R32热力学性质的蒙特卡罗模拟

建立在统计热力学和分子力学理论基础上的分子模拟方法逐渐运用于计算制冷剂的热力学性质。文中首先在NVT系综条件下,采用吉布斯蒙特卡罗模拟方法(GEMC),模拟了R32的气液相平衡的密度、饱和蒸汽压及蒸发焓;其次在NPT系综条件下,采用蒙特卡罗模拟方法(MC),模拟了R32在4MPa条件下的过冷液态密度。模拟结果同美国国家标准研究所(NIST)的Refprop 8.0相比,有很好的一致性。结果表明,运用该方法预测单一制冷剂的热力学性质是可行的。     

分子模拟预测制冷剂热物性研究

分子模拟在预测制冷剂流体热物性方面的应用发展至今已有三种方法:分子动力学(MD)和蒙特卡罗(MC)和基于量子化学的COSMO-RS法。分子模拟方法计算物质的热物性仅依赖于物质的分子性质,因而可先于试验预测其热物性质,尤其可方便地解决混合物质的热物性计算,是预测制冷工质热物性的一种精确、有效的方法。本文综合论述了采用分子模拟方法对已知、未知制冷纯工质、混合工质的汽液相平衡时饱和蒸汽压、密度、焓等参数的模拟结果。模拟结果证明,分子模拟的结果与Refprop 8.0或实验值相比较,最大相对误差在20%以内,基本能满足工程需要,因此可以用来预测制冷剂流体的热物性。     

制冷剂替代物相平衡性质的分子动力学模拟

本文利用分子动力学模拟算法研究制冷剂替代物的气液相平衡性质。通过将制冷剂替代物作为极性的2CLJ流体处理,建立了针对它们的 2CLJD 势能模型。利用 NPT+Test Particle 算法对五种制冷剂替代物 (HFC-152a,HFC-143a,HFC-134a,HCFC-142b和HFC-227ea)的气液相平衡性质进行了计算,同时验证了计算结果的热力学一致性。模拟结果与 NIST 的 REFPROP 数据库的最大相对偏差在2％以内。     

R134a/R125混合物制冷剂性能研究

基于制冷剂的气液相平衡特性,建立了R134a/R125混合制冷剂二元体系模型,对其物性进行了分析。建立了蒸气压缩式制冷系统仿真模型,对混合制冷剂的循环工况和性能进行了对比研究。通过物性分析及性能对比,得到了不同比例混合制冷剂的饱和曲线和导热系数的变化趋势。对于R134a/R125非共沸混合制冷剂,在0.1 MPa压力下混合质量比为0.5/0.5时,温度滑移达到最大5.33℃。R134a/R125混合制冷剂的液相导热系数低于R134a,且随着温度的升高而减小;气相导热系数的变化趋势与液相相反。当蒸发温度较低时,制冷剂R134a在蒸发器中为负压,对系统不利,采用混合制冷剂R134a/R125(0.9/0.1)可改善这一状况,虽然COP略低于R134a系统,但可获得比R134a更低的制冷温度,且系统运行更稳定。研究结果为R134a/R125混合制冷剂的应用提供数据。     

二元混合制冷剂CO2/R600的气液相平衡研究

采用PR状态方程及其混合法则对不同配比的二元混合制冷剂CO2/R600的气液相平衡进行了计算,将计算结果与相关文献数据进行了比对。结果表明,该文的计算结果与文献数据吻合较好,推算方法可用于CO2/R600混合制冷剂的基本热物性预测。     

一种基团贡献法的非共沸工质气液相平衡模型

本文引入一种基于基团贡献的逸度系数模型来解决常规模型对非共沸混合制冷剂气液相平衡预测精度不足的问题,该模型只使用较少的共性基团参数就能推算大量多元混合物的物性.使用PR状态方程结合vdW混合规则来描述气相逸度、UNI-FAC模型来描述液相逸度.通过对R32+R236fa非共沸混合工质对的气液相平衡饱和蒸汽压力进行计算,并将结果与实验数据、REFPROP 8.0商业软件计算值对比分析.结果 表明:本文模型比REFPROPS.0软件更适应于混合工质;此外,该模型只考虑基团间相互作用,简化了热物性计算.基于提出的模型,绘制了R32+R236fa非共沸混合工质在278.15～363.15 K的相图.     

纳米流体中气体水合物生成条件

为了研究纳米粒子对制冷剂水合物热力学生成条件的影响,建立了新型纳米流体中制冷剂水合物相平衡热力学模型,理论研究了HFC134a(R134a)气体水合物的热力学生成条件,并用实验验证了模拟结果.结果表明:新型纳米流体中制冷剂水合物热力学相平衡模型能够较好地反映纳米流体中制冷剂气体水合物相平衡特征,相同温度下相平衡最大相对误差为7.3%,平均相对误差为5.3%.     

HFC-161混合物作为HCFC-22替代制冷剂的实验研究

本文提出了以HFC-161混合物作为HCFC-22的新型替代制冷剂(HFC-161/125/32,15/42.5/42.5wt%),并在设计使用R410A的压缩机性能测试台上,进行了该混合制冷剂与R410A在若干工况下的性能对比实验.实验结果表明,该新制冷剂的COP略高于R410A,而排气温度略低于R410A.与R410A相比,该新制冷剂的蒸发压力和冷凝压力均低约10%,制冷量和压缩机功耗分别下降4.3%和4.8%.该混合制冷剂物性环境性能良好,ODP为零,GWP值小于R410A.可作为HCFC-22的较为理想的替代制冷剂之一.     

R125/R600a二元混合物的热力学性质

基于制冷剂物性计算软件NIST REFPROP 7.0及R125/R600a气液相平衡实验数据,使用MBWR状态方程结合Lemmon-Jacobsen混合物模型,对R125/R600a(50/50 mass%)混合替代工质的热力学性质进行了计算,并制作给出了其压焓图(p-h)、温熵图(T-s)和比定压热容图(c<,p>-T)及饱和性质表.最后理论计算分析了其空调制冷工况的性能,结果表明该混合工质具有与R22相当的压缩比和COP值,比R22较低的排气温度和较高的单位质量制冷量.     

PR状态方程的二元混合制冷剂热力性质计算

基于PR状态方程法和剩余函数法建立了混合制冷剂在过冷区、两相区以及过热区的热力学参数计算模型,将混合制冷剂R410A的计算结果与NIST数据库进行比对以验证模型精度.在此基础上,对二元混合制冷剂CO2/R600a(33.546/66.454,mass％)在不同相区的热力性质进行了推算,其计算结果可以用于以CO2/R600a为制冷剂的制冷系统的热力计算分析和性能优化.     

A new simulation method for the determination of phase equilibria in mixtures in the grand canonical ensemble

A grand canonical Monte Carlo (GCMC) simulation method is presented for the determination of the phase equilibria of mixtures. The coexistence is derived by expanding the pressure into a Taylor series as a function of the temperature and the chemical potentials that are the independent intensive variables of the grand canonical ensemble. The coefficients of the Taylor series can be calculated from ensemble averages and fluctuation formulae that are obtained from GCMC simulations in both phases. The method is able to produce the equilibrium data in a certain domain of the (T, p) plane from two GCMC simulations. The vapour-liquid equilibrium results obtained for a Lennard-Jones mixture agree well with the corresponding Gibbs ensemble Monte Carlo data.     

A hybrid Monte Carlo/fluid model of RF plasmas in aSiH4/H2 mixture

A fluid model for simulating the capacitively coupled steady RF plasmas in a monosilane-hydrogen mixture under plasma-processing conditions is developed. This model includes Monte Carlo treatment for electron transport properties in the mixture. The Monte Carlo simulation shows that the electron transport phenomena in the nonuniform RF field of 10 MHz are not in equilibrium with the local electric field. The nonequilibrium transport properties are incorporated in the fluid model (hybrid model) by modifying the equilibrium values of the swarm parameters using data from the Monte Carlo simulation. Using this model, the spatiotemporal variations of the charged species and the electric field in the sheath region of the RF plasma are calculated. For obtaining the steady RF plasma structure, ion-induced slow processes such as recombination and diffusion of ions are calculated by combining the hybrid model with rate equations for ions. Using the calculated steady RF plasma structure, a preliminary calculation of the silyl (SiH ₃) and hydrogen (H) radical distributions caused by the generation, diffusion, and reaction of the radicals are carried out. The effect of sticking on the profile of the radical distribution is presented     

Vapour—liquid equilibrium of the square-well fluid of variable range via a hybrid simulation approach

The equilibrium between vapour and liquid in a square-well system has been determined by a hybrid simulation approach combining chemical potentials calculated via the Gibbs ensemble Monte Carlo technique with pressures calculated by the standard NVT Monte Carlo method. The phase equilibrium was determined from the thermodynamic conditions of equality of pressure and chemical potential between the two phases. The results of this hybrid approach were tested by independent NPT and μPT calculations and are shown to be of much higher accuracy than those of conventional GEMC simulations. The coexistence curves, vapour pressures and critical points were determined for SW systems of interaction ranges λ = 1.25, 1.5, 1.75 and 2. The new results show a systematic dependence on the range λ, in agreement with results from perturbation theory where previous work had shown more erratic behaviour.     

Thermodynamic and transport properties of nitrogen and butane mixtures

A force field has been developed to describe the phase behaviour, interfacial, and transport properties of nitrogen and hydrocarbon mixtures under conditions relevant to those found in the high pressure extraction of oil from underground reservoirs. A Gibbs ensemble Monte Carlo method is used to parametrize intermolecular potentials for the pure components by matching experimental and simulated liquid and vapour coexisting densities. Also the surface tension, diffusion coefficient and shear viscosity of nitrogen and its mixtures with butane have been determined. The latter properties were obtained by canonical molecular dynamics simulations. The diffusion coefficient and shear viscosity were calculated by a Green-Kubo method. Results for pure nitrogen are given for temperatures ranging from 70 K to 110K. For mixtures of nitrogen with butane, results are presented at 339.4 K and 380.2 K. Good agreement is found between the results of simulations and available experimental data.     

Demixing and field-induced population inversion in a mixture of neutral and dipolar-hard spheres confined in a slit pore

We study the influence of an external field on demixing and its interplay with the field-induced population inversion for a binary mixture of neutral and dipolar non-additive hard-spheres confined in a slit pore. Demixing lines are determined by Gibbs Ensemble Monte Carlo simulations and population inversion paths by grand-canonical/canonical simulations of the pore/bulk equilibrium. Besides the demixing line in the bulk, results are given for two different pore widths and in parallel and normal fields. Similar to the effect of geometrical confinement, a normal field is found to favour the mixed state so that the population inversion does not interfere with demixing. A parallel field leads to more complex scenarios.     

The O-H distance in ice

The thermodynamic properties of normal and para-hydrogen are computed from multiple time-step path integral hybrid Monte Carlo (PIHMC) simulations. Four different isotropic pair potentials are evaluated by comparing simulation results with experimental data. The Silvera–Goldman potential is found to be the most accurate of the potentials tested for computing the density and internal energy of fluid hydrogen. Using the Silvera–Goldman potential, simulation and experimental data are compared on isobars ranging from 0.1 to 100 MPa and for temperatures from 18 to 300 K. The Gibbs free energy is calculated from the PIHMC simulations by an adaptation of Widom's particle insertion technique to a path integral fluid. A new method is developed for computing phase equilibria for quantum fluids directly by combining PIHMC with the Gibbs ensemble technique. This Gibbs–PIHMC method is used to calculate the vapour–liquid phase diagram of hydrogen from simulations. Agreement with experimental data is good.     

自复叠制冷循环中混合工质泄漏性能的研究

尽管自复叠制冷循环越来越广泛的用于深冷及普冷领域,对于循环中工质泄漏特性的研究却较少。以非共沸混合工质两相区的等温泄漏为基础,分析了在自复叠循环装置中各部件的泄漏对工质组成的影响,其中发生在冷凝器出口处、气液分离器和蒸发器出口处的泄漏对工质组成影响较大。以采用非共沸混合工质R600 a/CO2的自复叠低温冷冻箱为例,研究了不同泄漏点不同泄漏率的泄漏特性。结果说明当工质泄漏时混合工质组成和循环性能发生变化,在蒸发器出口处的泄漏对循环性能影响最大;当泄漏引起循环工质组成发生较大变化时,压缩机变容量和变压力比调节能力明显降低,此时系统不能满足设计工况的要求,工作性能变差。     

A simple effective pair potential for the molecular simulation of the thermodynamic properties of ammonia

A new optimized effective pair potential model is proposed, which is appropriate for the prediction of thermodynamic properties of fluid ammonia including vapour—liquid coexistence data. The phase behaviour is determined using a recently developed version of the Gibbs ensemble Monte Carlo method. Furthermore, liquid structure characteristics, the dielectric constant and supercritical properties are determined by Monte Carlo simulations in the isothermal—isobaric ensemble. The second virial coefficient of the pair potential model is calculated over a broad range of temperature. All properties are compared with experimental data or results of a multi-parameter equation of state for ammonia. The new model is found to yield coexistence properties and second virial coefficients in good agreement with experimental data and the results of the equation of state, respectively.