管漏产生的渗流水受挡水效应后对产生渗蚀的影响

为研究水力管线渗漏渗蚀产生的流动效应受地下构筑物阻挡后对形成渗蚀的影响,在流固双向耦合原理基础上,用离散单元法模拟管线渗漏流经地下构筑物的过程。用PFC模拟渗流水与地下构筑物相接触时产生的绕流现象,用Fluid Dynamics Simulation模拟绕流的完整过程。结果表明:渗流水在流经地下构筑物时会产生绕流现象;相同条件下渗流水流经构筑物时绕流速度不同,圆形构筑物的绕流速度最大,长方形构筑物的绕流速度最小;地下构筑物对渗流的阻挡效应强弱与构筑物的挡水宽度成正比,长方形构筑物的挡水效应最优,圆形构筑物的挡水效应最差。

不同细粒含量与间断比下不连续级配砂砾土渗蚀的CFD-DEM数值模拟

基于CFD-DEM方法,开展了不同细粒含量(质量分数,下同)与间断比下不连续级配砂砾土渗蚀数值试验,从宏细观角度研究了细粒含量与间断比对渗蚀的联合作用机理。研究结果表明:土体侵蚀率随间断比的增大而增加,当间断比G较小(G<3)时,土样内部未发生颗粒迁移现象,表现出较好的内部稳定性,与KéZDI判别准则吻合。当细粒含量不同时,间断比对渗蚀宏细观特征行为影响程度差异显著。当细粒含量较低(如15%)时,细颗粒较少参与土体内部应力传递,渗蚀过程中土样体积、强接触力链以及细颗粒应力分担系数几乎不发生改变,粗颗粒间孔隙直径增大,导致更多弱接触细颗粒流失,从而使得侵蚀率随间断比增加;相反,当细粒含量较高(如35%)时,参与应力传递的细颗粒比例相对较高,间断比增大导致粗颗粒数量减少,细颗粒流失易导致整个土体力链体系破坏,引起骨架颗粒重组变形,进而释放更多约束颗粒,加剧颗粒流失,故侵蚀率增加是孔隙直径增大和渗蚀过程中土骨架不断发生变形共同作用的结果。

基于计算流体力学-离散元耦合的相对密实度对断级配土体渗蚀效应的影响研究

土体几何因素(粒径比、细粒含量、相对密实度等)不仅影响土体内部稳定性,同时对侵蚀进程有着重要影响。基于离散单元法(discrete element method,简称DEM)与计算流体动力学(computational fluid dynamics,简称CFD)耦合方法,建立了内部不稳定土渗流-侵蚀三维计算模型。考虑细粒含量及相对密实度对内部侵蚀过程的耦合影响,对细观变量:土体孔径分布曲线、颗粒接触数目以及内部传力机制进行分析,以揭示细粒含量与相对密实度的影响的细观机制。研究结果表明:相对密实度的增加降低了侵蚀质量比,且该影响程度与细粒含量相关,细粒含量越高,相对密实度的影响更加明显;内部不稳定土体渗蚀后沿渗流方向可分为上游侵蚀区、中部稳定区及下游侵蚀区。土体渗透性在侵蚀过程中的增长幅度随相对密实度的降低而逐渐增加,随细粒含量的增加而增加;相对密实度对渗蚀过程的影响可归因于三方面:相同水力梯度下渗流流量不同,内部孔隙尺寸分布存在差异,以及细颗粒对土体内部应力传递的贡献不同。研究结果加深了土体几何因素对内部渗蚀过程影响的理解,并为宏观侵蚀本构的建立提供了参考。

间断级配土CFD-DEM内部渗蚀机理分析

渗蚀会导致土体内部颗粒流失,改变土体内部应力的分布。为了探究间断级配土内部随着细颗粒流失内部力链演化过程,利用CFD-DEM(流体力学-离散元方法)模拟了在0.1N的压力下(模拟近表层土体),在i=5、8、11水力梯度作用下颗粒流失过程,在恒定水力梯度i=8作用下0.1、5、10N作用力的作用下颗粒流失过程。结果表明在0.1N作用下,随着水力梯度的增加颗粒渗蚀速率越来越大,但是最终渗蚀量越来越小;在恒定水力梯度i=8作用下随着压力的增加,颗粒渗蚀速率越来越慢;土体内部渗蚀过程基本分为四个阶段;土体内部渗蚀后内部力链分布以粗颗粒-粗颗粒接触为主,文中通过CFD-DEM方法研究了土体内部渗蚀过程中微观机理,为后续渗蚀理论研究提供了基础。

不均匀级配砂土渗蚀过程的细观数值模拟

为了从细观角度研究不均匀级配砂土发生渗蚀的机理,采用离散元(DEM)与计算流体动力学(CFD)耦合模拟的方法,依据Kézdi判别准则,对9组不同颗粒级配的土样渗蚀过程进行数值模拟,分析内部稳定土体与易渗蚀土体在渗蚀过程中细观参数的变化,并探讨初始流速对渗蚀发展过程的影响。研究结果表明:土体的渗蚀过程可以分为起始、发展、过渡、延续4个阶段;土体的颗粒级配直接影响渗蚀的发展,间隙比越大,土体内部越不稳定,容易被侵蚀,当间隙比大于4时,土体渗蚀起始阶段时间短,发展阶段渗蚀速率大,延续阶段渗蚀率仍有稳定增长趋势;在相同间隙比的情况下,细粒质量分数不影响内部稳定土体渗蚀曲线的起始阶段和发展阶段,但细粒越多延续阶段的渗蚀率越大;流体边界条件是重要的外部因素,对于易渗蚀土体,超过临界流速,延续阶段的渗蚀率仍保持增长趋势,而对于内部稳定土体,流速对延续阶段渗蚀率逐渐稳定的趋势无影响。

不连续级配无黏性土渗蚀演变特征研究

渗蚀是指土体内部细颗粒在渗流作用下克服粒间作用力逐步脱离,在粗颗粒骨架孔隙中发生迁移,并可能引起土骨架应力重分布和变形的侵蚀过程。利用自制局部孔压可测的三轴渗蚀装置,研究了细颗粒含量和相对密度对不连续级配无黏性土渗蚀行为的影响,并根据渗流沿程局部水力梯度的时空演变揭示了土体渗蚀发展的演变特征。结果表明:不连续级配无黏性土启动和破坏水力梯度随细颗粒含量/相对密度的增大而增大;相对密度越大侵蚀程度越小,且等向应力下相对密度增大到一定值后,土体会由渗流不稳定状态转变为稳定状态;土体渗蚀启动的内部表现是局部水力梯度的突变及渗流沿程上的不均匀分布。土体渗蚀导致细颗粒流失,土体孔隙比增大,在等向应力条件下,会引发体积收缩现象。

渗蚀过程中孔结构参数对细颗粒流失的影响

利用核磁共振监测了渗蚀过程中孔隙分布的演变规律,并通过自编程序测量了流失细颗粒的粒度分布,最后,基于灰色系统理论讨论了孔结构参数对细颗粒流失的影响.结果表明,初始细颗粒百分数越小的试样渗蚀越严重.水力梯度恒定时,累计细粒流失百分比随时间呈指数增大,渗蚀过程可分为发展、缓蚀和平衡阶段.细粒对渗蚀的敏感性随粒径的减小而增强,而表面形态对细颗粒运移的影响程度随粒径的减小而减小.渗蚀过程中土-水相互作用的强度与细颗粒百分数和水力梯度分别呈正相关关系.总体而言,收缩尺寸分布的连通性和非均质性逐渐增强,复杂程度逐渐减弱,等效开口尺寸逐渐增大.孔结构参数对颗粒流失的影响程度从大到小依次为分形维数、不均匀系数、控制收缩尺寸、最可几孔径.

空调制冷铜管“铜铝渗蚀”开裂失效分析

空调换热器制冷铜管在安装数月后出现开裂泄漏制冷剂现象。对开裂的管材进行化学成分、力学性能、断口及金相检验分析。结果表明,Al-Cu渗蚀导致铜管底壁变薄,装配、运转应力导致开裂。追踪生产过程,发现火焰自动钎焊工艺中存在的Al-Cu渗蚀现象为铜管底壁变薄主要原因,据此提出相应的有效解决方案。

基于CFD-DEM的渗蚀研究:细粒含量与围压的影响

为了揭示渗透水流对砂土渗蚀破坏的规律,利用流固耦合方法(CFD-DEM)建立具有应力边界的长方体砂土试样,进行了不同围压和细粒含量条件下间断级配砂土的渗蚀模拟。结果表明,1)对于高细粒含量试样,围压越高渗蚀颗粒流失量越高,但对于低细粒含量试样,围压对渗蚀的促进作用并不明显。2)当细颗粒含量较高时,细粒流失会引起力链屈曲,促使颗粒间接触力减小,并且高围压下试样力链屈曲量越多,导致细颗粒流失增多。当细粒含量较低时,细颗粒不能填满粗颗粒间的空隙,承担外力的程度也较低。围压的增大主要由粗颗粒承担,细颗粒间接触力和接触数变化不大,因此围压对砂土细颗粒在渗蚀过程中的流失影响也较小。所得结论明晰了渗透水流作用下土体细颗粒在粗颗粒形成的骨架中选择性流失的过程,对控制砂土渗蚀破坏具有借鉴作用。

土体渗蚀对基坑工程的影响研究

富水地层中地下水渗流易造成土体颗粒迁移,引发土体内部侵蚀,进而改变土体内部结构及力学性质,甚至引起土体的渗蚀破坏。基坑工程中的开挖卸载和非稳定渗流场易引发土体内部的渗流侵蚀,但其发生和发展过程难以进行实时观察与精确测量,因而在当前基坑工程的设计与施工过程中较少考虑土体渗蚀的影响,存在一定的安全隐患。本文基于已有的文献研究成果,从基本理论、试验研究与计算模型3个方面揭示土体渗蚀发生和发展机理,再从土体土骨架结构、渗透系数和变形情况3个方面明确渗蚀对基坑工程土体受力与变形的影响,然后通过改造GDS三轴仪开展恒定剪应力渗透侵蚀试验,探究渗蚀对基坑工程土体力学特性的影响情况,最后对基坑工程中的土体渗蚀研究提出建议和展望。

细颗粒含量影响不连续级配砂砾土渗蚀特性的计算流体动力学-离散元耦合模拟

细颗粒含量是决定渗蚀是否发生的几何条件之一,对土体渗蚀特性的影响规律复杂。基于计算流体动力学-离散元(CFD-DEM)耦合方法,开展2种水力梯度、7种细颗粒含量的不连续级配砂砾土渗蚀数值模拟试验,从宏细观角度研究细颗粒含量对渗蚀影响的作用机制。结果表明:土体内部孔隙状态按细颗粒含量由低到高可分为欠填、满填、部分过填(细颗粒将部分粗颗粒分离)以及完全过填(细颗粒完全将粗颗粒分离)4种;欠填状态(细粒含量低于25%)时,细颗粒侵蚀率随细颗粒含量的增大而增大,但在欠填向满填(细含量为30%)转化过程中,侵蚀率随细颗粒含量增大而减小,其根本原因是满填时土样孔隙比达最小,细颗粒迁移难度增大;其后,侵蚀率随细颗粒含量继续增大,土体进入过填状态,但侵蚀类型从渗蚀逐渐转变为反向侵蚀,综合分析土体内部颗粒迁移分布和流失颗粒组成结果,判断细颗粒含量为35%时,属于部分过填状态,其内部稳定性处于过渡状态,本试验条件下表现为渗蚀类型;细颗粒含量为40%时,细颗粒已无法从粗颗粒间孔隙迁移通过,处于完全过填状态,表现为反向侵蚀类型。

Ablation behaviour and mechanical performance of ZrB_(2)-ZrC-SiC modified carbon/carbon composites prepared by vacuum infiltration combined with reactive melt infiltration

The development of advanced aircraft relies on high performance thermal-structural materials,and carbon/carbon com-posites(C/C)composited with ultrahigh-temperature ceramics are ideal candidates.However,the traditional routes of compositing are either inefficient and expensive or lead to a non-uniform distribution of ceramics in the matrix.Compared with the traditional C/C-ZrC-SiC composites prepared by the reactive melt infiltration of ZrSi_(2),C/C-ZrB_(2)-ZrC-SiC composites prepared by the vacuum infiltration of ZrB_(2) combined with reactive melt infiltration have the higher content and more uniform distribution of the introduced ceramic phases.The mass and linear ablation rates of the C/C-ZrB_(2)-ZrC-SiC composites were respectively 68.9%and 29.7%lower than those of C/C-ZrC-SiC composites prepared by reactive melt infiltration.The ablation performance was improved because the volatilization of B_(2)O_(3),removes some of the heat,and the more uniformly distributed ZrO_(2),that helps produce a ZrO2-SiO2 continu-ous protective layer,hinders oxygen infiltration and decreases ablation.

Numerical modelling for predicting corrosion initiation life of reinforced concrete square piles under reverse-seepage pressure

Square piles of reinforced concrete(RC)in marine environments are susceptible to chloride-inducedcorrosion.A novel reverse-seepage technique(RST)is applied to square piles to block the intrusion of chlorides.Thisresearch introduces a computational model designed to predict the lifespan of corrosion initiation in reinforced concretesquare piles when applied reverse-seepage pressure.The model considers the impacts of chloride binding and the tripletime-dependence property among the permeability,the corrected surface chloride concentration,and the diffusioncoefficient.The proposed numerical model is solved using the alternating direction implicit(ADI)approach,and itsaccuracy and reliability are evaluated by contrasting the computational outcomes with the analytical solution andexperimental results.Furthermore,the primary factors contributing to the corrosion of reinforced concrete square pilesare analyzed.The results indicate that applying RST can decrease the chloride penetration depth and prolong the lifespanof corrosion initiation in square piles.The water-cement ratio and reverse seepage pressure are the most influentialfactors.A water pressure of 0.4 MPa can double the life of concrete,and the durable life of concrete with a water-cementratio of 0.3 can reach 100 years.

Effect of surface diffusion alloying on erosion wear property of ZM5 magnesium alloy

Specimens of ZM5 magnesium alloy were dipped into the mixed powder of Al and Zn at （390±5） ℃ for 8 h in argon gas protective environment and subjected to surface diffusion alloying processing （SDAP）. The erosion wear behaviors of ZM5 magnesium alloy before and after SDAP were investigated in two different erosion wear environments： oil and quartz environment; tap water and quartz environment. The surfaces of erosion wear specimens exhibited cutting scratch grooves in the oil and quartz environment. Corrosive attack was weak and cutting wear mechanism was responsible for the mass loss. When the erosion wear medium was changed to tap water and quartz environment, corrosion pits and cracks were obviously observed after erosion wear test. The corrosion from tap water and the scour from quartz intensified mass losses. Compared with the untreated specimens, the application of SDAP improved the erosion wear resistance in the same environment.

Cyclic oxidation and hot corrosion of Al_2O_3 or Y_2O_3-dispersed low-temperature chromizing coating

Chromium coatings with and without Al_2O_3 or Y_2O_3 particles were prepared by chromizing the as-deposited Ni-film with and without Al_2O_3 or Y_2O_3 particles using a conventional pack-cementation method at 800 ℃. The cyclic oxidation at 800 ℃ and hot corrosion in molten 75% Na2SO4＋25% NaC1 at 800 ℃ of the three different chromizing coatings were investigated. The effects of Al_2O_3 or Y_2O_3 on the cyclic oxidation and hot corrosion behavior of the chromizing coatings were discussed. Microstructure results show that the codeposited Al_2O_3 or Y_2O_3 particles significantly retard the grain growth of the chromizing coating, which increases the cyclic oxidation and hot corrosion resistance of the chromizing coatings, due to the more rapid formation of purer and denser chromia scnle

西营河专用输水渠高地下水渠段衬砌技术的应用与探讨

西营河专用输水渠设计过程中,地下水位高、渠床含水量高段预制梯弧明渠设计是该工程的难点。分析渠道破坏成因:冻胀冻融破坏、地下水渗蚀破坏、地下水的浸湿软化渠床、盐胀破坏、渠道防渗材料自身的冻融破坏、冬季运行对渠道的影响和破坏。根据该渠段地理环境、排水条件,采用如下设计:渠道采用梯弧形断面形式;渠道在条件允许情况下抬高渠底;渠床全断面设置砂砾石层;砂砾石层底部设置通长塑料盲沟把地下水排向石羊河;预制板下设防渗土工布隔断渠内外水路;每5m设1道现浇横隔梁;渠道弧底采用现浇细粒混凝土砌卵石,坡面混凝土预制板采用干砌形式,以适应渠坡冻胀变形。

Microstructure and properties of alloying coating on AZ31B magnesium alloy

Vacuum thermal diffusion technique was applied to preparing alloying coating on AZ31 B magnesium alloy. The microstructure and phase composition of the coatings prepared at different holding time were investigated in detail using optical microscopy（OM）, scanning electron microscopy（SEM）, energy dispersive spectrometer（EDS） and X-ray diffraction（XRD）, and so on. The microhardness tester and electrochemical workstation（PS-168a） were used to measure the microhardness and corrosion resistance of the alloying coating. The results showed that the alloying coatings gradually generated with the extension of holding time under constant temperature. And the obvious bonding interface between the coating and substrate was observed, and the bonding interface was changed from smooth to zigzag. EDS and XRD analyses showed that the microstructure of alloying coating mainly consisted of eutectic α-Mg phase and continuous network β-Al（12）Mg（17） phase. The average microhardness of the coatings increased by 113% in comparison to the substrate, and the self-corrosion potential increased from-1.389 to-1.268 V at the same time.

Microstructure and properties of Al-coating on AZ31 magnesium alloy prepared by pack-cementation

Al-containing coatings were prepared on AZ31 magnesium alloy by pack-cementation technology.X-ray diffraction(XRD),backscattered electron imaging(BSEI)and energy dispersive spectroscopy(EDS)were jointly employed to characterize the phases,microstructure and composition of the coated samples.The results show that the feedstock composition has a significant impact on the phases,microstructure and thickness of the coatings.For the sample with AlCl3 powder as the activator,the coating is very thick and composed of gradient phases and structures from surface to inside,including small amount ofb-Mg2Al3,coarse eutectic-like structure ofγ-Mg17Al12+δ-Mg,and fineγ-Mg17Al12 precipitations.In contrast,for the sample with AlCl3 and pure Al composite powders as the activator,the coating is relatively thin and contains a thin Al2O3 layer and a small amount of fineγ-Mg17Al12 precipitates.For the pack-cementation aluminizing that is not protected by high-vacuum or inert gas,the addition of pure Al powders can easily introduce the Al2O3 layer into the coating to prevent active Al ions further penetrating into the magnesium matrix,resulting in the thin Al-containing coating.The microhardness and corrosion behavior of the two kinds of aluminized coatings were also studied and discussed.

Relationships Between Permeability and Erodibility of Cultivated Acrisols and Cambisols in Subtropical China

The relationships between soil erodibility factor （K） and soil saturated permeability （gfs） for cultivated Acrisols derived from Quaternary red clay and Cambisols derived from red sandstone were studied and quantified using a rainfall simulator and Guelph permeameter in a hilly area of subtropical China. A negative correlation existed between Kfs of the topsoil （0-5 cm） and K. The empirical expression K ≈ α × Kfs^-b＋c, where a, b and c are the structural coefficients related to soil properties, such as soil type, soil parent material, organic matter, pH and mechanical composition, best described the relationship between soil saturated permeability and soil erodibility.

Microstructure and hot corrosion resistance of Si-Al-Y coated TiAl alloy

In order to obtain a high-performance surface on TiAl alloy that can meet the requirements in hot corrosion environment,Si-Al-Y coatings were fabricated by pack cementation process at 1050℃for 4 h.Corrosion behaviors of the TiAl alloy with and without Si-Al-Y coatings are compared to illustrate the factors and corresponding mechanism in molten salt environment of 25 wt%K2SO4 and 75 wt%Na2SO4 at 900°C.The obtained Si-Al-Y coating was mainly composed of a TiSi2 outer layer,a(Ti,X)5Si4 and(Ti,X)5Si3(X represents Nb or Cr element)middle layer,a TiAl2 inner layer and a Al-rich inter-diffusion zone.The inter-phase selective corrosion containing corrosion pits extending alongα2 phase from lamellar interfaces in hot corrosion tested TiAl alloy was observed.However,by being coated with Si-Al-Y coating,the hot corrosion performance of TiAl alloy was improved remarkably.