摘要
超临界流体在工业技术领域得到越来越广泛的应用,从本质上研究超临界流体的结构特性具有重要意义.本文采用分子动力学方法模拟得到不同温度和压力下超临界流体局部密度波动的时序曲线变化规律,并进行非线性特性分析.结果表明,在近临界温度时,密度波动幅度在拟临界点处达到最大值,随着温度升高和偏离拟临界点程度的增大,波动幅度逐渐减小.超临界流体是具有显著密度波动的非均匀介质,可以看作是高密度(类液区)和低密度(类气区)流体的混合物,非均匀性随偏离拟临界点程度的增加而逐渐减弱.采用自相关函数法得到各工况延迟时间,结合时序曲线的吸引子相图和递归图定性分析发现,不同系统温度下,时序曲线的波动规律主要满足混沌和类随机两种特性.在近临界等温线上,较大密度范围内具有混沌特征;随着系统温度的升高,范围逐渐缩小,当T=1.075T_c时,混沌特征消失,各工况均符合类随机规律.混沌系统表现为较类随机系统更小的样本熵,在不同系统温度下,均在偏离拟临界点较远处得到较大值,并在一个小区域内收敛,此时样本熵的变化受系统温度的影响逐渐减小,密度起主导作用.
With the rapid development of supercritical fluid, it has been widely applied to various industrial technology fields such as supercritical fluid extraction, thermal and nuclear power generation systems, air conditioning and refrigeration, and solar power generation because of its unique physical and transport properties. At present, some studies have found that the speed of sound sharply changes near the supercritical point and the supercritical region can be divided into "liquid-like"and "gas-like" regions, and the concept of "pseudo-boiling" was proposed. However, the structural characteristics of supercritical fluids in different regions and their evolution laws versus time are rarely available in the literature. Therefore,in this paper, the density fluctuations in the "liquid-like" and "gas-like" regions of a supercritical fluid at different isotherms and densities have been investigated and analyzed by molecular dynamic simulation.The results show that near the critical temperature, the density time series curve has a large fluctuation at the "pseudocritical" point. As the system temperature and the deviation from the "pseudo-critical" point increase, the density fluctuation gradually decreases. In addition, with the density increase or decrease, the difference of density fluctuation between different isotherms decreases. This can indicate that the degree of density deviation from the "pseudo-critical"point is the main reason for density fluctuations. Moreover, supercritical fluid is a type of a heterogeneous medium with considerable density fluctuation and can be regarded as a mixture of high density("liquid-like") and low density("gaslike") fluids;therefore, the heterogeneous character declines with temperature, and the deviation from the "pseudo-critical"point increases.In this paper, nonlinear analysis has been based on the time series of density and the autocorrelation function, attractor phase diagram, and recurrence plots have been mainly used to conduct it. The studies confirm that the density fluct
作者
王艳
徐进良
李文
刘欢
Yan Wang;Jinliang Xu;Wen Li;Huan Liu(Beijing Key Laboratory of Multiphase Flow and Heat Transfer for Low Grade Energy Utilization,North China Electric Power University,Beijing 102206,China;Key Laboratory of Power Station Energy Transfer Conversion and System,Ministry of Education,North China Electric Power University,Beijing 102206,China)
出处
《科学通报》
EI
CAS
CSCD
北大核心
2020年第17期1694-1704,共11页
Chinese Science Bulletin
基金
国家重点研发计划(2017YFB0601801)
国家自然科学基金(51821004)资助。
关键词
超临界流体
分子动力学
密度波动
非线性
supercritical fluids
molecular dynamics
density fluctuation
non-linear
