 oC temperature range for 60EGW and 24% enhancement was observed by Azmi [22] for a particle concentration less than 1.0% at a working temperature of 70oC in 40EGW base fluids.
One of the prominent steps involves nanofluids preparation with stability, especially in relation to their application for industrial scale, the step remains challenging. From the previous investigation, the nano particles’ setting behavior accounts for the perceived challenges [23–26]. Two procedures through which nanofluids tend to be prepared include the two-step process, as well as the one-step process. In this investigation, the method that was used in nanofluids preparation entailed the two-step procedure. To modulate the stability of the materials that were used, the study relied on the aspect of pH control whereby the shape and size of the nano particles were controlled [27,28] similarly, the surfactant and dispersant were selected appropriately [29]. It is also imperative to highlight that different methods for dispersing nanofluids exist. Some of these techniques include homogenization, ultrasonication, and ball mining, which have an impact on the stability scenario [30,31].
In the aspect of heat transfer coefficient, one of the notable parameters that play an important role entails the target fluid’s state of thermal conductivity. From the majority of the previous experimental investigations, factors that affect the thermal conductivity of nanofluids include dispersion, the pH value, temperature, shape, size, volume concentration, and the base liquid; as well as material properties [32]. The transient hot-wire technique is mostly used to measure the state of thermal conductivity because of its high accuracy and quick response time – with the KD2 Pro instrument also playing a complementary role in realizing the merits. It is also notable that most of the previous experiments have seen the viscosity measured using viscometers. To take the viscosity readings, this investigation relied on the Anton Paar MCR 302 Rheometer. Given the shear rates, this tool determines the state of parameters such as viscosity and the shear stress.
In the experimental investigation by Namburu et al. [33], different particle sizes were used as the dependent variables for discerning the viscosities of SiO2 nanoparticles, hence a comparative analysis. The selected sizes included 100, 50, and 20 nm. These particles were dispersed in 60EGW while the range reflecting the concentration was between 0% and 10%. Also, the temperature range was between -36oC and +50oC. From the findings, the study indicated that at 8% concentration and with 100 nm-sized nano particles, SiO2 nanofluids exhibit the least velocity. In another investigation by Sundar et al. [34] in their experiments, investigated the influence of various EG/water base liquid mixture ratio , the main objective was to determine the impact of different water/EB base fluid mixture ratios on the viscosity of nanofluids; with particular reference to 60:40%, 40:60%, and 20:80% ratios by weight; having suspended the materials in Al2O3 nano particles. The range of temperature for these experimental conditions involved 20oC to 60oC. Also, Vajjha and Das [35] strived to unearth the state of thermal conductivity of different nanofluids. With the particle dispersal operating at 60EGW relative to the mass ratio, specific nanofluids that were investigated included ZnO, CuO and Al2O3. The range of temperature under which the experimental conditions were set was between 25oC and 90oC. Also, 10% was the maximum volume concentration under which the experimental study was conducted. The authors formulated a new relationship or correlation linked to \(\beta_{1}\) and\(f\left(T,\phi\right)\), which was expressed in the form:
\(f\left(T,\phi\right)=\left(2.8217\times 10^{-2}\phi+3.917\times 10^{-3}\right)\left(\frac{T}{T_{o}}\right)+(-3.0669\times 10^{-2}\phi-\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ 3.91123\times 10^{-3})\)(1)
The relations for \(\beta_{1}\) are listed in Table 1 for different nanofluids.