Mechanism Of Momentum And Heat Transfer Enhancement In Nanofluids By Molecular Dynamics Simulation

Nanofluids, by adding a certain proportion of nanoparticles into conventional working fluids, present significantly increased thermal conductivity, enhanced heat transfer performance, and meanwhile the flow resistance does not show significantly increases. By applying nanofluids, developing a new generation of enhanced heat transfer technology has an important practical significance. The macroscopic enhanced heat transfer performance of nanofluids is based on the microscopic strengthening mechanisms by nanoparticles, while macroscopic experimental and numerical simulation methods could not effectively reveal these microscopic mechanisms, which makes the heat transfer theory of nanofluids still needs to be determined. Therefore, by using Molecular Dynamics method, this dissertation is intended to reveal the microscopic strengthening mechanism of heat transfer in nanofluids. Thus, this research subject has an important theoretical research value.This dissertation attempts to reveal the strengthening mechanisms of heat transfer in nanofluids from two aspects, including:heat conduction and flow. Through Molecular Dynamics simulations, it is confirmed the absorption layer and micro-motions of nanoparticles are to be responsible for the heat conduction enhancement. By analyzing microscopic structure of nanofluids with radial distribution function, nanofluids are found to have a special microscopic structure similar to that of solid. Thus, the changed microscopic structure of nanofluids is suggested to be a strengthening mechanism in heat conduction as well. With consideration of these static and dynamic strengthening mechanisms, a renovated Jeffrey model for predicting thermal conductivity of nanofluids has been proposed, and the prediction results have been validated by experiments. Through statistical analysis for atomic potential energy distributions in nanoparticles, the ratio of energetic atoms in nanoparticles has been suggested to be a criterion for examining the ability of nanoparticles for the thermal conductivity enhancement. The rotating fluid element has been introduced, which is composed of one nanoparticle as the core, absorption layer, rotating fluid, and limited existence volume. Furthermore, the dynamics microscopic structure of nanofluids is analyzed. Enhanced heat conduction, strengthened internal blending, and changed heat transfer mode are proposed to be the mechanisms of convective heat transfer enhancement. Both the velocity gradient and temperature gradient are found to be increased. Meanwhile, the influencing laws for heat conduction and flow characteristics of nanofluids are summarized.This dissertation has investigated the mechanisms for enhanced thermal transport in nanofluids. By means of Molecular Dynamics simulation, the microscopic strengthening mechanisms in heat conduction and flow are investigated. Based on the mechanisms, a revised prediction model for thermal conductivity has been proposed. And the ratio of energetic atoms in nanoparticles has been suggested to be the criterion for examining the ability of nanoparticles for the thermal conductivity enhancement. The influencing laws for heat conduction and flow characteristics of nanofluids have been summarized. Both velocity gradient and temperature gradient of nanofluids near wall have been found to be increased.