Hereby, a comparative study of thermal and thermodynamic properties of nanofluids based on Multi-walled carbon nanotubes (MWCNT) and water is described. The first nanofluid includes pristine MWCNT while the second nanofluid prepared by MWCNT decorated with silver. To achieve the covalent functionalization, morphology of MWCNT-Ag was studied by transmission electron microscope. Subsequently, the value of the entropy generation and thermal performance of nanofluids (MWCNT/water and MWCNT-Ag/water) was inspected in a Two-Phase Closed Thermosyphon (TPCT). The results suggested as the concentration and input power increased, the thermal resistance decreased. Also in different concentrations, the thermal efficiency of nanofluids obeyed the sequence: MWCNT-Ag (1%wt) > MWCNT-Ag (0.5%wt) > MWCNT (1%wt) > MWCNT (0.5%wt) > Water. A variation of the vacuum pressure was also studied in the synthesized nanofluids as compared with the pure water. The results were shown a lower pressure drop of MWCNT-Ag/water than MWCNT/water and the water. Also it was found that, the higher extent of covalent functional groups which have higher thermal conductivity means the higher thermal performance. MWCNT-Ag/water can be an appropriate substitution for the water in the thermal equipment, due to the intensive thermal efficiency and/or low thermal resistance compared with pure water.

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