# Fewer Dimensions for Higher Thermal Performance: A Review on 2D Nanofluids
## 3 Application of 2D Nanofluids  
### Heat Exchangers  
Two-dimensional nanofluids are a good alternative to the traditional heat transfer fluids used in heat exchangers. This thermal management equipment has widespread application in diverse technologies including the cooling of electronics, heating/cooling of buildings, fabrication large-scale processes, energy recovery systems, energy conversion and production means, and the cooling of combustion engines. The most studied and used types of heat exchangers are the pinned-tube, shell and tube, plate, and double heat exchangers. Considering the application of 2D nanofluids in heat exchangers, the authors Arzani et al. [125] synthesized MWCNTs covalently functionalized with aspartic acid and suspended in water. The developed nanofluids had a good colloidal stability characterized by less than 20% sedimentation occurring for the highest nanotube weight fraction of 0.1%. The heat transfer rates of the nanofluids with different concentrations were studied in a horizontal annular heat exchanger under three different heat fluxes. In view of the experimental results, the investigation team concluded that the TC, HTC, viscosity, density, and Nusselt numbers of the nanofluids with MWCNTs fillers were enhanced compared with those of the water alone. However, the specific heat capacity was significantly reduced in comparison with that of the water alone. Additionally, the developed fluids entailed only a modest pressure drop increase, requiring little extra pumping power. Considering the findings of this experimental work, it was concluded that nanofluids possessed advantageous thermophysical properties to be employed in a wide range of large-scale applications using annular heat exchangers. Ghozatloo et al. [126] evaluated the heat transfer performance of Gr nanosheets dispersed in water flowing in a shell and a tube heat exchanger under a laminar regime. The results revealed that with an incorporation of 0.075% of Gr nanosheets, the TC of the water was enhanced by 31.8%. An increase was also observed in the HTC which depended on the conditions of the fluid flow, e.g., temperature of the fluid. The HTC of the prepared nanofluids was improved by a maximum of 35.6% compared to that of the water itself at 0.1% wt. of Gr nanosheets at 38 \({}^{\circ}\)C. The HTC was increased by 13.1% when the temperature was raised from 25 \({}^{\circ}\)C to 38 \({}^{\circ}\)C at a concentration of 0.1% wt. as a result of the impact of the Gr nanosheet concentration being more intense at higher temperatures. In this sense, the authors noticed that by augmenting the weight fractions of the Gr filler from 0.025% wt. to 0.1% wt., the HTC was enhanced by 15.3% at 25 \({}^{\circ}\)C and by 23.9% at 38 \({}^{\circ}\)C. On the other hand, Baby and Ramaprabhu [127] synthesized hydrogen-exfoliated Gr dispersed in ethylene glycol and deionized water. The Gr was synthesized through hydrogen-induced exfoliation of graphite oxide under a hydrogen atmosphere at 200 \({}^{\circ}\)C. It was found that a volume fraction of 0.05% vol. of hydrogen-exfoliated Gr in deionized water exhibited enhancements in the TC of 16% at 25 \({}^{\circ}\)C and of 75% at 50 \({}^{\circ}\)C compared with the base fluid alone. Moreover, the authors stated that the TC of the prepared nanofluids increased with increasing volume fraction and temperature. Additionally, the Nusselt number changed with the increasing concentration of hydrogen-exfoliated Gr and an increasing Reynolds number. Table 7 presents some of the recent experimental works on the application of 2D nanofluids in different types of heat exchangers.