Molecular Dynamics Simulation Of Bubble Nucleation And The Effect Of Nanoparaticle On Nucleation

Bubble nucleation is a basic phenomenon, which widely exists in natural scienceand engineering applications. Because the bubble nucleation process is hard to bedirectly observed in experiment, the mechanism of bubble nucleation is not fullyunderstood until now. This dissertation investigates the microscopic process of bubblenucleation by using molecular dynamics method. The effect of nanoparticle on bubblenucleation and boiling heat transfer is also studied, in order to analyze why thecritical heat flux of pool boiling heat transfer is significantly increased by a smalladdition of nanoparticles.Due to the limitation of computer ability, previous molecular dynamicssimulation only simulated the primary stage of bubble nucleation. This dissertationcarries out parallel molecular dynamics simulation to trace bubble nucleation in asystem containing 106 argon atoms. The simulation captures a complete bubblenucleation process which evolves from a small cavity to a bubble including a numberof vapor molecules inside. The results show that bubble is formed from a gradualgrowth of the low-density embryo caused by the density fluctuation in thesuperheated liquid. It coincides with the assumption of the bubble nucleationmechanism in classical nucleation theory. The bubble nucleation picture raised byKwak's molecular interaction model, which assumes that high energy moleculesaggregate to form cluster and the cluster expands to form bubble when the clusterreaches its instability, is not observed in current simulations. In addition, the bubblenucleation process is quantitatively analyzed. The results show that the shape ofsimulated critical vapor embryo is close to sphere and the radius of the critical vaporembryo qualitatively agrees with the prediction of classical nucleation theory, despitethat the crtical vapor embryo in our simulation is only in nanometer.The effect of nanoparticle on bubble nucleation process is also investigated bymolecular dynamics simulations and theoretical analyses. The results show that theeffect of the nanoparticle on the bubble nucleation increases with the increase of thenanoparticle's size and the hydrophobicity of the particle's surface. If the radius of thenanoparticle is less than half of the critical embryo size, the effect of naonparticle onbubble nucleation is not significant. For the pool boiling of nanofluids based on water,the nanoparticles' size is much less than the size of the critical vapor embryo, So thenanoparticles can not be new nucleation sites in the liquid.The pool boiling of SiO2 -water nanofluid is experimentally studied. Similar toprevieous experiments, the results show that the critical heat flux of pool boiling heattransfer is remarkably increased by a small addition of nanoparticles. The high -speed photograph of the boiling process and the TEM analysis of the heated surfaceshow that the nanoparticles deposite on the heat surface and form microstructure,which increases the density of the nucleation sites on the heated surface and is adominating contribution to the remarkable increase of critical heat flux.