1.4 Physiochemical properties of nano-additives
Raman spectroscopy is a powerful characteristic technique to acquire insights of structural disorders and defects of the materials. Raman spectrum of the raw GNPs is given in Fig. 1 (a). There are three prominent peaks represented as D, G, and 2D found at ~ 1340, ~1567, and ~ 2700 cm-1 respectively [36]. The D peak belongs to the breathing mode of sp2 carbon atoms, while its intensity relates to the degree of existing defects. The G peak corresponds to the doubly degenerate E2g phonon dispersion at the Brillouin zone center, its intensity and sharpness are attributed to the sp2 hybridized graphitic structure [37]. The 2D peak is the second-order Raman scattering process, originating from in-plane vibrational modes of carbon atoms. It is used to estimate the number of graphene layers.
The magnitude of the defects is estimated as ~ 0.70 using ID/IG ratio, its intensity is relatively lower than conventional graphene oxide flakes which normally lie above ~ 0.9 [38]. The sharp and intense G peak indicates the regular graphitic structure. Moreover, slightly blue shifting in the G peak as compared to the graphite is found which normally appears at ~ 1580 cm-1, this sifting is attributed to the modification that occurred due to its exfoliation. The 2D peak is broadened unlike graphite, the broadening and splitting of the 2D peak in multiple components are associated with the inter-layer interaction, which normally indicates few-layered graphene [39]. However, peak splitting is not significant. Therefore, can be concluded that several layers is quite lesser than graphite but a little bit higher than few-layered graphene which perfectly matches the characteristics of GNPs.
Figure 1 (b) shows the Raman spectrum of Ag NPs. The peak appeared around ~ 255 cm− 1 belongs to the vibration of atoms on the nanoparticle surface. Few more peaks are observed between 430 to 530 cm− 1 attributed to the stretching vibration of Ag-Ag bond inside nanoparticle clusters are relevant to surface plasmon resonance, indicating nanocrystalline structure. The other two sharp peaks around ~ 1156 and ~ 1654 cm− 1 are raised from the vibrational modes of atoms within the bulk of nanoparticles.
The morphology of raw nano-additives is analyzed by SEM. Figure 2 (a) shows the SEM image of GNPs. Figure 2 (b) shows Ag nanoparticles which are aggregated due to high surface area and dominant van der Walls forces. For closer details, nanofluids with and without surfactant have been characterized by HRTEM as shown in Fig. 3. Figure 3 (a) clearly shows that samples without surfactant suffer from agglomerations of Ag nanoparticles due to high surface area and dominant van der Walls forces. Figure 3 (b) demonstrates the nanofluid with Tween-80 surfactant which shows improved dispersion of the Ag nanoparticles. The particle size distribution mostly lies in the range of 650 to 750 nm.
1.5 Stability of GNPs/Ag hybrid nanofluid
The stability of prepared nanofluids has been analyzed using zeta potential values of nanofluids measured on the first day after preparation, then second day and seventh day. Zeta potential value is related to the charges present on the surface of particles. Figure 4 displays the zeta potentials of Ag and GNP-based nanofluids with the function of corresponding nano-additive concentrations. The thumb rule of analyzing stability with zeta potential is that if the absolute value of zeta potential is greater than ± 30 mV then nanofluids are said to be stable [40]. Figure 4 (a) indicates that Ag nanoparticles-based nanofluids without surfactant (T-80) are not stable as zeta potential values are below 30 mV. Moreover, the addition of surfactant drastically shifted zeta potential values on the positive side. However, it is effective till the lower concentration of 0.2 vol%. The diminishing in zeta potential values after one, second, and seventh day of preparation is minor which shows good stability of nanofluids.
Figure 4 (b) shows the zeta potential values of GNP-based nanofluids measured after the first, second, and seventh day of preparation. The trend is the same as observed for Ag-based nanofluids, however, relatively higher zeta potential values indicate better stability of GNPs than Ag nanoparticles, even nanofluids without surfactant show some stability that is, zeta potential higher than 30 mV consistently till the seventh day of preparation. The higher zeta potential value may be attributed to the higher surface area of GNPs. Unlike Ag nanofluids, the difference in the drop of zeta potential values on the second and seventh day is minimal, indicating good stability. Overall, GNP-based nanofluids remained stable at 0.2 vol% concentration.
Figure 5 shows the zeta potential values of GNPs/Ag hybrid nanofluid. It shows that nanofluid without surfactant is not stable at all. Zeta potential seems to be dropped with the increasing concentration of nano-additives. It indicates that only nanofluids with 0.1 vol% are stable, with an increase in the concentration of nano-additives zeta potential consistently decreases. Although the reduction between the second and seventh day of preparation is not significant but dropped below a threshold value. Therefore, the hybrid nanofluids are stable with 0.1 vol% concentration only.
The stability of GNPs/Ag hybrid nanofluids has been further analyzed using a UV-Vis spectrophotometer. Figure 6 shows the UV visible spectra of Ag/GNP hybrid nanofluids taken on day one and after a week of preparation. The comparison of absorbance on day one and seventh day revealed a minor drop in absorbance value of ~ 1 a.u indicating fair stability of nanofluids. Moreover, no shifting and variation in the symmetry of spectra over time is observed. The visual photography shows that after a week, there has been no significant clustering (Fig. 7). The UV-Vis spectrophotometer confirms these findings.
1.6 Thermal Conductivity of Base Fluid and Nanofluid
The thermal conductivity of a nanofluid is usually affected by the choice of the base fluid. In the present study, the thermal conductivity of the proposed base fluid (EG/GLY) with different mixing ratios of EG and GLY at various temperatures was determined as shown in Fig. 8. The temperature was varied between 40 to 80\(\text{℃}\) to validate the consistency of thermal conductivity measurements for the base fluid. Figure 8 reveals that increasing the temperature increases the thermal conductivities of the base fluid. This phenomenon can be explained by the fact that the distance between nanoparticles reduces with increasing volume fraction [41].
Moreover, the thermal conductivity of pure GLY is higher than pure EG and it is found that the thermal conductivity enhancement of GLY was 23.8% which is higher than that of EG at 80\(\text{℃}\). The enhancement is attributed to the low Brownian motion of the particles due to the high viscosity of GLY [42]. The thermal conductivity for the base fluid of (EG: GLY 60:40) is the highest compared to other mixtures and recorded a 27.5% enhancement ratio relative to the base fluids and adding 60% of EG caused the thermal conductivity of the resultant mixtures to increase by 15.54% at 40\(\text{℃}\). The comparison declares that 60% EG and 40% GLY added together will increase the resultant fluid thermal conductivity, but in the two cases (EG: GLY 40:60) and (EG: GLY 20:8), the thermal conductivity was relatively close to pure EG. Since better enhancement in thermal conductivity was found by the resultant mixture of (EG: GLY 60:40), this mixture was chosen as the optimal base fluid for Ag, GNP, and AG/GNP hybrid nanofluid.
The thermal conductivity of Ag and GNPs nanofluid - EG/GLY mixture 60:40 ratio (by volume) nanofluids with different concentrations of 0.1–0.3 vol.% have been carried out at different temperatures and the outcomes are shown in Fig. 9(a) and 9(b), respectively. It could be observed that thermal conductivity increases as the volume fraction of the nanoparticles increases. Furthermore, the optimal Ag and GNPs concentration is connected to surface modification, such as the addition of surfactants or the introduction of functional groups.
The thermal conductivity of hybrid nanofluid Ag/GNP- EG /GLY mixture 60:40 ratio (by volume) nanofluids with different volume concentrations of 0.1–0.3% have been carried out at different temperatures and the outcomes are shown in Fig. 10. Comparison for hybrid nanofluid-based EG/GLY nanofluids was not possible since relevant data was not available. It could be observed that the thermal conductivities of nanofluids have a nearly linear relationship with the volume fraction of particles and are significantly higher than that of the corresponding base fluids. The thermal conductivity of GNPs/Ag nanofluid was not able to be tested at concentrations higher than those shown due to agglomeration and settling. Once the concentration reaches a certain point, the particles may agglomerate and fall out of suspension, decreasing the overall heat conductivity.
Figure 10 shows that at an operating temperature of 60\(\text{℃}\), the thermal conductivity of the base fluid (EG/GLY) was shown 0.3267 W/mK. However, by adding 0.1 vol.% of GNP, the thermal conductivity increased 47.51% and when adding 0.1 vol.% of Ag, the thermal conductivity of fluid showed an increment of 67.46% (0.5471 W/m.K). Moreover, combining Ag with GNP as shown in Fig. 10, resulted in remarkable improvement when adding 0.1% increased the thermal conductivity of the base fluid (EG/GLY) by 102.85% (0.6627 W/m.K).
The increase in thermal conductivity is also induced by the Brownian motion of nanoparticles that disperse more quickly in hot regions, resulting in a higher thermal conductivity than the base fluid. A greater dramatic improvement in thermal conductivity of GNPs/Ag-EG-GLY nanofluid is seen for a low volume concentration of nanoparticles. Therefore, the heat transfer performance by using as low as 0.1 vol.% of nanoparticles has shown a significant impact on the thermal conductivity and at the same time a new generation heat transfer fluid will be developed by saving time and money. Then, other thermophysical properties of hybrid nanofluids (viscosity, density and specific heat capacity) were evaluated using concentrations of 0.1, 0.2, and 0.3 vol. % nanoparticles, as higher thermal conductivity was observed compared to the single base of Ag and GNPs nanofluid.
1.7 Viscosity of Ag-GNP Hybrid Nanofluid
The viscosity of Ag/GNP-(EG: GLY 60:40) nanofluids as a function of temperatures and volume concentration is depicted in Fig. 11. Viscosity is also increased with the increment in volume concentration of nanoparticles. For instance, a volume concentration of 0.2 vol.%, an increase of 68.9% compared to the base fluid was gained at a temperature of 60℃. Furthermore, as the temperature increased, the viscosity decreased rapidly due to the lower forces of inter-molecular and inter-particle bonding. The entire temperature range shows a decrease in viscosity. The experimental results also show that the viscosity of the nanofluid increases as the nanoparticle concentration increases. This is due to the increased Van der Walls force between base fluid and nanoparticle atoms or molecules [43].
The measured data revealed that the higher volume concentrations of Ag/GNPs nanoparticles cause a rise in viscosity due to an increase in the number of nanoparticles and consequently the number of collisions, which leads to an increase in the viscosity. This observation is well-supported by the research that has been done previously [44, 45].
The ideal nanofluid should not only have a high thermal conductivity but also have a low viscosity. However, hybrid nanofluid has higher viscosity compared to water and the viscosity of the nanoparticle-enriched fluids is found to be greater than the base fluid. The viscosity of a fluid has been the subject of research over the years because it has an impact not only on friction but also on pumping power and pressure drop. Even though water has low viscosity, studies performed by many authors suggest nanofluids could replace water as the working medium as the water has a low boiling point and high freezing point [46, 47]. Conventional fluids, particularly deionized water, have the disadvantage of having a low boiling point while having a high freezing point which is one of the limitations of their use in thermal system applications. Another reason is that the water may contain bacteria which can lead to corrosion of the heat thermal systems. Even though T80 has more influence on the viscosity of hybrid nanofluid than base fluid, the measured values are still low and T80 surfactant still can effectively weaken the aggregation of the hybrid nanoparticles, and considerable savings in the pumping power can be achieved at higher operating temperatures.
1.8 Density of Ag-GNP Hybrid Nanofluid
The density was measured for the base fluid of (EG: GLY 60:40) and Ag/GNP-based hybrid nanofluid at different temperatures and the results are displayed in Fig. 12. It can be seen that the density is decreasing with the increase in the fluid temperature. The decrease in the densities of the base fluid and Ag/GNP hybrid nanofluid is perhaps due to the fluid thermal expansion as the temperature rises. At 0.1%, 0.2%, and 0.3% particle concentration with T80, the density rise was (14.88%), (19.43%), and (22.70%) at 40°C; simultaneously, the density rise was (4.85%), (9.16%) and (13.12%) at 80°C, respectively compared to base fluid data as shown in Fig. 12. Therefore, it can be noted that the rise of the density is significant when the volume concentration of nanofluid increases. The results also show that the density of nanofluids increased with the volume concentration which is consistent with the literature [48]. The cause of this growth in density is related to the mass of nanoparticles which is greater than that of the base fluid. Other than that, the declination in the density values is attributed to the greater intermolecular spacing at higher temperatures.
1.9 Specific Heat (Cp) of Ag-GNP Hybrid Nanofluid
Specific heat capacity (Cp) is an important aspect of the thermal system, especially in the design and performance analysis of thermal energy storage systems. The specific heat capacity of (EG: GLY 60:40) based Ag/GNP nanofluids as a function of temperatures and volume concentration is shown in Fig. 13. It demonstrates that the specific heat capacity of nanofluid samples is lower than that obtained for base fluid of EG/GLY (60:40). Experimental results also show the maximum decrement in specific heat value is about 46.45% compared to base fluid at 40\(\text{℃}\).The experimental results indicate that all of the hybrid nanofluid samples exhibit unfavorable outcomes, as the specific heat of the nanofluids is observed to be lower compared to that of the base fluid. This implies that the heat energy needed to increase the temperature of nanofluids is comparatively lower than that of the base fluid of EG/GLY.
Moreover, the specific heat capacity of the nanofluid decreases with the increase in nanoparticle concentration. One possible explanation is that the surface free energy has a larger effect on the specific heat capacity of the GNP nanoparticles as a result of their larger surface area. However, the specific heat increases with the increase in temperature, which affects the specific heat more significantly than the mass fraction of the nanoparticle. This behavior is common since increasing the concentration of the dispersed nanoparticles or adding a surfactant to the base fluid will increase the fluid's heat transfer capabilities while reducing its initial thermal storage capacity [29, 49].