超流氦多孔塞相分离器的研究综述

超流氦恒温器是远红外探测器冷却系统的主要设备,气液相分离器则是超流氦恒温器的关键部件,能有效完成超流氦的气液分离,防止泄漏并实现空间液体的综合管理。文中介绍了在空间红外探测中应用较多的多孔塞相分离器的工作原理,描述了多孔塞的流量特性曲线,并对曲线上各工作区域的质量流量与温差(压差)的关系式进行了介绍。文中还对多孔塞的制备工艺进行了分析,并结合国内外研究进展给出了合理选择多孔塞所需要注意的相关参数。     

汽液界面动力学行为与热力学性质的分子动力学研究

本文采用分子动力学方法研究了热平衡条件下的汽液界面的动力学行为和热力学性质。统计获得了界面区的密度、压力张量及温度的分布,并且从分子层次观察分析了界面结构和动力学特性。研究表明汽液界面是一个随时间起伏涨落的曲面,界面层的分子并不是处于液相和蒸汽相之间的一种过渡状态,从汽相到液相密度的连续变化是长时间的统计结果,汽渡过渡区的厚度与汽液界面区的密度涨落的范围是一致的。对于平衡条件下的汽液界面,由于汽液相变的影响,在紧贴界面处存在一个分子平均动能非平衡分布的区域。此非平衡区域的存在与汽液两相的宏观热平衡并不矛盾,但可能对蒸发／凝结流率的估计有不可忽略的影响。     

Direct experimental verification of the isomorphism hypothesis for critical phenomena

The heat capacity at constant volume of a mixture of methane and 0.0345 mole fractions of heptane is experimentally studied over a wide range of densities and temperatures. In the case when the transition from a three-phase state takes place in the presence of a noncritical liquid phase, it is found that the behavior of the heat capacity in the vicinity of the upper end critical point is fully isomorphic with the behavior of the heat capacity in the vicinity of the liquid-vapor critical point of one-component fluid. It is shown that the measured quantity in this experiment is the heat capacity at constant volume and constant chemical potential μ of the heavy impurity component C _{v, μ}. Thus, it has been confirmed by direct measurements that the anomaly of this heat capacity completely coincides in character with the anomaly of the heat capacity at constant volume in the vicinity of the liquid-vapor critical point of one-component fluids.     

A study of near-critical states of metals in shock-wave experiments

Methods for creating near-critical states of the liquid-vapor phase transition during shock wave impact and optical methods for determining the positions of the boundaries of the two-phase region and the parameters of the critical point of the liquid-vapor phase transition (temperature, pressure) are described.     

Phase transitions in hadronic systems and thermodynamics of a system with an exponential spectrum

Both nuclear matter and hadronic matter at high excitations can be described by a liquid-vapor phase transition. For the hadronic systems, a system with an exponential mass spectrum (Hagedorn-like or bag-like) leads to a thermodynamics which is identical to that of a two phase coexistence at a fixed temperature.     

汽液界面热毛细波的分子动力学模拟

用分子动力学模拟方法研究汽液界面性质,在NVT系综中,以氩原子为对象,对长方形模拟盒中汽液平衡系统进行不同温度下的模拟计算。计算结果表明,汽液界面存在热毛细波。界面上的热涨落将使界面变宽,它不仅与界面张力有关,还与温度、界面的横截面积有关。可以把汽液界面上热毛细波的均分根涨落值,作为汽液界面的宽度和粗糙度的指标。     

平板多孔芯冷凝器内工质相变换热的EOF方法

本文采用一种新的方法-流体能量(EOF)法,来研究平板多孔芯冷凝器内工质流动与相变换热特性,重点讨论了冷凝器内汽液相变界面的位置及其影响因素,对其进行了理论建模与数值计算,得出了蒸汽冷凝时所形成的汽液相变界面位置,并与现有的实验结果进行了对比,最后定性地分析了不同入口速度和冷却热流量下的汽液界面情况。     

Finite-size scaling and particle-size cutoff effects in phase-separating polydisperse fluids

We study the liquid-vapor phase behavior of a polydisperse fluid using grand canonical simulations and moment free energy calculations. The strongly nonlinear variation of the fractional volume of liquid across the coexistence region prevents naive extrapolation from detecting the cloud point. We describe a finite-size scaling method which, nevertheless, permits accurate determination of cloud points from simulations of a single system size. By varying a particle-size cutoff, we find that the cloud point density is highly sensitive to the presence of rare large particles; this could affect the reproducibility of experimentally measured phase behavior in colloids and polymers.     

Field-theoretical treatment of the liquid-vapor phase transition in nuclear systems

The liquid-vapor phase transition in hot nuclear matter is investigated in a field-theoretical approach employing euclidean-space (imaginary time) path-integral techniques. This approach allows us to study the nucleation due to both quantum and thermodynamic fluctuations. The bubbles of the new phase appear as instanton solutions of the euclidean-space field equations. The critical bubble sizes and associated transition probabilities are calculated. We examine the temperature and density values for which a phase transition may develop in hot nuclear matter produced in the course of a heavy-ion reaction.     

汽液界面厚度和分维数

本文应用分子动力学模拟方法进行了汽液界面厚度和分维数的模拟研究。用统计系综方法,以氩原子为对象,对长方形模拟盒中粒子数目为2048个的汽液平衡共存系统进行模拟计算。研究了一定条件下汽液界面厚度的取定对确定界面分维数的影响,根据计算模拟结果,提出了可用15~85法则确定界面厚度和分维数。汽液界面在X方向、Y方向和Z方向具有不同的分维数,充分说明了汽液界面的各性异性性质。     

Phase behaviour of <Emphasis Type="Italic">n</Emphasis>-hexane/perfluoro-<Emphasis Type="Italic">n</Emphasis>-hexane binary thin wetting films

We present X-ray reflectivity investigations of the concentration distribution in binary liquid thin films on silicon substrates. The liquid-vapor coexistence of the binary mixture investigated, hexane and perfluorohexane, is far from criticality. Therefore, a sharp interface separates the liquid film from the vapor. The data reveal a separation of the film in layers parallel to the substrate. A phase diagram is constructed as a projection to the (composition difference, temperature) space, covering a temperature range corresponding to the one-phase and the two-phase regime of the bulk liquid. Although the composition data indicate a mixing gap similar to that of the bulk system, there are two major differences: i) only the near-surface phase changes its composition significantly, and ii) a composition gradient in the film exists also at higher temperatures where in the bulk system the one-phase regime exists.Received: 28 April 2004, Published online: 21 September 2004PACS: 61.10.Kw X-ray reflectometry (surfaces, interfaces, films) - 64.75. + g Solubility, segregation, and mixing; phase separation - 68.15. + e Liquid thin films     

Nonuniform van der Waals theory

The liquid-vapor interface of a confined fluid at the condensation phase transition is studied in a combined hydrostatic/mean-field limit of classical statistical mechanics. Rigorous and numerical results are presented. The limit accounts for strongly repulsive short-range forces in terms of local thermodynamics. Weak attractive longer-range ones, like gravitational or van der Waals forces, contribute a self-consistent mean potential. Although the limit is fluctuationfree, the interface is not a sharp Gibbs interface, but its structure is resolved over the range of the attractive potential. For a fluid of hard balls with –r ^–6 interactions the traditional condensation phase transition with critical point is exhibited in the grand ensemble: A vapor state coexists with a liquid state. Both states are quasiuniform well inside the container, but wall-induced inhomogeneities show up close to the boundary of the container. The condensation phase transition of the grand ensemble bridges a region of negative total compressibility in the canonical ensemble which contains canonically stable proper liquid-vapor interface solutions. Embedded in this region is a new, strictly canonical phase transition between a quasiuniform vapor state and a small droplet with extended vapor atmosphere. This canonical transition, in turn, bridges a region of negative total specific heat in the microanonical ensemble. That region contains subcooled vapor states as well as superheated very small droplets which are microcanonically stable.     

多孔介质内两相流汽相输运分析

本文结合可视化实验技术对多孔球层内汽液两相流动作深入观察和分析。根据汽泡与多孔球层的相互作用,定义汽泡输运的临界半径并建立汽泡输运的动力学模型。在实验观察和合理假设的基础上,定量讨论了汽相流动由分散态到连续态的转变,确定了临界转变点。     

气泡成核过程的格子Boltzmann方法模拟

利用精确差分格子Boltzmann模型探讨水在特定温度下的亚稳态及不稳定平衡态, 获得等温相变过程中形成气泡和液滴的条件, 模型预测结果与理论解符合良好. 在该等温模型的基础上耦合能量方程, 通过调节流体-壁面相互作用力获得不同的气泡与固壁间接触角, 从而建立了一种新的描述气液相变的格子Boltzmann理论模型. 利用该新模型模拟不同流体-壁面相互作用力下凹坑气泡成核过程, 再现了气泡成核过程中的三阶段特性; 探讨了接触角、曲率半径及气泡体积随气泡成核过程的变化关系, 获得了与文献结果定性符合的曲率-气泡体积关系曲线. 关键词： 格子Boltzmann方法 气泡成核过程 气液相变 接触角     

1/f Noise in a nonequilibrium phase transition: Experiment and mathematical model

The results of an experimental investigation of a high-power source of broad-band 1/f noise, which can be generated in a system of two interacting nonequilibrium phase transitions, are presented. This process takes place when a normal conductor-superconductor phase transition is superposed on the critical liquid-vapor transition in a boiling coolant. A mathematical model describing a nonequilibrium phase transition in a complicated nonlinear system with two interacting order parameters, which involves the conversion of white noise into stochastic fluctuations of the order parameters with 1/f and 1/f ² spectra, is proposed. The properties of the model fluctuations with a 1/f spectrum agree qualitatively with the experimentally observed properties. A characteristic difference between the model fluctuations with a 1/f ² spectrum and random walks is also noted. Zh. éksp. Teor. Fiz. 113, 1748–1757 (May 1998)     

Generic Two-Phase Coexistence and Nonequilibrium Criticality in a Lattice Version of Schlögl’s Second Model for Autocatalysis

A two-dimensional atomistic realization of Schlögl’s second model for autocatalysis is implemented and studied on a square lattice as a prototypical nonequilibrium model with first-order transition. The model has no explicit symmetry and its phase transition can be viewed as the nonequilibrium counterpart of liquid-vapor phase separations. We show some familiar concepts from study of equilibrium systems need to be modified. Most importantly, phase coexistence can be a generic feature of the model, occurring over a finite region of the parameter space. The first-order transition becomes continuous as a temperature-like variable increases. The associated critical behavior is studied through Monte Carlo simulations and shown to be in the two-dimensional Ising universality class. However, some common expectations regarding finite-size corrections and fractal properties of geometric clusters for equilibrium systems seems to be inapplicable.     

Hot and cold,nuclear and neutron matter

We report variational calculations of the equation of state of hot and cold, nuclear and neutron matter. The calculations cover a wide density range of interest in heavy-ion collisions and astrophysics. The “hot” calculations are limited to temperatures less than 20 MeV. A realistic nuclear hamiltonian that contains two- and three-nucleon interactions and fits the nucléon-nucléon scattering, as well as nuclear matter data, is used. Neutron star structure calculations are reported and their sensitivity to the three-neutron interactipn is examined. The liquid-vapor phase equilibrium, as well as the behavior of the effective mass in nuclear matter is discussed.     

Simulation of pulsed-laser-induced explosive boiling

Bulk evaporation process in absorbing condensed matter irradiated with laser pulses was studied using the one-dimensional thermal model with additional interfaces between different phases. Within this approach, it was shown that the repeated explosive boiling mode can be achieved using nanosecond laser pulses, if the nucleation time is shorter than 0.1 ns. This mode can be observed if the surface pressure is lower than the critical pressure P _c of the liquid-vapor phase transition. The dependences of these processes on the laser pulse intensity and duration, as well as on evaporation kinetics were studied. Explosive boiling and spallation of a transparent liquid film on a pulse-heated absorbing target, as well as the photoacoustic signal in the target before the explosion, were considered.     

Langevin dynamics simulations of phase separation in a one-component system

Langevin dynamics computer simulations have been performed for a two-dimensional Lennard-Jones fluid quenched into the coexistence region of its liquid-vapor phase diagram. For late stages of the phase-separation process, the average radius of the liquid clusters is found to grow proportional to (time)^1/4. This growth law is analyzed theoretically and compared to recent molecular dynamics and Monte Carlo results. Details of the different simulation methods are critically discussed.     

Modeling of the EHD effects on hydrodynamics and heat transfer within a flat miniature heat pipe including axial capillary grooves

A numerical model for an electrohydrodynamic (EHD) grooved Flat Miniature Heat Pipe (FMHP) is developed. Two microchannel shapes are considered as axial capillary structures: square and triangle grooves. For both groove shapes, the electric field affects the liquid-vapor radius of curvature which decreases in the condenser and increases in the evaporator under the action of the electric field. The liquid and vapor velocities are also affected by the EHD effects. The electric field effects on the velocities depend on the FMHP zone. It is also demonstrated that the electric field increases the vapor pressure drop; however, it decreases the liquid pressure drop. The liquid-wall and vapor-wall viscous forces as well as the shear liquid-vapor forces are affected by the electric field. The analysis of the electric forces shows that the dielectrophoretic forces which act on the liquid-vapor interface are predominant and their order of magnitude is much higher than the Coulomb forces. Finally, it is also demonstrated that the capillary limit increases with the electric field for both groove shapes.