Onsager-Machlup Theory for Nonequilibrium Steady States and Fluctuation Theorems

A generalization of the Onsager-Machlup theory from equilibrium to nonequilibrium steady states and its connection with recent fluctuation theorems are discussed for a dragged particle restricted by a harmonic potential in a heat reservoir. Using a functional integral approach, the probability functional for a path is expressed in terms of a Lagrangian function from which an entropy production rate and dissipation functions are introduced, and nonequilibrium thermodynamic relations like the energy conservation law and the second law of thermodynamics are derived. Using this Lagrangian function we establish two nonequilibrium detailed balance relations, which not only lead to a fluctuation theorem for work but also to one related to energy loss by friction. In addition, we carried out the functional integral for heat explicitly, leading to the extended fluctuation theorem for heat. We also present a simple argument for this extended fluctuation theorem in the long time limit. PACS numbers: 05.70.Ln, 05.40.-a, 05.10.Gg.     

烧结/堆积床多孔介质流动沸腾换热实验研究

本文实验对比研究了0.3 mm、0.5mm、0.7 mm三种粒径的铜颗粒烧结与堆积床多孔介质中的流动沸腾换热,主要研究了入口流速、热流密度、加热方位及粒径对流动沸腾换热的影响,以及多孔介质中的沸腾滞后。实验结果表明:大入口流速、低热流密度、下方加热以及小粒径时加热壁面的过热度较低,即有利于沸腾换热;本实验所用烧结多孔介质壁面过热度高于堆积床多孔介质,其原因是内部含有闭孔。     

多孔介质流动沸腾微观模型实验研究

多孔通道内流动沸腾广泛应用于热管、高效换热器、航天热防护等领域,认识孔隙尺度相变典型行为及特点有助于理解多孔介质内沸腾传热机理,进而改进多孔介质内部流体相变模型。基于此,本文搭建了二维多孔介质内流动沸腾实验台,对微米级多孔通道内部流动沸腾现象及气泡行为进行研究。实验发现在毛细力、黏性力和惯性力的共同作用下,多孔介质内的流动沸腾形态显著区别于直通道内的沸腾,存在局部气泡堵塞、合并和液膜蒸干、再润湿等行为。该研究有助于增强对相变过程的理解,并为多孔结构优化设计提供指导。     

基于格子Boltzmann方法的多孔介质流动模拟GPU加速

利用NVIDIA CUDA平台,在GPU上结合稀疏存贮算法实现基于格子Boltzmann方法的孔隙尺度多孔介质流动模拟加速,测试该算法相对基本算法的性能.比较该算法在不同GPU上使用LBGK和MRT两种碰撞模型及单、双精度计算时的性能差异.测试结果表明在GPU环境下采用稀疏存贮算法相对基本算法能大幅提高计算速度并节省显存,相对于串行CPU程序加速比达到两个量级.使用较新构架的GPU时,MRT和LBGK碰撞模型在单、双浮点数精度下计算速度相同.而在较上一代的GPU上,计算精度对MRT碰撞模型计算速度影响较大.     

A Micro-Mechanism Model for Porous Media

Based on the tortuous-expanding path/channel model, a micro-mechanism model for porous media is de- veloped. The proposed model is expressed as a function of tortuosity, porosity, resistance coefficient, and fluid properties. Every parameter in the proposed model has clear physical meaning. The results show that the model predictions are in good agreement with those from the existing experimental data.     

A New Numerical Solution of Fluid Flow in Stratigraphic Porous Media

A new numerical technique based on a lattice-Boltzmann method is presented for analyzing the fluid flow in stratigraphic porous media near the earth's surface. The results obtained for the relations between porosity, pressure, and velocity satisfy well the requirements of stratigraphic statistics and hence are helpful for a further study of the evolution of fluid flow in stratigraphic media.     

A Micro-Mechanism Model for Porous Media

Based on the tortuous-expanding path/channel model, a micro-mechanism model for porous media is developed. The proposed model is expressed as a function of tortuosity, porosity, resistance coefficient, and fluid properties. Every parameter in the proposed model has clear physical meaning. The results show that the model predictions are in good agreement with those from the existing experimental data.     

气体动理学格式模拟多孔介质流动的可行性

将气体动理学格式（GKS）拓展到模拟多孔介质内的低速渗流,并检验在孔隙尺度上模拟不可压缩低速流动的可行性与有效性.结果表明：GKS具有二阶空间精度,能够较精确地计算多孔介质的渗透率;相比于单松弛格子玻尔兹曼方法,GKS能够精确实现壁面无滑移边界条件,从而正确反映渗透率与黏性无关的特性;对于Berea砂岩切片结构中的复杂流动,模拟结果与实验吻合较好,能较精确地计算渗透率.给出GKS模拟达西渗流的马赫数选取准则,为研究多孔介质流动提供新的工具.     

二维非均匀多孔介质中不可压两相驱替的有限分析算法

为提高油藏数值模拟算法的计算效率,在求解单向稳态渗流的有限分析算法基础上,构建二维非均匀多孔介质中不可压两相渗流的有限分析算法.算法中,网格界面上的平均渗透率不是简单地取为相邻网格渗透率的调和平均值,而是通过奇点邻域解析解积分求得.相比于传统的数值算法,有限分析算法随着网格的加密,能够很快地收敛(仅需将原始网格细分至2×2或3×3),并且其计算精度和收敛性不依赖于介质的非均匀强度,从而计算效率得到提高.     

表征体元尺度渗流的离散统一动理学格式

结合表征体元尺度的通用渗流模型,提出离散统一动理学格式（DUGKS）渗流方法,分别用均匀网格和非均匀网格计算二维Poiseuille、Couette、方腔流等经典渗流问题,检验DUGKS渗流方法的有效性和非均匀网格应用的优势,将DUGKS渗流方法应用到裂缝系统中.     

Mixed Convective Viscoelastic Nanofluid Flow Past a Porous Media with Soret-Dufour Effects

The present study is carried out to see the thermal-diffusion (Dufour) and diffusion-thermo (Soret) effects on the mixed convection boundary layer flow of viscoelastic nanofluid flow over a vertical stretching surface in a porous medium. Optimal homotopy analysis method (OHAM) is best candidate to handle highly nonlinear system of differ-ential equations obtained from boundary layer partial differential equations via appropriate transformations. Graphical illustrations depicting different physical arising parameters against velocity, temperature and concentration distributions with required discussion have also been added. Numerically calculated values of skin friction coefficient, local Nusselt and Sherwood numbers are given in the form of table and well argued. It is found that nanofluid velocity increases with increase in mixed convective and viscoelastic parameters but it decreases with the increasing values of porosity parameter. Also, it is observed that Dufour number has opposite behavior on temperature and concentration profiles.     

具有多孔介质覆面层的柱体绕流流场分析

建立多孔结构覆面柱体绕流模型, 采用含Darcy-Brinkman-Forchheimer作用力项的格子Boltzmann方程对覆盖多孔介质层的方柱绕流进行数值模拟, 研究多孔介质对钝体绕流流场特性的影响。结果表明: 相比于不可渗透壁的柱体, 引入合适参数的多孔介质覆面层后可以有效降低其升力脉动幅值, 但阻力有所增加。同时, 较高雷诺数下多孔方柱的数值模拟表明: 多孔介质壁面使得尾迹区域的剪切层相距更远, 降低了尾流处湍动能, 并将雷诺应力的峰值移动到尾迹区域, 抑制了方柱两侧的动量交换, 使动量交换的位置发生在尾迹区域, 继而使得尾迹的涡街更加规则化。     

A Temperature-Mapping Method for TetraHydrofuran Hydrate Formation in Porous Media

Temperature mapping provides important information for thermal characteristic analysis. There is no effective method for temperature mapping in porous media. A magnetic resonance imaging method is proposed to obtain temperature maps for hydrate formation in porous media in this study. The experimental results show that there is a high-temperature region around the hydrate formation zones. A threshold value of the mean intensity is necessary for temperature mapping. The effects of temperature on the mean intensity are negligible after the tetrahydrofuran solution completely forms hydrates. It is also shown that hydrate dissociation is confined by heat transfer and that there are three parameters affecting the hydrate saturation.     

随机四参数法生成多孔介质及渗流模拟

本文基于多孔介质内流动的广泛应用背景,模拟了多孔介质渗流.采用随机四参数生长法生成了多孔介质模型,并对模型中的微小固相进行了过滤处理,将随机四参数生长法生成的多孔介质结合格子玻尔兹曼方法D2Q9模型模拟了压力驱动的多孔介质中的渗流.通过将模拟结果与著名的CK理论公式的计算结果进行比较,发现在孔隙率高时误差较小(cp=0...     

多孔媒质中液体的电声耦合

多孔媒质是充满连通管道的物质。在研究多孔媒质中液体的流动现象时发现，当流体相对于固－液界面处的带电表面运动时，由于液流和电磁场的耦合就出现了电声耦合（电动）现象。电动法同地震法结合，有可能成为油藏探测的潜在方法。     

复杂多孔介质腔体内自然对流换热的数值研究

采用曲线坐标系下压力与速度耦合的SIMPLEC算法,数值研究复杂多孔介质腔体内的自然对流换热问题.腔体的曲面温度分别保持恒定,上下表面绝热.在曲线坐标系中用有限容积法离散方程,并采用Brinkman扩展达西模型及局部非热平衡模型求解,综合研究Rayleigh数,Darcy数、孔隙率等参数对腔体内自然对流换热的影响.计算结果表明:Rayleigh数和Darcy数的影响最大而孔隙率的影响很小,同时存在使得腔体内换热达到最强的最佳纵横比.     

Pattern Structure of Deterministic Displacement in Random Porous Media with Dispersion Effect

A new model-model of random porous mediz degradation via several fluid displacing,freezing,and thawing cycles is introduced and investigated in this paper.The fluid transport is based on the deterministic method with dispersion effect.The result shows that the topology and the geometry of the porous media have a strong effect on displacement processes.The cluster size of viscous fingering (VF) pattern in percolation cluster increases with the increase of iteration parameter n.When iteration parameter n≥10,VF pattern does not change with n.We find that the displacement fluid forms trapping regions in random porous media with dispersion effect.And the trapping regions will expand with the increasing of the iteration parameter n.When r (throat size)→1 and n≥5,the peak value of the distribution Nmat(r) increases as n increases,where Nmat(r) is the normalized distribution of throat sizes after different displacement-damages but before freezing.The peak value of the distribution Ninv(r) reaches a maximum when n≥10 and r=1,where Ninv(r) is the normalized distribution of the size of invaded throat.This result is different from invasion percolation.It is found that the sweep efficiency E increases along with the increasing of iteration parameter n and decreases with the network size L,and E has a minimum as L increases to the maximum size of lattice.The VF pattern in percolation cluster has one frozen zone and one active zone.     

气体和气溶胶在多孔介质中迁移的数值模拟

为了研究多孔介质对气体和气溶胶的封闭能力,用数值方法模拟了气体和气溶胶在多孔介质中的迁移.考虑了渗透、吸附过滤、扩散、热传导等因素,在柱对称坐标下用双孔双渗模型进行了数值模拟,并与实验结果进行了比较.计算结果显示,多孔介质对气溶胶有较强的阻挡作用,模拟实验结果也证明了这一点.