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

Abstract

We studied the heat transfer mechanism of fluid with and without nanoparticles in the pipe flow using coarse-grained molecular dynamics. A nanopipe simulation model system was composed of a cylindrical Fe nanopipe (i.e., diameter of 40 nm) at 325 K and a mixture (50:50 wt%) of water and ethylene glycol (EG). For the nanofluids, functionalized graphene flakes (GFs) were added to the fluids. GF models with carboxyl (i.e., -COOH) and hydrogen (i.e., -H) terminal groups were compared to determine the effect of functional groups of nanoparticles on the heat transfer coefficient (HTC). From the nanopipe flow without GFs, we found that the heat transfer from the nanopipe to the fluid mainly occurred in thermal boundary layer (TBL). It was shown by a drastic elevating of HTC in an entrance region, where the TBL was formed. Estimated HTC of fluid was 3.08 × 108 W/m2∙K, and those of water and EG were 1.24 × 108 and 1.87 × 108 W/m2∙K, respectively. In nanofluids, HTC was increased by 1.1 and 1.06 times with COOH- and H-terminated GFs, respectively, compared to fluids without GFs. GFs were mainly flowed 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.