【作者】 王艳；

【导师】 徐进良；

【作者基本信息】 华北电力大学(北京) ， 动力工程及工程热物理， 2022， 博士

【摘要】 超临界流体(Supercritical fluid,SF)广泛存在于自然界,并作为多种动力循环及废水处理等的工作介质。传统热学理论将SF处理成无相变、无相界面的单相流体,难以精确描述SF能量传递转换。20世纪60-70年代,基于超临界传热和亚临界沸腾传热均可能出现壁温飞升的现象,提出了超临界类沸腾的概念,但直到如今,国际上并未给出超临界类沸腾的内涵和外延。因此,本文在超临界多相流框架下,研究了非受限空间和受限空间内SF相分布、类沸腾转换温度、传热特性等,在分子尺度下划分了类液、类两相和类气三个区域,重点探索了类两相区的特性及规律。本文的主要研究内容和创新点描述如下:近临界氩密度时空分布和时序曲线的非线性分析。目前纳米尺度的研究揭示了 SF具有异质结构特性。现有SF密度波动的研究主要集中在特定工况的定性分析,本文基于分子动力学模拟方法对临界温度Tc,密度在0.5ρc-1.4ρc(ρc为临界密度)范围内的近临界氩的密度波动进行定量分析,研究发现,随着平均密度增加,局部密度时序曲线的均方根偏差呈M型分布,在0.8ρc的工况偏差最大。在势函数φ(r)的作用范围内,原子间引力和斥力的作用会影响原子的排布,称为“势能诱导”机制;统计物理学指出涨落是一切物质的基本属性,涨落会产生微观的不均匀性,在临界区域内尤为明显,被称为“临界波动”机制。研究结果表明在上述两种机制的共同作用下,均方根偏差最大的工况出现在临界点以下(0.8ρc)。此外,通过非线性动力学分析,发现不同模拟工况呈现出混沌或随机特性。研究结果大大加深了对近临界流体的认识,为SF的实际应用提供理论基础。研究了超临界氩的类沸腾转换温度,发现了纳米尺度的类气泡。采用分子动力学模拟研究了非受限空间SF的相分布特征,模拟过程中控制体系的温度和压力,六面均使用周期性边界条件。亚临界沸腾发生在对应压力下的饱和温度,本文发现超临界类沸腾发生在一个温度区间内,将类沸腾起始温度和终止温度分别定义为Ts和Te,发现按近邻原子数、径向分布函数和二体过剩熵三种方法确定的Ts和Te相互吻合。进而,采用Ts和Te将超临界温压相图划分为类液、类两相和类气三个区域。在类两相区,发现纳米尺度类气泡结构,具有以下特征:(1)类气泡内的粒子分布稀疏,呈现出类气体属性。(2)类气泡具有明显的弯曲界面。引入非线性动力学分析方法,表明类两相区呈现混沌特性,与亚临界压力下的两相流类似。本文研究揭示了 SF具有显著的多相流特征,为建立超临界多相流理论奠定了基础。超临界水密度时空分布、相分布及氢键结构。孤立水分子的性质通过实验和计算已得到很好理解。水是由大量水分子组成,目前对其结构和动力学的理解比较有限。超临界水的密度波动、相分布和氢键结构等方面还存在很多未知的领域。本文基于刚性的SPC/E水模型,对超临界水的密度波动和相分布规律进行研究。发现随着温度和密度的降低,密度时序曲线的均方根偏差呈下降趋势,与氩流体M型的演化规律不同。根据近邻分子数和径向分布函数法建立了超临界水的三区模型。此外,揭示了超临界水氢键与密度、温度和压力之间的关系,在演化过程中会出现一个或多个氢键的断裂,断裂的氢键又会与新的分子形成新的氢键,或者以孤立分子存在,超临界水中氢键的断裂和重组主导了局部密度的波动,也是超临界水与氩密度波动规律存在差异的主要原因。相关研究加深了对超临界水结构的认识,证明SF广义的多相特征。受限纳米通道内SF的相分布和传热特性。随着纳米流体器件的不断发展,纳米尺度流-固界面附近的物理现象变得非常重要。页岩气在满足经济发展的同时减少了 CO2排放,SF压裂技术是纳米结构中提高页岩气采收率、碳捕集和储存的有效方法。因此,本文对受限空间内SF相分布和传热特性展开研究。主要考虑壁面润湿性,壁面温度和流体初始状态对相分布的影响。在壁面润湿性的作用下,不同初始状态的流体会自组织形成新的相分布,强润湿性壁面的近壁区会形成类液层,与亚临界压力下的环状流类似,弱润湿性壁面的近壁区会形成类气层,与亚临界压力下的Leidenfrost现象类似。受壁面温度的影响,在冷壁面处形成比热壁面厚(薄)的类液层(类气层)。SF的传热特性具有以下规律:(1)流-固界面处的温度阶跃和热阻长度随壁面润湿性的减弱而增大,与亚临界压力下的结论相反;(2)热流密度随壁面润湿性的减弱和初始系统密度的减小而减小,在弱壁面润湿性下,不同初始状态之间的差异减小。此外,不同模拟工况下,流体相分布的演化过程满足最小能量原理。研究揭示了受限空间内SF相分布和传热特性,为SF设备的设计和应用提供理论支撑。更多还原

【Abstract】 Supercritical fluids(SF)widely exists in nature and is used as a working medium for various power cycles and wastewater treatment,etc.The traditional thermal theory treats SF as a single-phase fluid without bubbles and interfacial phenomenon,and it is difficult to accurately describe the energy transfer and conversion of SF.In the 1960s to 1970s,the concept of supercritical pseudo-boiling was put forward based on the phenomenon that both supercritical and subcritical boiling heat transfer may have wall temperature peaks occur.But until now,the connotation and extension of supercritical pseudo-boiling have not been given internationally.This paper studies the SF phase distribution,pseudo-boiling transition temperature and heat transfer characteristics in unconfined and confined spaces.Three regimes of liquid-like,two-phase-like and gas-like are divided at the nanoscale,and the characteristics and laws of the two-phase-like region are explored.The main contents and innovations of this paper are described as follows:The density spatiotemporal distribution and nonlinear analysis of time series curves.At present,molecular-scale studies reveal that SF possesses heterostructure properties.In view of the existing research on SF density fluctuations mainly focused on the qualitative analysis of specific working conditions,the molecular dynamics simulation method is used to quantitatively analyze the density fluctuations,covering the ranges of densities 0.5ρc-1.4ρc(where pc is the critical density)at the critical temperature Tc.The deviations display M-shape distribution with increases of average densities,and the maximum deviation occurs at ρave=0.8ρc.In the range of the potential function φ(r),the potential energy and interaction force between two atoms depends on the interatomic distance and affects the arrangement of particles,which is called the "potential induction" mechanism.Statistical physics points out that fluctuation is the basic property of all matter,and fluctuations produce microscopic inhomogeneity,especially in the critical region called the "critical fluctuation" mechanism.Under the combined action of the two mechanisms,the strongest heterogeneity appears not at the critical point,but below the critical point atρave=0.8ρc.In addition,according to nonlinear dynamic analysis,it is found that different simulation conditions present chaos or random characteristics.The research results significantly deepen the fundamental understanding of SF and provide a guide theoretically for SF’s practical application.Pseudo-boiling transition temperature and nanoscale bubblelike in supercritical argon.The phase distribution characteristics of SF in unconfined space are investigated by molecular dynamics simulation.Pressure and temperature are well controlled and with periodic boundary conditions applied in all the box surfaces.Subcritical boiling takes place at a saturation temperature corresponding to subcritical pressure.This paper finds that supercritical pseudo-boiling occurs in a temperature range.An onset pseudo-boiling temperature and a termination pseudo-boiling temperature are defined as Ts and Te,respectively.We determine Ts and Te using three different approaches of neighboring molecules method,the radial distribution function method and the two-body excess entropy method,and find consistent outcomes.The two transition temperatures divide the whole phase diagram into three regimes of liquid-like,two-phase-like,and gas-like.In the two-phase-like regime,nano bubblelike are observed to have two distinct characteristics:(1)The particle distribution in the bubblelike is sparse,showing gas-like properties.(2)Bubblelike has an obvious curved interface.Nonlinear dynamics demonstrate chaotic behavior in the two-phase-like regime,similar to the two-phase regime in the subcritical domain.This study reveals that SF has significant multiphase flow characteristics,laying a foundation for establishing supercritical multiphase flow theory.The spatiotemporal density,phase distribution and hydrogen bond structure for supercritical water.Experiments and quantum calculations well understand the properties of a single water molecule,but a fluid containing a large number of water molecules,and its structure and dynamics are not understood yet.There is remains largely unknown territory in density fluctuation,phase distribution,and hydrogen bond structure of supercritical water.The density fluctuation and phase distribution of fluid containing hydrogen bonds are investigated based on the rigid SPC/E water model.Quantitative analysis of system density fluctuations is performed by square root error and maximum structure factor.The results show that the increase of average densities and temperatures suppresses the local density oscillation,different from the M-type evolution law of supercritical argon.The three-regime-model of supercritical water is established on the neighboring molecules method and the radial distribution function method.The relationship between hydrogen bonds and density,temperature and pressure is revealed.During the evolution process,one and more hydrogen bonds will be broken,and the broken hydrogen bonds will form new hydrogen bonds with new molecules or exist as isolated molecules.Hydrogen bonds are continuously broken up and re-organize in supercritical water dominate the local density fluctuation,which is also the main reason for the difference between the density fluctuation of supercritical water and argon.Our work deepens the fundamental understanding of supercritical water and demonstrates the generalized multiphase character of SF.The phase distribution and heat transfer characteristics of SF in confined space.With the rapid development of nano-device,the physical phenomena near the nanoscale fluid-solid interface have become very important.Shale gas reduces CO2 emissions while satisfying economic development,and SF fracturing technology effectively enhances shale gas recovery,carbon capture,and storage in nanostructures.Therefore,the SF phase distribution and heat transfer characteristics in confined space are investigated in this paper.The effects of wall wettability,wall temperature,and fluid’s initial state on phase distribution are considered.Under the action of wall wettability,the fluids in different initial states will self-organize to form new phase distribution.A liquid-like layer will form on the strong wettability wall surface,similar to the annular flow under subcritical pressure.At the same time,a gas-like layer is formed on the weakly wetting wall surface,similar to the Leidenfrost phenomenon under subcritical pressure.Affected by the temperature of the wall,a thicker(thinner)liquid-like(gas-like)layer is formed on the cold wall than hot wall.The teat transfer characteristics of SF have the following rules:(1)The temperature jump and thermal resistance length at the fluid-solid interface increase with the weakening of the wall wettability,contrary to the conclusions at subcritical pressure.(2)The heat flux decreases with the decrease of the wall wettability and the initial system density,and the difference between different initial states decreases at weak wall wettability.In addition,different simulation processes follow the principle of minimum energy.The evolution process of fluid phase distribution satisfies the minimum energy principle under different simulation conditions.This work aims to reveal the SF phase distribution and heat transfer characteristics in confined space and provide theoretical support for the design and application of SF equipment.更多还原