Nanoparticles are known to enhance the heat transfer of nanofluids in the heated pipe. Although positive effects on heat transfer ability of nanoparticles have been experimentally confirmed, it is difficult to empirically identify the mechanism of heat transfer. Accordingly, the role of nanoparticles was computationally investigated using graphene flakes (GFs), which have high thermal conductivity and no corrosion problem, in a nanopipe model system. In order to determine the effect of functional groups of nanoparticles on the heat transfer coefficient (HTC), two GF models with carboxyl (i.e., -COOH) and hydrogen (i.e., -H) terminal groups were compared. We investigated the heat transfer mechanism of nanofluids, a mixture (50:50 wt%) of water and ethylene glycol (EG), containing GFs (1 wt%) in Fe nanopipe (i.e., diameter of 40 nm) at 325 K, using coarse-grained molecular dynamics (CGMD). In the nanopipe flow system, where the flow rate was 0.19 Å/ps, we captured the temperature and velocity profiles and the number of GFs along the radial and pipe directions to elucidate the heat transfer mechanism by GFs. HTC of nanofluids was also estimated along the pipe length. We found that GFs acted to transfer the heated pipe to the nanofluids and increased the HTC value. GFs were located near the pipe wall and formed the GF-concentrated layer (i.e., radial distance of 100-200 Å). The layer was maintained in the thermally fully developed region, where the temperature profile of nanofluids showed the uniform distribution along the pipe length, consequently inducing the high HTC value. Especially, the GF-concentrated layer was thicker in the nanofluids containing COOH-terminated GFs because of higher miscibility with solvents, resulting in higher HTC value of COOH-terminated GFs compared to H-terminated GFs.

Theoretical study on heat transfer mechanism of nanofluids with functionalized graphene flakes in confined nanopipe system

ACS Spring Meeting 2021

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