Monte Carlo algorithms for simulating systems with adiabatic separation of electronic and nuclear degrees of freedom

Two Monte Carlo algorithms for the adiabatic sampling of nuclear and electronic degrees of freedom are introduced. In these algorithms the electronic degrees of freedom are subject to a secondary low-temperature thermostat in close analogy to the extended Lagrangian formalism used in molecular dynamics simulations. Numerical tests are carried out for two model systems of coupled harmonic oscillators, and for two more realistic systems: the water dimer and bulk liquid water. A statistical-mechanical discussion of the partition function for systems with adiabatic separation of electronic and nuclear degrees of freedom, but with finite electronic temperature, is presented. The theoretical analysis shows that the algorithms satisfy the adiabatic limit using suitable choices of the electronic temperature, T _elec, the number of electronic moves, R _elec, and the maximum step sizes used for displacements of nuclear coordinates. For quadratic coupling of the nuclear and electronic degrees of freedom, the electronic phase-space volume is independent of the nuclear coordinates. In this case, the sampling of the nuclear coordinate phase-space recovers the correct Born-Oppenheimer limit independent of T _elec, but each electronic degree of freedom contributes an offset of 0.5k _B T′_elec (with T _elec ^′−1=T ⁻¹+T _elec ⁻¹) to the average total energy. For nonquadratic coupling, satisfactory sampling of the nuclear coordinate phase-space requires a low T _elec to limit the ratio of the electronic phase-space volumes at T _elec and T′_elec to be close to unity. Received: 22 December 1998 / Accepted: 5 January 1999 / Published online: 21 June 1999     

A new calorimeter for simultaneous measurement of isotherms and heats of adsorption

A new calorimeter designed for simultaneous measurements of heats and isotherms of gas adsorption and desorption systems is presented. It consists of a volumetric/manometric gas adsorption instrument, the adsorption vessel of which is placed within a second vessel filled with inert gas. This gas acts as a sensor, as not only its temperature but also its pressure is increased if heat is released from the adsorption vessel via the sensor gas to its thermostated surroundings. Indeed, the time integral of the sensor gas pressure signal turned out to be strongly related to the total heat released from the adsorption vessel.A basic theoretical equation of the measurement procedure is given. Results of numerous calibration measurements are presented. The question of what type and amount of sensor gas should be used to achieve high sensitivity of the instrument is discussed.Two examples of measurements of heats of adsorption and adsorption isotherms are given, namely adsorption of N₂ on alumina oxide (CRM-BAM-PM-104) at 77 K and CO₂ on zeolite Na13X and wessalite DAY both at 298 K.     

室温离子液体的热化学研究1.EMIES溶解热和Pitzer参数

在303．150K下,利用等温溶解反应量热计测定了不同浓度硫酸乙酯-1-甲基．3-乙基咪唑(EMIES)室温离子液体在水中的摩尔溶解热(△sHm),根据Pitzer理论得到了EMIES的无限稀释摩尔溶解热(△sHm^0)和Pitzer焓参数：βMX^(0)L,βMX(1)L和CMX^ФL,并计算了表观相对摩尔焓和溶质、溶剂的相对偏摩尔焓．     

Cell asymmetry correction for temperature modulated differential scanning calorimetry

The quality of measurement of heat capacity by differential scanning calorimetry (DSC) is based on strict symmetry of the twin calorimeter. This symmetry is of particular importance for temperature-modulated DSC (TMDSC) since positive and negative deviations from symmetry cannot be distinguished in the most popular analysis methods. The heat capacities for sapphire-filled and empty aluminum calorimeters (pans) under designed cell imbalance caused by different pan-masses were measured. In addition, the positive and negative signs of asymmetry have been explored by analyzing the phase-shift between temperature and heat flow for sapphire and empty runs. The phase shifts change by more than 180° depending on the sign of the asymmetry. Once the sign of asymmetry is determined, the asymmetry correction for temperature-modulated DSC can be made.On leave from Toray Research Center, Inc., Otsu, Shiga 520, JapanThis work was supported by the Division of Materials Research, National Science Foundation, Polymers Program, Grant # DMR 90-00520 and the Division of Materials Sciences, Office of Basic Energy Sciences, U.S. Department of Energy at Oak Ridge National Laboratory, managed by Lockheed Martin Energy Research Corp. for the U.S. Department of Energy, under contract number DE-AC05-960R22464.     

The Eckert number phenomenon: Experimental investigations on the heat transfer from a moving wall in the case of a rotating cylinder

The Eckert number phenomenon was investigated theoretically by Geropp in 1969 and describes a reversal in heat transfer from a moving wall at an Eckert number Ec ≈ 1. In this report the Eckert number phenomenon is confirmed experimentally for the first time. For that purpose the heat transfer from a heated, vertically rotating cylinder in a crossflow was investigated. In order to perform the experiments in a range where the predicted phenomenon occurs, extreme rotational speeds were necessary. A heating concept had to be developed which allowed an input of heating power independent of the speed and which therefore had to be contact-free. The results show, among other things, that the temperature difference between the wall and the surrounding fluid has a significant effect on the predicted reversal of heat transfer at the wall. Moreover, maximum heat transfer occurs at an Eckert number Ec ≈ 0.3, which is of great importance for the cooling of hot surfaces in a gas-flow.     

The formation of multiple adiabatic shear bands

In a previous paper, Zhou et al. [2006. A numerical methodology for investigating adiabatic shear band formation. J. Mech. Phys. Solids, 54, 904-926] developed a numerical method for analyzing one-dimensional deformation of thermoviscoplastic materials. The method uses a second order algorithm for integration along characteristic lines, and computes the plastic flow after complete localization with high resolution and efficiency. We apply this numerical scheme to analyze localization in a thermoviscoplastic material where multiple shear bands are allowed to form at random locations in a large specimen. As a shear band develops, it unloads neighboring regions and interacts with other bands. Beginning with a random distribution of imperfections, which might be imagined as arising qualitatively from the microstructure, we obtain the average spacing of shear bands through calculations and compare our results with previously existing theoretical estimates. It is found that the spacing between nucleating shear bands follows the perturbation theory due to Wright and Ockendon [1996. A scaling law for the effect of inertia on the formation of adiabatic shear bands. Int. J. Plasticity 12, 927-934], whereas the spacing between mature shear bands is closer to that predicted by the momentum diffusion theory of Grady and Kipp [1987. The growth of unstable thermoplastic shear with application to steady-wave shock compression in solids. J. Mech. Phys. Solids 35, 95-119]. Scaling laws for the dependence of band spacing on material parameters differ in many respects from either theory.     

Thermoelastic Stress Analysis of Structures Under Natural Vibrations

The paper focuses on stress analyses of structures subjected to excitation forces operating at resonant frequencies. The structures are analysed experimentally using the Thermoelastic Stress Analysis (TSA) technique. Experiments are carried out for fixed-free beams of different dimensions and materials, and also for a steel rectangular plate with clamped edges. These structures are excited by a shaker via a stinger. For materials with low thermal conductivity, the agreement between the theory, numerical results and experimental results is excellent. As the thermal conductivity of the material is increased, the correspondence is not as close. This is because of non-adiabatic behaviour. The implications of these results are discussed in detail in the paper and a means of deriving the severity of heat transfer is provided. Other factors that influence the TSA results from structures under natural loading are also discussed.     

Rosetta Stone Experiments

Dynamic failure events such as armor penetration and explosive fragmentation are too complex to be treated by classical single-crack continuum fracture mechanics. In such cases deformation and fracture result from multiple cracks, voids, and shear bands acting simultaneously and influencing one another’s evolution. An alternative “meso” fracture mechanics is needed that treats microfailure activity while permitting fast and inexpensive predictive computations. This paper discusses the approach and experiments that elucidate and quantify failure physics on the micron level. “Rosetta Stone” experiments that isolate a damage mode, produce statistical distributions of damage features, and “freeze in” damage at various stages of development are described and illustrated. The observations and data lead to equations describing nucleation and growth of cracks, voids, and shear bands. The resulting mesomechanical material failure models link the microworld with the macroworld and can be used in continuum hydrocodes for fast, efficient simulations of dynamic fracture scenarios.

Adiabatic and diabatic two-phase venting flow in a microchannel

The growth and advection of the vapor phase in two-phase microchannel heat exchangers increase the system pressure and cause flow instabilities. One solution is to locally vent the vapor formed by capping the microchannels with a porous, hydrophobic membrane. In this paper we visualize this venting process in a single 124 μm by 98 μm copper microchannel with a 65 μm thick, 220 nm pore diameter hydrophobic Teflon membrane wall to determine the impact of varying flow conditions on the flow structures and venting process during adiabatic and diabatic operation. We characterize liquid velocities of 0.14, 0.36 and 0.65 m/s with superficial air velocities varying from 0.3 to 8 m/s. Wavy-stratified and stratified flow dominated low liquid velocities while annular type flows dominated at the higher velocities. Gas/vapor venting can be improved by increasing the venting area, increasing the trans-membrane pressure or using thinner, high permeability membranes. Diabatic experiments with mass flux velocities of 140 and 340 kg/s/m² and exit qualities up to 20% found that stratified type flows dominate at lower mass fluxes while churn-annular flow became more prevalent at the higher mass-flux and quality. The diabatic flow regimes are believed to significantly influence the pressure-drop and heat transfer coefficient in vapor venting heat exchangers.     

三聚氰胺的生成焓、热容和熵

通过多种热化学方法研究了三聚氰胺的热力学性质. 首先用氧弹式燃烧热量计测定了三聚氰胺在298.15 K 时的燃烧热, 根据燃烧热结果, 计算出三聚氰胺的标准摩尔燃烧焓和标准摩尔生成焓, 分别为: △cHΘm=(-2455.17±4.65) kJ·mol-1; △fHΘm =(-763.38±5.16) kJ·mol-1. 然后根据键焓与燃烧焓之间的关系, 估算出三聚氰胺中的C≈N(此键介于单键与双键之间)键能为458.30 kJ·mol-1, 此值介于碳氮单键键能和双键键能之间. 通过绝热热量计测定了三聚氰胺从80到385 K的低温热容. 根据热容值, 计算了此温度区间的标准摩尔生成焓, 其与温度呈线性关系. 另外, 三聚氰胺的热稳定性也用热重-差示扫描量热(TG-DSC)法进行了分析, 确定其分解的DSC曲线的峰顶温度为603.37 K.