Molecular Dynamics Simulation On Thermal Conductivity Of Ionic Liquid-Based Nanofluids

Absorption heat pump, which can realize the transport of heat from low to high temperature, is a kind of important energy-saving and environment-friendly equipment for utilizing waste heat. The common working pairs of the absorption cycle have many shortcomings, such as toxicity (NH3/H2O), corrosion and easy crystallization (LiBr/H2O). Ionic liquids are a new type of green solvent and own many excellent properties, such as negligible vapor pressure, wide range of liquid process, high thermal stabilities and no corrosive. These properties of ionic liquids make some of them excellent candidates for use as absorbents of refrigerant in absorption heat pump or absorption refrigerator. With the in-depth research on heat transfer enhancement, heat transfer performance of refrigerant is the main factor which affects the equipment with high efficiency and compact. In recent years, nanofluids have proved to be a very good heat transfer fluid. Nanofluids are a new type of heat transfer fluid engineered by uniform and stable suspension of nanometer-sized particles into liquids. Nanofluids show fantastic heat transfer properties that are much better than those of conventional fluids due to the small size effect of nanoparticles.In this paper, starting from the study of water-based nanofluids, an equilibrium molecular dynamics simulation was used to compute thermal conductivity of some ionic liquid-based nanofluids. The main research works are summarized as follows:In the first part, the density, shear viscosity, self-diffusion coefficient and thermal conductivity of water are evaluated for four rigid water models with equilibrium molecular dynamics simulation. Then the thermal conductivity of copper-water nanofluids with SPC/E water model as base fluid are computed. The results showed that the addition of copper nanoparticles into the ionic liquids can enhance its thermal conductivity.In the second part, the stable geometries, vibration frequency, energy and atomic charge of the ionic liquid 1-ethyl-3-methylimidazolium dimethyl phosphate ([EMIM] [DMP]) have been investigated by performing quantitative calculations. Then the force field is established through mixing and matching Amber and OPLS force field and the density, self-diffusion and thermal conductivity of the ionic liquid is evaluated to verify the accuracy of force field. Lastly the thermal conductivity of the base fluid and nanofluids with various mass fractions of nanoparticle loadings and different materials were computed using the Green-Kubo method. The results showed that the addition of nanoparticles into the ionic liquids can obviously enhance their thermal conductivity.In the third part, the density, self-diffusion coefficient and thermal conductivity of the ionic liquid aqueous solutions with different mole fraction of ionic liquid are computed. After that, we also simulate the thermal conductivity of ionic liquid aqueous solutions nanofluids and compare it with the base liquid.In the fourth part, we also investigated the enhanced mechanisms of thermal transport of nanofluids at the atomic level by analyzing the motion of particles, microscopic structure and energy contributions of each of correlation functions to heat flux. The results indicate that the virial self-correlation term has a maximum positive effect on the thermal transport enhancement of nanofluids.