Prediction of specific heat and thermal conductivity of nanofluids by a combined equilibrium and non-equilibrium molecular dynamics simulation

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Abstract Nanofluids, a new thermal fluids, have scientific challenges because the existing theories underpredict their thermal conductivity. One way to calculate this parameter is equilibrium molecular dynamics (EMD). In the previous studies of EMD, the thermal conductivity of nanofluids was calculated by the autocorrelation function of the heat current through the Green-Kubo formula. The convergence of this function requires a large time, nevertheless convergence of integral may still be slow or not well behaved. In this study, a new method based on combination of equilibrium and non-equilibrium molecular dynamics simulation in a non-periodic boundary conditions was used to calculate the thermal conductivity. In this method, first the specific heat and the thermal diffusivity of a nanofluid were determined by EMD and non-equilibrium (NEMD) respectively. Then the thermal conductivity was calculated from the relation of thermal diffusivity with the constant volume specific heat. This approach was tested by the nanofluid of silicon nitride nanoparticles in a liquid argon. The CHARMM22 force field and the force field of silicon nitride were combined to perform the simulation. The nanoparticle was generated according to the data of X-ray crystallography. The results of simulation for the base fluid at different temperatures were compared with experimental data to check the accuracy of the MD modeling. The effects of temperature and nanoparticle loadings on the thermal conductivity were investigated. The results showed that the thermal conductivity increases with increasing the loadings and decreasing the temperature. The calculation of root mean square displacements for liquid argon showed that the thermal transport enhancement of the nanofluid was mostly due to the increased movement of liquid phase atoms in the presence of non-metallic nanoparticle. This finding was also confirmed by the analysis of the density profile of liquid atoms near the interface. Finally, the comparison of the results of this study with other researchers showed the kind of nanoparticle could not significant to increase the thermal conductivity of nanofluids.