【作者】 王金照；

【导师】 过增元； 陈民；

【作者基本信息】 清华大学 ， 动力工程及工程热物理， 2005， 博士

【摘要】 汽泡成核是自然界和工程领域中广泛存在的一类基本物理现象。由于难以对汽泡成核过程进行直接的实验观测,到目前为止汽泡成核机理的认识中仍存在一些有争议的问题。本文采用分子动力学方法模拟了汽泡成核过程,从分子水平上观察和分析了汽泡成核的过程和机理。同时,还研究了纳米颗粒对汽泡成核及沸腾换热的影响,并从成核的角度来分析实验中观察到的纳米颗粒显著提高池沸腾临界热流密度的原因。以往汽泡成核过程的分子模拟因受计算能力的限制,只能得到汽泡成核过程的前期阶段。本文采用分子动力学并行计算,模拟了分子数为106的氩流体汽泡成核,得到了一个比较完整的汽泡成核过程。模拟结果表明汽泡是由过热液体中密度涨落形成的低密度汽核逐渐生长得到的,这一模拟结果与经典成核理论关于汽泡成核机理的假设一致。Kwak分子相互作用模型所假设的高能量分子聚集形成团簇,团簇达到不稳定状态后自发膨胀形成汽泡的成核机制在本文模拟中没有观察到。另外,对成核过程的定量分析表明在本文模拟中虽然临界汽核尺寸在纳米量级,但其形状仍接近于球形,其半径仍可由宏观的经典成核理论近似描述。通过分子动力学模拟和理论分析研究了纳米颗粒对流体内部汽泡成核的影响。结果表明纳米颗粒尺寸越大、表面厌水性越强,颗粒对汽泡成核的影响越大。当纳米颗粒的半径不到均质成核临界核半径的1 2时,纳米颗粒对汽泡成核的影响已不明显。在常压下以水为母液的纳米流体池沸腾中,纳米颗粒的尺寸远小于临界核的尺寸,颗粒不能在流体内部成为新的核化点。论文还对二氧化硅—水纳米流体的池沸腾换热进行了实验研究,和已有的纳米流体沸腾换热实验一样,发现加入少量的纳米颗粒可以显著提高流体沸腾换热的临界热流密度。为了分析其原因,本文对沸腾过程和加热表面进行了高速摄影和电镜分析,结果表明尽管纳米颗粒不能在流体内部成为新的核化点,但纳米颗粒沉积在加热丝表面上形成了微结构,提高了加热丝上的核化点密度,并对临界热流密度的提高起主要作用。更多还原

【Abstract】 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.更多还原