Characterization of Thermophysical and Electrical Properties of SiC and BN Nanofluids

Abstract

Experimental data associated with the thermophysical properties (TPPs) of various nanofluids (NFs) are essential for their diverse applications in energy storage and conversion, as well as thermal management. This study experimentally investigated important TPPs such as thermal conductivity (TC), thermal diffusivity, density and viscosity, as well as the electrical conductivity of two new types of NFs, namely silica (SiC) and boron nitride (BN) nanofluids. The NFs are prepared at five low concentrations of nanoparticles from 0.01 to 0.05 vol.% dispersed into a mixture of ethylene glycol (EG) and distilled water (DW). The TPPs are measured, and their enhancements are evaluated in comparison with their base fluids. The results show a good increase in TC and thermal diffusivity for both types of nanofluids with increasing concentrations until reaching the maximum enhancement of about 4.4% for the SiC nanofluid and about 7.0% for the BN nanofluid at the same concentration (0.05 vol.%). On other hand, a Newtonian rheological behaviour is observed, and viscosity and density are also found to increase for both types of NFs, where the maximum increase in viscosity and density at 0.05 vol.% are found to be 5.2% and 0.3%, respectively. The electrical conductivity also increases by up to 3.2 times for SiC nanofluids and 2.8 times for BN nanofluids at the maximum concentration of these nanoparticles (0.05 vol.%) compared with the base fluid (EG/DW). The overall evaluation of the obtained results demonstrates the great potential of these nanofluids in heat transfer applications.

1. Introduction

Heat transfer fluids are essential for most applications in industry for cooling and heating processes by numerous types of heat transfer systems (e.g., heat exchangers). In most of the applications, traditional thermal fluids such as water and oils are used, and inherently low thermal properties (TPs) of these fluids are the main barrier to the development of advanced heat transfer systems with improved performance [1,2]. The enhancement in the TPPs of the thermal fluids plays a significant role in improving the overall performance of many heat transfer processes, systems and applications in a wide variety of industries. Mixing NPs with such common thermal fluids can result in enhanced TPPs and features [3,4], and such mixtures of NPs and fluids are widely known as ‘‘nanofluids’’. NFs have been found to improve the heat transfer performance in thermal systems [5,6] for numerous applications. Different types of metallic and non-metallic NPs have been mixed with conventional thermal fluids to prepare various NFs (e.g., [7]) and their TPPs, based on different conditions such as particles concentrations and type, base fluid type, temperature and so on, have been investigated [8]. So far, TC has been considered the most significant property of NFs as it greatly impacts heat transfer performances of thermal fluids in real applications. Investigations on the TC of various types of NFs showed significant increase in TC by the addition of various kinds of NPs to the base fluids [9,10,11,12,13]. However, there are still controversies on the enhancement and underlying mechanisms of the TC of nanofluids [14]. Some analytical models such as the correlations of Timofeeva et al. [15], Nan et al. [16], and Murshed et al. [17] were also proposed to predict the TC of the NFs. These models have considered various factors such as the shape, size and fraction of the NPs, and the nanolayer. Further details and more information about studies on the TC of NFs can be found in the literature [18].

As the industry trend is leading towards more rational use of energy, in addition to minimizing the sizes of systems and devices [19,20], compact heat exchangers have appeared, with further challenges regarding the NF features [21]. The rise of pressure drop of the NF flow in mini/microchannels has been considered a disadvantage as it requires an additional pumping power in the system [6,22]. Therefore, the enhancement in the convectional heat transfer of NFs for heat exchanger systems also requires the use of fluids with a small value of viscosity and a greater value of thermal conductivity for improving the thermal performance and minimising the resultant pressure drop in the applications involving heating and cooling [5,23]. However, the viscosity of NFs rises significantly with the rising NPs concentrations, which is not desired for heat transfer in any cooling or heating systems. More information about the progress on the viscosity of NFs can be found in the review research conducted by Murshed and Estelle [23]. Nevertheless, it is always recommended to maintain the viscosity of NFs as low as possible by employing low NP concentrations.

In addition, some important points should not be overlooked. For example, the boiling and freezing temperature points are considered important when preparing the NFs, and can be set in some thermal fluids such as EG/DW mixtures by adjusting the mixing of EG in water [24,25,26]. The effects of minimising the scales of the heat exchangers have also been mentioned by researchers in the field [27] and it is reported the importance of investigating the properties of the thermal fluid with the variation of temperature [28,29]. Furthermore, CFD tools have been commonly used for numerical investigations of the thermal behaviour of NFs in heat exchangers [30]. So far, numerical approaches have been adapted to investigate the behaviour of NFs for heat transfer performance in minichannels [31] and microchannels [32], and a good agreement with experimental data for numerous situations has been presented, for example, the NFs with non-Newtonian rheology behaviour [33]. Furthermore, various computational techniques were introduced in previous investigations on NFs for different heat transfer applications, for instance solar thermal energy applications where optical properties of NFs are also important [34], cooling electronics and automotive vehicles, etc. [8,35]. In the literature, several numerical approaches were adapted based on the TPPs of NFs and their operation conditions [36,37]. The temperature independence of PPTs for the modelled nanofluids was found in the numerical approaches in previous studies with no consideration for their rheology [38,39], which shows the lack of data regarding PPTs for various temperatures as well as for the possibility of changing the rheological performance to a Non-Newtonian one. The latter led to the need for a wide range of investigations on the TPPs of NFs toward the optimisation of their performance for specific applications [6]. Yasmin et al. [40] reviewed recent studies on NFs and highlighted the importance of preparation parameters related to stirring, sonication and the surfactant on the TPPs of NFs regardless of particle type and base fluid. In addition, only a handful of studies can be found investigating the SiC and BN NFs. For instance, a study by Mohammadi et al. [41] dealt with the flow of SiC NFs in a mini-heat sink for cooling electronic elements and reported an enhancement of heat transfer around 55% for 0.5 vol.% SiC, compared with the base fluid. In addition, Chen et al. [42] investigated the TPPs and optical properties of SiC-based ionic liquid NFs for solar collector applications. Their results indicated an advanced solar absorption performance, as well as enhancements in TC and specific heat of base fluid of about 10% and 5%, respectively. Li et al. [43] found good stability and enhancement of TC up to 16.21% for SiC/EG NFs at particle concentrations between 0.2 and 1.0 vol.%. Moreover, the TPPs of SiC/(EG/DW) NFs were determined by Li and Zou [44] and an increase in TC up to 33% at 1.0 vol.% was found with Newtonian behaviour for the NF samples. Another investigation by Song et al. [45] for SiC/DW NF in pool boiling reported an enhancement in CHF up to around 105% by adding 0.01 vol.% of NP to the BF. On another hand, Gómez-Villarejo et al. [46] experimentally studied BN/DW NFs and found an increase of 8% in specific heat and about 10% in TC compared with the BF, and their NFs were also found to be Newtonian. Later, Gómez-Villarejo et al. [47] investigated the thermal properties of BN NFs to improve solar thermal performance. The results showed good stability and Newtonian behaviour with a minor rise in viscosity for the NFs. The TC and heat transfer coefficient increased up to 33% and 18%, respectively, compared with the BF. Krishnam et al. [48] also studied the BN/DW NFs for cooling purposes in thermal management systems, and an increase of about 16% was observed in the TC at 0.1 vol.% compared with BF.

Consequently, TC and thermal diffusivity are considered the key properties responsible for assessing the heat transfer performance of NFs, where enhancing those properties leads to a better heat transfer efficiency of the thermal systems [5,8,32]. Most studies in the field have focused on investigating the TC of NFs [49]. Nevertheless, some types of NFs such as SiC and BN are still not adequately investigated to explore their real thermophysical properties under different conditions and factors such as concentrations and temperatures. Furthermore, some TPPs such as density, rheological behaviour and viscosity of NFs can negatively influence the thermal performance of NFs, which must be also investigated well [32]. One of the important factors that have an impact on the heat transfer performance of NFs is the density, which can be changed based on the type of base fluid and NP, and NP size, as well as the nanolayer, which is an interfacial layer between the NPs and the base fluids [50]. On the other hand, electrical conductivity is considered one of the important properties of NFs that can be influenced by diverse parameters such as NP type, stability, temperature, etc. [51,52]. The electrical conductivity of the thermal fluid is important, especially for electronic applications where its increase is unfavourable [53]. Furthermore, thermoelectric materials have received important attention for enhancing their performance by adding components such as Zn [54]. However, electrical conductivity has mostly been ignored in the investigations on NFs by the researchers.

Therefore, it becomes clear that there is still a lack of investigations on some types of NFs based on the NP types and the use of base fluids. In addition, apart from the TC, there is a need for a careful investigation of other different TPPs of diverse NFs for their optimum performance in applications. Therefore, this research intends to study experimentally five important properties, namely TC, thermal diffusivity, density, rheology (including viscosity) and electrical conductivity of NFs produced by adding SiC and BN NPs to an EG/DW mixture at low concentrations. The obtained results of this study can fill the gap in knowledge, providing the necessary information for employing NFs for heat transfer systems used in thermal management [46,47,48,55,56,57,58,59].

2. Nanofluids Preparation and Experimental Procedures

The samples for both NF types were produced using the SiC (silicon carbide) NPs of 50–60 nm size and BN (boron nitride) NPs of 70 nm size with 99.9% purity provided by the supplier (IoLiTec, Heilbronn, Germany). A mixture of 70 vol.% distilled water (DW) and 30 vol.% ethylene glycol (EG) was used as the base fluid. The mixture of EG/DW base fluid was selected as it is widely used in various cooling and heating applications [26,60,61]. EG/DW mixtures can result in special advantages for the prepared NFs considering their freezing and boiling points, which are significant for industrial heat transfer purposes [26,62]. The samples were prepared by dispersing five low concentrations from 0.01 to 0.05 vol.% of each type of NP into the EG/DW base fluid. Each NF sample was first mixed with a magnetic stirrer for about 25 min, and then the sample was sonicated for about 20 min with the continuous mode (amplitude 60% and 40 kHz frequency) using a probe-type ultrasonicator (Hielscher UP200Ht, Berlin, Germany). The mentioned steps of preparing NFs are to ensure good dispersion of the NPs into the base fluids and thus achieve good stability. The resultant 10 samples of the BN and SiC NFs are shown in Figure 1. The photos in Figure 1 confirm an excellent dispersion status of NPs and a good homogeneity of the samples. It can also be noticed that the opaqueness of the samples increases with an increase in particle concentration (the colours become stronger). Additionally, to increase the level of the measurement’s accuracy, the TPPs of NF samples were measured on the same day of the preparation in order to avoid any agglomeration of particles that can happen over time.

In this study, the thermal properties of the NFs were evaluated at room temperature (20 °C) by a thermal properties analyzer (METER Group, Pullman, WA, USA), which works based on the transient hot-wire (THW) technique (which is well-recognised and appropriate and thus commonly used for the measurement of TC of NFs), using a single-needle type KS-3 sensor (length of 60 mm and diameter of 1.3 mm) for the TC and a dual-needle type SH-3 sensor (length of 30 mm, diameter of 1.3 mm and the spacing between the two needles is 6 mm) for the thermal diffusivity. The SH-3 experimental measurements need 2 min, where it starts to heat for 30 s with measuring and concurrently recording temperatures for 90 s. The needle sensors were oriented to be vertical and centred inside the vial that contained the NFs, ensuring that there were no bubbles in the NF medium or around the needle.

Several measurements for each sample (NF) were performed, with a sample resting time between the measurements of 25 min. Moreover, the used experimental device was calibrated with the base fluid, and the maximum variation was found to be within 1.8% of the standard referenced values.

Then, the rheology and viscosity of NF samples were measured at several temperatures from 22 °C to 41 °C and shear rates from 200 to 800 1/s. The used device was a viscometer from AMETEK Brookfield with a cone and spindle type CPE-40. In addition, an uncertainty of about 2.5% was defined for the experimental measurements by the used viscometer. The viscometer was linked to a thermostatic bath together with a small pump and thermocouple to set the required temperature value for the NF sample. The experiments were performed at several values of shear rates and temperature points for each sample, at an amount of 0.5 mL, which is the volume of fluid required for the proper working of the viscometer. The rheological performance was assessed by evaluating the data of shear as a function of the shear rate for all NFs at different NP concentrations.

Furthermore, the density of the samples was measured at room temperature by a density meter from Kem Kyoto Electronics (Kyoto, Japan). The density device was calibrated with the DW and the base fluid as mentioned in the user manual before conducting the experimental measurements of NFs. It was ensured that the measuring tube cell of the density meter was clean and without bubbles before the measurements, in addition to considering the stabilization of the NF sample temperature. The electrical conductivity of the NF samples was determined using a precise multi-range conductivity meter (HI8633, HANNA, Woonsocket, RI, USA). The accuracy of the utilised electrical conductivity probe was first checked using DW, and the temperature stabilisation was ensured during the measurements. Each NF sample was put in a suitable vial and five measurements were performed for the NF sample, taking the average value for the measurements.

It should be mentioned that the TPPs of the NF samples were measured in a thermally and physically isolated environment. In addition, several experimental readings were taken for each NF sample in each type of measurement, and the average values are presented. Moreover, the experimental apparatuses were validated, and a calibration procedure was followed to ensure the accuracy of the measurements. The maximum measurement uncertainties for the viscosity, density and electrical conductivity were 2.5%, 1.5% and 1.0%, respectively.

3. Results and Discussion

3.1. Thermal Properties

As mentioned before, the key thermal properties (TPs) such as TC and diffusivity of these two NFs were measured at different concentrations. The results presented in Figure 2 refer to the higher TC of the two types of NFs compared with the base fluids. The TC was found to increase with increasing NP concentrations until reaching the maximum enhancement of 4.4% for SiC NF and 7.0% for BN NF at 0.05 vol.%. The higher values of BN NFs are due to the higher (almost double) TC of BN NPs compared with that of SiC NPs. However, given that the SiC NP is abundantly available and much cheaper compared with BN, such enhancements are also very significant for the heat transfer applications of their NFs.

Moreover, thermal diffusivity was also found to increase with the addition of NPs, up to 4.2% for SiC NF and 7.2% for BN NF at 0.05 vol.%, as shown in Figure 3. Although the enhancements were moderate, they were obtained at very low concentrations. Interestingly, while the maximum increase in thermal diffusivity of BN NFs was slightly higher compared with their TC enhancement, SiC NFs showed a slightly lower enhancement of thermal diffusivity in comparison with their maximum TC increase. This could be due to the volumetric heat capacity of SiC NFs. It is to be noted that very limited studies [47,63,64] have been reported in the literature where the thermal diffusivity of NFs, particularly present types (BN and SiC), are measured and reported. As the TC of NFs was found to be higher than that of the base fluids, it is anticipated that the thermal diffusivity would also be higher and increase with increasing concentrations of NPs. The current results present a consistency for the enhancements of TC and thermal diffusivity of both NF types, and the findings agree with the conclusions of literature studies for similar NFs [47,65].

However, using a percentage of EG with DW as a base fluid decreases the TC, as EG has a lower TC than DW [5]. Therefore, numerous factors may influence the TPs of NFs, for instance the viscosity of the base fluid, temperature and interfacial nanolayer around the NPs, in addition to the stability and dispersion of the NPs [66]. In addition, Brownian movement of the NPs at the molecular level can be a significant factor for the enhanced TPs of NFs in relation to the used base fluid (e.g., EG/DW) along with the NP concentrations (e.g., the low concentrations used of SiC and BN) [67].

3.2. Density

Density is an important TPP of the NFs that influences heat transfer efficiency, and is also essential in the calculations of flow and convective heat transfer parameters in experimental and numerical investigations [31,68]. Typically, the density of NFs has been defined by means of general mixture correlations (e.g., [69]), which may not reach the high level of accuracy required in the investigation of heat transfer. Therefore, the current study presents experimental density results for SiC and BN NFs at different concentrations and at room temperature, as presented in Figure 4. Moreover, the values of density are calculated using the mixture correlation, as presented by Pak and Cho [70] (Equation (1)), and compared with the experimental results in Figure 4.

$${\rho }_{nf}=\phi {\rho }_p+\left(1-\phi \right){\rho }_{bf}$$

(1)

According to the acquired data, density slightly increased with the addition of NPs, reaching the highest increase of 0.3% at 0.05 vol.% for SiC NFs and slightly lower for BN NFs. The low increase in density can be explained by the lower particle concentrations. However, density has an important impact on the pressure drop and pumping power of the fluid flows through heat exchangers, and having low increases in density (such as the results of the current study of a maximum of 0.3%) is desirable for NFs.

3.3. Viscosity and Rheological Characteristics

The rheology and viscosity of NFs are significant for heat transfer applications due to the impact on the flow, pressure drop and the convective-based heat transfer performance. Therefore, the rheological characteristics of the SiC and BN NFs were determined at a relatively moderate shear rate range, and are provided in Figure 5.

It can be noticed from Figure 5 that the shear stress results are linear relative to the shear rates for both types of particles (SiC and BN) and the particle concentrations. The latter refers to a Newtonian behaviour for these SiC and BN NFs. The present finding is consistent with the conclusions of related studies in the literature for NFs [44,46]. However, the change of the rheological behaviour of NFs is expected in many cases, especially with higher particle concentrations and different shear rate ranges where they may show non-Newtonian behaviour [33]. The rheological characteristics of NFs can vary based on numerous factors related to the nature and amount of NPs [71,72] in addition to the base fluids.

Furthermore, the increased viscosity of the NFs has been considered one of the disadvantages that should be diminished or minimised. A higher viscosity of the fluid results in a higher pressure drop of the flow and thus a higher required pumping power in the heat exchanger or flow systems, resulting in lower energy efficiency and higher cost. Therefore, the viscosity of SiC and BN NFs are measured as a function of the temperature and concentration of NPs. The higher values of viscosity are obtained with the increase of NP concentrations, and they reduce with increasing temperature.

Figure 6 shows that the viscosity of SiC and BN NFs increased by increasing the concentration of the particles and decreased considerably with the temperature rise. There was also a similar increase in viscosity for both SiC and BN NFs with the addition of NPs, with an increase of 1.05% for 0.01 vol.% to 5.2% for 0.05 vol.%. In addition, the reduction of viscosity due to the temperature rise from 22 °C to 41 °C was about 32.2% at 0.05 vol.%, which agreed with the related findings in the literature [23,73]. Such a large decrease in the viscosity of these NFs shows great importance as they can perform better at elevated temperature conditions. Data of viscosity of NFs found in the literature [23,73] show that viscosities are generally evaluated at various temperatures for several particle concentrations, and values are observed to be greater than the base fluid and are reduced by raising the temperature. However, the results of viscosity, TC and density presented in the current study are commensurate with the data found in the literature [23,73], where those properties are raised with an increase in NP concentration [74,75].

3.4. Electrical Conductivity

The effect of the addition of SiC and BN NPs to the base fluid on the electrical conductivity was also investigated in this study. The results in Figure 7 demonstrate that the electrical conductivity of the two types of NFs increased with the addition of NPs. Moreover, the SiC NFs showed higher values of electrical conductivity than BN NFs, which can be explained by the higher conductivity of the SiC materials. The maximum increase was obtained at 0.05 vol.%, where electrical conductivity increased 3.2 times for SiC NFs and 2.8 times for BN NFs, compared with the base fluid. This trend of increasing the electrical conductivity of NFs agrees with the results in the literature [76], and this should be considered in the investigations of the properties of NFs due to its importance in many engineering applications.

4. Conclusions

In this work, two less studied but very important NFs, namely SiC and BN NFs, were studied experimentally for the characterization of their thermophysical, rheological and electrical properties. Five low concentrations (0.01–0.05 vol.%) of these NPs were used to prepare sample NPs, and their TPs such as TC and thermal diffusivity were measured, and their other important properties including viscosity, and density electrical conductivity were investigated. The enhancement of the thermal properties of the two types of NPs at different concentrations was presented, reporting a maximum enhancement of TC of about 7.0%, and the maximum thermal diffusivity was found to be about 7.2% for BN NF at a 0.05 vol.% concentration. The measured TC and thermal diffusivity were found to increase with increasing NP concentration in a similar trend for both types of NFs, with a lower enhancement for SiC NFs (around 4.2% maximum enhancement). Such enhancements of these two important TPs of these NFs demonstrate their potential for heat transfer applications.

Furthermore, the higher (compared with base fluid) electrical conductivity of these two NFs (SiC and BN) was found to further increase with increasing NP concentration, and the maximum increase of about 3.2 times for SiC NFs and 2.8 times for BN NFs at a 0.05% particle concentration was observed. In addition, the viscosity and density of the NFs were measured and analysed, and both properties were found to increase with increasing particle concentration for both NFs. The SiC and NB NFs presented a Newtonian behaviour at all concentrations and the studied shear rate range. Finally, this study confirmed that the use of low concentrations of SiC and BN for NF-based EG/DW (that led to a low increase in the viscosity and density) was efficient and the overall evaluation of the obtained properties drives to the conclusion of the importance of these NFs in heat transfer applications. Therefore, the findings of this study provide an important database for new types of nanofluids that can be built upon for further experimental and numerical investigations regarding their promising thermal performance in applications such as in heat exchangers, and energy storage and conversion systems.