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1.
Hydrogen attack (HA) is the development of grain-boundary porosity by cavities filled with high-pressure methane that originates from the reaction of carbides with hydrogen at high temperatures. The cavities grow by grain-boundary diffusion and by creep of the adjacent grain material till they coalesce with neighbouring cavities to form a microcrack. Earlier work on HA has focussed on unit cells containing a single cavity, using average cavitation properties. Here, non-uniform cavitation properties on the grain-size scale are assumed in a polycrystalline aggregate, and unit cell analyses are performed to investigate the influence of the adjacent grains on the development of the grain-boundary HA. The numerical results are explained in terms of two simplified models which highlight the key parameters governing the grain deformation-grain boundary cavitation interaction process. 相似文献
2.
初生空化与液体抗拉强度的关系 总被引:3,自引:1,他引:2
杨志明 《水动力学研究与进展(A辑)》1990,5(4):27-34
自1983年开始以液体的抗拉强度作为其空化临界压强研究初生空化的比尺效应以来,该方法已得到进一步的应用和完善。本文就研究中遇到的一些问题,如关于空化灵敏度仪的校正,“零抗拉强度”的概念,试验设备中液体抗拉强度的动态变化以及在空化试验中使用该方法的技术和意义等作详细的阐述。 相似文献
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茂名乙烯乙二醇装置关键设备碳酸盐泵 ,1 996年 8月投料试车后 ,泵及进出口管线存在异常噪声和振动 ,管线的振动范围入口端约为 1 8m ,出口端约为 8m ,管道最大振幅可达 2 5~ 3 5mm ,噪音最高达到 1 1 6db ,严重影响设备的安全运行。从装置设计、实际操作状况、试验入手分析 ,找出了产生振动的直接原因是泵汽蚀。根据装置现有实际情况 ,采取在泵入口设置增能器方法 ,来提高泵的吸入压头 ,进而提高泵装置汽蚀余量。安装增能器后 ,管线振动基本消除 ,噪音也降到安全环保规定范围内 ,使碳酸盐泵的运行周期由 3个月延长至 1年以上 相似文献
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Computational cavitation flows at inception and light stages on an axial-flow pump blade and in a cage-guided control valve 总被引:1,自引:1,他引:0
Cavitation flows induced around an axial-flow pump blade and inside a high pressure cage-type valve are simu-lated by a two-dimensional unsteady Navier-Stokes analysis with the simplest treatment of bubble dynamics.Thefluid is assumed as a continuum of homogeneous dispersed mixture of water and vapor nuclei.The analysis isaimed to capture transient stages with high amplitude pressure change during the birth and collapse of the bubbleespecially at the stage of cavitation inception.By the pump blade analysis,in which the field pressure is moderate,cavitation number of the inception and locations of developed cavitation are found to agree with experimental re-suits in a wide flow range between high incidence and negative incidence.In the valve flow analysis,in which thewater pressure of 5MPa is reduced to 2MPa,pressure change responding to the bubble collapse between the vaporpressure lower than 1 KPa and the extreme pressure of higher than 10~4 KPa is captured through a stable computa-tion.Location of the inception bubble and pressure force to the valve plug is found agree well with the respectiveexperimental features. 相似文献
7.
In Part I of the failure analysis on abnormal wall thinning of heat-transfer titanium tubes used in condensers in nuclear power plant, we analyzed the causes and mechanisms of abnormal thinning that commonly happened at the contact part between the tubes and the support plates. This kind of failure was the mainstream failure type in our case and the main causes were found to be eccentric contact wear and three-body contact wear rooted in processing defect of internal borings, corrosion products deposit and sagging, and foreign particles. However, there were still some individual failure tubes with different failure sites and modes and were located under the bypass pipes at the shoulder of the tube tower instead of in its lower part, obviously telling another failure story. In Part II of the failure analysis, material analysis, metallographic examination, mechanical performance tests, macro- and microstructure analysis and composition analysis were conducted. The failure causes were found to be erosion and cavitation corrosion and the synergetic effect of them. Finally, corresponding countermeasures were suggested. 相似文献
8.
This study investigates two algorithms proposed to solve a new cavitation model. This new cavitation model is based on a compressible Reynolds equation in which the density–pressure relation is obtained from a barotropic–isentropic assumption. It can be viewed as an approximation of the Jakobson–Floberg–Olsson/Elrod Adams cavitation model. Two algorithms are proposed to solve it. The first one is explicit and needs an important number of nodes. The second is implicit and can be used for steady-state and unsteady problems. Its implementation is easy and needs only minor modifications for a computer code in which cavitation is ignored. It can also be used to compute the solution of the usual J.F.O./E.A. model. Faster convergence is obtained using a relaxation parameter. 相似文献
9.
Qi-Ning Song Nan Xu Yao Tong Chen-Ming Huang Shou-Yu Sun Chen-Bo Xu Ye-Feng Bao Yong-Feng Jiang Yan-Xin Qiao Zhi-Yuan Zhu Zheng-Bin Wang 《金属学报(英文版)》2019,32(12):1470
The effect of sulphide(Na_2S) concentration(SC) on the corrosion and cavitation erosion behaviours of a cast nickel aluminium bronze(NAB) in 3.5% NaCl solution is investigated in this study.The results show that when the SC exceeds 50 ppm,the hydrogen evolution reaction dominates the cathodic process,and a limiting current region appears in the anodic branch of the polarisation curve due to the formation of a copper sulphide film,which is a diffusion-controlled process.After longterm immersion,the increased mass loss rate of NAB with the sulphide additions of 20 and 50 ppm is attributed to the less protective films,which contains a mixture of copper oxides and sulphides.Moreover,NAB undergoes severe localised corrosion(selective phase corrosion,SPC) at the β' phases and eutectoid microstructure α+κ_Ⅲ.By comparison,NAB undergoes general corrosion and a copper sulphide film is formed in 100 and 200 ppm sulphide solutions.Cavitation erosion greatly increases the corrosion rate of NAB in all solutions and causes a negative potential shift in 3.5% NaCl solution due to the film destruction.However,a positive potential shift occurs in the solutions with SC higher than 50 ppm due to the accelerated mass transfer of the cathodic process.The cavitation erosion mass loss rate of NAB increases with the increase of SC.The occurrence of severe SPC decreases the phase boundary cohesion and causes brittle fracture under the cavitation impact.The corrosion-enhanced erosion is the most predominant factor for the cavitation erosion damage when the SC exceeds 50 ppm. 相似文献
10.
Cavitation phenomenon is observed during deformation in many semicrystalline polymers above their glass transition temperature. Numerous voids (cavities) both nanometer and micrometer size are formed inside amorphous phase between lamellae during deformation of a polymer. The cavitation is observed only in tension, never during compression or shearing. Most often used methods of voids detection are: microscopies (SEM, TEM, AFM and light microscopy), small angle X-ray scattering and measurements of density. Usually the voids are detected close to yielding or at yielding, strongly suggesting that yielding is often caused by cavitation. However, there is a competition between two processes: breaking of amorphous phase leading to cavitation and plastic deformation of lamellar crystals. Which process occurs first depends on the relation between compliances of those two phases. If the crystals are weak and defected their deformation occurs (mostly by chain slips mechanism) without cavitation. If the crystals in a polymer are thick and more perfect then the barrier for their deformation, represented by shear yielding stress, is increased and the cavitation sets in first and yielding is determined by the stress needed for cavitation. Further deformation involves deformation of crystals due to rapid local change of stress around voids. The influence of different morphological factors: crystal thickness, crystallinity degree, arrangement of crystalline elements (e.g. in spherulites), morphology of amorphous phase (free volume, entanglements, tie molecules) were analyzed. Experimental factors, such as temperature of deformation and rate of deformation influence remarkably the formation of cavities. Cavitation is generated at points where a high local triaxial state of stress is developed. Triaxiality of stress can be amplified by a notch, even very mild notch with large radius of curvature stimulates generation of cavities. Evolution of nano-cavities into micro-cavities and change of their shapes with increasing deformation were evidenced by SAXS. Initially voids are oriented perpendicularly to deformation direction, however, with increasing elongation they become oriented along deformation direction. Stress whitening is visual sign of cavitation and is caused be light scattering either by microvoids or by assemblies of nanovoids. 相似文献