Influence of functional groups on the thermophysical properties of CNT- CO2 nanofluids: A molecular dynamics study

WL Xu and WS Tian and J Xia and GZ Qin and X Zheng and B Liu, DIAMOND AND RELATED MATERIALS, 155, 112325 (2025).

DOI: 10.1016/j.diamond.2025.112325

Supercritical Brayton cycles (SBCs) show strong potential for waste heat recovery, yet their efficiency is limited by inadequate thermophysical properties of conventional supercritical fluids. We propose a carbon nanotube (CNT)-COQ nanofluid and employ molecular dynamics simulations to investigate how CNT functionalization (OH/COOH groups) optimizes stability and thermophysical performance. The nanofluid achieves temperature-responsive thermal conductivity enhancement, rising from 28.8 % (300K) to 163.5 % (400 K), while pressure elevation from 10 to 15 MPa reduces this enhancement by 29 %. Functionalization effectively suppresses viscosity, with CNT-OH and CNT-COOH nanofluids exhibiting 13.3 % and 23.6 % reductions (478.76 to 422.11 mu Pa & sdot;s), respectively. Temperature elevation enhances conductivity but reduces viscosity, whereas pressure increases both parameters. Microscopic analysis reveals functionalization mechanisms. Formation of ordered COQ nanolayers around CNTs facilitates heat transfer, modified vibrational spectra enhance energy propagation, and increased molecular mobility (quantified via mean square displacement) reduces flow resistance. These synergistic effects enable simultaneous thermal conductivity improvement (up to 163.5 %) and viscosity reduction (up to 23.6 %), outperforming traditional supercritical fluids. Our findings establish CNT-COQ nanofluids as next-generation working fluids for SBCs, addressing critical efficiency limitations through molecular-level design. The fundamental insights into functionalization-property relationships provide a blueprint for developing highperformance thermal management systems.

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