裂隙岩体交叉水流-传热对温度影响的数值分析

裂隙岩体内水流-传热特征是评价高放废物处置库安全运行的重要组成部分。采用3DEC离散元软件建模,着重分析热源温度、裂隙水流速及裂隙开度对裂隙岩体温度的影响。在设定条件下,计算分析表明:(1)由于热传导与水流传热的不规则性,瞬态到稳态,上层岩石形成从下向上为主的传热路径,中、下层岩石形成从左向右为主的传热路径;岩石的温度梯度逐渐减小,裂隙两侧岩石等温线的不连续性逐渐增大;(2)斜裂隙水流与岩石热传导的耦合迟于竖裂隙水流与岩石热传导的耦合,岩石热传导与斜裂隙水流和邻近热源侧的竖裂隙水流对温度分布起控制作用;(3)热源温度越低,裂隙水流速越高,裂隙开度越大,岩石温度越低,系统达到稳态所需要的时间越短;(4)低流速裂隙水的热传导占主导作用,高流速裂隙水的对流传热占主导作用,裂隙交汇处存在局部热对流。     

无填充裂隙岩体水流–传热模型实验研究

 裂隙岩体内的水流–传热特征是评价高放射核废物处置库安全运行的重要组成部分。选取中国高放射核废物地下处置库重点预选场区–甘肃北山地区的花岗岩,加工组合成尺寸为150.25 cm×90.4 cm×30 cm(高度×宽度×厚度)的规则裂隙岩体模型,进行室内实验,研究热源温度和裂隙水流速对多裂隙岩体内水流–传热过程的影响。模型由18块花岗岩组成,包含开度不同的2条水平和2条垂直裂隙,在模型一侧设置电热板并在裂隙以及岩石内部布置温度和压力传感器。实验结果表明：(1) 由于热源和垂直裂隙之间的距离,岩石热传导和裂隙水流动约在实验开始一小时后耦合;(2) 在水平裂隙与垂直裂隙的交汇处存在局部热对流,使垂直裂隙水的温度在该处明显升高;(3) 热源温度越高,裂隙水流速越低,系统温度场达到稳定的时间越长,热源的影响距离越远;(4) 邻近热源侧的垂直裂隙对整个温度场的分布起控制作用,水平裂隙会增加热源的影响距离;(5) 实验中裂隙水温度低于100 ℃,热源温度变化对裂隙水压力变化的影响很小。     

裂隙岩体水流-传热模型的理论计算和参数分析

在前期选用中国高放射核废物处置库重要预选场区——甘肃北山地区的花岗岩,设计制作裂隙岩体水流-传热试验装置,在进行模型试验的基础上,给出了试验模型的理论计算方法,对试验结果进行了验证,并分析了试验参数的敏感性。研究结果表明,理论计算得到的裂隙水温度时间变化规律和模型稳态温度分布与试验数据基本一致;垂直裂隙的出水温度对不同参数的敏感程度与具体的参数值有关,其中热源温度、岩石的热传导系数、水流速度、垂直裂隙的开度等均属于高敏感参数;邻近热源的垂直裂隙对模型温度分布起控制作用,明确其结构和水流特征对研究裂隙岩体内的热量运移和温度分布具有十分重要的意义。     

考虑双重孔隙–裂隙岩体中强度异向性的THM耦合有限元分析

计入裂隙连通率,进一步完善了确定双重孔隙–裂隙介质岩体的黏聚力及内摩擦角的方法,将其引入独立开发的热–水–应力耦合二维弹塑性有限元程序中。假定一个位于饱和岩体中的实验室尺度的核废物地质处置库模型,以此为计算背景,拟定裂隙组正交和斜交的二种情况,进行热–水–应力耦合有限元分析。结果表明:与裂隙组正交时的各场量呈轴对称分布相比,两组裂隙斜交时,孔隙水压力及其流速呈轴对称分布,但岩体中的应力场、裂隙水压力及其流速、塑性区均呈非轴对称分布;裂隙组正交时岩体中的塑性区大面积出现,而裂隙组斜交时岩体中几乎无塑性区产生;岩体中裂隙组的产状强烈地影响到应力场、塑性区和裂隙水的状态。     

热学参数对裂隙岩体水流-传热温度影响的数值分析

针对裂隙岩体概念模型,采用3DEC离散元模拟计算不同热学参数下裂隙岩体的温度场和裂隙出水口水温变化。结果表明:1)岩石热传导与裂隙V_1水流约在热源放热2.5 d后开始耦合,裂隙V_2约在热源放热5 d后开始耦合。模型达到稳态时,由于裂隙V_1更加靠近热源,裂隙V_1比V_2水温高约10℃。2)由于裂隙水流动传热作用,使模型下端温度高于上端,岩体等温线形成从左向右的温度梯度。3)模型达到稳态时,岩石的比热增大200 J/(kg·℃)时,岩体温度降低约1℃;岩石的导热系数增大0.5 W/(m·℃)时,岩体温度升高约3℃;水的比热增大0.8 J/(kg·℃)时,岩体温度降低约2℃;水的导热系数增大0.05 W/(m·℃)时,岩体温度降低约0.001℃;岩石与水的对流换热系数增大5 W/(m~2·℃)时,岩体温度降低约1℃。模型达到稳态所需时间随各参数的增大而缩短。     

二维裂隙岩体渗流传热的离散裂隙网络模型数值计算方法

 针对裂隙岩体渗流传热问题,用解析方法,比较2种不同岩石基质与裂隙水界面热交换假设下的计算结果,对一般裂隙岩体,2种假设下的计算结果相同。基于离散裂隙网络模型的思想,在商业有限元软件COMSOL中实现一种计算已知裂隙网络的裂隙岩体渗流和传热过程的数值方法,该方法可以同时计算岩石基质与裂隙中的渗流和传热过程及二者间的交换,并与解析解比较进行验证。用该方法对一随机生成的二维裂隙岩体进行计算,得到的出口温度曲线,可以反映裂隙岩体渗流传热的早期热突破和长尾效应等特点,并分析岩石基质渗透率、热传导系数的不同取值对裂隙岩体渗流和传热过程的影响。     

一种双重孔隙介质水–应力耦合模型及其有限元分析

建立了一种双重孔隙介质水–应力耦合模型,其特点是可考虑裂隙的组数、间距、方向、连通率和刚度的变化的影响,并研制出相应的二维有限元程序。在假定裂隙的渗透性与裂隙间距无关的前提下,通过算例考察了不同的裂隙间距对双重介质岩体中的变形、主应力、孔隙水压力及裂隙水压力的作用,并与单重介质岩体的相应情况作了对比。结果显示:裂隙间距对双重介质岩体的位移影响很大,但对岩体主应力及孔隙与裂隙水压力的影响很小,岩体水压力主要取决于孔隙与裂隙的孔隙率与渗透系数。     

基于随机裂隙网络研究溶洞方位对隧洞渗流场影响

在喀斯特地区,岩体常伴有不同发育程度的裂隙。裂隙的存在弱化了围岩的连续性,当隧洞穿越岩溶区时,裂隙成为地下水渗流的通道,影响隧洞稳定性。通过离散元软件,利用蒙特卡罗随机原理,根据实际裂隙几何参数统计数据,构建随机裂隙网络,基于随机裂隙网络分析隧洞穿越岩溶区时溶洞的方位对隧洞渗流场的影响,通过洞周渗流量及裂隙水压力分布规律,提出了溶洞恒压区与隧洞低压区裂隙水渗流模式,以此为基础进一步分析不同间距下溶洞对隧洞渗流场的影响。     

钻孔岩体裂隙几何参数确定方法研究及其应用

 裂隙岩体渗透性是高放废物处置库场址选择和评价的重要考虑因素。钻孔岩体渗透性特征主要依靠钻孔电视获得的裂隙几何参数来确定。针对传统统计法确定裂隙几何参数存在的不足和误差,提出了新的统计方法和计算公式。研究认为：(1) 由于岩体裂隙空间分布多变性,提出了以裂隙密度和产状为基础的岩体均质区划分方法;(2) 钻孔岩体裂隙发育优势产状采用聚类分析方法来统计;(3) 裂隙间距的确定考虑了测量误差的影响并提出了修正公式;(4) 裂隙有效水力隙宽提出按照力学成因来确定。将获得的裂隙几何参数代入渗透张量计算程序中,得到钻孔岩体的渗透张量以及渗透主值和主渗透方向,从而为钻孔岩体渗透性分析提供依据。     

钻孔岩体裂隙几何参数确定方法及其应用

裂隙岩体渗透性是高放废物处置库场址选择和评价的重要因素。钻孔岩体渗透性特征主要依靠钻孔电视获得的裂隙几何参数来确定。针对传统统计法确定裂隙几何参数存在的不足和误差,提出新的统计方法和计算公式。研究结果如下:(1)由于岩体裂隙空间分布多变性,提出以裂隙密度和产状为基础的岩体均质区划分方法;(2)钻孔岩体裂隙发育优势产状采用聚类分析方法来统计;(3)裂隙间距的确定考虑了测量误差的影响,并提出修正公式;(4)裂隙有效水力隙宽提出按照力学成因来确定。将获得的裂隙几何参数代入渗透张量计算程序中,得到钻孔岩体的渗透张量以及渗透主值和主渗透方向,从而为钻孔岩体渗透性分析提供依据。     

Energy consumption and thermal comfort in dwelling-cells: A zonal-model approach

In this study, we present a simplified model of the thermal behaviour of dwelling-cells, with a view to evaluating the performances of various heating systems that are commonly used in such environments. This model is based on a zonal-method representation of thermal exchanges in enclosed spaces.Following the validation of the model, we carried out a numerical study on two types of heat source, i.e. localized (a hot-water radiator and an electrical convector) and distributed (a hot-water heated floor and an electrical heated ceiling).The models were used to predict the heat losses specific to each system, as well as the indoor thermal ambience that the different systems induced. It was found that, for the configurations studied, the distributed heat sources presented a slight advantage over the localized sources, with regard to the criteria of energy consumption and thermal comfort.     

A numerical study of THM effects on the near-field safety of a hypothetical nuclear waste repository—BMT1 of the DECOVALEX III project. Part 3: Effects of THM coupling in sparsely fractured rocks

As a part of the international DECOVALEX III project, and the European BENCHPAR project, the impact of thermal–hydrological–mechanical (THM) couplings on the performance of a bentonite-back-filled nuclear waste repository in near-field crystalline rocks is evaluated in a Bench-Mark Test problem (BMT1) and the results are presented in a series of three companion papers in this issue. This is the third paper with focuses on the effects of THM processes at a repository located in a sparsely fractured rock. Several independent coupled THM analyses presented in this paper show that THM couplings have the most significant impact on the mechanical stress evolution, which is important for repository design, construction and post-closure monitoring considerations. The results show that the stress evolution in the bentonite-back-filled excavations and the surrounding rock depends on the post-closure evolution of both fields of temperature and fluid pressure. It is further shown that the time required to full resaturation may play an important role for the mechanical integrity of the repository drifts. In this sense, the presence of hydraulically conducting fractures in the near-field rock might actually improve the mechanical performance of the repository. Hydraulically conducting fractures in the near-field rocks enhances the water supply to the buffers/back-fills, which promotes a more timely process of resaturation and development of swelling pressures in the back-fill, thus provides timely confining stress and support to the rock walls. In one particular case simulated in this study, it was shown that failure in the drift walls could be prevented if the compressive stresses in back-fill were fully developed within 50 yr, which is when thermally induced rock strain begins to create high differential (failure-prone) stresses in the near-field rocks.     

Effective thermal conductivity of an intact heterogeneous limestone

The Cobourg limestone is a very low porosity rock consisting of lighter nodular regions that are predominantly calcite and darker regions consisting of calcite,quartz,dolomite,and an appreciable clay fraction.This paper presents the application of the theory of multi-phasic composites to estimate the possible maximum effective thermal conductivity of the heterogeneous rocks.The thermal conductivity estimates are expected to be representative of the intact rock,without fractures or fissures that can influence the heat co nduction process.The estimates are therefore indicative of the thermal properties of the rock in undisturbed regions unaffected by the influences of stress relief and excavation damage during construction of deep ground repositories for the disposal of heat-emitting nuclear waste.     

Foundation heat loss from heated concrete slab-on-grade floors

In recent years, there has been a renewed interest in heated concrete slab floors to provide for space heating in both residential and commercial buildings. The existing design procedures for these heating systems are based on simplified thermal models with several assumptions. In particular, the simplified models assume that both the upper and the lower concrete slab surface are isothermal and that the heat transfer through the bottom of the slab surfaces is uniformly distributed over the entire surface. In this paper, a more realistic and flexible model for heated or cooled concrete slab floors is considered to determine the heat transfer between the concrete slab and the ground. In particular, steady-state and steady-periodic semi-analytical solutions are developed to determine the temperature field within the ground medium and within the concrete slab-on-grade floor where hot or chilled water pipes are embedded. The solution presented in this paper is applied to determine the ground heat loss/gain for a heated or cooled floor under various design conditions including the level of floor insulation, and the temperature of the water pipes. These solutions are obtained using the interzone temperature profile estimation (ITPE) procedure. Detailed analysis is presented to determine the effect of the slab insulation configuration on soil and slab temperature field and on the monthly variation of the total slab heat loss.     

裂隙岩体等效渗透系数张量数值法研究

由于核废料地质储存、地热开采、深部油气开采的工程需求,裂隙岩体渗透性及其随着应力、温度的影响受到广泛关注。通过温度-渗流-应力耦合三轴仪对大理岩人工裂隙渗透率随应力及温度变化规律进行了试验研究,获得了大理岩闭合裂隙渗透率随应力、温度的变化趋势及受影响程度。在试验基础上,通过数值方法研究了裂隙岩体等效渗透系数的尺寸效应及各向异性,获得了该裂隙岩体的等效渗透系数REV及渗透张量。     

Thermally induced mechanical and permeability changes around a nuclear waste repository—a far-field study based on equivalent properties determined by a discrete approach

A numerical investigation is conducted on the impacts of the thermal loading history on the evolution of mechanical response and permeability field of a fractured rock mass containing a hypothetical nuclear waste repository. The geological data are extracted from the site investigation results at Sellafield, England.A combined methodology of discrete and continuum approaches is presented. The results of a series of simulations based on the DFN–DEM (discrete fracture network–distinct element method) approach provide the mechanical and hydraulic properties of fractured rock masses, and their stress-dependencies. These properties are calculated on a representative scale that depends on fracture network characteristics and constitutive models of intact rock and fractures. In the present study, data indicate that the large scale domain can be divided into four regions with different property sets corresponding to the depth. The results derived by the DFN–DEM approach are then passed on to a large-scale analysis of the far-field problem for the equivalent continuum analysis.The large-scale far-field analysis is conducted using a FEM code, ROCMAS for coupled thermo-mechanical process. The results show that the thermal stresses of fractured rock masses vary significantly with mechanical properties determined at the representative scale. Vertical heaving and horizontal tensile displacement are observed above the repository. Observed stress and displacement fields also shows significant dependency on how the mechanical properties are characterized. The permeability changes induced by the thermal loading show that it generally decreases close to the repository. However, change of permeability is small, i.e., a factor of two, and thermally induced dilation of fracture was not observed. Note that the repository excavation effects were not considered in the study.The work presented in this paper is the result of efforts on a benchmark test (BMT2) within the international co-operative projects DECOVALEX III and BENCHPAR.     

基于DTS的土体分布式导热系数测试方法

土体导热系数的确定对地源热泵的优化设计起着至关重要的作用。为了获得更精细的土体导热系数,在常规热响应试验的基础上,引入分布式光纤测温系统(DTS),对换热孔的温度进行分布式测量,得到孔内竖直方向岩土初始地温分布和在热响应试验过程中换热孔温度分布及其随时间的变化情况,通过与热电阻温度传感器的对比,验证了DTS测温的准确性。在线热源传热模型的基础上,建立了以换热孔为线热源的传热模型。分别进行了双U型管和单U型管恒热流热响应测试,根据DTS监测的温度分布及其变化,得到了土体导热系数随深度的分布。与综合导热系数相比,分布式导热系数不仅精度满足要求,而且可以更精细地反映土体导热系数分布的不均匀性,对于地源热泵的优化设计和换热能力的评价具有重要的意义。     

隧道横向偏置火源顶棚温度纵向分布特性研究

为研究隧道横向火源位置对隧道顶棚温度沿纵向分布过程的影响,采用数值模拟与全尺寸模型实验相结合的方法,分析3 种火源功率多种横向偏移位置火源燃烧产生的顶棚温升与对应中心火源工况沿隧道纵向不同位置的温度分布特性。结果表明：对于多种横向偏置火源位置,火源所处纵向的顶棚温升衰减仍可用指数形式描述,越靠近隧道侧壁,温升衰减速度越快。火源与横向中心的偏距和纵向距离的耦合影响对温升衰减规律可以用相对独立的公式形式进行描述。火源功率越大,不同偏距火源下影响温升纵向衰减的范围越小。