共查询到19条相似文献,搜索用时 578 毫秒
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使用分子动力学方法,研究过热水系统均质沸腾核化过程.采用Langevin 动力学方法控制体积可变系统的温度与压力,更好地模拟了沸腾实际物理过程.得到了液相系统体积连续膨胀、分子间距逐渐增大,最终稳定在汽相的现象学规律.当过热温度较高时,亚稳态液相系统可能在局部形成不同大小的近球形区域:气核,这些气核是不稳定的,处于不断演化之中.通过分析分子所受引力与斥力的共同作用,得到了气核形成与消亡以及多个气核融合的机理.比较了模拟结果与经典沸腾理论的差异,提出了气核生长是比气泡生长更为微观过程的认识.通过研究不同过热
关键词:
过热水系统
分子动力学
气核
临界过热度 相似文献
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针对水在垂直矩形通道内的流动沸腾,对空穴核化的机理进行了实验研究.不同表面物性的沸腾对比发现良好湿润性表面成核更为持续稳定.空穴成核过程中形成的低过热区域超出了气泡直径大小,影响因数在1.3~1.8范围内变化.微液膜蒸发模型分析液膜厚度在活化核心处最小,而热流密度刚好相反.流动条件加强了气泡脱离运动,致使主流对流冷却作用增强,影响范围超出了气泡直径区域.核心间的相互作用导致核心状态出现间断性,同时主流对流冷却也是重要原因. 相似文献
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采用分子动力学方法模拟纳米尺度下液体在固体壁面上发生核化沸腾的过程,主要研究壁面浸润性对气泡初始核化过程和气泡生长速率的影响以及固-液界面效应在液体核化沸腾的能量传递过程中所起到的作用.研究结果发现:壁面浸润性越强,气泡在固壁处越容易核化.该结果与经典核化理论中“疏水壁面易于产生气泡”的现象产生了明显的区别.其根本原因是在纳米尺度下,固-液界面热阻效应不能被忽略.一方面,在相同的壁温下,通过增强固-液相互作用,可以显著降低界面热阻,使得热量传递效率提高,导致靠近壁面处的流体温度升高,气泡核化等待时间缩短,有利于液体沸腾核化.另一方面,气泡的生长速率随着壁面浸润性的增强而明显升高.当气泡体积生长到一定程度时,会在壁面处形成气膜,从而导致壁面传热性能恶化.因此,通过壁面的热流密度呈现出先增大后减小的规律. 相似文献
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《工程热物理学报》2016,(7)
多孔表面在强化沸腾领域有重要应用。本文制备了系列孔径相近但厚度不同的铜基微纳双尺度多孔表面,这些样品表面上都有一系列直径约为130μm的微孔,而孔壁上则是纳米(亚微米)孔隙。以纯水为工质的池沸腾实验显示,当热流密度较低时,存在最优厚度使得沸腾换热性能最佳;样品CHF随着厚度增加而增加。双尺度多孔表面有着区别于一般的多孔表面的重要特性,当壁面过热度较低时,只有大孔可以形成活化中心;但当壁面过热度到达一定温度后,其孔壁上的纳米尺度(亚微米)尺度结构形成大量的活化中心,其壁面过热度几乎不再随着热流密度的上升而上升。随着厚度的增加,其壁面可以形成的活化穴直径在减小,造成活化所需要的壁面过热度升高。 相似文献
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根据实验所得沸腾曲线,对纳米颗粒悬浮液进行稳态数值模拟,计算了不同过热度下活化核心的密度.计算结果表明一对于不同浓度的纳米颗粒悬浮液,在考虑了其物性变化对沸腾传热的影响外,颗粒的加入对活化核心密度产生的影响是主要的因素,并且影响效果随颗粒浓度的变化不呈单向趋势. 相似文献
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The formation of nuclei from a supersaturated vapor on a surface with active centers is treated within standard self-consistent classical model with exhaustion of active centers. Basic characteristics of nucleation process (total number of nuclei, nucleation rate, time lag and size distribution of nuclei) are determined by numerical solution of kinetic equations. It is shown that standard approach to nucleation on active centers based on Avrami model coincides with our approach in the values of time lag of nucleation process, but it differs in the total number of nuclei. 相似文献
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Water with small volume (a few microlitres or less) often maintains its liquid state even at temperatures much lower than 0 °C. In this study, we examine the onset of ice nucleation in micro-sized water droplets with immersed solid particles under weak ultrasonic vibrations. The experimental results show that ice nucleation inside the water droplets can be successfully induced at relatively high temperatures. The experimental observations indicate that the nucleation sites are commonly encountered in the region between the particle and the substrate. A numerical study is conducted to gain insight into the possible underlying phenomenon for ice nucleation in such systems. The simulation results show that the collapse of cavitation bubbles in the crevice at the particle surface is structure sensitive with the hemisphere-shape crevice generating pressures as high as 1.63 GPa, which is theoretically suitable for inducing ice nucleation. 相似文献
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ABSTRACTThe rate of nucleation processes such as the freezing of a supercooled liquid or the condensation of supersaturated vapour is mainly determined by the height of the nucleation barrier and the diffusion coefficient for the motion across it. Here, we use a Bayesian inference algorithm for Markovian dynamics to extract simultaneously the free energy profile and the diffusion coefficient in the nucleation barrier region from short molecular dynamics trajectories. The specific example we study is the nucleation of vapour bubbles in liquid water under strongly negative pressures, for which we use the volume of the largest bubble as a reaction coordinate. Particular attention is paid to the effects of discretisation, the implementation of appropriate boundary conditions and the optimal selection of parameters. We find that the diffusivity is a linear function of the bubble volume over wide ranges of volumes and pressures, and is mainly determined by the viscosity of the liquid, as expected from the Rayleigh–Plesset theory for macroscopic bubble dynamics. The method is generally applicable to nucleation processes and yields important quantities for the estimation of nucleation rates in classical nucleation theory. 相似文献
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Pre-nucleation relaxation phenomenon in electric field induced nucleation in liquids has been observed at subcritical fields. The relaxation times τ in the processes of the application of electric field and induced nucleation have been measured. τ is found to be function of the degree of superheat of the liquid. 相似文献