制冷剂R410a的气液相平衡的吉布斯蒙特卡罗模拟

制冷剂的气液相平衡性能,对制冷循环的研究至关重要。文中基于UFF(universal force field)力场,运用吉布斯蒙特卡罗方法模拟了混合制冷剂R410a的气液相平衡性质,模拟结果同Refprop8.0提供的数据有很好的一致性。并根据模拟结果得到了R410a的临界状态点的温度和密度,与Refprop8.0的相对误差分别在0.4%和3.3%。表明采用合理的分子力场,运用吉布斯蒙特卡罗模拟方法模拟混合制冷剂的气液相平衡是可行的。     

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%.     

R290/R134a在水平微肋管中沸腾传热的数值模拟

为探究混合制冷剂R290/R134a(4/6)在水平微肋管中的沸腾传热特性,采用了CFD软件数值模拟的方法,对混合制冷剂R290/R134a(4/6)分别在外径为7 mm,长为500 mm的水平光滑管和微肋管中进行数值模拟与理论分析。分析了质量流量、热流密度,以及干度对混合制冷剂在水平微肋管中换热特性的影响。结果表明：两种管型的沸腾换热系数随质量流量、热流密度和干度的增大出现先增大后减小的趋势;热流密度对制冷剂沸腾换热系数的影响最大,在质量流量保持不变,改变热流密度的条件下,微肋管最大传热系数分别为光滑管的1.30、1.31、1.26倍;质量流量的增加提高了制冷剂的临界干度,光滑管与微肋管最大临界干度分别为0.57、0.63。     

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。     

超高压下碳结构的第一性原理研究

采用第一性原理密度泛函理论,结合平面波赝势和广义梯度近似(GGA)方法计算了多种碳结构在超高压条件下的热力学稳定性及晶体结构性质.计算结果表明,碳结构的质量密度和基态能量与其对称性高低没有很强的依赖关系;在超高压条件下,随着压强的增加,热力学相对稳定的碳结构依次是石墨、金刚石、BC-8和SC碳结构,Clark等人所提出的R8结构并不是一种真正的高压稳定碳结构.     

相对湿度对空调中翅片管蒸发器性能的影响

基于换热器几何结构参数以及流路布置等相同的3HP商用空调,在空调工况下,研究了相对湿度对R22替代制冷剂R407C、R410A、R32以及R290在蒸发器内的流动和传热性能的影响规律。研究表明:在一定的条件下,增加环境相对湿度,这五种制冷剂对应蒸发器换热量、压降和质量流量均增加,其中R407C蒸发器换热量最大,R290和R32蒸发器压降和制冷剂质量流量均小于R22蒸发器。在低湿度环境下(约20—30%相对湿度),显热换热占主导因素。随着相对湿度的增加,显然换热减少,潜热换热增强且起主导因素。在满足换热量的要求下,R290在协调压降与质量流量具有一定的优势且满足现行工质替代理念,可以作为现有商用空调的环保型替代工质。     

R1234yf/RE170混合工质用于汽车空调的可行性

目前汽车空调常用的制冷剂四氟乙烷(R134a)存在较高的温室效应指数(GWP=1 300),会对环境造成不利的影响。将R1234yf和RE170以质量配比为70:30组成新型混合制冷剂(代号NCUR04)应用于汽车空调中,通过热力学性质、环境特性、循环性能、安全特性、润滑特性方面综合考虑替代R134a的可行性,并与潜在替代制冷剂R1234yf进行对比。研究表明:NCUR04环境性能、热力学性质较R134a和R1234yf优异;单位质量制冷量分别比R134a和R1234yf高24.7%和66.2%;单位容积制冷量比R134a低4.5%,比R1234yf高5.9%;能效比(COP)分别比R134a和R1234yf高1.4%和8.8%;排气温度比R134a低2.4℃,比R1234yf高5.5℃;与POE润滑油互溶性良好。因此,NCUR04替代R134a用于汽车空调具有可行性。     

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.     

双温度氩-氮等离子体热力学和输运性质计算

获得覆盖较宽温度和压力范围内的等离子体热力学和输运性质是开展等离子体传热和流动过程数值模拟的必要条件.本文通过联立Saha方程、道尔顿分压定律以及电荷准中性条件求解等离子体组分;采用理想气体动力学理论计算等离子体热力学性质;基于Chapman-Enskog方法求解等离子体输运性质.利用上述方法计算了压力为0.1, 1.0和10.0 atm (1 atm=101325 Pa),电子温度在300—30000 K范围内,非局域热力学平衡(电子温度不等于重粒子温度)条件下氩-氮等离子体的热力学和输运性质.结果表明压力和非平衡度会影响等离子体中各化学反应过程,从而对氩-氮等离子体的热力学及输运性质有较大的影响.在局域热力学平衡条件下,计算获得的氩-氮等离子体输运性质和文献报道的数据符合良好.     

R32/油混合物管内两相流动摩擦压降模型预测研究

R32具有很好的热物性,且GWP值相对较低,在制冷剂的替代进程中有潜力得到推广应用。在蒸汽压缩制冷系统中,润滑油难免随制冷剂混入换热部件,恶化制冷剂侧的流动特性。基于文献R32/油混合物两相流动摩擦压降实验数据,对比文献中四种模型的预测值和实验值。结果表明:在低质流密度工况下,文献模型对实验的结果预测精度较高,在高质流密度工况下,预测结果偏离了实验结果,且预测值均大于实验值,模型需进一步研究和优化。     

Monte Carlo simulations in the isothermal—isobaric ensemble: the requirement of a ‘shell’ molecule and simulations of small systems

A modification of the widely used Monte Carlo method for determining thermophysical properties in the isothermal-isobaric ensemble is presented. The new Monte Carlo method, now consistent with recent derivations describing the proper statistical mechanical formulation of the constant pressure ensemble for small systems, requires a ‘shell’ molecule to uniquely identify the volume of the system, thereby avoiding the redundant counting of configurations. Ensemble averages obtained with the new algorithm differ from averages calculated with the old Monte Carlo method, particularly for small system sizes, although both sets of averages become equal in the thermodynamic limit. Monte Carlo simulations in the constant pressure ensemble applied to the Lennard-Jones fluid demonstrate these differences for small systems. Peculiarities of small systems are also discussed, revealing that ‘shape’ is an important thermodynamic variable. Finally, an extension of the Monte Carlo method to mixtures is presented.     

Structure of polymer solutions at interfaces: a Monte Carlo simulation study

The canonical ensemble Monte Carlo method is applied to study the structure of polymer solutions confined between surfaces. The polymer molecules are modeled as fused-sphere freely rotating chains with fixed bond length and bond angles and the solvent as hard spheres. The simulation results for the configurational and conformational properties of the chains are presented with varying interfacial distances, chain concentrations, and chain lengths. The chains are depleted at the wall at lower density, which, however, becomes less at higher density. With an increase in the interfacial distance, the enhancement/depletion of the chains at the wall becomes more marked. At all interfacial distances and chain lengths, increasing the concentration of the solvent makes the oscillation in the density profile of the chains more pronounced. Conformational properties provide important indications regarding the behaviour of chains as they approach surfaces.     

Thermodynamic properties for the triangular-well fluid

The thermodynamic properties of the triangular-well fluid with a well range of up to twice the hard sphere diameter were studied by means of a new developed equation of state and molecular simulation. This EoS is based on the perturbation theory of Barker and Henderson with the first and second-order perturbation terms evaluated by molecular simulation and then a fit with a simple function based on the radial distribution function of the reference fluid. The thermodynamic properties for the triangular-well fluid were also obtained directly by Gibbs ensemble and NPT Monte Carlo simulations. Good agreement is observed between the proposed EoS and the molecular simulation results. A model for the triangular-well solid is also presented; this has been used to calculate the solid–liquid transition line. Very good agreement is obtained with previously report values for this line and for the triple point temperature and pressure.     

Simulation of phase equilibria and interfacial properties of binary mixtures on the liquid-vapour interface using lattice sums

Molecular dynamics simulations of Lennard-Jones binary mixtures were performed to obtain phase equilibria and thermodynamic properties for the liquid—vapour interface. The dispersion interactions were handled using the lattice sum method where the full interaction is obtained and there is no requirement for any long range correction to the properties. The application of the method using the Lorentz—Berthelot combining rule for unlike interactions is discussed. The coexisting densities, adsorption of molecules at the interface and surface tension are the main results of this work. Coexisting properties were compared with Gibbs ensemble Monte Carlo results and with those of the grand canonical Monte Carlo method combined with the histogram reweighting technique, and good agreement was found. The lattice sum method results were compared with those of the spherically truncated and shifted potential to analyse the truncation effect. The adsorption of molecules at the interface and surface tension increase with interaction.     

A perturbation density functional theory for hydrogen bonding cyclic molecules

Using the framework of Wertheim's thermodynamic perturbation theory, a new polyatomic density functional theory is developed to account for the intermolecular association of cyclic molecules in interfacial systems. To test the theory, Monte Carlo simulations in the canonical ensemble were performed for the specific case of an associating triatomic ring with one association site next to a hard wall. The theory and simulation results were found to be in good agreement.     

Temperature and density extrapolations in canonical ensemble monte carlo simulations

We show how to use the multiple histogram method to combine canonical ensemble Monte Carlo simulations made at different temperatures and densities. The method can be applied to study systems of particles with arbitrary interaction potential and to compute the thermodynamic properties over a range of temperatures and densities. The calculation of the Helmholtz free energy relative to some thermodynamic reference state enables us to study phase coexistence properties. We test the method on the Lennard-Jones fluids for which many results are available.     

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.     

Cavity-bias sampling in reaction ensemble Monte Carlo simulations

A methodology is presented to sample efficiently configurations of reacting mixtures in the reaction ensemble Monte Carlo simulation technique. A cavity-biasing scheme is used, which more effectively samples configurations than conventional random sampling. Akin to other biasing schemes that are implemented into insertion-based Monte Carlo methods such as Gibbs ensemble Monte Carlo, the method presented here searches for space in the reacting mixture whereby the insertion of a product molecule is energetically favoured. This sampling bias is then corrected in the acceptance criteria. The approach allows for the study of reacting mixtures at high density as well as for polyatomic molecular species. For some cases, the method is shown to increase the efficiency of the reaction steps by a factor greater than 20. The approach is shown to be readily generalized to other biasing schemes such as orientational-biasing of polar molecules and configurational-biasing of chain-like molecules.