多相混合渗流理论研究

针对现有多相渗流理论假设各相均为连续相、无相间交换,不能表征相对渗透率端点附近出现非连续相,未能考虑多相混合、界面作用、相间传质传输等多相掺混复杂流动的问题,本文把多相渗流流体作为一个总体即混合流体,研究多相流体在多孔介质中传输,包含不相溶、相界面变化、相间传质传输、混合相,搞清各相间交换关系和流动机制,即多相混合流动规律.首先基于平衡热力学第一、第二定律,考虑渗流过程中的多相体系平衡条件,推导出了渗流过程中多相体系平衡热力学关系式,之后运用多相流体全质量守恒定律和渗流过程中多相体系平衡热力学公式,建立了多相流体混合渗流理论模型,分析了多相混合渗流理论与传统多相渗流理论的关系,提出了多相混合渗流的理论.指出多相体系流体总的渗流速度不仅与压力梯度成正比,还与多相体系混合渗流程度有密切关系,其中混合渗流程度是饱和度、界面张力、压力梯度和孔隙度的函数.研究结果表明,多相混合渗流理论深刻地反映了多相流体混合渗流的本质,揭示了多相流体混合渗流的内在作用变化规律,弥补了多相渗流理论用单相达西定律推广到了多相渗流中的不足,多相混合渗流理论涵盖了传统多相渗流理论,具有重大的理论意义和应用价值.     

非达西渗流拟启动压力梯度推算

岩体启动压力梯度的大小是石油开采,工程防渗处理等方面需要考虑的影响因素.非线性渗流现象已被广泛所认同,低渗透性岩渗流存在启动压力梯度,它基本代表了流体产生渗流时的压力梯度大小.试验对试样进水端水压进行长期稳定控制,出水端采用精确测定流出水体积变化量的方法进行流量测定;当渗出端水体积的变化量与时间呈直线关系时,即认为该时段渗透特征符合达西定律.通过对具有不同渗透性的软弱岩系统的试验测试,根据实测渗透系数与压力梯度的正相关关系,提出了拟启动压力梯度推算的"三次平均法",即:首先统一调整实测渗透系数值,然后确定绝对渗透系数值,再根据该值确定Vo值,3次取平均推算拟启动压力梯度的方法.分析表明:拟启动压力梯度随围压升高而增大,随岩石的绝对渗透系数降低而增大;在渗透压差增大过程测得的拟启动压力梯度大于降低过程的拟启动压力梯度,这些都与理论分析结果相吻合.实例说明拟启动压力梯度推算"三次平均法"较为合理,易于推广使用.这为通过室内试验确定工程岩体启动渗流压力梯度的大小,提出了一种新的思路.     

岩质边坡渗流场中等效连续介质模型的应用

在运用等效连续介质模型分析了裂隙岩体渗流场时,综合考虑并比较了单孔压水试验和节理裂隙统计取得的渗透系数,获得了能较好反映原位地质环境的修正渗透张量.将这种确定渗透张量的方法应用于某铁矿边坡工程渗流场分析中,取得了较好的结果.     

单轴压缩条件下裂隙粗糙度对渗透系数的影响

裂隙粗糙度是影响裂隙岩体渗流特性和流体流动复杂性的重要因素,为了深入研究单轴压缩条件下粗糙度对渗透系数的影响,采用3D打印技术和数字建模方法制备了粗糙度不同的裂隙试样,通过自制的试验装置对不同法向压力下的裂隙试样进行了试验.结果表明,在没有法向压力的条件下,随着粗糙度的增加,渗透系数以负指数函数形式减小,采用Forchheimer方程定量的分析了渗流流量与水力梯度之间的非线性关系,Forchheimer方程可以很好地描述粗糙裂隙表面的流动过程,线性项系数随着粗糙度的增大而减小,非线性项系数随着粗糙度的增大而增大;在恒定法向压力且大于水压的条件下,裂隙试样的渗透系数随着粗糙度的增大线性减小,随着水压的增大,粗糙度对渗透系数的影响作用增强;定义了系数δ,分析了在有无法向压力条件下,粗糙度对渗透系数影响的差异性,δ随着水力梯度的增加而增加,随着法向压力的增加而减小.研究结果可以加深对粗糙裂隙表面流体流动的认识,为进一步研究岩体流动特性奠定坚实的基础.     

广西武宣铅锌矿地下水渗流应力耦合数值模拟分析

本文通过地下水渗流应力耦合数值模拟分析,对深部开采排水疏干三维流场所及裂隙渗流围岩变形与破坏分析评价,得出在实际采掘过程中,应注意运输巷道与采掘工作面连接处以及采掘面转角处岩体的稳定性,加强支护,防止发生岩体失稳。     

应力—渗流耦合作用下损伤岩石渗流特性

地下矿产资源开采过程中,频繁的应力扰动会对深部硬岩造成一定的损伤.硬岩内部孔隙和裂隙在高地应力和高渗透压的状态下迅速扩展贯通,严重威胁岩体工程的安全和稳定性.针对损伤花岗岩开展了一系列应力—渗流耦合试验,结合CT扫描,考虑渗透压、围压和损伤程度等因素的影响,综合分析了损伤岩石在应力—渗流耦合作用下的力学和渗流特性.试验...     

低、特低渗透油藏压裂水平井产能计算方法

考虑低、特低渗透油藏裸眼压裂水平井流体从基质-裂缝-水平井筒及基质-水平井筒的耦合流动,以等值渗流阻力法及叠加原理为基础,将裸眼压裂水平井渗流区域划分为三个流动区域:流体在水平井压裂裂缝内的达西线性流动区域、水力压裂裂缝泄流引起的非达西椭圆渗流区域和流体在地层中的径向渗流区域.建立裸眼水平井水力压裂多条横向裂缝相互干扰的非达西产能预测模型,分析水力裂缝参数对产能影响,揭示裸眼压裂水平井开采变化规律.模拟结果表明:裂缝条数越多,裂缝干扰越强,水平井压裂存在一最优裂缝条数;裂缝沿水平井筒排布靠近两端、中间相对较稀,且两端长、中间短的开发效果最好.     

高炉转鼓式水渣分离装置过滤网的探讨

通过对各种型式过滤网的对比分析,从过滤能力、使用寿命和检修维护多方面论述了各自的优缺点;针对涟钢3200m3高炉的转鼓规格,采用传统的比较过滤面积和有限元模拟分析两种方法从理论上论证了中冶南方开发的皱褶式过滤网良好的过滤性能;最后,借助达西渗流理论提出了对转鼓过滤能力的粗略估算方法。     

非常规油气藏多场耦合渗流理论研究进展

非常规油气藏是目前世界油气开发的重点领域,由于非常规油气藏的储层条件差,渗流场与应力场、温度场耦合作用,流体的流动更为复杂,以往对多场耦合理论的运用存在简易化和适应性缺陷,工程中缺少理论指导下的更有效的开采工艺与开发方法,制约了这类油气藏的大规模高效开发,亟需对多场耦合渗流力学理论进行深入认识,以对工程问题提供有效指导.从实验认识、理论分析、仿真模拟三个方面阐述了我国非常规油气资源开发领域的多场耦合渗流力学理论的研究现状,重点围绕多尺度介质力学行为特性表征、岩体和流体的温度响应机制、耦合作用的概念及数学模型、多场耦合模拟仿真方法等关键问题的最新成果、认识展开论述.在此基础上对地下真实应力及温度环境的模拟、烃类吸附及置换的热量测试等问题进行分析,建议针对岩石的塑性应变、重复压裂后的应力环境变化、混合烃类的输运模型、以及流动条件随应力和温度变化的模型等科学问题进一步深化.旨在为进一步阐明我国非常规油气藏开发的动用机理、确定高效开发方法提供指导,同时希望能够促进渗流力学的学科发展.     

单轴荷载下裂隙岩体超声波传播速度研究

从断裂力学的理论出发,分析了弹性波在裂隙岩体介质中的传播特征;以裂隙岩体的裂隙密度参数在单轴加卸载过程中的不同表达式为基础,建立了裂隙岩体在单轴循环加载过程中各个阶段裂纹重分布后超声波波速的理论公式;同时采用声波实验和单轴循环加载实验相结合的方法,以山东某矿勘探区岩体为试验对象,对取自该矿区的岩样进行了实验。实验结果与理论模型计算结果基本一致,说明对于具有一定分布规律的裂隙岩体介质,弹性波在其中传播的速度可以通过裂隙岩体的裂隙密度参数求解得到。     

Multicrack Detection on Semirigidly Connected Beams Utilizing Dynamic Data

The problem of crack detection has been studied by many researchers, and many methods of approaching the problem have been developed. To quantify the crack extent, most methods follow the model updating approach. This approach treats the crack location and extent as model parameters, which are then identified by minimizing the discrepancy between the modeled and the measured dynamic responses. Most methods following this approach focus on the detection of a single crack or multicracks in situations in which the number of cracks is known. The main objective of this paper is to address the crack detection problem in a general situation in which the number of cracks is not known in advance. The crack detection methodology proposed in this paper consists of two phases. In the first phase, different classes of models are employed to model the beam with different numbers of cracks, and the Bayesian model class selection method is then employed to identify the most plausible class of models based on the set of measured dynamic data in order to identify the number of cracks on the beam. In the second phase, the posterior (updated) probability density function of the crack locations and the corresponding extents is calculated using the Bayesian statistical framework. As a result, the uncertainties that may have been introduced by measurement noise and modeling error can be explicitly dealt with. The methodology proposed herein has been verified by and demonstrated through a comprehensive series of numerical case studies, in which noisy data were generated by a Bernoulli–Euler beam with semirigid connections. The results of these studies show that the proposed methodology can correctly identify the number of cracks even when the crack extent is small. The effects of measurement noise, modeling error, and the complexity of the class of identification model on the crack detection results have also been studied and are discussed in this paper.     

煤田露头自燃的渗流-热动力耦合模型及应用

分析了煤田露头介质内气体渗流及露头自燃热动力系统特征,建立了煤田露头自燃渗流-热动力耦合模型,推导了煤自燃过程中挥发分计算式.对新疆某煤田自燃火区进行了数值模拟.结果表明:开采引发煤田露头自燃时,燃烧中心集中在顶板附近,燃烧沿顶板向露头方向蔓延较快;高温区域靠近顶板,燃烧区域以外岩石内温度梯度变化不明显;自燃生成气体主要集中于露头自燃点下风侧方向,上风侧方向只有很小范围内有自燃气体存在;高温区域分布与自燃气体渗流方位位置一致;自燃加速了气体在煤岩介质内的渗流,有助于自燃的发展.根据模拟结果对煤田火区治理时火源探测、治理及监测等相关措施进行了具体分析.     

流体理化特性对土体渗流规律影响研究进展

土体渗流规律对工程建设和安全生产至关重要,渗透特性影响因素研究是岩土工程的热点问题。流体在土中的运动规律与土体自身性质、流体的理化性质、弥散和对流等密切相关,是土体液固相互作用的结果。文中基于流体理化特性,结合土体渗透性能的相关研究,分析了流体的化学组成、浓度、pH值和温度等因素对土体渗流规律的影响及内在机理。流体主要通过改变土体的微细观结构、双电层厚度、结合水含量、产生化学势等方式对土体的渗透特性产生影响。文中对土体液固相互作用研究进行了展望,可为相关工程实践提供参考。      

复合型疲劳试验与折线裂纹强度因子计算

本文用三点弯曲偏裂纹试样研究了Ⅰ—Ⅱ复合型裂纹的疲劳扩展。由于裂纹扩展呈折线形,所以首先研究了折线裂纹应力强度因子的计算方法,并对计算结果用有机玻璃进行了实验验证。从而可以对疲劳裂纹扩展的数据进行有效的整理,得以说明一些判据的可用性,并对有关问题进行讨论。     

Crack propagation directions in unfilled resins

Posterior composite restorative materials undergo accelerated wear in the occlusal contact area, primarily through a fatigue mechanism. To facilitate the timely development of new and improved materials, a predictive wear model is desirable. The objective of this study was to develop a finite element model enabling investigators to predict crack propagation directions in resins used as the matrix material in composites, and to verify these predictions by observing cracks formed during the pin-on-disc wear of a 60:40 BISGMA:TEGDMA resin and an EBPADMA resin. Laser confocal scanning microscopy was used to measure crack locations. Finite element studies were done by means of ABAQUS software, modeling a cylinder sliding on a material with pre-existing surface-breaking cracks. Variables included modulus, cylinder/material friction coefficient, crack face friction, and yield behavior. Experimental results were surprising, since most crack directions were opposite previously published observations. The majority of surface cracks, though initially orthogonal to the surface, changed direction to run 20 to 30 degrees from the horizontal in the direction of indenter movement. Finite element modeling established the importance of subsurface shear stresses, since calculations provided evidence that cracks propagate in the direction of maximum K(II)(theta), in the same direction as the motion of the indenter, and at an angle of approximately 20 degrees. These findings provide the foundation for a predictive model of sliding wear in unfilled glassy resins.     

Crack detection by resonant frequency measurements

The resonant frequency of 304 stainless steel specimens with a center-drilled hole has been measured as a function of fatigue cycles during crack initiation and propagation. Simultaneous measurements of crack lengths by scanning electron microscopy yield the resonant frequencyvs crack length. The change of resonant frequency is equivalent to the change of an effective elastic modulus. Analytical results for a “spring” model to predict the change in effective modulus due to the presence of cracks have been compared with results derived from resonant tests. In the model, the load transfer across the plane of the crack is represented by a distribution of springs of stiffness that depends on the crack length. Good agreement is observed between theory and experiment for cracks up to 500-μm long. The model may be used to obtain the crack length from resonance measurements. Formerly with the Materials Science and Engineering Department, Northwestern University, Evanston, IL 60208,     

Propagation of surface cracks on beam blank in the mould during continuous casting

ABSTRACT

We focus on crack propagation to investigate surface cracks in the mould during continuous casting, based on the crack initiation mechanism proposed in previous studies. The temperature and stress data of a solidified shell were extracted, and an extended finite element model based on the continuous damage theory of elastic–plastic materials was developed to simulate surface crack propagation. The results showed that, in the cracked area, stress concentration occurred at the crack tip, and the element split open and the crack propagated when the maximum principal stress in the stress concentration area reached the critical value. Prefabricated cracks in the fillet and web mainly developed into longitudinal cracks in the mould. The theoretical mechanism of this study was found to be the same as the crack propagation mechanism observed during the actual production of beam blanks. Thus, this study reveals the theoretical principle of crack initiation and propagation and can provide theoretical guidance for controlling surface cracks during beam blank continuous casting.     

Theoretical Modeling of Signal Loss versus Crack Opening for a Novel Crack Sensor

The degradation of concrete structures is always accompanied by the formation of cracks. Crack monitoring is hence useful in assessing the “health condition” of the structure. In our previous investigation on an optical-fiber crack sensor, we have successfully demonstrated the possibility of crack detection and monitoring without requiring prior knowledge of crack locations. Also, a single fiber can be employed to monitor a number of cracks. In practical applications, the signal loss versus crack opening relationship of the sensor needs to be properly “designed” to satisfy performance requirements on sensitivity to small cracks and/or the total number of detectable cracks (which is limited by the maximum loss at each crack). In this paper, a theoretical model for the signal loss versus crack opening relationship is developed through a combination of mechanical and optical analyses. Using concrete beams with embedded sensors, crack monitoring experiments are conducted. Test results are found to be in reasonably good agreement with predictions from the model. With the verified model, simulations are carried out to study the effect of various parameters on sensor performance. The potential application of the theoretical model to generate sensor design guidelines is hence demonstrated.     

Response of Quasi-Brittle Materials Reinforced by Short, Aligned Fibers

Pseudo-strain-hardening behavior of quasi-brittle materials reinforced by discontinuous, aligned fibers subjected to uniaxial tension is studied in this paper. An R-curve was used to describe transverse cracking of fiber-reinforced composites. The equivalent inclusion method and Mori-Tanaka theory were used to derive the strain energy release rate of the composites. The proposed model takes into account the influence of fiber distribution. When properties of fiber and matrix are known, the proposed model is capable of predicting whether or not a composite will fail with a single crack or multiple cracks. If a composite fails due to multiple cracks, the model can further predict its pseudo-strain-hardening behavior after the formation of the first transverse crack. The theoretical calculation reasonably matched the experimental results.