# Fewer Dimensions for Higher Thermal Performance: A Review on 2D Nanofluids
## 2 Types of 2D Nanofluids
### Carbon Nanotubes
The CNT aspect ratio can be defined as the ratio between the size of the nanotube along a symmetry rotation axis and its perpendicular diameter. For CNTs, the aspect ratio parameter is the ratio between the nanotube length and its diameter and was found to be much larger than one. The authors concluded that the experimentally measured viscosity of the dilute CNTs could be fitted into the Maron-Pierce model [83] as a function of the concentration of the nanotubes and working temperature. The experimental data obtained for the CNTs agree well with the theoretical predictions through first principles that determine the intrinsic viscosity as a function of the aspect ratio. It was argued that the aspect ratio is the main factor in the evaluated range. The surface functionality implied only a minor correction, as it does not provoke considerable aspect ratio modifications. Table 3 summarizes some studies on CNT 2D nanofluids. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Reference** & **Authors/Year** & **Concentration** & **Base Fluid** & **Main Findings** & **Remarks** \\ \hline
[84] & Kumaresan and Verlaj, 2012 & MWCNTs at 0.15\%, 0.30\%, and 0.45\% vol. & Water and ethylene glycol mixtures & Approximate 19.8\% increase in the TC. & Maximum specific heat capacity increase at 0.15\% vol. \\ \hline
[85] & Glory et al., 2013 & MWCNTs from 0.01\% to 3\% wt. & Water & 64\% increase in the TC at 3\% wt. & The TC increase was temperature independent up to 2\% wt. of MWCNTs. \\ \hline
[86] & Fadhillahanafi et al., 2013 & MWCNTs at 0.5\% wt. & Water & 22.2\% increase in the TC. & Addition of 0.01\% wt. of PVP surfactant. \\ \hline
[87] & Tong et al., 2015 & MWCNTs at 0.25\% wt. & Water & \
\begin{tabular}{c} 4\% increase in the efficiency of an enclosed-type U-tube \\ solar collector. \\ \end{tabular} &
\begin{tabular}{c} Considerable decrease in CO\({}_{2}\) emissions. \\ \end{tabular} \\ \hline
[88] & Delfani et al., 2016 & MWCNTs at 25, 50, and 100 p.p.m. & Water and ethylene glycol mixtures & From 10\% to 29\% increase in a direct absorption solar & The efficiency of the collector increased with an increasing concentration and flow rate. \\ \hline
[89] & Esfe et al., 2017 & MWCNTs-SiO\({}_{2}\) at 0.05\% to 1.95\% wt. & Ethylene glycol & Increase of 22.2\% in the TC at 1.94\% wt. and at 50 \({}^{\circ}\)C. & A correlation was proposed based on the temperature and concentration for the TC of the hybrid nanofluids. \\ \hline
[90] & Mahbubul et al., 2018 & SWCNTs at 0.05\%, 0.1\%, and 0.2\% vol. & Water & 10\% increase in the efficiency of a tube solar collector in reference to the water alone. & Critical solar irradiance of 900 \(W/\text{m}^{2}\), after that no considerable enhancement was verified. \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Some studies on 2D CNT nanofluids and hybrids. Further and conclusive experimental works on the inclusion of hybrid CNT formulations in solar thermal energy conversion and harvesting systems and in the nanofiltration of PV/T systems are welcome. There is a lack of studies on the use of CNT nanofluids in some specific heat transfer technologies, such as solar saline water desalination. Furthermore, research must prioritize the search for innovative synthesis procedures using new surfactants to mitigate the absence of the stability over time in the CNT nanofluids. Further studies regarding the impact of the size of CNTs on the thermophysical characteristics of the corresponding nanofluids are required. This factor should be comprehensively analyzed within the scope of the energy and exergy efficiencies of solar thermal energy systems, and not only with regard to its influence on heat transfer enhancement features. Moreover, further studies on the potential miniaturization and entropy generation of heat transfer systems should be experimentally and numerically undertaken to encourage some conclusive research on this matter.