Experimental and modeling studies of N-doped carbon quantum dot nanofluids for heat transfer systems.

The thermal performance of the internal heat transfer fluid was an important factor affecting the heat transfer efficiency of the exchanger system. This study reports a nanofluid that achieves long-term stability without any additives. The synthesized environmentally friendly N-doped carbon quantum dots nanoparticles (N-CQDs) had a uniform particle size distribution, with particle sizes concentrated between 2 and 2.5 nm. The stability, thermophysical properties and rheological characteristics of N-CQDs nanofluids were investigated. The results showed that the Turbiscan stability index of N-CQDs nanofluid was about 1.5 and the ΔT value fluctuated within 0.5 %, showing excellent stability. The thermal conductivity of 0.1 vol% of N-CQDs nanofluid increased by 43.21 % at 50 °C compared to the base fluid. The specific heat capacity decreases with increasing nanofluid loading. The viscosity of N-CQDs nanofluid behaves as a Newtonian fluid. In addition, the models for estimating the viscosity and thermal conductivity of N-CQDs nanofluids are proposed, respectively. Due to the different particle sizes and shapes of nanoparticles, the proposed model can better predict the thermal conductivity and viscosity of N-CQDs nanofluids compared with other empirical models. [Display omitted] • The prepared N-CQDs nanofluids have excellent long-term stability. • The thermal properties of N-CQDs nanofluids have been enhanced. • The viscosity of N-CQDs nanofluids with different concentrations was measured. • The new models are proposed for estimating the thermal conductivity and viscosity of N-CQDs nanofluids. [ABSTRACT FROM AUTHOR]