Effects of Surface Wettability on Rapid Boiling and Bubble Nucleation: A Molecular Dynamics Study

Molecular dynamics simulation is conducted to study the effects of surface wettability on rapid boiling and bubble nucleation over smooth surface. The simple L-J liquid is heated by smooth metal surface with different conditions of wettability in cuboid simulation box. The results show that surface wettability has significant impact on phase transition of liquid film. When the heating temperature is 200 K, the rapid boiling occurs above strongly hydrophilic and weakly hydrophilic surfaces; however, only slow evaporation phenomenon occurs above weakly hydrophobic surface within 2.5-ns simulation time. The reason is that the interaction between argon and platinum atoms is stronger over hydrophilic surface, which has higher efficiency in heat transfer. Furthermore, based on the difference of surface wettability in heat transfer efficiency, the surface with nonuniform wettability is constructed, and the central region is more hydrophilic than surrounding region. The growing process of bubble nucleus can be completely observed above the more hydrophilic region.     

Molecular Dynamics Study on Thermal Conductivity of Carbon Nanotubes

The thermal conductivity of a (5, 5) carbon nanotube at room temperature is studied by non‐equilibrium molecular dynamics simulations. The thermal conductivity increases from 30 W/(m·K) to 1000 W/(m·K) as the tube length increases from 6 nm to 4 µm. It is proportional to the tube length when the tube length is less than 40 nm, which indicates that the heat conduction is in the ballistic transport regime. The thermal conductivity relates to the tube length by an exponential function as the tube length increases, and the length dependence exponent decreases and approaches zero, which indicates that the phonon transport changes from the ballistic regime to the diffusive regime. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; 39(7): 455–459, 2010; Published online in Wiley Online Library ( wileyOnlinelibrary.com ). DOI 10.1002/htj.20311     

A Molecular Dynamics Simulation of Brownian Motion of a Nanoparticle in a Nanofluid

Brownian motion of nanoparticles is one of the mechanisms proposed to explain how nanoparticles improve the thermal conductivity of nanofluids where it has been assumed that a large volume of the surrounding base fluid is dragged by the moving nanoparticle, which leads to the heat transfer enhancement. Therefore, the necessary condition for this mechanism to play a key role is that the velocity vector of the moving nanoparticle points in the same direction as the velocity vectors of the surrounding base fluid. Using equilibrium molecular dynamics simulation and modeling a nanofluid, this mechanism is examined where the angles between the velocity vector of the nanoparticle and the velocity vectors of the surrounding base fluid are considered. The results for these angles indicate that the moving nanoparticle cannot considerably drag the surrounding base fluid even with increasing temperature. Furthermore, it is found that the moving nanoparticle may cause a weak thermal dispersion (fluctuation) around itself.     

发动机气门的动力学及温度场三维数值模拟

采用有限元法对发动机气门进行了动力学及温度场的三维数值模拟，分别得出了气门在动载荷、温度载荷作用下的应力分布规律，研究结果为气门的失效分析和气门的优化设计提供了理论依据。     

Influence of $$NO_2^ - $$ on the Microscopic Structure and Physical Properties of the Binary Nitrate Salts: a Molecular Dynamics Simulation Study

Molecular dynamics simulation method was used to study the influence of NO₂^- on both the structure and properties of the binary nitrate salts(60 wt.% NaNO₃ + 40 wt.% KNO₃). The density and viscosity of the mixtures were experimentally measured and the simulation results met well with the experimental ones. The simulation results showed that, with the addition of NO₂^-, the ionic clusters tended to loose and the mobilities of all th...     

Thermal resistance of crystal interface: Molecular dynamics simulation

Non‐equilibrium molecular dynamics computer simulation was carried out to investigate a thermal resistance phenomenon at interfaces of Lennard‐Jones particle crystals. Inside an FCC crystal, one‐dimensional steady thermal energy flow was realized using a pair of temperature‐controlled heat baths. Four types of sharp and flat interfaces between the following crystals were investigated: (i) crystals with different particle masses, (ii) crystals with different interaction energy parameters, (iii) crystals with various surface interaction parameters, and (iv) crystals with different size parameters. Except for case (iv), observed temperature profiles have a discontinuity at the interface, from which the temperature gap is evaluated. Due to the temperature discontinuity, the thermal energy flux is smaller than that through a single crystal without an interface. The observed reduction of the energy flux is larger than that predicted with a simple acoustic model. Interfaces with finite width, or composition gradient, were also examined. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(3): 135–146, 2005; Published online in Wiley InterScience ( www.interscience. wiley.com ). DOI 10.1002/htj.20058     

微狭缝中甲烷吸附特性的分子动力学模拟

为了研究甲烷在纳米尺度狭缝中的吸附特性,采用分子动力学模拟的方法研究了温度、压力以及狭缝表面的水滴对甲烷吸附情况的影响。研究表明:降低温度和增大压力均能使吸附相的密度增大、吸附层的层数增加、吸附区扩大;升高压力将使吸附作用减弱,温度较低时,升高温度将使吸附作用增强,温度较高时,升高温度将使吸附强度减弱;狭缝表面存在水滴时,狭缝对甲烷的吸附作用被明显削弱。     

液态碳氢燃料高压低温粘度测量实验研究

在Hagen-Poiseuille定律的基础上,利用双毛细管法,设计了一套适用于液态碳氢燃料高压低温粘度测量实验系统,测量压力:0.1~10 MPa,温度测量范围:233.35~313.75 K,扩展相对不确定度为:2.10%~5.08%(置信因子k=2)。实验在利用纯物质正己烷和质量比1:1正庚烷-正辛烷二元混合物对测量系统可靠性进行验证的基础上,测试了两种碳氢燃料A、B压力0.7、1.5、3和6 MPa,温度:233.55~313.65 K下的粘度值,结果表明:在相同压力下,两种燃料的粘度值随温度的增加而减小;在233.55~273.25 K附近,两种燃料粘度值随温度的变化率要大于273.25~313.65 K,表明低温区碳氢燃料A、B粘度受温度的影响比高温区大,且温度越低温度的影响程度越大;在233.55~273.25 K低温区范围内,燃料A粘度值受温度的影响程度要小于燃料B。实验中还发现,当温度接近231.25 K时,燃料B在毛细管中瞬间凝固,系统压力急剧升高,而燃料A在此温度下依然处于液态。在相同温度下,两种燃料的粘度值随着压力的增加稍有增大。此套高压低温粘度测量系统简单可靠,测量精度高,能够实现液态碳氢燃料高压低温粘度的在线测量。     

Lattice Thermal Conductivity of Pristine Si Nanowires: Classical Nonequilibrium Molecular Dynamics Study

We investigate lattice thermal conductivities of Si nanowires (SiNWs) based on classical nonequilibrium molecular dynamics (NEMD) simulations. The SiNWs are supposed to have a crystalline diamond structure and extend along the [001] axial direction with the {100} sidewall facets on which the surface Si atoms are all dimerized and fully passivated by hydrogen. Various sizes in the square cross section are considered, and the lengths are varied to find the diffusive limits of the lattice thermal conductivities. The upper limits of the diffusive lattice thermal conductivities of SiNWs are 11.2, 16.0, 22.8, and 28.0 W/mK for SiNWs with widths of 2.7, 4.9, 8.1, and 11.3 nm, respectively. The mode-averaged mean free paths of phonons are 50, 52, 63, and 80 nm for the SiNWs, respectively, which are 7–19 times longer than the SiNW widths.     

煤中吡咯与吡啶类氮热解的分子动力学模拟

采用基于ReaxFF反应力场的分子动力学研究了不同温度下吡咯与吡啶的热解机理。结果表明,两者的主要含氮产物与中间产物均为 HCN 和 CN,其他主要产物为 H2、C2,H2以及 C3,H4等;随着温度的升高和时间的增长,两者热解的产物数量与种类也越来越多,产物中的氮逐渐从HCN向NH3和N2转移,但吡啶的热解产物要远少于吡咯,且其热解时间也大大晚于吡咯。模拟结果与相关实验数据一致,说明反应力场分子动力学计算可以合理有效地用于吡啶与吡咯热解机理的研究。     

微尺度下固壁面对氩的饱和压力影响

采用分子动力学模拟方法研究了液体氩与固体界面的相互作用性质，得到了一定温度下氩的饱和压力参数与固壁尺度的关系。数值计算结果表明，微尺度下液体的饱和性质存在尺度效应。固体壁面尺度越小，液体氩的饱和压力越低；当固体壁面尺度增加时，液体氩的饱和压力也随之增加；固体壁面尺度超过一定值后，液体氩的饱和压力与大容积下液体氩的饱和压力一致。     

Molecular Dynamics Study of Interactions between the Water/ice Interface and a Nanoparticle in the Vicinity of a Solid Surface

ABSTRACT

In this study, non-equilibrium molecular dynamics simulations were conducted for a coexistence system of water and ice on a wall surface with a single nanoparticle to reveal the effects of water solidification on the nanoparticle in the vicinity of a wall surface. We further investigated the effect of the presence and size of particles on the density profile of water in the vicinity of the wall surface and the force acting on particles from water molecules, when the solidification interface contacted the wall and the particles. The results revealed that a strong mutual influence exists between the solidification interface and the nanoparticle on the wall’s surface; the nanoparticle on the wall prevents water solidification in proximity to the wall. Moreover, the force acting on the nanoparticle from water molecules changes as the solidification interface approaches; the cooling temperature is shown to affect the direction of this force. It indicates that the solidification process is a key influential factor which affects nanoparticle movements on a wall surface at molecular scales.     

乌东德大坝低热水泥混凝土温控防裂效果研究

鉴于目前国内高混凝土坝尚无全坝应用低热水泥的先例,结合混凝土性能特点,研究了乌东德大坝中热、低热水泥混凝土的温度控制标准、最高温度控制措施及大坝抗裂安全系数,并采用有限单元法仿真计算了典型坝段施工期温度场和温度应力场。结果表明,大坝在采用低热水泥混凝土后,最高温度明显降低,富裕度增大,同时抗裂安全系数可提高至2.0以上。结果为乌东德拱坝全面采用低热水泥混凝土提供了参考依据。     

洞庭湖出流丰枯变化与全球海温变异的关系研究

为分析洞庭湖出流丰枯变化与全球海温变异的关系,基于1961~2012年洞庭湖逐月出流资料,分析洞庭湖出流特征,并由此求解与COBE-SST全球逐月海温资料的相关性。结果表明,洞庭湖出流有缓慢减少的趋势,且在不同时间尺度上存在明显的丰枯交替情况;洞庭湖出流量与海温的相关关系显著,枯季尤为明显,最高相关系数达0.7;出流异常年的海温变异特征与我国气候影响因子息息相关,当西太平洋海温较低时,洞庭湖1月出流较多,反之较少;7月出流异常较多的年份北太平洋温度较高,而出流异常较少的年份东太平洋海温较高。     

Effect of Graphite Layers on the Conformation and Thermal Conductivity of n-octadecane:A Molecular Dynamics Study

To understand the thermal conductivity improvement of the paraffin and graphite composite PCMs in micro-scale,the conformation characteristics of molecules with...     

地下厂房岩锚梁结构温控防裂研究

由于施工段长度较长,混凝土标号较高、浇筑量较大,现代大型地下厂房中的岩锚梁结构较容易发生温度裂缝,影响结构安全稳定。对此,采用有限元法数值模拟了岩锚梁温度及应力场,并通过与实测结果的对比,验证了数值方法的有效性,在此基础上,采用数值模拟方法进一步研究了岩锚梁温度应力发展过程及主要影响因素。结果表明,最高温度是决定岩锚梁最大拉应力的主要因素之一,初期最高温度越高,后期降低至稳定温度时所形成的拉应力越大,开裂风险也越大。施工中应采取适当的温控措施降低最高温度,并合理控制分缝尺寸,以提高岩锚梁结构的抗裂安全。研究成果可为类似地下工程提供参考。     

环境温度和压力对坦克柴油机冷却空气影响的CFD仿真研究

为了掌握不同环境温度、压力下坦克柴油机冷却空气的流动与传热性能,将计算流体力学应用于坦克柴油机冷却空气的仿真。在仿真值与试验值对比结果符合较好的前提下,对不同环境温度、压力对冷却空气流动与传热的影响进行了数值仿真,基于仿真结果,拟合得到了冷却空气平均温度随环境温度、压力变化的简便计算公式,适合于工程实用。     

低温烟气凝结换热及对天然气热效率的影响实验研究

为研究烟气露点附近及以下的低温烟气对流凝结换热规律与烟气换热对天然气利用热效率的影响,建立了烟气在翅片管换热器内对流凝结换热实验系统,研究了不同烟气温度、水蒸气含量对烟气凝结换热的影响,得出了烟气凝结换热实验准则关联式,分析对比了天然气利用热效率实验值与理论值.实验结果表明:当被加热水温度为23℃,烟气出口温度为73℃...     

半导体制冷系统非稳态温度工况的模型及实验分析

文章分析了半导体制冷系统中电偶极在电场与温度场的耦合作用下非稳态温度变化特性，建立了半导体制冷过程的传热微分方程式，进行了数值模拟和计算，并通过实验对该模型进行验证。其非稳态分析结果可以为半导体制冷的深入研究和设计应用提供理论基础。     

变几何多级轴流压气机动态仿真模型的研究与应用

首先采用基于级平均直径上的一维流动逐级叠加法建立了变几何多级轴流压气机全工况性能预估模型,在此基础上,将压气机的级作为控制体,运用一维非稳态质量、动量和能量平衡微分方程来描述级出口截面处热力学参数动态特性,采用模块化方法建立各级集总参数模块,按照工质流程将各级的仿真模块连接起来即形成压气机通流部分机理性动态仿真模型.仿真试验表明:该仿真模型能够正确反映燃气轮机启动过程中与压气机有关的热力学参数变化过程,模型的动态响应特性与实际压气机热力参数的变化趋势基本一致.