超临界流体物性畸变特性的多尺度研究

Multi-scale Study on Property Distortion Mechanism of Supercritical Fluid

为填补超临界流体物性畸变特性的理论空白,本文分别从微观尺度的分子动力学模拟、介观尺度的平均场理论和标度理论及宏观尺度的物性测量实验等方面展开研究。应用分子动力学模拟方法得到了跨越拟临界点时二氧化碳体系的微观结构特性,通过分析双体分布函数等结构参数的变化规律得到了物性畸变的分子动力学机理。基于朗道二级相变理论和标度理论,提出了一种考虑外场的气液相变模型,定量预测了超临界流体在拟临界区附近和趋于临界点时密度、定压比热等热力学量的畸变规律。在宏观尺度方面,本文开展了超临界二氧化碳密度、定压比热、黏度的高精度测量实验,指出了现有物性计算模型的潜在缺陷。研究结果表明,对于不同的超临界流体,临界点和拟临界区附近的物性畸变特性存在普适规律。在相图上,超临界流体区可进一步划分为4个区域:临界点邻域、类气区、类液区和拟临界区。不同区域内,超临界流体特性由不同尺度下的影响机制所主导。

To fill the theoretical gap of the property distortion mechanism of supercritical fluids,the molecular dynamics simulation at the micro scale,mean field theory and scale theory at the mesoscopic scale,and property measurement experiments at the macro scale were analyzed.The molecular dynamics simulation was used to obtain the microstructure characteristics of carbon dioxide system when crossing pseudo-critical points,and the molecular dynamics mechanism of property distortion was obtained by analyzing the variation law of structural parameters such as pair radial distribution function.The simulation result proves the existence of gas-like region,liquid-like region and the rapid transitions between them.Based on Landau theory and scale theory,a new model was proposed to quantitatively predict the distortion laws of thermodynamic quantities such as density and isobaric specific heat of supercritical fluids near the pseudo-critical region and approaching the critical point.Compared with the classical Landau theory,the new model is based on a redefined order parameterηand adds a first order term ofη.This term represents the effect of the external field on the thermodynamic potential,and also corresponds to the effect of temperature and pressure on the fluid system.At the macro scale,high-precision measurement experiments of density,isobaric specific heat and viscosity of supercritical carbon dioxide was carried out,and the potential defects of existing models were pointed out in this paper.The accuracy of the existing thermodynamic property calculation models of carbon dioxide are good outside the pseudo-critical region,but there are still defects inside pseudo-critical region or near the critical point.Moreover,the calculation accuracy needs to be verified by experiment and further improved.The results show that for different supercritical fluids,there is a general law of property distortion near the critical point and in the pseudo-critical region.On the phase diagram,the supercritical fluid region can be further divided into four regions:The vicinity of the critical point,gas-like region,liquid-like region and pseudo-critical region.In different regions,the characteristics of supercritical fluids are dominated by the laws at different scales.In the vicinity of the critical point,the fluctuation within the system plays a dominant role,which leads to the inapplicability of both mean field theory and continuum hypothesis.Therefore,the study of fluid in this region must start with more basic assumptions,based on molecular dynamics simulation or lattice Boltzmann method.However,the vicinity of the critical point is so small that the classical descriptions are valid in most conditions.